HUMAN HEALTH

THE IMPACTS OF CLIMATE CHANGE ON

IN THE UNITED STATES

A Scientific Assessment

U.S. Global Change Research Program

HUMAN HEALTH

THE IMPACTS OF CLIMATE CHANGE ON

IN THE UNITED STATES

A Scientific Assessment

U.S. Global Change Research Program
ii

Recommended Citation: USGCRP, 2016: The Impacts of Climate Change on Human Health in the United States: A Scientific 

Assessment. Crimmins, A., J. Balbus, J.L. Gamble, C.B. Beard, J.E. Bell, D. Dodgen, R.J. Eisen, N. Fann, M.D. Hawkins, S.C. Herring, 

L. Jantarasami, D.M. Mills, S. Saha, M.C. Sarofim, J. Trtanj, and L. Ziska, Eds. U.S. Global Change Research Program, Washington, DC, 

312 pp. http://dx.doi.org/10.7930/J0R49NQX

To read the full report, go to: health2016.globalchange.gov

This report was produced by the U.S Global Change Research Program. 

1800 G Street, NW, Suite 9100 

Washington, D.C. 20006 USA

www.globalchange.gov

First Published April 2016

ISBN: 978-0-16-093241-0

This report is in the public domain. Some materials in the report are copyrighted and permission was granted for their 

publication in this report. For subsequent uses that include such copyrighted materials, permission for reproduction 

must be sought from the copyright holder. In all cases, credit must be given for copyrighted materials. All other 

materials are free to use with credit to this report.
iii

April 2016

Dear Colleagues:

On behalf of the National Science and Technology Council and the U.S. Global Change Research Program, I am pleased 

to share this report, The Impacts of Climate Change on Human Health in the United States: A Scientific Assessment. It advances scientific understanding of the impacts of climate change on public health, highlights social and environmental disparities that make some communities particularly vulnerable to climate change, and confirms that climate change is a significant threat to the health of all Americans.

This report was developed by over 100 experts from across the Nation representing eight Federal agencies. I want to thank 

in particular the efforts of the U.S. Environmental Protection Agency (EPA), the U.S. Department of Health and Human 

Services (HHS), and the National Oceanic and Atmospheric Administration (NOAA) for leading in the development of 

this report. It was called for under the President’s Climate Action Plan and is a major contribution to the sustained National Climate Assessment process. The report was informed by input gathered in listening sessions and scientific and technical 

information contributed through open solicitations. It underwent rigorous reviews by the public and by scientific experts 

inside and outside of the government, including a special committee of the National Academies of Sciences, Engineering, 

and Medicine. 

I applaud the authors, reviewers, and staff who have developed this scientific assessment. Their dedication over the past 

three years has been remarkable and their work has advanced our knowledge of how human health is impacted by climate 

change now and in the future.

Combating the health threats from climate change is a top priority for President Obama and a key driver of his Climate 

Action Plan. I strongly and respectfully urge decision makers across the Nation to use the scientific information contained 

within to take action and protect the health of current and future generations. 

Dr. John P. Holdren 

Assistant to the President for Science and Technology 

Director, Office of Science and Technology Policy 

Executive Office of the President 

 

 
iv

About the USGCRP Climate and Health Assessment

The U.S. Global Change Research Program (USGCRP) Climate and Health 

Assessment has been developed to enhance understanding and inform decisions 

about the growing threat of climate change to the health and well-being of residents 

of the United States. This scientific assessment is part of the ongoing efforts of 

USGCRP’s sustained National Climate Assessment (NCA) process and was called 

for under the President’s Climate Action Plan.1 USGCRP agencies identified human 

health impacts as a high-priority topic for scientific assessment. 

This assessment was developed by a team of more than 100 experts from 8 U.S. 

Federal agencies (including employees, contractors, and affiliates) to inform public 

health officials, urban and disaster response planners, decision makers, and 

other stakeholders within and outside of government who are interested in better 

understanding the risks climate change presents to human health. 

The USGCRP Climate and Health Assessment draws from a large body of scientific 

peer-reviewed research and other publicly available sources; all sources meet the 

standards of the Information Quality Act (IQA). The report was extensively reviewed 

by the public and experts, including a committee of the National Academies of 

Sciences, Engineering, and Medicine,2 the 13 Federal agencies of the U.S. Global 

Change Research Program, and the Federal Committee on Environment, Natural 

Resources, and Sustainability (CENRS).

About the National Climate Assessment

The Third National Climate Assessment (2014 NCA)3 assessed the science of 

climate change and its impacts across the United States, now and throughout this 

century. The report documents climate change related impacts and responses for 

various sectors and regions, with the goal of better informing public and private 

decision making at all levels. The 2014 NCA included a chapter on human health 

impacts,4 which formed the foundation for the development of this assessment.

THE IMPACTS OF CLIMATE CHANGE ON 

HUMAN HEALTH IN THE UNITED STATES

A Scientific Assessment
v

TABLE OF CONTENTS

About this Report...............................................................................................................................vi

Guide to the Report........................................................................................................................... ix

List of Contributors ..........................................................................................................................xii

CHAPTERS

Executive Summary.....................................................................................................1

1. Introduction: Climate Change and Human Health....................................................25

2. Temperature-Related Death and Illness...................................................................43

3. Air Quality Impacts...................................................................................................69

4. Impacts of Extreme Events on Human Health...........................................................99

5. Vector-Borne Diseases...........................................................................................129

6. Climate Impacts on Water-Related Illness.............................................................157

7. Food Safety, Nutrition, and Distribution..................................................................189

8. Mental Health and Well-Being................................................................................217

9. Populations of Concern ..........................................................................................247

Appendix 1: Technical Support Document: Modeling Future Climate Impacts on Human Health ..287

Appendix 2: Process for Literature Review ....................................................................................301

Appendix 3: Report Requirements, Development Process, Review, and Approval .........................303

Appendix 4: Documenting Uncertainty: Confidence and Likelihood................................................305 

Appendix 5: Glossary and Acronyms..............................................................................................307
U.S. Global Change Research Program vi Impacts of Climate Change on Human Health in the United States

ABOUT THIS REPORT

Climate change threatens human health and well-being in the United States. The U.S. Global Change Research 

Program (USGCRP) Climate and Health Assessment has been developed to enhance understanding and inform decisions about this growing threat. This scientific assessment, called for under the President’s Climate Action Plan,1

is a major report of the sustained National Climate Assessment (NCA) process. The report responds to the 1990 

Congressional mandate5 to assist the Nation in understanding, assessing, predicting, and responding to human-induced and natural processes of global change. The agencies of the USGCRP identified human health impacts as a 

high-priority topic for scientific assessment.

The purpose of this assessment is to provide a comprehensive, evidence-based, and, where possible, quantitative 

estimation of observed and projected climate change related health impacts in the United States. The USGCRP 

Climate and Health Assessment has been developed to inform public health officials, urban and disaster response 

planners, decision makers, and other stakeholders within and outside of government who are interested in better 

understanding the risks climate change presents to human health. 

The authors of this assessment have compiled and assessed current research on human health impacts of climate 

change and summarized the current state of the science for a number of key topics. This assessment provides a 

comprehensive update to the most recent detailed technical assessment for the health impacts of climate change, 

the 2008 Synthesis and Assessment Product 4.6 (SAP 4.6), Analyses of the Effects of Global Change on Human 

Health and Welfare and Human Systems.

6

 It also updates and builds upon the health chapter of the 2014 NCA.4

While Chapter 1: Introduction: Climate Change and Human Health includes a brief overview of observed and 

projected climate change impacts in the United States, a detailed assessment of climate science is outside the 

scope of this report. This report relies on the 2014 NCA3 and other peer-reviewed scientific assessments of climate 

change and climate scenarios as the basis for describing health impacts.

Each chapter of this assessment summarizes scientific literature on specific health outcomes or climate change related exposures that are important to health. The chapters emphasize research published between 2007 and 2015 

that quantifies either observed or future health impacts associated with climate change, identifies risk factors for 

health impacts, and recognizes populations that are at greater risk. In addition, four chapters (Temperature-Related Death and Illness, Air Quality Impacts, Vector-Borne Disease, and Water-Related Illness) highlight recent 

modeling analyses that project national-scale impacts in these areas. 

The geographic focus of this assessment is the United States. Studies at the regional level within the United States, 

analyses or observations in other countries where the findings have implications for potential U.S. impacts, and 

studies of global linkages and implications are also considered where relevant. For example, global studies are 

considered for certain topics where there is a lack of consistent, long-term historical monitoring in the United 

States. In some instances it is more appropriate to consider regional studies, such as where risk and impacts vary 

across the Nation.

While climate change is observed and measured on long-term time scales (30 years or more), decision frameworks for public health officials and regional planners are often based on much shorter time scales, determined by 

epidemiological, political, or budgeting factors. This assessment focuses on observed and current impacts as well 

as impacts projected in 2030, 2050, and 2100. 

The focus of this assessment is on the health impacts of climate change. The assessment provides timely and 

relevant information, but makes no policy recommendations. It is beyond the scope of this report to assess the 

peer-reviewed literature on climate change mitigation, adaptation, or economic valuation or on health co-bene-
U.S. Global Change Research Program vii Impacts of Climate Change on Human Health in the United States

fits that may be associated with climate mitigation, adaptation, and resilience strategies. The report does assess 

scientific literature describing the role of adaptive capacity in creating, moderating, or exacerbating vulnerability 

to health impacts where appropriate. The report also cites analyses that include modeling parameters that make 

certain assumptions about emissions pathways or adaptive capacity in order to project climate impacts on human 

health. This scientific assessment of impacts helps build the integrated knowledge base needed to understand, 

predict, and respond to these changes, and it may help inform mitigation or adaptation decisions and other strategies in the public health arena. 

Climate and health impacts do not occur in isolation, and an individual or community could face multiple threats 

at the same time, at different stages in one’s life, or accumulating over the course of one’s life. Though important 

to consider as part of a comprehensive assessment of changes in risks, many types of cumulative, compounding, or secondary impacts are beyond the scope of this report. Though this assessment does not focus on health 

research needs or gaps, brief insights gained on research needs while conducting this assessment can be found at 

the end of each chapter to help inform research decisions.

The first chapter of this assessment provides background information on observations and projections of climate 

change in the United States and the ways in which climate change, acting in combination with other factors 

and stressors, influences human health. It also provides an overview of the approaches and methods used in 

the quantitative projections of health impacts of climate change conducted for this assessment. The next seven 

chapters focus on specific climate-related health impacts and exposures: Temperature-Related Death and Illness; 

Air Quality Impacts; Extreme Events; Vector-Borne Diseases; Water-Related Illness; Food Safety, Nutrition, and 

Distribution; and Mental Health and Well-Being. A final chapter on Populations of Concern identifies factors that 

create or exacerbate the vulnerability of certain population groups to health impacts from climate change. That 

chapter also integrates information from the topical health impact chapters to identify specific groups of people in 

the United States who may face greater health risks associated with climate change.

The Sustained National Climate Assessment 

The Climate and Health Assessment has been developed as part of the U.S. Global Change Research Program’s 

(USGCRP’s) sustained National Climate Assessment (NCA) process. This process facilitates continuous and transparent participation of scientists and stakeholders across regions and sectors, enabling new information and 

insights to be synthesized as they emerge. The Climate and Health Assessment provides a more comprehensive 

assessment of the impacts of climate change on human health, a topic identified as a priority for assessment by 

USGCRP and its Interagency Crosscutting Group on Climate Change and Human Health (CCHHG) and featured in 

the President’s Climate Action Plan.1

Report Sources

The assessment draws from a large body of scientific, peer-reviewed research and other 

publicly available resources. Author teams carefully reviewed these 

sources to ensure a reliable assessment of the state of scientific 

understanding. Each source of information was determined 

to meet the four parts of the Information Quality Act (IQA): 

utility, transparency and traceability, objectivity, and integrity and security (see Appendix 2: Process for Literature 

Review). More information on the process each chapter 

author team used to review, assess, and determine whether 

a literature source should be cited can be found in the Supporting Evidence section of each chapter. Report authors made use of 

the findings of the 2014 NCA, peer-reviewed literature and scientific assessments, and government statistics (such as population census reports). Authors 

also updated the literature search7 conducted by the National Institute of Environmental 

Health Sciences (NIEHS) as technical input to the Human Health chapter of the 2014 NCA.
U.S. Global Change Research Program viii Impacts of Climate Change on Human Health in the United States

Overarching Perspectives

Five overarching perspectives, derived from decades of 

observations, analysis, and experience, have helped to 

shape this report: 1) climate change is happening in the 

context of other ongoing changes across the United States 

and around the globe; 2) there are complex linkages and 

important non-climate stressors that affect individual and 

community health; 3) many of the health threats described 

in this report do not occur in isolation but may be cumulative, compounding, or secondary; 4) climate change impacts 

can either be amplified or reduced by individual, community, and societal decisions; and 5) climate change related 

impacts, vulnerabilities, and opportunities in the United 

States are linked to impacts and changes outside the United 

States, and vice versa. These overarching perspectives are 

briefly discussed below.

Global Change Context

This assessment follows the model of the 2014 NCA, which 

recognized that climate change is one of a number of global 

changes affecting society, the environment, the economy, 

and public health.3 While changes in demographics, socioeconomic factors, and trends in health status are discussed 

in Chapter 1: Introduction: Climate Change and Human 

Health, discussion of other global changes, such as land-use 

change, air and water pollution, and rising consumption of 

resources by a growing and wealthier global population, are 

limited in this assessment.

Complex Linkages and the Role of Non-Climate 

Stressors 

Many factors may exacerbate or moderate the impact of climate change on human health. For example, a population’s 

vulnerability 1) may be affected by direct climate changes 

or by non-climate factors (such as changes in population, 

economic development, education, infrastructure, behavior, 

technology, and ecosystems); 2) may differ across regions 

and in urban, rural, coastal, and other communities; and 3) 

may be influenced by individual vulnerability factors such 

as age, socioeconomic status, and existing physical and/or 

mental illness or disability. These considerations are summarized in Chapter 1: Introduction: Climate Change and Human 

Health and Chapter 9: Populations of Concern. There are 

limited studies that quantify how climate impacts interact 

with the factors listed above or how these interactions can 

lead to many other compounding, secondary, or indirect 

health effects. However, where possible, this assessment 

identifies key environmental, institutional, social, and behavioral influences on health impacts. 

Cumulative, Compounding, or Secondary Impacts

Climate and health impacts do not occur in isolation and an 

individual or community could face multiple threats at the 

same time, at different stages in one’s life, or accumulating 

over the course of one’s life. Some of these impacts, such as 

the combination of high ozone levels on hot days (see Ch. 

3: Air Quality Impacts) or cascading effects during extreme 

events (see Ch. 4: Extreme Events), have clear links to one 

another. In other cases, people may be threatened simultaneously by seemingly unconnected risks, such as increased 

exposure to Lyme disease and extreme heat. These impacts 

can also be compounded by secondary or tertiary impacts, 

such as climate change impacts on access to or disruption of 

healthcare services, damages to infrastructure, or effects on 

the economy. 

Societal Choices and Adaptive Behavior

Environmental, cultural, and socioeconomic systems are 

tightly coupled, and as a result, climate change impacts can 

either be amplified or reduced by cultural and socioeconomic decisions.3

 Adaptive capacity ranges from an individual’s 

ability to acclimatize to different meteorological conditions 

to a community’s ability to prepare for and recover from 

damage, injuries, and lives lost due to extreme weather 

events. Awareness and communication of health threats to 

the public health community, practitioners, and the public is an important factor in the incidence, diagnosis, and 

treatment of climate-related health outcomes. Recognition 

of these interactions, together with recognition of multiple 

sources of vulnerability, helps identify what information 

decision makers need as they manage risks.

International Context

Climate change is a global phenomenon; the causes and the 

impacts involve energy-use, economic, and risk-management decisions across the globe.3

 Impacts, vulnerabilities, 

and opportunities in the United States are related in complex and interactive ways with changes outside the United 

States, and vice versa. The health of Americans is affected 

by climate changes and health impacts experienced in other 

parts of the world. 
U.S. Global Change Research Program Impacts of Climate Change on Human Health in the United States ix

The following describes the format of the report and the 

structure of each chapter.

Executive Summary

The Executive Summary describes the impacts of climate 

change on the health of the American public. It summarizes 

the overall findings and represents each chapter with a brief 

overview, the Key Findings, and a figure from the chapter.

Chapters

Key Findings and Traceable Accounts

Topical chapters include Key Findings, which are based on 

the authors’ consensus expert judgment of the synthesis of 

the assessed literature. The Key Findings include confidence 

and likelihood language as appropriate (see “Documenting 

Uncertainty” below and Appendix 4: Documenting 

Uncertainty). 

Each Key Finding is accompanied by a Traceable Account 

which documents the process and rationale the authors 

used in reaching these conclusions and provides additional information on sources of uncertainty. The Traceable 

Accounts can be found in the Supporting Evidence section of 

each chapter. 

Chapter Text

Each chapter assesses the state of the science in terms of 

observed and projected impacts of climate change on human health in the United States, describes the link between 

climate change and health outcomes, and summarizes the 

authors’ assessment of risks to public health. Both positive 

and negative impacts on health are reported as supported 

by the scientific literature. Where appropriate and supported by the literature, authors include descriptions of 

critical non-climate stressors and other environmental and 

institutional context; social, behavioral, and adaptive factors 

that could increase or moderate impacts; and underlying 

trends in health that affect vulnerability (see “Populations 

of Concern” below). While the report is designed to inform decisions about climate change, it does not include 

an assessment of literature on climate change mitigation, 

adaptation, or economic valuation, nor does it include policy 

recommendations. 

GUIDE TO THE REPORT
U.S. Global Change Research Program x Impacts of Climate Change on Human Health in the United States

Figure 1: The center boxes include selected examples of climate drivers, the primary pathways by which humans are exposed 

to health threats from those drivers, and the key health outcomes that may result from exposure. The left gray box indicates 

examples of the larger environmental and institutional context that can affect a person’s or community’s vulnerability to health 

impacts of climate change. The right gray box indicates the social and behavioral context that also affects a person’s vulnerability 

to health impacts of climate change. This path includes factors such as race, gender, and age, as well as socioeconomic factors 

like income and education or behavioral factors like individual decision making. The examples listed in these two gray boxes 

can increase or reduce vulnerability by influencing the exposure pathway (changes in exposure) or health outcomes (changes in 

sensitivity or adaptive capacity). The diagram shows that climate change can affect health outcomes directly and by influencing 

the environmental, institutional, social, and behavioral contexts of health.

Understanding the Exposure Pathway Diagrams

Exposure Pathway Diagram

Each topical chapter includes an exposure pathway diagram 

(see Figure 1). These conceptual diagrams illustrate a key 

example by which climate change affects health within 

the area of interest of that chapter. These diagrams are 

not meant to be comprehensive representations of all the 

factors that affect human health. Rather, they summarize 

the key connections between climate drivers and health 

outcomes while recognizing that these pathways exist within the context of other factors that positively or negatively 

influence health outcomes. 

The exposure pathway diagram in Chapter 1: Introduction: 

Climate Change and Human Health is a high-level overview of the main routes by which climate change affects 

health, summarizing the linkages described in the following 

chapters. Because the exposure pathway diagrams rely on 

examples from a specific health topic area, a diagram is not 

included in Chapter 9: Populations of Concern, as that chapter describes crosscutting issues relevant to all health topics.

Research Highlights

Four chapters include research highlights: Temperature-Related Death and Illness, Air Quality Impacts, Vector-Borne 

Disease, and Water-Related Illness. Six research highlight 

sections across these four chapters describe the findings of 

recently published quantitative analyses of projected impacts 

conducted for inclusion in this report. Each analysis is summarized with a brief description of the study’s 1) Importance, 

2) Objectives, 3) Methods, 4) Results, and 5) Conclusions. The 

analyses are all published in external peer-reviewed sources, 

and the full description of modeling methods and findings 

can be found in those citations. While authors of these analyses were provided with modeling guidance and conferred on 

opportunities for consistency in approach, no comprehensive 

set of assumptions, timeframes, or scenarios were applied 

across modeling analyses. Therefore, these six studies do not 

represent an integrated modeling assessment. The findings of 

these analyses are considered as part of the overall assessment of the full body of literature when developing the chapter Key Findings. For more information on modeling methods 

see Appendix 1: Technical Support Document.
U.S. Global Change Research Program xi Impacts of Climate Change on Human Health in the United States

Populations of Concern

One of the main goals of this assessment was to identify populations that are particularly vulnerable to specific health impacts associated with climate change. Each chapter includes 

discussion of this topic in addition to the full chapter devoted 

to populations of concern. In these discussions, the authors 

identify segments of the general population that the peer-reviewed literature has identified as being at increased risk for 

health-related climate impacts, now or in the future. 

Emerging Issues

The Emerging Issues sections briefly describe emerging areas 

of research including areas of potential future concern; health 

impacts not currently prevalent or severe in the United States 

but with potential to become a health concern; or areas 

where the links between climate change and a human health 

outcome are in early stages of study and for which a more 

comprehensive synthesis is outside the scope of this report. 

Research Needs 

While the goal of this assessment is to highlight the current state of the science on climate impacts on health, 

research needs identified through the development of this 

assessment are briefly summarized in each chapter. These 

research needs could inform research beyond the current 

state of the science or outside the scope of this report. 

Supporting Evidence

The Traceable Accounts supporting each Key Finding are provided at the end of each chapter in the Supporting Evidence 

section.

Documenting Uncertainty: 

Confidence and Likelihood

Two kinds of language are used when describing the 

uncertainty associated with specific statements in this report: 

confidence language and likelihood language (see table below 

and Appendix 4: Documenting Uncertainty). Confidence in 

the validity of a finding is based on the type, amount, quality, 

strength, and consistency of evidence and the degree of 

expert agreement on the finding. Confidence is expressed 

qualitatively and ranges from low confidence (inconclusive 

evidence or disagreement among experts) to very high 

confidence (strong evidence and high consensus). 

Likelihood language describes the likelihood of occurrence 

based on measures of uncertainty expressed probabilistically 

(in other words, based on statistical analysis of observations 

or model results or based on expert judgment). Likelihood, or 

the probability of an impact, is a term that allows a quantitative estimate of uncertainty to be associated with projections. 

Thus, likelihood statements have a specific probability associated with them, ranging from very unlikely (less than or equal 

to a 1 in 10 chance of the outcome occurring) to very likely 

(greater than or equal to a 9 in 10 chance).

Likelihood and Confidence Evaluation

All Key Findings include a description of confidence. Where 

it is considered scientifically justified to report the likelihood 

of particular impacts within the range of possible outcomes, Key Findings also include a likelihood designation. 

Confidence and likelihood levels are based on the expert 

assessment and consensus of the chapter author teams. The 

author teams determined the appropriate level of confidence or likelihood by assessing the available literature, 

determining the quality and quantity of available evidence, 

and evaluating the level of agreement across different studies. For specific descriptions of the process by which each 

chapter author team came to consensus on the Key Findings 

and assessment of confidence and likelihood, see the Traceable Account section for each chapter. More information is 

also available in Appendix 1: Technical Support Document 

and Appendix 4: Documenting Uncertainty.

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few sources, limited consistency, models 

incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts

Likelihood

Very Likely

≥ 9 in 10

Likely

≥ 2 in 3

As Likely As Not

≈ 1 in 2

Unlikely

≤ 1 in 3

Very Unlikely

≤ 1 in 10
U.S. Global Change Research Program xii Impacts of Climate Change on Human Health in the United States

Report Steering Committee 

Lead Coordinator

Allison Crimmins, U.S. Environmental Protection Agency

Committee Members

John Balbus, National Institutes of Health

Charles B. Beard, Centers for Disease Control and Prevention

Rona Birnbaum, U.S. Environmental Protection Agency

Neal Fann, U.S. Environmental Protection Agency

Janet L. Gamble, U.S. Environmental Protection Agency

Jada Garofalo, Centers for Disease Control and Prevention

Vito Ilacqua, U.S. Environmental Protection Agency

Lesley Jantarasami, U.S. Environmental Protection Agency

George Luber, Centers for Disease Control and Prevention

Shubhayu Saha, Centers for Disease Control and Prevention

Paul Schramm, Centers for Disease Control and Prevention

Mark M. Shimamoto, U.S. Global Change Research Program, 

National Coordination Office

Kimberly Thigpen Tart, National Institutes of Health

Juli Trtanj, National Oceanic and Atmospheric Administration

Chapter Authors

Carl Adrianopoli, U.S. Department of Health and Human Services 

Allan Auclair, U.S. Department of Agriculture 

John Balbus, National Institutes of Health 

Christopher M. Barker, University of California, Davis  

Charles B. Beard, Centers for Disease Control and Prevention 

Jesse E. Bell, Cooperative Institute for Climate and Satellites–North 

Carolina 

Kaitlin Benedict, Centers for Disease Control and Prevention 

Martha Berger, U.S. Environmental Protection Agency 

Karen Bouye, Centers for Disease Control and Prevention 

Terry Brennan, Camroden Associates, Inc. 

Joan Brunkard, Centers for Disease Control and Prevention 

Vince Campbell, Centers for Disease Control and Prevention 

Karletta Chief, The University of Arizona 

Tracy Collier, National Oceanic and Atmospheric Administration and 

University Corporation for Atmospheric Research 

Kathryn Conlon, Centers for Disease Control and Prevention 

Allison Crimmins, U.S. Environmental Protection Agency 

Stacey DeGrasse, U.S. Food and Drug Administration 

Daniel Dodgen, U.S. Department of Health and Human Services, 

Office of the Assistant Secretary for Preparedness and Response 

Patrick Dolwick, U.S. Environmental Protection Agency 

Darrin Donato, U.S. Department of Health and Human Services, 

Office of the Assistant Secretary for Preparedness and Response 

David R. Easterling, National Oceanic and Atmospheric 

Administration  

Kristie L. Ebi, University of Washington 

Rebecca J. Eisen, Centers for Disease Control and Prevention 

Vanessa Escobar, National Aeronautics and Space Administration 

Neal Fann, U.S. Environmental Protection Agency 

Barry Flanagan, Centers for Disease Control and Prevention 

Janet L. Gamble, U.S. Environmental Protection Agency  

Jada F. Garofalo, Centers for Disease Control and Prevention 

Cristina Gonzalez-Maddux, formerly Institute for Tribal Environmental 

Professionals 

Micah Hahn, Centers for Disease Control and Prevention 

Elaine Hallisey, Centers for Disease Control and Prevention 

Michelle D. Hawkins, National Oceanic and Atmospheric 

Administration 

Mary Hayden, National Center for Atmospheric Research  

Stephanie C. Herring, National Oceanic and Atmospheric 

Administration 

Jeremy Hess, University of Washington

Radley Horton, Columbia University 

Sonja Hutchins, Centers for Disease Control and Prevention 

Vito Ilacqua, U.S. Environmental Protection Agency 

John Jacobs, National Oceanic and Atmospheric Administration 

Lesley Jantarasami, U.S. Environmental Protection Agency 

Ali S. Khan, University of Nebraska Medical Center 

Patrick Kinney, Columbia University 

Laura Kolb, U.S. Environmental Protection Agency 

Nancy Kelly, U.S. Department of Health and Human Services, 

Substance Abuse and Mental Health Services Administration 

Samar Khoury, Association of Schools and Programs of Public 

Health 

Max Kiefer, Centers for Disease Control and Prevention, National 

Institute for Occupational Safety and Health 

Jessica Kolling, Centers for Disease Control and Prevention 

Kenneth E. Kunkel, Cooperative Institute for Climate and Satellite–

North Carolina,  

Annette La Greca, University of Miami 

Erin Lipp, The University of Georgia 

Irakli Loladze, Bryan College of Health Sciences 

Jeffrey Luvall, National Aeronautics and Space Administration 

Kathy Lynn, University of Oregon 

Arie Manangan, Centers for Disease Control and Prevention 

Marian McDonald, Centers for Disease Control and Prevention 

LIST OF CONTRIBUTORS
U.S. Global Change Research Program xiii Impacts of Climate Change on Human Health in the United States

Sandra McLellan, University of Wisconsin-Milwaukee 

David M. Mills, Abt Associates 

Andrew J. Monaghan, National Center for Atmospheric Research 

Stephanie Moore, National Oceanic and Atmospheric Administration 

and University Corporation for Atmospheric Research 

Rachel Morello-Frosch, University of California, Berkeley 

Joshua Morganstein, Uniformed Services University of the Health 

Sciences 

Christopher G. Nolte, U.S. Environmental Protection Agency 

Nicholas H. Ogden, Public Health Agency of Canada 

Hans Paerl, The University of North Carolina at Chapel Hill 

Adalberto A. Pérez de León, U.S. Department of Agriculture 

Carlos Perez Garcia-Pando, Columbia University 

Dale Quattrochi, National Aeronautics and Space Administration 

John Ravenscroft, U.S. Environmental Protection Agency 

Margaret H. Redsteer, U.S. Geological Survey 

Joseph Reser, Griffith University 

Jennifer Runkle, Cooperative Institute for Climate and Satellites–

North Carolina 

Josef Ruzek, U.S. Department of Veterans Affairs 

Shubhayu Saha, Centers for Disease Control and Prevention 

Marcus C. Sarofim, U.S. Environmental Protection Agency 

Paul J. Schramm, Centers for Disease Control and Prevention 

Carl J. Schreck III, Cooperative Institute for Climate and Satellites–

North Carolina 

Shulamit Schweitzer, U.S. Department of Health and Human 

Services, Office of the Assistant Secretary for Preparedness and 

Response 

Mario Sengco, U.S. Environmental Protection Agency 

Mark M. Shimamoto, U.S. Global Change Research Program, 

National Coordination Office 

Allan Showler, U.S. Department of Agriculture 

Tanya L. Spero, U.S. Environmental Protection Agency 

Joel Schwartz, Harvard University 

Perry Sheffield, Icahn School of Medicine at Mount Sinai, New York 

Alexis St. Juliana, Abt Associates 

Kimberly Thigpen Tart, National Institutes of Health  

Jeanette Thurston, U.S. Department of Agriculture 

Juli Trtanj, National Oceanic and Atmospheric Administration 

Robert Ursano, Uniformed Services University of the Health 

Sciences  

Isabel Walls, U.S. Department of Agriculture 

Joanna Watson, Centers for Disease Control and Prevention, 

National Institute for Occupational Safety and Health 

Kyle Powys Whyte, Michigan State University 

Amy F. Wolkin, Centers for Disease Control and Prevention 

Lewis Ziska, U.S. Department of Agriculture

Chapter Coordinators 

Allison Crimmins, U.S. Environmental Protection Agency

Jada F. Garofalo, Centers for Disease Control and Prevention

Lesley Jantarasami, U.S. Environmental Protection Agency

Andrea Maguire, U.S. Environmental Protection Agency

Daniel Malashock, U.S. Department of Health and Human Services, 

Public Health Service 

Jennifer Runkle, Cooperative Institute for Climate and Satellites–

North Carolina

Marcus C. Sarofim, U.S. Environmental Protection Agency 

Mark M. Shimamoto, U.S. Global Change Research Program, 

National Coordination Office

United States Global Change Research Program

Michael Kuperberg, Executive Director, USGCRP, White House Office 

of Science and Technology Policy (OSTP)

Ben DeAngelo, Deputy Executive Director, USGCRP, White House 

OSTP

Subcommittee on Global Change Research Leadership and 

Executive Committee

Chair

Thomas Karl, U.S. Department of Commerce

Vice Chairs

Michael Freilich, National Aeronautics and Space Administration

Gerald Geernaert, U.S. Department of Energy

Richard Spinrad, U.S. Department of Commerce

Roger Wakimoto, National Science Foundation

Jeffrey Arnold, U.S. Army Corps of Engineers (Adjunct)

Principals

John Balbus, U.S. Department of Health and Human Services

William Breed, U.S. Agency for International Development (Acting)

Joel Clement, U.S. Department of the Interior

Pierre Comizzoli, Smithsonian Institution

Wayne Higgins, U.S. Department of Commerce

Scott Harper, U.S. Department of Defense (Acting)

William Hohenstein, U.S. Department of Agriculture

Jack Kaye, National Aeronautics and Space Administration

Dorothy Koch, U.S. Department of Energy

C. Andrew Miller, U.S. Environmental Protection Agency

Craig Robinson, National Science Foundation

Arthur Rypinski, U.S. Department of Transportation (Acting)

Trigg Talley, U.S. Department of State
U.S. Global Change Research Program xiv Impacts of Climate Change on Human Health in the United States

Executive Office of the President Liaisons

Tamara Dickinson, Principal Assistant Director for Environment and 

Energy, White House OSTP

Afua Bruce, Executive Director, National Science and Technology 

Council, White House OSTP (from June 2015)

Jayne Morrow, Executive Director, National Science and Technology 

Council, White House OSTP (through June 2015)

Richard Duke, White House Council on Environmental Quality

Kimberly Miller, White House Office of Management and Budget

Fabien Laurier, Director (Acting), National Climate Assessment, 

White House OSTP (from December 2013)

USGCRP Climate and Health Assessment Staff

USGCRP National Coordination Office

Michael Kuperberg, Executive Director, USGCRP, White House OSTP

Ben DeAngelo, Deputy Executive Director, USGCRP, White House 

OSTP

Katharine Jacobs, Director, National Climate Assessment, White 

House OSTP (through December 2013)

Thomas Armstrong, Executive Director, USGCRP NCO, White House 

OSTP (through December 2014)

Christopher P. Weaver, Executive Director (Acting, through August 

2015), formerly Deputy Director, USGCRP NCO, White House 

OSTP

Glynis C. Lough, Chief of Staff, National Climate Assessment 

Bradley Akamine, Chief Digital Officer

Mark M. Shimamoto, Health Program Lead

Ilya Fischhoff, Senior Scientist, National Climate Assessment

Emily Therese Cloyd, Engagement and Outreach Lead

Steve Aulenbach, GCIS Content Curator (through September 2015)

Samantha Brooks, SGCR Executive Secretary (through July 2015)

Tess Carter, Student Assistant, National Climate Assessment

Brian Duggan, GCIS Lead System Engineer (through September 

2015)

Bryce Golden-Chen, Coordinator, National Climate Assessment 

(through September 2015)

Justin Goldstein, Advance Science Climate Data and Observing 

Systems Coordinator

Alexa Jay, Science Writer (from December 2015)

Amanda Jensen, Student Assistant, The George Washington 

University (January-May 2015)

Amanda McQueen, SGCR Executive Secretary (from July 2015)

Alena Marovitz, Student Assistant, Amherst College (June-August 

2015)

Tanya Maslak, Chief of Operations (through May 2015)

Julie Morris, Associate Director of Implementation and Strategic 

Planning

Brent Newman, GCIS Data Coordinator (from January 2015)

Katie Reeves, Engagement Support Associate (from December 

2015)

Catherine Wolner, Science Writer (through June 2015)

Robert Wolfe, Technical Lead for the Global Change Information 

System (GCIS), NASA (through March 2016)

NOAA Technical Support Unit, National Centers for 

Environmental Information

David R. Easterling, NCA Technical Support Unit Director, NOAA 

National Centers for Environmental Information (NCEI)

Paula Ann Hennon, NCA Technical Support Unit Deputy Director, 

Cooperative Institute for Climate and Satellites–North Carolina 

(CICS-NC) (through December 2015)

Kenneth E. Kunkel, Lead Scientist, CICS-NC

Sara W. Veasey, Creative Director, NOAA NCEI

Andrew Buddenberg, Software Engineer/Scientific Programmer, 

CICS-NC

Sarah Champion, Data Architect, CICS-NC

Daniel Glick, Editor, CICS-NC

Jessicca Griffin, Lead Graphic Designer, CICS-NC

Angel Li, Web Developer, CICS-NC

Liz Love-Brotak, Graphic Designer, NOAA NCEI

Tom Maycock, Project Manager/Editor, CICS-NC

Deborah Misch, Graphic Designer, LMI Consulting

Susan Osborne, Copy Editor, LMI Consulting

Deborah B. Riddle, Graphic Designer, NOAA NCEI

Jennifer Runkle, Editor, CICS-NC

April Sides, Web Developer, CICS-NC

Mara Sprain, Copy Editor, LAC Group

Laura E. Stevens, Research Scientist, CICS-NC

Brooke C. Stewart, Science Editor, CICS-NC

Liqiang Sun, Research Scientist/Modeling Support, CICS-NC

Devin Thomas, Metadata Specialist, ERT Inc.

Kristy Thomas, Metadata Specialist, ERT Inc.

Teresa Young, Print Specialist, ERT Inc.

UNC Asheville’s National Environmental Modeling and 

Analysis Center (NEMAC)

Karin Rogers, Director of Operations/Research Scientist

Greg Dobson, Director of Geospatial Technology/Research Scientist

Caroline Dougherty, Principal Designer

John Frimmel, Applied Research Software Developer

Ian Johnson, Geospatial and Science Communications Associate
U.S. Global Change Research Program xv Impacts of Climate Change on Human Health in the United States

USGCRP Interagency Crosscutting Group on Climate 

Change and Human Health (CCHHG) 

Co-Chairs

John Balbus, National Institutes of Health

George Luber, Centers for Disease Control and Prevention

Juli Trtanj, National Oceanic and Atmospheric Administration

Coordinator

Mark M. Shimamoto, U.S. Global Change Research Program, 

National Coordination Office

National Aeronautics and Space Administration

Sue Estes, Universities Space Research Association 

John Haynes, Science Mission Directorate 

U.S. Department of Agriculture

Isabel Walls, National Institute of Food and Agriculture

U.S. Department of Commerce

Michelle Hawkins, National Oceanic and Atmospheric 

Administration

Hunter Jones, National Oceanic and Atmospheric Administration

Juli Trtanj, National Oceanic and Atmospheric Administration

U.S. Department of Defense

Jean-Paul Chretien, Armed Forces Health Surveillance Center

James Persson, U.S. Army Research Institute of Environmental 

Medicine

U.S. Department of Health and Human Services

John Balbus, National Institutes of Health

Charles B. Beard, Centers for Disease Control and Prevention

Ross Bowling, Office of the Assistant Secretary for Administration

Kathleen Danskin, Office of the Assistant Secretary for Preparedness 

and Response

Stacey Degrasse, Food and Drug Administration

Renee Dickman, Office of the Assistant Secretary for Planning and 

Evaluation 

Caroline Dilworth, National Institutes of Health

Jada F. Garafalo, Centers for Disease Control and Prevention

Christine Jessup, National Institutes of Health

Maya Levine, Office of Global Affairs

George Luber, Centers for Disease Control and Prevention

Joshua Rosenthal, National Institutes of Health

Shubhayu Saha, Centers for Disease Control and Prevention

Bono Sen, National Institutes of Health

Paul J. Schramm, Centers for Disease Control and Prevention

Joanna Watson, Centers for Disease Control and Prevention - NIOSH

Kimberly Thigpen Tart, National Institutes of Health

U.S. Department of Homeland Security

Jeffrey Stiefel, Office of Health Affairs

U.S. Department of Housing and Urban 

Development

J. Kofi Berko, Jr., Office of Lead Hazard Control & Healthy Homes

U.S. Department of the Interior

Patricia Bright, U.S. Geological Survey

Joseph Bunnell, U.S. Geological Survey

U.S. Department of State

Joshua Glasser, Bureau of Oceans and International Environmental 

and Scientific Affairs

U.S. Environmental Protection Agency

Martha Berger, Office of Children’s Health Protection

Rona Birnbaum, Office of Air and Radiation

Bryan Bloomer, Office of Research and Development

Allison Crimmins, Office of Air and Radiation

Amanda Curry Brown, Office of Air and Radiation

Janet L. Gamble, Office of Research and Development

Vito Ilacqua, Office of Research and Development

Michael Kolian, Office of Air and Radiation

Marian Rutigliano, Office of Research and Development

White House National Security Council

David V. Adams

Review Editors

Rupa Basu, California Office of Environmental Health Hazard 

Assessment

Paul English, Public Health Institute, Oakland, CA

Kim Knowlton, Columbia University Mailman School of Public Health

Patricia Romero-Lankao, National Center for Atmospheric Research

Bart Ostro, University of California, Davis

Jan Semenza, European Centre for Disease Prevention and Control

Fran Sussman, ICF International

Felicia Wu, Michigan State University

Acknowledgements

The authors acknowledge RTI International, ICF International, 

Abt Associates, and Abt Environmental Research (formerly 

Stratus Consulting) for their support in the development of this 

report. 
U.S. Global Change Research Program xvi Impacts of Climate Change on Human Health in the United States

References:

1. Executive Office of the President, 2013: The President’s 

Climate Action Plan. Washington, D.C. https://http://www.

whitehouse.gov/sites/default/files/image/president27sclimateactionplan.pdf

2. National Academies of Sciences Engineering and Medicine, 

2015: Review of the Draft Interagency Report on the Impacts 

of Climate Change on Human Health in the United States. 

National Academies Press, Washington, D.C. http://www.

nap.edu/catalog/21787/review-of-the-draft-interagency-report-on-the-impacts-of-climate-change-on-human-health-inthe-united-states

3. 2014: Climate Change Impacts in the United States: The Third 

National Climate Assessment. Melillo, J.M., T.C. Richmond, 

and G.W. Yohe, Eds. U.S. Global Change Research Program, 

Washington, D.C., 842 pp. http://dx.doi.org/10.7930/

J0Z31WJ2

4. Luber, G., K. Knowlton, J. Balbus, H. Frumkin, M. Hayden, 

J. Hess, M. McGeehin, N. Sheats, L. Backer, C.B. Beard, 

K.L. Ebi, E. Maibach, R.S. Ostfeld, C. Wiedinmyer, E. 

Zielinski-Gutiérrez, and L. Ziska, 2014: Ch. 9: Human 

health. Climate Change Impacts in the United States: The Third 

National Climate Assessment. Melillo, J.M., T.C. Richmond, 

and G.W. Yohe, Eds. U.S. Global Change Research Program, 

Washington, D.C., 220-256. http://dx.doi.org/10.7930/

J0PN93H5

5. GCRA, 1990: Global Change Research Act of 1990, Pub. 

L. No. 101-606, 104 Stat. 3096-3104. http://www.gpo.gov/

fdsys/pkg/STATUTE-104/pdf/STATUTE-104-Pg3096.pdf

6. CCSP, 2008: Analyses of the Effects of Global Change on 

Human Health and Welfare and Human Systems. A Report 

by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. 205 pp. Gamble, J. 

L., (Ed.), Ebi, K.L., F.G. Sussman, T.J. Wilbanks, (Authors). 

U.S. Environmental Protection Agency, Washington, D.C. 

http://downloads.globalchange.gov/sap/sap4-6/sap4-6-finalreport-all.pdf

7. USGCRP, 2012: National Climate Assessment Health Sector 

Literature Review and Bibliography. Technical Input for the 

Interagency Climate Change and Human Health Group. 

National Institute of Environmental Health Sciences. http://

www.globalchange.gov/what-we-do/assessment/nca-activities/available-technical-inputs

PHOTO CREDITS

cover and title page–Manhattan skyline: © iStockPhoto.com/

stockelements; Farmer: © Masterfile/Corbis; Girl getting 

checkup: © Rob Lewine/Tetra Images/Corbis

Pg. vii–Elderly Navajo woman and her niece, image by © Alison 

Wright/Corbis; Doctor showing girl how to use stethoscope: 

©John Fedele LLC/Corbis; Senior man watering the flowers in 

the garden: © iStockPhoto.com/Alexander Raths

Pg. ix– Large crowd of people: © iStockPhoto.com/Ints Vikmanis
U.S. Global Change Research Program 1 Impacts of Climate Change on Human Health in the United States

EXECUTIVE SUMMARY

U.S. Global Change Research Program

THE IMPACTS OF CLIMATE CHANGE ON HUMAN HEALTH IN THE UNITED STATES

A Scientific Assessment

Climate change threatens human health and well-being in the United States. The U.S. Global Change Research 

Program (USGCRP) Climate and Health Assessment has been developed to enhance understanding and inform 

decisions about this growing threat. This scientific assessment, called for under the President’s Climate Action 

Plan, is a major report of the sustained National Climate Assessment (NCA) process. The report responds to the 

1990 Congressional mandate to assist the Nation in understanding, assessing, predicting, and responding to 

human-induced and natural processes of global change. The agencies of the USGCRP identified human health 

impacts as a high-priority topic for scientific assessment.

The purpose of this assessment is to provide a comprehensive, evidence-based, and, where possible, quantitative 

estimation of observed and projected climate change related health impacts in the United States. The USGCRP 

Climate and Health Assessment has been developed to inform public health officials, urban and disaster 

response planners, decision makers, and other stakeholders within and outside of government who are interested 

in better understanding the risks climate change presents to human health.

Recommended Citation: Crimmins, A., J. Balbus, J.L. Gamble, C.B. Beard, J.E. Bell, D. Dodgen, R.J. Eisen, N. Fann, M.D. 

Hawkins, S.C. Herring, L. Jantarasami, D.M. Mills, S. Saha, M.C. Sarofim, J. Trtanj, and L. Ziska, 2016: Executive Summary. The 

Impacts of Climate Change on Human Health in the United States: A Scientific Assessment. U.S. Global Change Research 

Program, Washington, DC, page 1–24. http://dx.doi.org/10.7930/J00P0WXS

Lead Authors

Allison Crimmins

U.S. Environmental Protection Agency

John Balbus

National Institutes of Health

Janet L. Gamble

U.S. Environmental Protection Agency

Charles B. Beard

Centers for Disease Control and Prevention

Jesse E. Bell

Cooperative Institute for Climate and Satellites–North Carolina

Daniel Dodgen

U.S. Department of Health and Human Services, Office of the 

Assistant Secretary for Preparedness and Response

Rebecca J. Eisen

Centers for Disease Control and Prevention

Neal Fann

U.S. Environmental Protection Agency

On the web: health2016.globalchange.gov

Michelle D. Hawkins

National Oceanic and Atmospheric Administration

Stephanie C. Herring

National Oceanic and Atmospheric Administration

Lesley Jantarasami

U.S. Environmental Protection Agency

David M. Mills

Abt Associates

Shubhayu Saha

Centers for Disease Control and Prevention

Marcus C. Sarofim

U.S. Environmental Protection Agency

Juli Trtanj

National Oceanic and Atmospheric Administration

Lewis Ziska

U.S. Department of Agriculture 
U.S. Global Change Research Program 2 Impacts of Climate Change on Human Health in the United States

Executive Summary of

THE IMPACTS OF CLIMATE CHANGE ON 

HUMAN HEALTH IN THE UNITED STATES

Climate change is a significant threat to the health of the 

American people. The impacts of human-induced climate 

change are increasing nationwide. Rising greenhouse gas 

concentrations result in increases in temperature, changes in 

precipitation, increases in the frequency and intensity of some 

extreme weather events, and rising sea levels. These climate 

change impacts endanger our health by affecting our food and 

water sources, the air we breathe, the weather we experience, 

and our interactions with the built and natural environments. 

As the climate continues to change, the risks to human health 

continue to grow. 

Current and future climate impacts expose more people in 

more places to public health threats. Already in the United 

States, we have observed climate-related increases in our 

exposure to elevated temperatures; more frequent, severe, or 

longer-lasting extreme events; degraded air quality; diseases 

transmitted through food, water, and disease vectors (such as 

ticks and mosquitoes); and stresses to our mental health and 

well-being.

Almost all of these threats are expected to worsen with continued climate change. Some of these health threats will occur 

over longer time periods, or at unprecedented times of the 

year; some people will be exposed to threats not previously 

experienced in their locations. Overall, instances of potentially beneficial health impacts of climate change are limited 

in number and pertain to specific regions or populations. For 

example, the reduction in cold-related deaths is projected to 

be smaller than the increase in heat-related deaths in most 

regions.

Every American is vulnerable to the health impacts associated 

with climate change. Increased exposure to multiple health 

threats, together with changes in sensitivity and the ability to 

adapt to those threats, increases a person’s vulnerability to climate-related health effects. The impacts of climate change on 

human health interact with underlying health, demographic, 

and socioeconomic factors. Through the combined influence 

of these factors, climate change exacerbates some existing 

health threats and creates new public health challenges. While 

all Americans are at risk, some populations are disproportionately vulnerable, including those with low income, some 

communities of color, immigrant groups (including those with 

limited English proficiency), Indigenous peoples, children 

and pregnant women, older adults, vulnerable occupational 

groups, persons with disabilities, and persons with preexisting 

or chronic medical conditions.

© iStockPhoto.com/LouieBaxter 

Changes in aquatic habitats and species may affect subsistence 

fishing among Indigenous populations.

© Mike Kane/Aurora Photos/Corbis
U.S. Global Change Research Program 3 Impacts of Climate Change on Human Health in the United States

In recent years, scientific understanding of how climate change 

increases risks to human health has advanced significantly. 

Even so, the ability to evaluate, monitor, and project health 

effects varies across climate impacts. For instance, information 

on health outcomes differs in terms of whether complete, 

long-term datasets exist that allow quantification of observed 

changes, and whether existing models can project impacts at 

the timescales and geographic scales of interest. Differences 

also exist in the metrics available for observing or projecting 

different health impacts. For some health impacts, the available metrics only describe changes in risk of exposure, while 

for others, metrics describe changes in actual health outcomes 

(such as the number of new cases of a disease or an increase 

in deaths).

This assessment strengthens and expands our understanding 

of climate-related health impacts by providing a more definitive description of climate-related health burdens in the United States. It builds on the 2014 

National Climate Assessment1

and reviews and synthesizes key 

contributions to the published 

literature. Acknowledging the 

rising demand for data that can 

be used to characterize how climate change affects health, this report assesses recent analyses that quantify observed and projected health impacts. Each 

chapter characterizes the strength of the scientific evidence 

for a given climate–health exposure pathway or “link” in the 

Every American is vulnerable to the health 

impacts associated with climate change

While all Americans are at risk, some populations are 

disproportionately vulnerable, including children and pregnant 

women.

Los Angeles, California, May 22, 2012. Unless offset by additional emissions reductions of ozone precursors, climate-driven 

increases in ozone will cause premature deaths, hospital visits, lost school days, and acute respiratory symptoms.

causal chain between a climate change impact and its associated health outcome. This assessment’s findings represent 

an improvement in scientific confidence in the link between 

climate change and a broad 

range of threats to public health, 

while recognizing populations of 

concern and identifying emerging issues. These considerations 

provide the context for understanding Americans’ changing 

health risks and allow us to identify, project, and respond 

to future climate change health threats. The overall findings 

underscore the significance of the growing risk climate change 

poses to human health in the United States.

© John Fedele/Blend Images/Corbis © Ringo Chiu/ZUMA Press/Corbis
EXECUTIVE SUMMARY

U.S. Global Change Research Program 4 Impacts of Climate Change on Human Health in the United States

CLIMATE CHANGE AND

HUMAN HEALTH

The influences of weather and climate on human health are significant and varied. Exposure to health hazards related to climate 

change affects different people and different communities to 

different degrees. While often assessed individually, exposure to 

multiple climate change threats can occur simultaneously, resulting in compounding or cascading health impacts. 

With climate change, the frequency, severity, duration, and 

location of weather and climate phenomena—like rising temperatures, heavy rains and droughts, and some other kinds of 

severe weather—are changing. This means that areas already 

experiencing health-threatening weather and climate phenomena, such as severe heat or hurricanes, are likely to experience 

worsening impacts, such as higher temperatures and increased 

storm intensity, rainfall rates, and storm surge.

Conceptual diagram illustrating the exposure pathways by which climate change affects human health. Here, the center boxes list 

some selected examples of the kinds of changes in climate drivers, exposure, and health outcomes explored in this report. Exposure 

pathways exist within the context of other factors that positively or negatively influence health outcomes (gray side boxes). Some of 

the key factors that influence vulnerability for individuals are shown in the right box, and include social determinants of health and 

behavioral choices. Some key factors that influence vulnerability at larger scales, such as natural and built environments, governance 

and management, and institutions, are shown in the left box. All of these influencing factors can affect an individual’s or a community’s 

vulnerability through changes in exposure, sensitivity, and adaptive capacity and may also be affected by climate change. 

1

It also means that some locations will experience new climate-related health threats. For example, areas previously unaffected by toxic algal blooms or waterborne diseases because of 

cooler water temperatures may face these hazards in the future 

as increasing water temperatures allow the organisms that cause 

these health risks to thrive. Even areas that currently experience 

these health threats may see a shift in the timing of the seasons 

that pose the greatest risk to human health.

Climate change can therefore affect human health in two main 

ways: first, by changing the severity or frequency of health 

problems that are already affected by climate or weather factors; 

and second, by creating unprecedented or unanticipated health 

problems or health threats in places where they have not previously occurred. 

Climate Change and Health
EXECUTIVE SUMMARY

U.S. Global Change Research Program 5 Impacts of Climate Change on Human Health in the United States

Examples of Climate Impacts on Human Health

The diagram shows specific examples of how climate change can affect human health, now and in the future. These effects 

could occur at local, regional, or national scales. The examples listed in the first column are those described in each underlying 

chapter’s exposure pathway diagram. Moving from left to right along one health impact row, the three middle columns show 

how climate drivers affect an individual’s or a community’s exposure to a health threat and the resulting change in health 

outcome. The overall climate impact is summarized in the final gray column. For a more comprehensive look at how climate 

change affects health, and to see the environmental, institutional, social, and behavioral factors that play an interactive role 

in determining health outcomes, see the exposure pathway diagrams in chapters 2–8 in the full report. 
EXECUTIVE SUMMARY

U.S. Global Change Research Program 6 Impacts of Climate Change on Human Health in the United States

Climate change will increase the frequency and severity of future 

extreme heat events while also resulting in generally warmer 

summers and milder winters, with implications for human health.

Increasing concentrations of greenhouse gases 

lead to an increase of both average and extreme 

temperatures. This is expected to lead to an increase in deaths and illness from heat and a potential decrease in deaths from cold, particularly for 

a number of communities especially vulnerable to 

these changes, such as children, the elderly, and 

economically disadvantaged groups. 

Days that are hotter than the average seasonal temperature in the summer or colder than the average 

seasonal temperature in the winter cause increased 

levels of illness and death by compromising the 

body’s ability to regulate its temperature or by 

inducing direct or indirect health complications. 

Loss of internal temperature control can result in a 

cascade of illnesses, including heat cramps, heat 

exhaustion, heatstroke, and hyperthermia in the 

presence of extreme heat, and hypothermia and 

frostbite in the presence of extreme cold. 

Temperature extremes can also worsen chronic 

conditions such as cardiovascular disease, 

respiratory disease, cerebrovascular disease, and 

diabetes-related conditions. Prolonged exposure 

to high temperatures is associated with increased 

hospital admissions for cardiovascular, kidney, and 

respiratory disorders.

Future Increases in Temperature-Related 

Deaths 

Key Finding 1: Based on present-day sensitivity 

to heat, an increase of thousands to tens of 

thousands of premature heat-related deaths in 

the summer [Very Likely, High Confidence] and 

a decrease of premature cold-related deaths in 

the winter [Very Likely, Medium Confidence] are 

projected each year as a result of climate change 

by the end of the century. Future adaptation will 

very likely reduce these impacts (see the Changing 

Tolerance to Extreme Heat Finding). The reduction 

in cold-related deaths is projected to be smaller 

than the increase in heat-related deaths in most 

regions [Likely, Medium Confidence].

Even Small Differences from Seasonal 

Average Temperatures Result in Illness 

and Death

Key Finding 2: Days that are hotter than usual in 

the summer or colder than usual in the winter 

are both associated with increased illness and 

death [Very High Confidence]. Mortality effects are 

observed even for small differences from seasonal 

average temperatures [High Confidence]. Because 

small temperature differences occur much more 

frequently than large temperature differences, not 

accounting for the effect of these small differences 

would lead to underestimating the future impact of 

climate change [Likely, High Confidence].

TEMPERATURE-RELATED 2 DEATH AND ILLNESS © iStockPhoto.com/jpbcpa © iStockPhoto.com/Dreef
EXECUTIVE SUMMARY

U.S. Global Change Research Program 7 Impacts of Climate Change on Human Health in the United States

Projected Changes in Deaths in U.S. Cities by Season

Outdoor workers spend a great deal of time exposed to 

temperature extremes, often while performing vigorous 

activities.

This figure shows the projected increase in deaths due to warming in the summer 

months (hot season, April–September), the projected decrease in deaths due to 

warming in the winter months (cold season, October–March), and the projected net 

change in deaths compared to a 1990 baseline period for the 209 U.S. cities examined, 

using the GFDL–CM3 and MIROC5 climate models (see Ch. 2: Temperature-Related 

Deaths and Illness). (Figure source: adapted from Schwartz et al. 2015)2

Changing Tolerance to Extreme Heat

Key Finding 3: An increase in population tolerance 

to extreme heat has been observed over time 

[Very High Confidence]. Changes in this tolerance 

have been associated with increased use of air 

conditioning, improved social responses, and/or 

physiological acclimatization, among other factors 

[Medium Confidence]. Expected future increases 

in this tolerance will reduce the projected increase 

in deaths from heat [Very Likely, Very High 

Confidence].

Some Populations at Greater Risk

Key Finding 4: Older adults and children have a 

higher risk of dying or becoming ill due to extreme 

heat [Very High Confidence]. People working 

outdoors, the socially isolated and economically 

disadvantaged, those with chronic illnesses, 

as well as some communities of color, are also 

especially vulnerable to death or illness [Very High 

Confidence].

© Fotosearch
EXECUTIVE SUMMARY

U.S. Global Change Research Program 8 Impacts of Climate Change on Human Health in the United States

Ragweed pollen frequently triggers hay fever 

and asthma episodes during the fall.

Changes in the climate affect the air we breathe, both indoors 

and outdoors. The changing climate has modified weather patterns, which in turn have influenced the levels and location of 

outdoor air pollutants such as ground-level ozone (O3) and fine 

particulate matter. Increasing carbon dioxide (CO2) levels also 

promote the growth of plants that release airborne allergens 

(aeroallergens). Finally, these changes to outdoor air quality and 

aeroallergens also affect indoor air quality as both pollutants and 

aeroallergens infiltrate homes, schools, and other buildings. Poor 

air quality, whether outdoors or indoors, can negatively affect 

the human respiratory and cardiovascular systems. Higher pollen 

concentrations and longer pollen seasons can increase allergic 

sensitization and asthma episodes and thereby limit productivity 

at work and school. 

The air quality response to climate change can vary substantially by region across scenarios. Two downscaled global climate 

model projections using two greenhouse gas concentration pathways estimate increases in average daily maximum temperatures 

of 1.8°F to 7.2°F (1°C to 4°C) and increases of 1 to 5 parts per billion (ppb) in daily 8-hour maximum ozone in the year 2030 

relative to the year 2000 throughout the continental United States. Unless reductions in ozone precursor emissions offset the 

influence of climate change, this “climate penalty” of increased ozone concentrations due to climate change would result in tens 

to thousands of additional ozone-related premature deaths per year, shown here as incidences per year by county (see Ch. 3: 

Air Quality Impacts). (Figure source: adapted from Fann et al. 2015)3

AIR QUALITY 3 IMPACTS

© Roy Morsch/Corbis

Projected Change in Temperature, Ozone, and Ozone-Related Premature Deaths in 2030
EXECUTIVE SUMMARY

U.S. Global Change Research Program 9 Impacts of Climate Change on Human Health in the United States

(Top) Dampness and mold in U.S. homes are linked to approximately 4.6 million cases of worsened asthma. (Left) Wildfires 

are a major source of airborne particulate matter, especially in the western United States during summer. Climate change has 

already led to an increased frequency of large wildfires, as well as longer durations of individual wildfires and longer wildfire 

seasons in the western United States. (Right) Nearly 6.8 million children in the United States are affected by asthma, making it 

a major chronic disease of childhood. 

Exacerbated Ozone Health Impacts 

Key Finding 1: Climate change will make it harder 

for any given regulatory approach to reduce groundlevel ozone pollution in the future as meteorological 

conditions become increasingly conducive to forming 

ozone over most of the United States [Likely, High 

Confidence]. Unless offset by additional emissions 

reductions of ozone precursors, these climate-driven 

increases in ozone will cause premature deaths, 

hospital visits, lost school days, and acute respiratory 

symptoms [Likely, High Confidence].

Increased Health Impacts from Wildfires

Key Finding 2: Wildfires emit fine particles and ozone 

precursors that in turn increase the risk of premature 

death and adverse chronic and acute cardiovascular 

and respiratory health outcomes [Likely, High 

Confidence]. Climate change is projected to increase 

the number and severity of naturally occurring wildfires 

in parts of the United States, increasing emissions of 

particulate matter and ozone precursors and resulting 

in additional adverse health outcomes [Likely, High 

Confidence]. 

Worsened Allergy and Asthma Conditions 

Key Finding 3: Changes in climate, specifically rising 

temperatures, altered precipitation patterns, and 

increasing concentrations of atmospheric carbon 

dioxide, are expected to contribute to increases in 

the levels of some airborne allergens and associated 

increases in asthma episodes and other allergic 

illnesses [High Confidence].

Photo credit goes here

© Stephen Welstead/LWA/Corbis Courtesy of Bart Everson/flickr© Erich Schlegel/Corbis
EXECUTIVE SUMMARY

U.S. Global Change Research Program 10 Impacts of Climate Change on Human Health in the United States

 Estimated Deaths and Billion Dollar Losses 

from Extreme Events in the U.S., 2004–2013

Heat

Waves Tornadoes Hurricanes Floods Wind

Storms Lightning Cold

Waves

Winter

Storms

10-year Total Fatalaties 

0

200

400

600

800

1000

1200

1400 Billion Dollar Losses

from Disasters

(2004-2013)

$392 Billion 

Hurricanes

$78 Billion 

Heat Waves/Droughts 

$46 Billion

Tornadoes/Severe Storms

$30 Billion

Flooding/Severe Storms

Climate change projections show that there will 

be continuing increases in the occurrence and 

severity of some extreme events by the end of the 

century, while for other extremes the links to climate change are more uncertain. Some regions of 

the United States have already experienced costly 

impacts—in terms of both lives lost and economic 

damages—from observed changes in the frequency, intensity, or duration of certain extreme events. 

While it is intuitive that extremes can have health 

impacts such as death or injury during an event 

(for example, drowning during floods), health 

impacts can also occur before or after an extreme 

event, as individuals may be involved in activities that put their health at risk, such as disaster 

preparation and post-event cleanup. Health risks 

may also arise long after the event, or in places 

outside the area where the event took place, as a 

result of damage to property, destruction of assets, 

loss of infrastructure and public services, social 

and economic impacts, environmental degradation, 

and other factors. 

Extreme events also pose unique health risks if 

multiple events occur simultaneously or in succession in a given location. The severity and extent 

of health effects associated with extreme events 

depend on the physical impacts of the extreme 

events themselves as well as the unique human, 

societal, and environmental circumstances at the 

time and place where events occur. 

This figure provides 10-year estimates of fatalities related to extreme events from 2004 to 2013,4 as well as estimated economic 

damages from 58 weather and climate disaster events with losses exceeding $1 billion (see Smith and Katz 2013 to understand 

how total losses were calculated).5 These statistics are indicative of the human and economic costs of extreme weather events 

over this time period. Climate change will alter the frequency, intensity, and geographic distribution of some of these extremes,1

which has consequences for exposure to health risks from extreme events. Trends and future projections for some extremes, 

including tornadoes, lightning, and wind storms are still uncertain (see Ch. 4: Extreme Events).

IMPACTS OF EXTREME EVENTS 4 ON HUMAN HEALTH

Estimated Deaths and Billion Dollar Losses from Extreme Events

in the United States 2004–2013
EXECUTIVE SUMMARY

U.S. Global Change Research Program 11 Impacts of Climate Change on Human Health in the United States

(Top) A truck gets stuck in the storm surge covering Highway 90 in Gulfport, Mississippi, during Hurricane Isaac. (Bottom) Power 

lines damaged in Plaquemines Parish, Louisiana, by Hurricane Isaac. September 3, 2012.

Family farmer in drought-stressed peanut field, Unadilla,

Georgia. July 24, 2012.

Disruption of Essential Infrastructure

Key Finding 2: Many types of extreme events 

related to climate change cause disruption of 

infrastructure, including power, water, transportation, 

and communication systems, that are essential to 

maintaining access to health care and emergency 

response services and safeguarding human health 

[High Confidence].

Vulnerability to Coastal Flooding

Key Finding 3: Coastal populations with greater 

vulnerability to health impacts from coastal flooding 

include persons with disabilities or other access and 

functional needs, certain populations of color, older 

adults, pregnant women and children, low-income 

populations, and some occupational groups [High 

Confidence]. Climate change will increase exposure 

risk to coastal flooding due to increases in extreme 

precipitation and in hurricane intensity and rainfall 

rates, as well as sea level rise and the resulting 

increases in storm surge [High Confidence].

© Mike Theiss/National Geographic Creative/Corbis © Julie Dermansky/Corbis

© Erik S. Lesser/epa/epa/Corbis

Increased Exposure to Extreme Events

Key Finding 1: Health impacts associated 

with climate-related changes in exposure to 

extreme events include death, injury, or illness; 

exacerbation of underlying medical conditions; 

and adverse effects on mental health [High 

Confidence]. Climate change will increase 

exposure risk in some regions of the United States 

due to projected increases in the frequency and/or 

intensity of drought, wildfires, and flooding related 

to extreme precipitation and hurricanes [Medium 

Confidence].
EXECUTIVE SUMMARY

U.S. Global Change Research Program 12 Impacts of Climate Change on Human Health in the United States

Vector-borne diseases are illnesses that are 

transmitted by vectors, which include mosquitoes, ticks, and fleas. These vectors can carry 

infective pathogens such as viruses, bacteria, 

and protozoa, which can be transferred from one 

host (carrier) to another. The seasonality, distribution, and prevalence of vector-borne diseases 

are influenced significantly by climate factors, 

primarily high and low temperature extremes 

and precipitation patterns. 

Climate change is likely to have both short- and 

long-term effects on vector-borne disease transmission and infection patterns, affecting both 

seasonal risk and broad geographic changes in 

disease occurrence over decades. While climate 

variability and climate change both alter the 

transmission of vector-borne diseases, they will 

likely interact with many other factors, including 

how pathogens adapt and change, the availability of hosts, changing ecosystems and land use, 

demographics, human behavior, and adaptive 

capacity. These complex interactions make it 

difficult to predict the effects of climate change 

on vector-borne diseases.

In the eastern United States, Lyme disease is transmitted to 

humans primarily by blacklegged (deer) ticks. 

© Science Stills/ARS/Visuals Unlimited/Corbis

VECTOR-BORNE 5 DISEASES

Maps show the reported cases of Lyme disease in 2001 and 2014 for the areas of the country where Lyme disease is most 

common (the Northeast and Upper Midwest). Both the distribution and the numbers of cases have increased (see Ch. 5: VectorBorne Diseases). (Figure source: adapted from CDC 2015)6 © iStockPhoto.com/powerofforever

2001 2014

Changes in Lyme Disease Case Report Distribution
EXECUTIVE SUMMARY

U.S. Global Change Research Program 13 Impacts of Climate Change on Human Health in the United States

Changing Distributions of Vectors and 

Vector-Borne Diseases

Key Finding 1: Climate change is expected to 

alter the geographic and seasonal distributions of 

existing vectors and vector-borne diseases [Likely, 

High Confidence].

Earlier Tick Activity and Northward Range 

Expansion

Key Finding 2: Ticks capable of carrying the 

bacteria that cause Lyme disease and other 

pathogens will show earlier seasonal activity and 

a generally northward expansion in response to 

increasing temperatures associated with climate 

change [Likely, High Confidence]. Longer seasonal 

activity and expanding geographic range of these 

ticks will increase the risk of human exposure to 

ticks [Likely, Medium Confidence].

Changing Mosquito-Borne Disease 

Dynamics

Key Finding 3: Rising temperatures, changing 

precipitation patterns, and a higher frequency 

of some extreme weather events associated with 

climate change will influence the distribution, 

abundance, and prevalence of infection in the 

mosquitoes that transmit West Nile virus and 

other pathogens by altering habitat availability and 

mosquito and viral reproduction rates [Very Likely, 

High Confidence]. Alterations in the distribution, 

abundance, and infection rate of mosquitoes will 

influence human exposure to bites from infected 

mosquitoes, which is expected to alter risk for 

human disease [Very Likely, Medium Confidence].

Emergence of New Vector-Borne 

Pathogens

Key Finding 4: Vector-borne pathogens are 

expected to emerge or reemerge due to the 

interactions of climate factors with many other 

drivers, such as changing land-use patterns 

[Likely, High Confidence]. The impacts to human 

disease, however, will be limited by the adaptive 

capacity of human populations, such as vector 

control practices or personal protective measures

[Likely, High Confidence]. Birds such as the house finch are the natural host of West 

Nile virus. Humans can be infected from a bite of a mosquito 

that has previously bitten an infected bird. 

© CDC/Science Faction/Corbis © Pete Oxford/Minden Pictures/Corbis © iStockPhoto.com/leekris
EXECUTIVE SUMMARY

U.S. Global Change Research Program 14 Impacts of Climate Change on Human Health in the United States

Precipitation and temperature changes affect fresh and marine water quantity and quality primarily through urban, 

rural, and agriculture runoff. This runoff in turn affects human exposure to water-related illnesses primarily through 

contamination of drinking water, recreational water, and fish or shellfish (see Ch. 6: Water-Related Illness).

Across most of the United States, climate change 

is expected to affect fresh and marine water 

resources in ways that will increase people’s 

exposure to water-related contaminants that cause 

illness. Water-related illnesses include waterborne 

diseases caused by pathogens, such as bacteria, 

viruses, and protozoa. Water-related illnesses 

are also caused by toxins produced by certain 

harmful algae and cyanobacteria and by chemicals 

introduced into the environment by human 

activities. Exposure occurs through ingestion, 

inhalation, or direct contact with contaminated 

drinking or recreational water and through 

consumption of contaminated fish and shellfish. 

Factors related to climate change—including 

temperature, precipitation and related runoff, 

hurricanes, and storm surge—affect the growth, 

survival, spread, and virulence or toxicity of agents 

(causes) of water-related illness. Whether or not 

illness results from exposure to contaminated 

water, fish, or shellfish is dependent on a complex 

set of factors, including human behavior and social 

determinants of health that may affect a person’s 

exposure, sensitivity, and adaptive capacity. 

Water resource, public health, and environmental 

agencies in the United States provide many public 

health safeguards to reduce risk of exposure and 

illness even if water becomes contaminated. These 

include water quality monitoring, drinking water 

treatment standards and practices, beach closures, 

and issuing advisories for boiling drinking water 

and harvesting shellfish.

Links between Climate Change, Water Quantity and 

Quality, and Human Exposure to Water-Related Illness

CLIMATE IMPACTS ON 6 WATER-RELATED ILLNESSES
EXECUTIVE SUMMARY

U.S. Global Change Research Program 15 Impacts of Climate Change on Human Health in the United States

Seasonal and Geographic Changes in 

Waterborne Illness Risk

Key Finding 1: Increases in water temperatures 

associated with climate change will alter the 

seasonal windows of growth and the geographic 

range of suitable habitat for freshwater toxinproducing harmful algae [Very Likely, High 

Confidence], certain naturally occurring Vibrio

bacteria [Very Likely, Medium Confidence], and 

marine toxin-producing harmful algae [Likely, 

Medium Confidence]. These changes will increase 

the risk of exposure to waterborne pathogens and 

algal toxins that can cause a variety of illnesses 

[Medium Confidence].

Runoff from Extreme Precipitation 

Increases Exposure Risk

Key Finding 2: Runoff from more frequent 

and intense extreme precipitation events will 

increasingly compromise recreational waters, 

shellfish harvesting waters, and sources of 

drinking water through increased introduction of 

pathogens and prevalence of toxic algal blooms 

[High Confidence]. As a result, the risk of human 

exposure to agents of water-related illness will 

increase [Medium Confidence].

Water Infrastructure Failure

Key Finding 3: Increases in some extreme weather 

events and storm surges will increase the risk 

that infrastructure for drinking water, wastewater, 

and stormwater will fail due to either damage 

or exceedance of system capacity, especially in 

areas with aging infrastructure [High Confidence]. 

As a result, the risk of exposure to water-related 

pathogens, chemicals, and algal toxins will 

increase in recreational and shellfish harvesting 

waters, and in drinking water where treatment 

barriers break down [Medium Confidence].

Red tide bloom, Hood Canal, Puget Sound, Washington State.

Young women walk through floodwater in the historic district 

of Charleston, South Carolina, as Hurricane Joaquin passes 

offshore. October 4, 2015.

© Don Paulson/SuperStock/Corbis © Richard Ellis/Corbis
EXECUTIVE SUMMARY

U.S. Global Change Research Program 16 Impacts of Climate Change on Human Health in the United States

The food system involves a network of interactions with our physical and biological environments as food 

moves from production to consumption, or from “farm to table.” Rising CO2 and climate change will affect the 

quality and distribution of food, with subsequent effects on food safety and nutrition (see Ch. 7: Food Safety).

Farm to Table

The Potential Interactions of Rising CO2 and Climate Change on Food Safety

A safe and nutritious food supply is a vital component of food security. The impacts of climate 

change on food production, prices, and trade for 

the United States and globally have been widely 

examined, including in the recent report “Climate 

Change, Global Food Security, and the U.S. Food 

System.”7 An overall finding of that report was 

that “climate change is very likely to affect global, 

regional, and local food security by disrupting food 

availability, decreasing access to food, and making 

utilization more difficult.” 

This chapter focuses on some of the less reported 

aspects of food security, specifically the impacts 

of climate change on food safety, nutrition, and 

distribution. There are two overarching means by 

which increasing carbon dioxide (CO2) and climate 

change alter safety, nutrition, and distribution of 

food. The first is associated with rising global temperatures and the subsequent changes in weather 

patterns and extreme climate events. Current and 

anticipated changes in climate and the physical 

environment have consequences for contamination, spoilage, and the disruption of food distribution. The second pathway is through the direct 

CO2 “fertilization” effect on plant photosynthesis. 

Higher concentrations of CO2 stimulate growth and 

carbohydrate production in some plants, but can 

lower the levels of protein and essential minerals 

in a number of widely consumed crops, including 

wheat, rice, and potatoes, with potentially negative 

implications for human nutrition.

FOOD SAFETY, NUTRITION, 7 AND DISTRIBUTION
EXECUTIVE SUMMARY

U.S. Global Change Research Program 17 Impacts of Climate Change on Human Health in the United States

Increased Risk of Foodborne Illness

Key Finding 1: Climate change, including 

rising temperatures and changes in weather 

extremes, is expected to increase the exposure 

of food to certain pathogens and toxins [Likely, 

High Confidence]. This will increase the risk 

of negative health impacts [Likely, Medium 

Confidence], but actual incidence of foodborne 

illness will depend on the efficacy of practices 

that safeguard food in the United States [High 

Confidence]. 

Chemical Contaminants in the Food 

Chain

Key Finding 2: Climate change will increase 

human exposure to chemical contaminants in 

food through several pathways [Likely, Medium 

Confidence]. Elevated sea surface temperatures 

will lead to greater accumulation of mercury 

in seafood [Likely, Medium Confidence], 

while increases in extreme weather events will 

introduce contaminants into the food chain 

[Likely, Medium Confidence]. Rising carbon 

dioxide concentrations and climate change 

will alter incidence and distribution of pests, 

parasites, and microbes [Very Likely, High 

Confidence], leading to increases in the use of 

pesticides and veterinary drugs [Likely, Medium 

Confidence].

Rising Carbon Dioxide Lowers Nutritional 

Value of Food

Key Finding 3: The nutritional value of 

agriculturally important food crops, such as 

wheat and rice, will decrease as rising levels of 

atmospheric carbon dioxide continue to reduce the 

concentrations of protein and essential minerals in 

most plant species [Very Likely, High Confidence].

Extreme Weather Limits Access to Safe 

Foods

Key Finding 4: Increases in the frequency or 

intensity of some extreme weather events 

associated with climate change will increase 

disruptions of food distribution by damaging 

existing infrastructure or slowing food shipments 

[Likely, High Confidence]. These impediments lead 

to increased risk for food damage, spoilage, or 

contamination, which will limit availability of and 

access to safe and nutritious food depending on 

the extent of disruption and the resilience of food 

distribution infrastructure [Medium Confidence].

(Left) The risk of foodborne illness is higher when food is prepared outdoors. (Right) Crop dusting of a corn field in Iowa. © Hill Street Studios/Blend Images/Corbis

© Lucas Payne/AgStock Images/Corbis
EXECUTIVE SUMMARY

U.S. Global Change Research Program 18 Impacts of Climate Change on Human Health in the United States

Children are at particular risk for distress, anxiety, and 

other adverse mental health effects in the aftermath of 

an extreme event.

The effects of global climate change on mental 

health and well-being are integral parts of the overall 

climate-related human health impacts. Mental health 

consequences of climate change range from minimal 

stress and distress symptoms to clinical disorders, 

such as anxiety, depression, post-traumatic stress, 

and suicidality. Other consequences include effects 

on the everyday life, perceptions, and experiences of 

individuals and communities attempting to understand and respond appropriately to climate change 

and its implications. The mental health and well-being consequences of climate change related impacts 

rarely occur in isolation, but often interact with other 

social and environmental stressors. The interactive 

and cumulative nature of climate change effects on 

health, mental health, and well-being are critical 

factors in understanding the overall consequences of 

climate change on human health.

The Impact of Climate Change on Physical, Mental, and Community Health

At the center of the diagram are human figures representing adults, children, older adults, and people with disabilities. The left 

circle depicts climate impacts including air quality, wildfire, sea level rise and storm surge, heat, storms, and drought. The right 

circle shows the three interconnected health domains that will be affected by climate impacts: Medical and Physical Health, 

Mental Health, and Community Health (see Ch. 8: Mental Health). (Figure source: adapted from Clayton et al. 2014)7

MENTAL HEALTH AND 8 WELL-BEING

© Aurora/Aurora Photo/Corbis
EXECUTIVE SUMMARY

U.S. Global Change Research Program 19 Impacts of Climate Change on Human Health in the United States

(Top) Rescue worker receives hug from Galveston, TX, 

resident after Hurricane Ike, September 2008. (Bottom) 

People experience the threat of climate change through 

frequent media coverage.

Residents and volunteers in the Rockaways section of 

Queens in New York City filter through clothes and food 

supplies from donors following Superstorm Sandy. 

November 3, 2012.

Exposure to Disasters Results in Mental 

Health Consequences

Key Finding 1: Many people exposed to 

climate-related or weather-related disasters 

experience stress and serious mental health 

consequences. Depending on the type of the 

disaster, these consequences include posttraumatic stress disorder (PTSD), depression, 

and general anxiety, which often occur at the 

same time [Very High Confidence]. The majority 

of affected people recover over time, although 

a significant proportion of exposed individuals 

develop chronic psychological dysfunction [High 

Confidence].

Specific Groups of People Are at 

Higher Risk 

Key Finding 2: Specific groups of people are at 

higher risk for distress and other adverse mental 

health consequences from exposure to climaterelated or weather-related disasters. These groups 

include children, the elderly, women (especially 

pregnant and post-partum women), people with 

preexisting mental illness, the economically 

disadvantaged, the homeless, and first responders 

[High Confidence]. Communities that rely on the 

natural environment for sustenance and livelihood, 

as well as populations living in areas most 

susceptible to specific climate change events, 

are at increased risk for adverse mental health 

outcomes [High Confidence].

Climate Change Threats Result in Mental 

Health Consequences and Social Impacts

Key Finding 3: Many people will experience adverse 

mental health outcomes and social impacts from 

the threat of climate change, the perceived direct 

experience of climate change, and changes to 

one’s local environment [High Confidence]. Media 

and popular culture representations of climate 

change influence stress responses and mental 

health and well-being [Medium Confidence].

Extreme Heat Increases Risks for People 

with Mental Illness

Key Finding 4: People with mental illness are at 

higher risk for poor physical and mental health 

due to extreme heat [High Confidence]. Increases 

in extreme heat will increase the risk of disease 

and death for people with mental illness, including 

elderly populations and those taking prescription 

medications that impair the body’s ability to 

regulate temperature [High Confidence].

© U.S. Air Force/Staff Sgt. James L. Harper Jr. Courtesy of flickr/flash.pro

© Alec McClure/Demotix/Corbis
EXECUTIVE SUMMARY

U.S. Global Change Research Program 20 Impacts of Climate Change on Human Health in the United States

Determinants of Vulnerability

Defining the determinants of vulnerability to health impacts associated with climate change, including 

exposure, sensitivity, and adaptive capacity (see Ch. 9: Populations of Concern). (Figure source: adapted 

from Turner et al. 2003)8

Climate change is already causing, and is expected 

to continue to cause, a range of health impacts 

that vary across different population groups in 

the United States. The vulnerability of any given 

group is a function of its sensitivity to climate 

change related health risks, its exposure to those 

risks, and its capacity for responding to or coping 

with climate variability and change. Vulnerable 

groups of people, described here as populations 

of concern, include those with low income, some 

communities of color, immigrant groups (including 

those with limited English proficiency), Indigenous peoples, children and pregnant women, older 

adults, vulnerable occupational groups, persons 

with disabilities, and persons with preexisting or 

chronic medical conditions. Characterizations of 

vulnerability should consider how populations of 

concern experience disproportionate, multiple, 

and complex risks to their health and well-being in 

response to climate change.

Vulnerability Varies Over Time and Is 

Place-Specific

Key Finding 1: Across the United States, people 

and communities differ in their exposure, their 

inherent sensitivity, and their adaptive capacity to 

respond to and cope with climate change related 

health threats [Very High Confidence]. Vulnerability 

to climate change varies across time and location, 

across communities, and among individuals within 

communities [Very High Confidence].

Health Impacts Vary with Age and Life 

Stage

Key Finding 2: People experience different inherent 

sensitivities to the impacts of climate change at 

different ages and life stages [High Confidence]. 

For example, the very young and the very old are 

particularly sensitive to climate-related health 

impacts. 

POPULATIONS OF 9 CONCERN
EXECUTIVE SUMMARY

U.S. Global Change Research Program 21 Impacts of Climate Change on Human Health in the United States

Social Determinants of Health Interact 

with Climate Factors to Affect Health Risk

Key Finding 3: Climate change threatens the 

health of people and communities by affecting 

exposure, sensitivity, and adaptive capacity [High 

Confidence]. Social determinants of health, 

such as those related to socioeconomic factors 

and health disparities, may amplify, moderate, 

or otherwise influence climate-related health 

effects, particularly when these factors occur 

simultaneously or close in time or space [High 

Confidence]. 

Mapping Tools and Vulnerability Indices 

Identify Climate Health Risks 

Key Finding 4: The use of geographic data and 

tools allows for more sophisticated mapping of risk 

factors and social vulnerabilities to identify and 

protect specific locations and groups of people 

[High Confidence].

(Left) Persons with disabilities often rely on medical 

equipment (such as portable oxygen) that requires an 

uninterrupted source of electricity. (Right) Climate-related 

exposures may lead to adverse pregnancy and newborn 

health outcomes.

© JGI/Tom Grill/Blend Images/Corbis

© iStockPhoto.com/ozqurkeser

Because of existing vulnerabilities, Indigenous people, especially those who are dependent on the environment for sustenance 

or who live in geographically isolated or impoverished communities, are likely to experience greater exposure and lower 

resilience to climate-related health effects.
EXECUTIVE SUMMARY

U.S. Global Change Research Program 22 Impacts of Climate Change on Human Health in the United States

1. Melillo, J.M., T.C. Richmond, and G.W. Yohe, Eds., 2014:

Climate Change Impacts in the United States: The Third 

National Climate Assessment. U.S. Global Change Research 

Program, Washington, D.C., 842 pp. http://dx.doi.

org/10.7930/J0Z31WJ2

2. Schwartz, J.D., M. Lee, P.L. Kinney, S. Yang, D. Mills, M. 

Sarofim, R. Jones, R. Streeter, A. St. Juliana, J. Peers, and 

R.M. Horton, 2015: Projections of temperature-attributable 

premature deaths in 209 U.S. cities using a cluster-based 

Poisson approach. Environmental Health, 14. http://dx.doi.

org/10.1186/s12940-015-0071-2 

3. Fann, N., C.G. Nolte, P. Dolwick, T.L. Spero, A. Curry 

Brown, S. Phillips, and S. Anenberg, 2015: The geographic 

distribution and economic value of climate change-related 

ozone health impacts in the United States in 2030. Journal 

of the Air & Waste Management Association, 65, 570-580. 

http://dx.doi.org/10.1080/10962247.2014.996270 

4. NOAA, 2015: Natural Hazard Statistics: Weather Fatalities. 

National Oceanic and Atmospheric Administration, National Weather Service, Office of Climate, Water, and Weather 

Services. http://www.nws.noaa.gov/om/hazstats.shtml

5. Smith, A.B. and R.W. Katz, 2013: US billion-dollar weather and climate disasters: Data sources, trends, accuracy 

and biases. Natural Hazards, 67, 387-410. http://dx.doi.

org/10.1007/s11069-013-0566-5 

6. CDC, 2015: Lyme Disease: Data and Statistics: MapsReported Cases of Lyme Disease – United States, 2001-

2014. Centers for Disease Control and Prevention. http://

www.cdc.gov/lyme/stats/

7. Brown, M.E., J.M. Antle, P. Backlund, E.R. Carr, W.E. 

Easterling, M.K. Walsh, C. Ammann, W. Attavanich, C.B. 

Barrett, M.F. Bellemare, V. Dancheck, C. Funk, K. Grace, 

J.S.I. Ingram, H. Jiang, H. Maletta, T. Mata, A. Murray, M. 

Ngugi, D. Ojima, B. O’Neill, and C. Tebaldi, 2015: Climate 

Change, Global Food Security, and the U.S. Food System. 

146 pp. U.S. Global Change Research Program. http://www.

usda.gov/oce/climate_change/FoodSecurity2015Assessment/FullAssessment.pdf

References

8. Clayton, S., C.M. Manning, and C. Hodge, 2014: Beyond 

Storms & Droughts: The Psychological Impacts of Climate Change. 51 pp. American Psychological Association 

and ecoAmerica, Washington, D.C. http://ecoamerica.org/

wp-content/uploads/2014/06/eA_Beyond_Storms_and_

Droughts_Psych_Impacts_of_Climate_Change.pdf

9. Turner, B.L., R.E. Kasperson, P.A. Matson, J.J. McCarthy, 

R.W. Corell, L. Christensen, N. Eckley, J.X. Kasperson, A. 

Luers, M.L. Martello, C. Polsky, A. Pulsipher, and A. Schiller, 

2003: A framework for vulnerability analysis in sustainability 

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8074-8079. http://dx.doi.org/10.1073/pnas.1231335100 

End
U.S. Global Change Research Program 23 Impacts of Climate Change on Human Health in the United States

© Richard T. Nowitz/Corbis 
U.S. Global Change Research Program 24 Impacts of Climate Change on Human Health in the United States

© iStockPhoto.com/ © Ints Vikmanis 
INTRODUCTION: CLIMATE 1 CHANGE AND HUMAN HEALTH

On the web: health2016.globalchange.gov

THE IMPACTS OF CLIMATE CHANGE ON HUMAN HEALTH IN THE UNITED STATES

A Scientific Assessment

U.S. Global Change Research Program

Lead Authors

John Balbus

National Institutes of Health

Allison Crimmins*

U.S. Environmental Protection Agency

Janet L. Gamble

U.S. Environmental Protection Agency 

Contributing Authors

David R. Easterling

National Oceanic and Atmospheric Administration 

Kenneth E. Kunkel

Cooperative Institute for Climate and Satellites–North Carolina 

Shubhayu Saha

Centers for Disease Control and Prevention

Marcus C. Sarofim

U.S. Environmental Protection Agency

Recommended Citation: Balbus, J., A. Crimmins, J.L. Gamble, D.R. Easterling, K.E. Kunkel, S. Saha, and M.C. Sarofim, 2016: Ch. 1: 

Introduction: Climate Change and Human Health. The Impacts of Climate Change on Human Health in the United States: A Scientific 

Assessment. U.S. Global Change Research Program, Washington, DC, 25–42. http://dx.doi.org/10.7930/J0VX0DFW

*Chapter Coordinator
26

INTRODUCTION: CLIMATE CHANGE 1 AND HUMAN HEALTH

Human health has always been influenced by climate and weather. Changes in climate and climate 

variability, particularly changes in weather extremes, affect the environment that provides us with clean 

air, food, water, shelter, and security. Climate change, together with other natural and human-made health 

stressors, threatens human health and well-being in numerous ways. Some of these health impacts are 

already being experienced in the United States. 

Given that the impacts of climate change are projected to increase over the next century, certain existing 

health threats will intensify and new health threats may emerge. Connecting our understanding of how 

climate is changing with an understanding of how those changes may affect human health can inform 

decisions about mitigating (reducing) the amount of future climate change, suggest priorities for protecting 

public health, and help identify research needs.

1.1 Our Changing Climate 

Observed Climate Change

The fact that the Earth has warmed over the last century is 

unequivocal. Multiple observations of air and ocean temperatures, sea level, and snow and ice have shown these changes to 

be unprecedented over decades to millennia. Human influence 

has been the dominant cause of this observed warming.1 The 

2014 U.S. National Climate Assessment (2014 NCA) found that 

rising temperatures, the resulting increases in the frequency or 

intensity of some extreme weather events, rising sea levels, and 

melting snow and ice are already disrupting people’s lives and 

damaging some sectors of the U.S. economy.2

The concepts of climate and weather are often confused. 

Weather is the state of the atmosphere at any given time and 

place. Weather patterns vary greatly from year to year and 

from region to region. Familiar aspects of weather include 

temperature, precipitation, clouds, and wind that people experience throughout the course of a day. Severe weather conditions include hurricanes, tornadoes, blizzards, and droughts.

Climate is the average weather conditions that persist over 

multiple decades or longer. While the weather can change in 

minutes or hours, identifying a change in climate has required 

observations over a time period of decades to centuries or longer. Climate change encompasses both increases and decreases in temperature as well as shifts in precipitation, changing 

risks of certain types of severe weather events, and changes to 

other features of the climate system.

Observed changes in climate and weather differ at local and 

regional scales (Figure 1). Some climate and weather changes 

already observed in the United States include:2, 3

• U.S. average temperature has increased by 1.3°F to 1.9°F 

since record keeping began in 1895; most of this increase 

has occurred since about 1970. The first decade of the 

2000s (2000–2009) was the warmest on record throughout 

the United States. 
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U.S. Global Change Research Program 27 Impacts of Climate Change on Human Health in the United States

• Average U.S. precipitation has increased since 1900, but 

some areas have experienced increases greater than the national average, and some areas have experienced decreases. 

• Heavy downpours are increasing nationally, especially over 

the last three to five decades. The largest increases are in 

the Midwest and Northeast, where floods have also been 

increasing. Figure 2 shows how the annual number of heavy 

downpours, defined as extreme two-day precipitation 

events, for the contiguous United States has increased, 

particularly between the 1950s and the 2000s. 

• Drought has increased in the West. Over the last decade, the Southwest has experienced the most persistent 

droughts since record keeping began in 1895.4 Changes 

in precipitation and runoff, combined with changes in 

consumption and withdrawal, have reduced surface and 

groundwater supplies in many areas.

• There have been changes in some other types of extreme 

weather events over the last several decades. Heat waves 

have become more frequent and intense, especially in the 

West. Cold waves have become less frequent and intense 

across the nation. 

Major U.S. Climate Trends

Figure 1: Major U.S. national and regional climate trends. Shaded areas are the U.S. regions defined in the 2014 NCA.2, 4

Change in Number of Extreme 

Precipitation Events

Figure 2: Time series of 5-year averages of the number of 

extreme 2-day duration precipitation events, averaged over the 

United States from 1900 to 2014. The number is expressed as 

the percent difference from the average for the entire period. 

This is based on 726 stations that have precipitation data for 

at least 90% of the days in the period. An event is considered 

extreme if the precipitation amount exceeds a threshold for a 

once-per-year recurrence. (Figure source: adapted from Melillo 

et al. 2014)2
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U.S. Global Change Research Program 28 Impacts of Climate Change on Human Health in the United States

• The intensity, frequency, and duration of North Atlantic hurricanes, as well as the frequency of the strongest (category 

4 and 5) hurricanes, have all increased since the early 1980s. 

The relative contributions of human and natural causes to 

these increases remain uncertain. 

Projected Climate Change

Projections of future climate conditions are based on results 

from climate models—sophisticated computer programs that 

simulate the behavior of the Earth’s climate system. These 

climate models are used to project how the climate system is 

expected to change under different possible scenarios. These 

scenarios describe future changes in atmospheric greenhouse 

gas concentrations, land use, other human influences on 

climate, and natural factors. The most recent set of coordinated climate model simulations use a set of scenarios called 

Representative Concentration Pathways (RCPs), which describe four possible trajectories in greenhouse gas concentrations.1

 Actual future greenhouse gas concentrations, and the 

resulting amount of future climate change, will still largely be 

determined by choices society makes about emissions.2 The 

RCPs, and the temperature increases associated with these 

scenarios, are described in more detail in Appendix 1: Technical Support Document and in the 2014 NCA.3, 5, 6

Some of the projected changes in climate in the United States 

as described in the 2014 NCA are listed below:2, 3

• Temperatures in the United States are expected to continue 

to rise. This temperature rise has not been, and will not 

be, uniform across the country or over time (Figure 3, top 

panels).

• Increases are also projected for extreme temperature conditions. The temperature of both the hottest day and coldest 

night of the year are projected to increase (Figure 4, top 

panels). 

• More winter and spring precipitation is projected for the 

northern United States, and less for the Southwest, over this 

century (Figure 3, bottom panels).

• Increases in the frequency and intensity of extreme precipitation events are projected for all U.S. areas (Figure 4, 

bottom panels).

• Short-term (seasonal or shorter) droughts are expected to 

intensify in most U.S. regions. Longer-term droughts are 

expected to intensify in large areas of the Southwest, the 

southern Great Plains, and the Southeast. Trends in reduced 

surface and groundwater supplies in many areas are expected to continue, increasing the likelihood of water shortages 

for many uses.

Projected Changes in Temperature and Precipitation by Mid-Century

Figure 3: Projected changes in annual average 

temperature (top) and precipitation (bottom) for 2021–

2050 (left) and 2041–2070 (right) with respect to the 

average for 1971–2000 for the RCP6.0 scenario. 

The RCP6.0 pathway projects an average global 

temperature increase of 5.2°F in 2100 over the 

1901–1960 global average temperature (the RCPs 

are described in more detail in Appendix 1: Technical 

Support Document). Temperature increases in the 

United States for this scenario (top panels) are in 

the 2°F to 3°F range for 2021 to 2050 and 2°F to 4°F 

for 2041 to 2070. This means that the increase in 

temperature projected in the United States over the 

next 50 years under this scenario would be larger 

than the 1°F to 2°F increase in temperature that has 

already been observed over the previous century. 

Precipitation is projected to decrease in the Southwest 

and increase in the Northeast (bottom panels). These 

projected changes are statistically significant (95% 

confidence) in small portions of the Northeast, as 

indicated by the hatching. (Figure source: adapted 

from Sun et al. 2015)54
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U.S. Global Change Research Program 29 Impacts of Climate Change on Human Health in the United States

• Heat waves are projected to become more intense, and cold 

waves less intense, everywhere in the United States. 

• Hurricane-associated storm intensity and rainfall rates are 

projected to increase as the climate continues to warm. 

1.2 How Does Climate Change Affect Health?

The influences of weather and climate on human health are 

significant and varied. They range from the clear threats of 

temperature extremes and severe storms to connections that 

may seem less obvious. For example, weather and climate 

affect the survival, distribution, and behavior of mosquitoes, 

ticks, and rodents that carry diseases like West Nile virus or 

Lyme disease. Climate and weather can also affect water and 

food quality in particular areas, with implications for human 

health. In addition, the effects of global climate change on 

mental health and well-being are integral parts of the overall 

climate-related human health impact.

A useful approach to understand how climate change affects 

health is to consider specific exposure pathways and how they 

can lead to human disease. The concept of exposure pathways 

is adapted from its use in chemical risk assessment, and in this 

context describes the main routes by which climate change affects health (see Figure 5). Exposure pathways differ over time 

and in different locations, and climate change related exposures can affect different people and different communities to 

different degrees. While often assessed individually, exposure 

to multiple climate change threats can occur simultaneously, 

resulting in compounding or cascading health impacts. Climate 

change threats may also accumulate over time, leading to 

longer-term changes in resilience and health.

Whether or not a person is exposed to a health threat or 

suffers illness or other adverse health outcomes from that 

exposure depends on a complex set of vulnerability factors. 

Vulnerability is the tendency or predisposition to be adversely 

affected by climate-related health effects, and encompasses 

three elements: exposure, sensitivity or susceptibility to harm, 

and the capacity to adapt or to cope (see also Figure 1 in Ch. 

9: Populations of Concern). Because multiple disciplines use 

these terms differently and multiple definitions exist in the 

literature, the distinctions between them are not always clear.7

All three of these elements can change over time and are 

place- and system-specific.8 In the context of this report, we 

define the three elements of vulnerability as follows (definitions adapted from IPCC 2014 and NRC 2012)9, 10

• Exposure is contact between a person and one or more 

biological, psychosocial, chemical, or physical stressors, 

including stressors affected by climate change. Contact may 

occur in a single instance or repeatedly over time, and may 

occur in one location or over a wider geographic area.

• Sensitivity is the degree to which people or communities 

are affected, either adversely or beneficially, by climate 

variability or change. 

Projected Changes in the Hottest/Coldest and Wettest/Driest Day of the Year

Figure 4: Projected changes in several climate 

variables for 2046–2065 with respect to the 1981–

2000 average for the RCP6.0 scenario. These include 

the coldest night of the year (top left) and the hottest 

day of the year (top right). By the middle of this 

century, the coldest night of the year is projected to 

warm by 6°F to 10°F over most of the country, with 

slightly smaller changes in the south. The warmest 

day of the year is projected to be 4°F to 6°F warmer 

in most areas. Also shown are projections of the 

wettest day of the year (bottom left) and the annual 

longest consecutive dry day spell (bottom right). 

Extreme precipitation is projected to increase, with 

an average change of 5% to 15% in the precipitation 

falling on the wettest day of the year. The length of 

the annual longest dry spell is projected to increase 

in most areas, but these changes are small: less than 

two days in most areas. (Figure source: adapted from 

Sun et al. 2015)54
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U.S. Global Change Research Program 30 Impacts of Climate Change on Human Health in the United States

• Adaptive capacity is the ability of communities, institutions, 

or people to adjust to potential hazards, to take advantage 

of opportunities, or to respond to consequences. A related 

term, resilience, is the ability to prepare and plan for, absorb, recover from, and more successfully adapt to adverse 

events.

Vulnerability, and the three components of vulnerability, are 

factors that operate at multiple levels, from the individual and 

community to the country level, and affect all people to some 

degree.8 For an individual, these factors include human behavioral choices and the degree to which that person is vulnerable 

based on his or her level of exposure, sensitivity, and adaptive 

capacity. Vulnerability is also influenced by social determinants 

of health (see Ch. 9 Populations of Concern), including those 

that affect a person’s adaptive capacity, such as social capital 

and social cohesion (for example, the strength of interpersonal 

networks and social patterns in a community). 

At a larger community or societal scale, health outcomes are 

strongly influenced by adaptive capacity factors, including 

those related to the natural and built environments (for example, the state of infrastructure), governance and management 

(health-protective surveillance programs, regulations and 

enforcement, or community health programs), and institutions 

(organizations operating at all levels to form a national public 

health system).11, 12 For example, water resource, public health, 

and environmental agencies in the United States provide many 

public health safeguards, such as monitoring water quality 

and issuing advisories to reduce risk of exposure and illness 

if water becomes contaminated. Some aspects of climate 

change health impacts in the United States may therefore be 

FPO

Climate Change and Health

Figure 5: Conceptual diagram illustrating the exposure pathways by which climate change affects human health. Exposure pathways 

exist within the context of other factors that positively or negatively influence health outcomes (gray side boxes). Key factors that 

influence vulnerability for individuals are shown in the right box, and include social determinants of health and behavioral choices. 

Key factors that influence vulnerability at larger scales, such as natural and built environments, governance and management, and 

institutions, are shown in the left box. All of these influencing factors can affect an individual’s or a community’s vulnerability through 

changes in exposure, sensitivity, and adaptive capacity and may also be affected by climate change.
1–INTRODUCTION: CLIMATE CHANGE AND HUMAN HEALTH

U.S. Global Change Research Program 31 Impacts of Climate Change on Human Health in the United States

mediated by factors like strong social capital, fully functional governance/management, and institutions that maintain 

the Nation’s generally high level of adaptive capacity. On the 

other hand, the evidence base regarding the effectiveness of 

public health interventions in a climate change context is still 

relatively weak.13 Current levels of adaptive capacity may not 

be sufficient to address multiple impacts that occur simultaneously or in close succession, 

or impacts of climate change 

that result in unprecedented 

damages.2, 12

The three components of 

vulnerability (exposure, sensitivity, and adaptive capacity) 

are associated with social and 

demographic factors, including 

level of wealth and education, as well as other characteristics 

of people and places, such as the condition of infrastructure 

and extent of ecosystem degradation. For example, poverty can leave people more exposed to climate and weather 

threats, increase sensitivity because of associations with higher rates of illness and nutritional deficits, and limit people’s 

adaptive capacity. As another example, people living in a city 

with degraded coastal ecosystems and inadequate water and 

wastewater infrastructure may be at greater risk of health 

consequences from severe storms. Figure 5 demonstrates the 

interactions among climate drivers, health impacts, and other 

factors that influence people’s vulnerability to health impacts. 

We are already experiencing changes in the frequency, 

severity, and even the location of some weather and climate 

phenomena, including extreme temperatures, heavy rains 

and droughts, and some other kinds of severe weather, and 

these changes are projected to continue. This means that 

areas already experiencing health-threatening weather and 

climate phenomena, such as severe heat or hurricanes, are 

likely to experience worsening impacts, such as even higher 

temperatures and increased storm intensity, rainfall rates, and 

storm surge. It also means that 

some areas will experience new 

climate-related health threats. 

For example, areas previously unaffected by toxic algal blooms or 

waterborne diseases because of 

cooler water temperatures may 

face these hazards in the future 

as increasing water temperatures allow the organisms that 

cause these health risks to thrive. Even areas that currently 

experience these health threats may see a shift in the timing 

of the seasons that pose the greatest risk to human health.

Climate change can therefore affect human health in two main 

ways: first, by changing the severity or frequency of health 

problems that are already affected by climate or weather factors; and second, by creating unprecedented or unanticipated 

health problems or health threats in places where they have 

not previously occurred. 

1.3 Our Changing Health

In order to understand how climate change creates or exacerbates health problems, assessments of climate change health 

impacts must start with what is known about the current state 

and observed trends in a wide array of health conditions. In 

addition, because preexisting health conditions, socioeconomic status, and life stage all contribute to vulnerability to climate-related and weather-related health effects, assessments 

of climate change health impacts should be informed by projected changes in these factors. In cases where people’s health 

or socioeconomic status is getting worse, climate change may 

accentuate the health burdens associated with those worsening trends. Conversely, in cases where people’s health or socioeconomic status is improving, the effect of climate change 

may be to slow or reduce that improvement. Where the state 

of scientific understanding allows, the inclusion of projected 

trends in health and socioeconomic conditions into models of 

climate change impacts on health can provide useful insights 

into these interactions between non-climate factors and climate change effects.

Demographic and Socioeconomic Trends

The United States is in the midst of several significant demographic changes: the population is aging, growing in number, 

becoming more ethnically diverse, and demonstrating greater 

disparities between the wealthy and the poor. Immigration is 

Current levels of adaptive capacity may not 

be sufficient to address multiple impacts 

that occur simultaneously or in close succession, 

or impacts of climate change that result 

in unprecedented damages.

Storm-damaged home after Hurricane Sandy.
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U.S. Global Change Research Program 32 Impacts of Climate Change on Human Health in the United States

having a major influence on both the size and age distribution 

of the population.14 Each of these demographic trends has 

implications for climate change related human health impacts 

(see Ch. 9: Populations of Concern). Some of these trends and 

projections are summarized below:

Trends in population growth

• The total U.S. population has more than doubled since 1950, 

from 151,325,798 persons in 1950 to 308,745,538 in 2010.15

• The Census Bureau projects that the U.S. population will grow 

to almost 400 million by 2050 (from estimates of about 320 

million in 2014).16

Trends in the elderly population

• The nation’s older adult population (ages 65 and older) will 

nearly double in number from 2015 through 2050, from 

approximately 48 million to 88 million.17 Of those 88 million 

older adults, a little under 19 million will be 85 years of age 

and older.18

Trends in racial and ethnic diversity

• As the United States becomes more diverse, the aggregate 

minority population is projected to become the majority by 

2042.17 The non-Hispanic or non-Latino White population will 

increase, but more slowly than other racial groups. Non-Hispanic Whites are projected to become a minority by 2050.19

• Projections for 2050 suggest that nearly 19% of the population will be immigrants, compared with 12% in 2005.19

• The Hispanic population is projected to nearly double from 

12.5% of the U.S. population in 2000 to 24.6% in 2050.20

Trends in economic disparity

• Income inequality rose and then stabilized during the last 

30 years, and is projected to resume rising over the next 20 

years, though at a somewhat slower overall rate that declines 

to near zero by 2035.21 For example, the Gini coefficient, a 

measure of income inequality, is estimated to have risen by 

18% between 1984 and 2000, and is projected to rise by an 

additional 17% for all workers between 2009 and 2035.21

• America’s communities of color have disproportionately 

higher poverty rates and lower income levels. While racial 

disparities in household wealth were higher in the late 1980s 

than now, trends in more recent years have been toward 

greater inequality. The ratio of the median net household 

worth of White, non-Hispanic versus non-White or Hispanic 

households increased from 6.0 to 7.8 between 2007 and 

2013.22 In 2009, 25.8% of non-Hispanic Blacks and 25.3% of 

Hispanics had incomes below the poverty level as compared 

to 9.4% of non-Hispanic Whites and 12.5% of Asian Americans.23 In 2014, the median income level for a non-Hispanic 

Black household was approximately $35,000, $25,000 lower 

than a non-Hispanic White household.24

Population growth and migration in the United States may place 

more people at risk of the health impacts of climate change, 

especially as more people are located in and around vulnerable 

areas, such as coastal, low-lying, or flood-prone zones;25 densely 

populated urban areas;26 and drought-stricken or wildfire-prone 

regions. Increases in racial and ethnic diversity and in the number of persons living near the poverty line may increase the risk 

of health impacts from climate change. Economic disparity can 

make it difficult for some populations to respond to dangerous 

weather conditions, especially when evacuation is necessary or 

when the aftermath requires rebuilding of homes and businesses not covered by home or property insurance. 

Trends in Health Status 

As a nation, trends in the population’s health are mixed. Some 

major indicators of health, such as life expectancy, are consistently improving, while others, such as rate and number of diabetes deaths, are getting worse. Changes in these metrics may 

differ across populations and over time. For example, though 

rates of obesity have increased in both children and adults over 

the last 30 years or more, rates over just the last decade have 

remained steady for adults but increased among children.27

Incidence: A measure of the frequency with which 

an event, such as a new case of illness, occurs in a 

population over a period of time.

Morbidity: A disease or condition that reduces 

health and the quality of life. The morbidity rate is 

a measure of the frequency of disease among a 

defined population during a specified time period.

Mortality: Death as a health outcome. The mortality 

rate is the number of deaths in a defined population 

during a specified time period.

Premature (early) mortality or death: Deaths that 

occur earlier than a specified age, often the average 

life expectancy at birth. 

Prevalence: A measure of the number or proportion 

of people with a specific disease or condition at a 

specific point in time.

Surveillance: The collection, analysis, interpretation, 

and dissemination of health data. 

Terminology
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U.S. Global Change Research Program 33 Impacts of Climate Change on Human Health in the United States

Climate change impacts to human health will act on top of 

these underlying trends. Some of these underlying health conditions can increase sensitivity to climate change effects such 

as heat waves and worsening air quality (see Ch. 2: Temperature-Related Death and Illness; Ch. 3: Air Quality Impacts; Ch. 

9: Populations of Concern). Understanding the trends in these 

conditions is therefore important in considering how many 

people are likely to experience illness when exposed to these 

climate change effects. Potential climate change related health 

impacts may reduce the improvements that would otherwise 

be expected in some indicators of health status and accentuate 

trends towards poorer health in other health indicators.1, 28

Examples of health indicators that have been improving between 2000 and 2013 include the following:

• Life expectancy at birth increased from 76.8 to 78.8 years.29

• Death rates per 100,000 people from heart disease and cancer decreased from 257.6 to 169.8 and from 199.6 to 163.2, 

respectively.29

• The percent of people over age 18 who say they smoke decreased from 23.2% to 17.8%.29

At the same time, some health trends related to the prevalence of chronic diseases, self-reported ill health, and disease 

risk factors have been getting worse. For example:

• The percentage of adult (18 years and older) Americans describing their health as “poor or fair” increased from 8.9% in 

2000 to 10.3% in 2012.29

• Prevalence of physician-diagnosed diabetes among adults 

aged 20 and over increased from 5.2% in 1988–1994 to 8.4% 

in 2009-2012.29

• The prevalence of obesity among adults (aged 20–74) 

increased by almost three-fold from 1960–1962 (13.4% of 

adults classified as obese) to 2009–2010 (36.1% of adults 

classified as obese).

30

• In the past 30 years, obesity has more than doubled in children and quadrupled in adolescents in the United States. The 

percentage of children aged 6–11 who were obese increased 

from 7% in 1980 to nearly 18% in 2012. Similarly, the percentage of adolescents aged 12–19 years who were obese 

increased from 5% to nearly 21% over the same period. In 

2012, approximately one-third of American children and adolescents were overweight or obese.31

Table 1 shows some examples of underlying health conditions 

that are associated with increased vulnerability to health 

effects from climate change related exposures (see Ch. 9: Populations of Concern for more details) and provides information 

on current status and future trends.

Health status is often associated with demographics and socioeconomic status. Changes in the overall size of the population, 

racial and ethnic composition, and age distribution affect the 

health status of the population. Poverty, educational attainment, access to care, and discrimination all contribute to disparities in the incidence and prevalence of a variety of medical 

conditions (see Ch. 9: Populations of Concern). Some examples 

of these interactions include: 

Older Adults. In 2013, the percentage of adults age 75 

and older described as persons in fair or poor health 

totaled 27.6%, as compared to 6.2% for adults age 18 to 

44.29 Among adults age 65 and older, the number in nursing homes or other residential care facilities totaled 1.8 

million in 2012, with more than 1 million utilizing home 

health care.32

Children. Approximately 9.0% of children in the United 

States have asthma. Between 2011 and 2013, rates for 

Black (15.3%) and Hispanic (8.6%) children were higher 

than the rate for White (7.8%) children.29 Rates of asthma 

were also higher in poor children who live below 100% of 

the poverty level (12.4%).29

Diabetes increases sensitivity to heat stress.
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U.S. Global Change Research Program 34 Impacts of Climate Change on Human Health in the United States

Table 1: Current estimates and future trends in chronic health conditions that interact with the health 

risks associated with climate change.

Health 

Conditions Current Estimates Future Trends Possible Influences of Climate Change

Alzheimer's 

Disease

Approximately 5 million 

Americans over 65 had 

Alzheimer's disease in 2013.33

Prevalence of Alzheimer’s is 

expected to triple to 13.8 million 

by 2050.33

Persons with cognitive 

impairments are vulnerable to 

extreme weather events that 

require evacuation or other 

emergency responses.

Asthma

Average asthma prevalence in 

the U.S. was higher in children 

(9% in 2014)29 than in adults 

(7% in 2013).34 Since the 1980s, 

asthma prevalence increased, 

but rates of asthma deaths and 

hospital admissions declined.35, 36

Stable incidence and increasing 

prevalence of asthma is 

projected in the U.S. in coming 

decades.

Asthma is exacerbated by 

changes in pollen season and 

allergenicity and in exposures 

to air pollutants affected by 

changes in temperature, 

humidity, and wind.28

Chronic 

Obstructive 

Pulmonary Disease 

(COPD)

In 2012, approximately 6.3% of 

adults had COPD. Deaths from 

chronic lung diseases increased 

by 50% from 1980 to 2010.37, 38

Chronic respiratory diseases are 

the third leading cause of death 

and are expected to become 

some of the most costly illnesses 

in coming decades.37

COPD patients are more 

sensitive than the general 

population to changes in 

ambient air quality associated 

with climate change.

Diabetes

In 2012, approximately 9% of 

the total U.S. population had 

diabetes. Approximately 18,400 

people younger than age 20 

were newly diagnosed with type 

1 diabetes in 2008–2009; an 

additional 5,000 were diagnosed 

with type 2.39

New diabetes cases are 

projected to increase from about 

8 cases per 1,000 in 2008 to 

about 15 per 1,000 in 2050. 

If recent increases continue, 

prevalence is projected to 

increase to 33% of Americans by 

2050.40

Diabetes increases sensitivity 

to heat stress; medication and 

dietary needs may increase 

vulnerability during and after 

extreme weather events.

Cardiovascular 

Disease

Cardiovascular disease (CVD) 

is the leading cause of death in 

the U.S.41

By 2030, approximately 41% of 

the U.S. population is projected 

to have some form of CVD.42

Cardiovascular disease 

increases sensitivity to heat 

stress.

Mental Illness

Depression is one of the most 

common types of mental illness, 

with approximately 7% of adults 

reporting a major episode in the 

past year. Lifetime prevalence 

is approximately twice as high 

for women as for men.43 Lifetime 

prevalence is more than 15% for 

anxiety disorders and nearly 4% 

for bipolar disorder.44

By 2050, the total number of 

U.S. adults with depressive 

disorder is projected to increase 

by 35%, from 33.9 million to 45.8 

million, with those over age 65 

having a 117% increase.43

Mental illness may impair 

responses to extreme events; 

certain medications increase 

sensitivity to heat stress.

Obesity

In 2009–2010, approximately 

35% of American adults were 

obese.31 In 2012, approximately 

32% of youth (aged 2–19) were 

overweight or obese.45, 46

By 2030, 51% of the U.S. 

population is expected to be 

obese. Projections suggest a 

33% increase in obesity and 

a 130% increase in severe 

obesity.47

Obesity increases sensitivity to 

high ambient temperatures.

Disability

Approximately 18.7% of the U.S. 

population has a disability. In 

2010, the percent of American 

adults with a disability was 

approximately 16.6% for those 

age 21–64 and 49.8% for 

persons 65 and older.48

The number of older adults with 

activity limitations is expected to 

grow from 22 million in 2005 to 

38 million in 2030.49

Persons with disabilities may 

find it hard to respond when 

evacuation is required and when 

there is no available means of 

transportation or easy exit from 

residences
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U.S. Global Change Research Program 35 Impacts of Climate Change on Human Health in the United States

Non-Hispanic Blacks. In 2014, the percentage of non-Hispanic Blacks of all ages who were described as persons in 

fair or poor health totaled 14.3% as compared to 8.7% for 

Whites. Health risk factors for this population include high 

rates of smoking, obesity, and hypertension in adults, as 

well as high infant death rates.29

Hispanics. The percentage of Hispanics of all ages who 

were described as persons in fair or poor health totaled 

12.7% in 2014. Health disparities for Hispanics include 

moderately higher rates of smoking in adults, low birth 

weights, and infant deaths.29

The impacts of climate change may worsen these health 

disparities by exacerbating some of the underlying conditions 

they create. For example, disparities in life expectancy may be 

exacerbated by the effects of climate change related heat and 

air pollution on minority populations that have higher rates of 

hypertension, smoking, and diabetes. Conversely, public health 

measures that reduce disparities and overall rates of illness in 

populations would lessen vulnerability to worsening of health 

status from climate change effects.

1.4 Quantifying Health Impacts

For some changes in exposures to health risks related to climate 

change, the future rate of a health impact associated with any 

given environmental exposure can be estimated by multiplying 

three values: 1) the baseline rate of the health impact, 2) the 

expected change in exposure, and 3) the exposure–response 

function. An exposure–response function is an estimate of how 

the risk of a health impact changes with changes in exposures, 

and is related to sensitivity, one of the three components of 

vulnerability. For example, an exposure–response function 

for extreme heat might be used to quantify the increase in 

heat-related deaths in a region (the change in health impact) for 

every 1°F increase in daily ambient temperature (the change in 

exposure).

The ability to quantify many types of health impacts is dependent on the availability of data on the baseline incidence or 

prevalence of the health impact, the ability to characterize 

the future changes in the types of exposures relevant to that 

health impact, and how well the relationship between these 

exposures and health impacts is understood. Health impacts 

with many intervening factors, like infectious diseases, may 

require different and more complex modeling approaches. 

Where our understanding of these relationships is strong, 

some health impacts, even those occurring in unprecedented places or times of the year, may in fact be predictable. 

Where there is a lack of data or these relationships are poorly 

understood, health impacts are harder to project. For more 

information on exposure–response (also called dose–response 

or concentration–response) functions, see the Exposure–Response section in Appendix 1: Technical Support Document. 

Information on trends in underlying health or background 

rates of health impacts is summarized in Section 1.3, “Our 

Changing Health.” Data on the incidence and prevalence of 

health conditions are obtained through a complicated system of state- and city-level surveillance programs, national 

health surveys, and national collection of 

data on hospitalizations, emergency room 

visits, and deaths. For example, data on 

the incidence of a number of infectious 

diseases are captured through the National 

Notifiable Diseases Surveillance System.50

This system relies first on the mandatory reporting of specific 

diseases by health care providers to state, local, territorial, 

and tribal health departments. These reporting jurisdictions 

then have the option of voluntarily providing the Centers 

for Disease Control and Prevention (CDC) with data on a set 

of nationally notifiable diseases. Because of challenges with 

getting health care providers to confirm and report specific 

diagnoses of reportable diseases in their patients, and the 

lack of requirements for reporting a consistent set of diseases 

and forwarding data to CDC, incidence of infectious disease is 

generally believed to be underreported, and actual rates are 

uncertain.51

Asthma affects approximately 9% of children in the United 

States.
1–INTRODUCTION: CLIMATE CHANGE AND HUMAN HEALTH

U.S. Global Change Research Program 36 Impacts of Climate Change on Human Health in the United States

Characterizing certain types of climate change related exposures can be a challenge. Exposures can consist of temperature changes and other weather conditions, inhaling air 

pollutants and pollens, consuming unsafe food supplies or 

contaminated water, or experiencing trauma or other mental 

health consequences from weather disasters. For some health 

impacts, the ability to understand the relationships between 

climate-related exposures and health impacts is limited by 

these difficulties in characterizing exposures or in obtaining 

accurate data on the occurrence of illnesses. For these health 

impacts, scientists may not have the capability to project 

changes in a health outcome (like incidence of diseases), and 

can only estimate how risks of exposure will change. For example, modeling capabilities allow projections of the impact of 

rising water temperatures on the concentration of Vibrio bacteria, which provides an understanding of geographic changes 

in exposure but does not capture how people may be exposed 

and how many will actually become sick (see Ch. 6: Water-Related Illness). Nonetheless, the ability to project changes in 

exposure or in intermediate determinants of health impacts 

may improve understanding of the change in health risks, 

even if modeling quantitative changes in health impacts is not 

possible. For example, seasonal temperature and precipitation 

projections may be combined to assess future changes in ambient pollen concentrations (the exposure that creates risk), 

even though the potential associated increase in respiratory 

and allergic diseases (the health impacts) cannot be directly 

modeled (see Ch. 3: Air Quality Impacts). 

Modeling Approaches Used in this Report

Four chapters within this assessment—Ch. 2: Temperature-Related Death and Illness, Ch. 3: Air Quality Impacts, Ch. 5: 

Figure 6: Examples of sources of uncertainty in projecting impacts of climate change on human health. The left column illustrates 

the exposure pathway through which climate change can affect human health. The right column lists examples of key sources of 

uncertainty surrounding effects of climate change at each stage along the exposure pathway.

Sources of Uncertainty
1–INTRODUCTION: CLIMATE CHANGE AND HUMAN HEALTH

U.S. Global Change Research Program 37 Impacts of Climate Change on Human Health in the United States

Adverse health effects attributed to climate 

change can have many economic and social 

consequences, including direct medical costs, 

work loss, increased care giving, and other 

limitations on everyday activities

Vector-Borne Diseases, and Ch. 6: Water-Related Illness—include new peer-reviewed, quantitative analyses based on 

modeling. The analyses highlighted in these chapters mainly 

relied on climate model output from the Coupled Model 

Intercomparison Project Phase 5 (CMIP5). Due to limited data 

availability and computational resources, the studies highlighted in the four chapters analyzed only a subset of the full 

CMIP5 dataset, with most of 

the studies including at least 

one analysis based on RCP6.0, 

an upper midrange greenhouse 

gas concentration pathway, to 

facilitate comparisons across 

chapters. For example, the air 

quality analysis examined results 

from two different RCPs, with a 

different climate model used for 

each, while the waterborne analyses examined results from 

21 of the CMIP5 models for a single RCP. See the Guide to the 

Report and Appendix 1: Technical Support Document for more 

on modeling and scenarios. 

Adverse health effects attributed to climate change can have 

many economic and social consequences, including direct 

medical costs, work loss, increased care giving, and other limitations on everyday activities. Though economic impacts are a 

crucial component to understanding risk from climate change, 

and may have important direct and secondary impacts on 

human health and well-being by reducing resources available 

for other preventative health measures, economic valuation of 

the health impacts was not reported in this assessment.

Uncertainty in Health Impact Assessments

Figure 6 illustrates different sources of uncertainty along the 

exposure pathway.

Two of the key uncertainties in projecting future global 

temperatures are 1) uncertainty about future concentrations 

of greenhouse gases, and 2) uncertainty about how much 

warming will occur for a given increase in greenhouse gas concentrations. The Intergovernmental Panel on Climate Change’s 

Fifth Assessment Report found that the most likely response of 

the climate system to a doubling of carbon dioxide concentrations lies between a 1.5°C and 4.5°C (2.7°F to 8.1°F) increase 

in global average temperature.1 Future concentrations depend 

on both future emissions and how long these emissions 

remain in the atmosphere (which can vary depending on how 

natural systems process those emissions). To capture these 

uncertainties, climate modelers often use multiple models, 

analyze multiple scenarios, and conduct sensitivity analyses to 

assess the significance of these uncertainties.

Uncertainty in current and future estimates of health or socioeconomic status is related to several factors. In general, estimates are more uncertain for less-prevalent health conditions 

(such as rare cancers versus cardiovascular disease), smaller 

subpopulations (such as Hispanic subpopulations versus White 

adults), smaller geographic areas (census tracts versus state 

or national scale), and time periods further into the future 

(decades versus seasons or years). Most current estimates of 

disease prevalence or socioeconomic status have uncertainty 

expressed as standard errors or confidence intervals that are 

derived from sampling methods 

and sample sizes. When modeling health impacts using data on 

health prevalence or socioeconomic status, these measures 

of uncertainty are typically 

included in the analysis to help 

establish a range of plausible 

results. Expert judgment is typically used to assess the overall 

effects of uncertainty from estimates of health or socioeconomic status when assessing the scientific literature.

The factors related to uncertainty in exposure–response functions are similar to those for the projections of health or socioeconomic status. Estimates are more uncertain for smaller 

subpopulations, less-prevalent health conditions, and smaller 

geographic areas. Because these estimates are based on 

observations of real populations, their validity when applied 

to populations in the future is more uncertain the further into 

the future the application occurs. Uncertainty in the estimates 

of the exposure–outcome relationship also comes from factors 

related to the scientific quality of relevant studies, including 

appropriateness of methods, source of data, and size of study 

populations. Expert judgment is used to evaluate the validity 

of an individual study as well as the collected group of relevant studies in assessing uncertainty in estimates of exposure–

outcome relationships.

Approach to Reporting Uncertainty in Key Findings

Despite the sources of uncertainty described above, the current state of the science allows an examination of the likely direction of and trends in the health impacts of climate change. 

Over the past ten years, the models used for climate and 

health assessments have become more useful and more accurate (for example, Melillo et al. 2014).6, 52, 53 This assessment 

builds on that improved capability. A more detailed discussion 

of the approaches to addressing uncertainty from the various 

sources can be found in the Guide to the Report and Appendix 

1: Technical Support Document.

Two kinds of language are used when describing the uncertainty associated with specific statements in this report: 

confidence language and likelihood language. Confidence in 

the validity of a finding is expressed qualitatively and is based 

on the type, amount, quality, strength, and consistency of 

evidence and the degree of expert agreement on the finding. 

Likelihood, or the projected probability of an impact occurring, 
1–INTRODUCTION: CLIMATE CHANGE AND HUMAN HEALTH

U.S. Global Change Research Program 38 Impacts of Climate Change on Human Health in the United States

is based on quantitative estimates or measures of uncertainty 

expressed probabilistically (in other words, based on statistical analysis of observations or model results, or on expert 

judgment). Whether a Key Finding has a confidence level 

associated with it or, where findings can be quantified, both 

a confidence and likelihood level associated with it, involves 

the expert assessment and consensus of the chapter author 

teams. 

PHOTO CREDITS

Pg. 25–Los Angeles, California skyline: © Lisa Romerein/Corbis

Pg. 26– Doctor showing girl how to use stethoscope: © John 

Fedele/Blend Images/Corbis

Pg. 31–Collapsed house after Hurricane Sandy: © iStockPhoto.

com/Aneese

Pg. 33– Woman checking blood sugar levels: © Monkey Business 

Images/Corbis

Pg. 35–Girl suffering from asthma: © Stephen Welstead/LWA/

Corbis

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few sources, limited consistency, models 

incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts

Likelihood

Very Likely

≥ 9 in 10

Likely

≥ 2 in 3

As Likely As Not

≈ 1 in 2

Unlikely

≤ 1 in 3

Very Unlikely

≤ 1 in 10
U.S. Global Change Research Program 39 Impacts of Climate Change on Human Health in the United States

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http://www.cdc.gov/lyme/stats/survfaq.html

52. Post, E.S., A. Grambsch, C. Weaver, P. Morefield, J. Huang, 

L.-Y. Leung, C.G. Nolte, P. Adams, X.-Z. Liang, J.-H. Zhu, 

and H. Mahone, 2012: Variation in estimated ozone-related 

health impacts of climate change due to modeling choices 

and assumptions. Environmental Health Perspectives, 120,

1559-1564. http://dx.doi.org/10.1289/ehp.1104271

53. Tamerius, J.D., E.K. Wise, C.K. Uejio, A.L. McCoy, and 

A.C. Comrie, 2007: Climate and human health: Synthesizing environmental complexity and uncertainty. Stochastic 

Environmental Research and Risk Assessment, 21, 601-613. 

http://dx.doi.org/10.1007/s00477-007-0142-1

54. Sun, L., K.E. Kunkel, L.E. Stevens, A. Buddenberg, J.G. 

Dobson, and D.R. Easterling, 2015: Regional Surface Climate Conditions in CMIP3 and CMIP5 for the United 

States: Differences, Similarities, and Implications for the 

U.S. National Climate Assessment. NOAA Technical Report 

NESDIS 144, 111 pp. National Oceanic and Atmospheric Administration, National Environmental Satellite, Data, 

and Information Service. http://www.nesdis.noaa.gov/technical_reports/NOAA_NESDIS_Technical_Report_144.pdf

End
TEMPERATURE-RELATED DEATH 2 AND ILLNESS

On the web: health2016.globalchange.gov

THE IMPACTS OF CLIMATE CHANGE ON HUMAN HEALTH IN THE UNITED STATES

A Scientific Assessment

Lead Authors

Marcus C. Sarofim* 

U.S. Environmental Protection Agency

Shubhayu Saha

Centers for Disease Control and Prevention

Michelle D. Hawkins 

National Oceanic and Atmospheric Administration

David M. Mills 

Abt Associates

Contributing Authors

Jeremy Hess 

University of Washington

Radley Horton 

Columbia University

Patrick Kinney 

Columbia University

Joel Schwartz 

Harvard University

Alexis St. Juliana 

Abt Associates

Recommended Citation: Sarofim, M.C., S. Saha, M.D. Hawkins, D.M. Mills, J. Hess, R. Horton, P. Kinney, J. Schwartz, and A. St. Juliana, 

2016: Ch. 2: Temperature-Related Death and Illness. The Impacts of Climate Change on Human Health in the United States: A 

Scientific Assessment. U.S. Global Change Research Program, Washington, DC, 43–68. http://dx.doi.org/10.7930/J0MG7MDX 

*Chapter Coordinator

U.S. Global Change Research Program
44

TEMPERATURE-RELATED DEATH 

AND ILLNESS

FPO

2

Key Findings

Future Increases in Temperature-Related Deaths 

Key Finding 1: Based on present-day sensitivity to heat, an increase of thousands to tens of thousands of 

premature heat-related deaths in the summer [Very Likely, High Confidence] and a decrease of premature coldrelated deaths in the winter [Very Likely, Medium Confidence] are projected each year as a result of climate 

change by the end of the century. Future adaptation will very likely reduce these impacts (see Changing 

Tolerance to Extreme Heat Finding). The reduction in cold-related deaths is projected to be smaller than the 

increase in heat-related deaths in most regions [Likely, Medium Confidence].

Even Small Differences from Seasonal Average Temperatures Result in Illness and Death

Key Finding 2: Days that are hotter than usual in the summer or colder than usual in the winter are both 

associated with increased illness and death [Very High Confidence]. Mortality effects are observed even 

for small differences from seasonal average temperatures [High Confidence]. Because small temperature 

differences occur much more frequently than large temperature differences, not accounting for the effect 

of these small differences would lead to underestimating the future impact of climate change [Likely, High 

Confidence].

Changing Tolerance to Extreme Heat

Key Finding 3: An increase in population tolerance to extreme heat has been observed over time [Very High 

Confidence]. Changes in this tolerance have been associated with increased use of air conditioning, improved 

social responses, and/or physiological acclimatization, among other factors [Medium Confidence]. Expected 

future increases in this tolerance will reduce the projected increase in deaths from heat [Very Likely, Very High 

Confidence].

Some Populations at Greater Risk

Key Finding 4: Older adults and children have a higher risk of dying or becoming ill due to extreme heat [Very 

High Confidence]. People working outdoors, the socially isolated and economically disadvantaged, those with 

chronic illnesses, as well as some communities of color, are also especially vulnerable to death or illness [Very 

High Confidence].
2–TEMPERATURE-RELATED DEATH AND ILLNESS

U.S. Global Change Research Program 45 Impacts of Climate Change on Human Health in the United States

2.1 Introduction

The Earth is warming due to elevated concentrations of greenhouse gases, and will continue to warm in the future. U.S. average temperatures have increased by 1.3°F to 1.9°F since record 

keeping began in 1895, heat waves have become more frequent 

and intense, and cold waves have become less frequent across 

the nation (see Ch. 1: Introduction). Annual average U.S. temperatures are projected to increase by 3°F to 10°F by the end 

of this century, depending on future emissions of greenhouse 

gases and other factors.1 These temperature changes will have 

direct effects on human health.

Days that are hotter than the average seasonal temperature in 

the summer or colder than the average seasonal temperature 

in the winter cause increased levels of illness and death by 

compromising the body’s ability to regulate its temperature 

or by inducing direct or indirect health complications. Figure 

1 provides a conceptual model of the various climate drivers, 

social factors, and environmental and institutional factors that 

can interact to result in changes in illness and deaths as a result 

of extreme heat. Increasing concentrations of greenhouse gases 

lead to an increase of both average and extreme temperatures, 

leading to an increase in deaths and illness from heat and a 

potential decrease in deaths from cold. Challenges involved in 

determining the temperature–death relationship include a lack 

of consistent diagnoses on death certificates and the fact that 

the health implications of extreme temperatures are not absolute, differing from location to location and changing over time. 

Both of these issues can be partially addressed through the 

use of statistical methods. Climate model projections of future 

temperatures can be combined with the estimated relationships between temperatures and health in order to assess how 

deaths and illnesses resulting from temperature could change 

in the future. The impact of a warming climate on deaths and 

illnesses will not be realized equally as a number of populations, 

such as children, the elderly, and economically disadvantaged 

groups, are especially vulnerable to temperature. 

Climate Change and Health—Extreme Heat

Figure 1: This conceptual diagram illustrates the key pathways by which climate change influences human health during an extreme 

heat event, and potential resulting health outcomes (center boxes). These exposure pathways exist within the context of other factors 

that positively or negatively influence health outcomes (gray side boxes). Key factors that influence vulnerability for individuals are 

shown in the right box, and include social determinants of health and behavioral choices. Key factors that influence vulnerability at 

larger scales, such as natural and built environments, governance and management, and institutions, are shown in the left box. All 

of these influencing factors can affect an individual’s or a community’s vulnerability through changes in exposure, sensitivity, and 

adaptive capacity and may also be affected by climate change. See Chapter 1: Introduction for more information.
2–TEMPERATURE-RELATED DEATH AND ILLNESS

U.S. Global Change Research Program 46 Impacts of Climate Change on Human Health in the United States

2.2 Contribution of Extreme Temperatures to Death 

and Illness

Temperature extremes most directly affect health by compromising the body’s ability to regulate its internal temperature. 

Loss of internal temperature control can result in a cascade of 

illnesses, including heat cramps, heat exhaustion, heatstroke, 

and hyperthermia in the presence of extreme heat, and 

hypothermia and frostbite in the presence of extreme cold. 

Temperature extremes can also worsen chronic conditions 

such as cardiovascular disease, respiratory disease, cerebrovascular disease, and diabetes-related conditions. Prolonged 

exposure to high temperatures is associated with increased 

hospital admissions for cardiovascular, kidney, and respiratory 

disorders. Exposures to high minimum temperatures may also 

reduce the ability of the human body to recover from high 

daily maximum temperatures. 

2.3 Defining Temperature Exposures

Extreme temperatures are typically defined by some measure, 

for example, an ambient temperature, heat index (a combination of temperature and humidity), or wind chill (a combination of temperature and wind speed), exceeding predefined 

thresholds over a number of days.2, 3, 4, 5, 6, 7, 8 Extremes can be 

defined by average, minimum, or maximum daily temperatures, by nighttime temperatures, or by daytime temperatures. However, there is no standard method for defining a 

heat wave or cold wave. There are dramatic differences in 

the observed relationships between temperature, death, and 

illness across different regions and seasons; these relationships vary based on average temperatures in those locations 

and the timing of the heat or cold event. For example, a 95°F 

day in Vermont will have different implications for health 

than a 95°F day in Texas, and 

similarly, a 95°F day in May will 

have different implications than 

one in August9, 10, 11, 12 (this is 

further discussed in"Evidence 

of Adaptation to Temperature 

Extremes" on page 49). Therefore, in some cases, temperature extremes are defined by 

comparison to some local average (for example, the top 1% of 

warmest days recorded in a particular location) rather than to 

some absolute temperature (such as 95°F). While temperature 

extremes are generally determined based on weather station records, the exposure of individuals will depend on their 

location: urban heat islands, microclimates, and differences 

between indoor and outdoor temperatures can all lead to 

differences between weather station data and actual exposure. The indoor environment is particularly important as most 

people spend the majority of their time inside. 

One exception to using relative measures of temperature is 

that there are some critical physical and weather condition 

thresholds that are absolute. For example, one combined 

measure of humidity and temperature is known as the wet 

bulb temperature. As the wet bulb temperature reaches or 

exceeds the threshold of 35°C (95°F), the human body can no 

longer cool through perspiration, and recent evidence suggests that there is a physical heat tolerance limit in humans 

to sustained temperatures above 35°C that is similar across 

diverse climates.13 The combined effects of temperature and 

humidity have been incorporated in tools such as heat index 

tables, which reflect how combinations of heat and relative 

humidity “feel.” The heat index in these tools is often presented with notes about the potential nature and type of health 

risks different combinations of temperature and humidity may 

pose, along with confounding conditions such as exposure to 

direct sunlight or strong winds.

Variations in heat wave definitions make it challenging to compare results across studies or determine the most appropriate 

public health warning systems.8, 14 This is important as the 

associations between deaths and illnesses and extreme heat 

conditions vary depending on the methods used for defining 

the extreme conditions.2, 15, 16

2.4 Measuring the Health Impact of Temperature

Two broad approaches are used to study the relationship between temperatures and illness and death: direct attribution 

and statistical methods.17, 18 

Direct Attribution Studies

With direct attribution, researchers link health outcomes to 

temperatures based on assigned diagnosis codes in medical 

records such as hospital admissions and death certificates. 

For example, the International 

Classification of Diseases (ICD10) contains specific codes for 

attributing deaths to exposure 

to excessive natural heat (X30) 

and excessive natural cold 

(X31).19 However, medical 

records will not include information on the weather conditions 

at the time of the event or preceding the event. It is generally 

accepted that direct attribution underestimates the number of 

people who die from temperature extremes. Reasons for this 

include difficulties in diagnosing heat-related and cold-related 

deaths, lack of consistent diagnostic criteria, and difficulty in 

identifying, or lack of reporting, heat or cold as a factor that 

worsened a preexisting medical condition.9, 17 Heat-related 

deaths are often not reported as such if another cause of 

death exists and there is no well-publicized heat wave. An 

additional challenging factor in deaths classified as X31 (cold) 

deaths is that a number of these deaths result from situations 

involving substance use/abuse and/or contact with water, 

both of which can contribute to hypothermia.20, 21 

Temperature extremes most directly affect 

health by compromising the body's ability to 

regulate its internal temperature.
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U.S. Global Change Research Program 47 Impacts of Climate Change on Human Health in the United States

exacerbating the cause of death, only recording the ultimate 

cause, such as a stroke or a heart attack (see, for example, 

Figure 2, where the excess deaths during the 1995 Chicago 

heat wave clearly exceeded the number of deaths recorded as 

heat-related on death certificates). Statistical methods focus 

on determining how temperature contributes to premature 

deaths and illness and therefore are not susceptible to this 

kind of undercount, though they face potential biases due to 

time-varying factors like seasonality. Both methods depend on 

temperatures measured at weather stations, though the actual temperature exposure of individuals may differ. In short, 

while the focus on temperature is consistent in both methods, 

the methods potentially evaluate very different combinations 

of deaths and weather conditions. 

2.5 Observed Impact of Temperature on Deaths

A number of extreme temperature events in the United States 

have led to dramatic increases in deaths, including events in 

Kansas City and St. Louis in 1980, Philadelphia in 1993, Chicago in 1995, and California in 2006. (See Figure 2 for more on 

the July 1995 heat wave in Chicago).28, 29, 30, 31, 32

Recent U.S. studies in specific communities and for specific extreme temperature events continue to conclude that extreme 

temperatures, particularly extreme heat, result in premature 

deaths.7, 30, 36, 37 This finding is further reinforced by a growing 

suite of regional- and national-scale studies documenting an 

increase in deaths following extreme temperature conditions, 

using both direct attribution17 and statistical approaches.9, 10, 

12, 15, 38 The connection between heat events and deaths is also 

evident internationally. The European heat wave of 2003 is an 

especially notable example, as it is estimated to have been responsible for between 30,000 and 70,000 premature deaths.39

However, statistical approaches find that elevated death 

rates are seen even for less extreme temperatures. These 

approaches find an optimal temperature, and show that there 

are more deaths at any temperatures that are higher or lower 

than that optimal temperature.11, 40 Even though the increase 

in deaths per degree are smaller near the optimum than at 

more extreme temperatures, because the percentage of days 

that do not qualify as extreme are large,41 it can be important 

to address the changes in deaths that occur for these smaller 

temperature differences. 

A recent analysis of U.S. deaths from temperature extremes 

based on death records found an average of approximately 

1,300 deaths per year from 2006 to 2010 coded as resulting 

from extreme cold exposures, and 670 deaths per year coded 

as resulting from exposure to extreme heat.17 These results, 

and those from all similar studies that rely solely on coding 

within medical records to determine cause of deaths, will 

underestimate the actual number of deaths due to extreme 

temperatures.17, 42 For example, some statistical approaches 

estimate that more than 1,300 deaths per year in the United 

Statistical Studies

Statistical studies measure the impact of temperature on 

death and illness using methods that relate the number of 

cases (for example, total daily deaths in a city) to observed 

weather conditions and other socio-demographic factors. 

These statistical methods determine whether the temperature 

conditions were associated with increased deaths or illness 

above longer-term average levels. These associations establish 

the relationship between temperature and premature deaths 

and illness. In some cases, particularly with extreme temperature conditions, the increase in premature deaths and illness 

can be quite dramatic and the health impact may be referred 

to in terms of excess deaths or illnesses. Methods for evaluating the impact of temperature in these models vary. 

Many studies include all the days in the study period, which 

makes it possible to capture changes in deaths resulting from 

small variations of temperatures from their seasonal averages. Other methods restrict the analysis to days that exceed 

some threshold for extreme heat or cold conditions.22 Some 

studies incorporate methods that determine different health 

relationships for wind, air pressure, and cloud cover as well 

as the more common temperature and humidity measures.15

Another approach is to identify a heat event and compare 

observed illness and deaths during the event with a carefully 

chosen comparison period.23, 24, 25 Many of these methods also 

incorporate socio-demographic factors (for example, age, race, 

and poverty) that may affect the temperature–death relationship. 

Comparing Results of Direct Attribution and Statistical 

Studies

Comparing death estimates across studies is therefore 

complicated by the use of different criteria for temperature 

extremes, different analytical methods, varying time periods, 

and different affected populations. Further, it is widely accepted that characteristics of extreme temperature events such as 

duration, intensity, and timing in season directly affect actual 

death totals.2, 12 Estimates of the average number of deaths 

attributable to heat and cold considering all temperatures, 

rather than just those associated with extreme events, provide 

an alternative for considering the mortality impact of climate 

change.26, 27 Statistical studies can also offer insights into what 

aspects of a temperature extreme are most important. For 

example, there are indications that the relationship between 

high nighttime temperatures and mortality is more pronounced than the relationship for daytime temperatures.12, 16

These two methods (direct attribution and statistical approaches) yield very different results for several reasons. 

First, statistical approaches generally suggest that the actual 

number of deaths associated with temperature is far greater 

than those recorded as temperature-related in medical records. Medical records often do not capture the role of heat in 
2–TEMPERATURE-RELATED DEATH AND ILLNESS

U.S. Global Change Research Program 48 Impacts of Climate Change on Human Health in the United States

Heat-Related Deaths During the 1995 Chicago Heat Wave

Figure 2: This figure shows the relationship between high temperatures and deaths observed during the 1995 

Chicago heat wave. The large spike in deaths in mid-July of 1995 (red line) is much higher than the average 

number of deaths during that time of year (orange line), as well as the death rate before and after the heat wave. 

This increase in the rate of deaths occurred during and after the heat wave, as shown here by temperatures 

exceeding 100°F during the day (green line). Humidity and high nighttime temperatures were also key contributing 

factors to this increase in deaths.33 The number of excess deaths has been estimated to be about 700 based on 

statistical methods, but only 465 deaths in Cook County were classified as “heat-related” on death certificates 

during this same period,29 demonstrating the tendency of direct attribution to undercount total heat-related deaths. 

(Figure source: EPA 2014)34

Heat-Related Deaths in Chicago in the Summer of 1995

Figure 2 illustrates an example of excess deaths 

following an extreme heat event. In this case, excess 

deaths are determined by calculating the difference 

between daily observed deaths in Chicago during the 

worst of the heat wave (starting on July 11) and longerterm daily averages for this time of year. The period of 

extreme heat extended from June 21 through August 

10, 1995. Research into the event suggests it was the 

combination of high humidity, high daily maximum 

temperatures, and high daily minimum temperatures 

that made this event truly exceptional.33 This event 

is estimated to have resulted in nearly 700 excess 

deaths in Chicago, based on a statistical approach.35

By comparison, a direct attribution approach based 

on death certificates found only 465 deaths were 

attributed to extreme heat during this time period.29

This kind of underestimate resulting from relying 

on death certificates is common. It is reasonable 

to expect that deaths may be even less likely to be 

attributed to extreme heat during a heat wave that, 

unlike the Chicago event, does not receive a great deal 

of public attention.
2–TEMPERATURE-RELATED DEATH AND ILLNESS

U.S. Global Change Research Program 49 Impacts of Climate Change on Human Health in the United States

States are due to extreme heat.15, 43 Different approaches to 

attributing cause of death lead to differences in the relative 

number of deaths attributed to heat and cold.44 Studies based 

on statistical approaches have found that, despite a larger 

number of deaths being coded as related to extreme cold 

rather than extreme heat, and a larger mortality rate in winter 

overall, the relationship between mortality and an additional 

day of extreme heat is generally much larger than the relationship between mortality and an additional day of extreme 

cold.12

Confounding Factors and Effect Modifiers

While the direct attribution approach underestimates the 

number of deaths resulting from extreme temperature events, 

there are a few ways in which the statistical approach may 

lead to an overestimation. However, any overestimation due 

to these potential confounding factors and effect modifiers is 

thought to be much smaller than the direct attribution underestimation.12

The first potential overestimation results from the connection 

between elevated temperatures and other variables that correlate with temperature, such as poor air quality. This connection involves a combination of factors, including stagnant air 

masses and changes in the atmospheric chemistry that affect 

the concentrations of air pollutants such as ozone or particulate matter (see Ch. 3: Air Quality Impacts). If some portion of 

the deaths during extreme heat 

events are actually a result of 

the higher levels of atmospheric 

pollution that are correlated 

with these events, then including those deaths in a statistical 

analysis to determine the relationship of increased heat on 

human health would result in double counting deaths.10, 45, 46, 47

However, this issue is often addressed by including air pollution and other correlated variables in statistical modeling.26

A second consideration when using statistical approaches to 

determine the relationship between temperature and deaths 

is whether some of the individuals who died during the temperature event were already near death, and therefore the 

temperature event could be considered to have “displaced” 

the death by a matter of days rather than having killed a person not otherwise expected to die. This effect is referred to as 

mortality displacement. There is still no consensus regarding 

the influence of mortality displacement on premature death 

estimates, but this effect generally accounts for a smaller portion of premature deaths as events become more extreme.7, 12, 

48, 49, 50

Evidence of Adaptation to Temperature Extremes 

The impact on human health of a given temperature event 

(for example, a 95°F day) can depend on where and when it 

occurs. The evidence also shows larger changes in deaths and 

hospitalizations in response to elevated temperatures in cities 

where temperatures are typically cooler as compared with 

warmer cities.9, 11, 40, 51, 52 This suggests that people can adapt, 

at least partially, to the average temperature that they are 

used to experiencing. Some of this effect can be explained by 

differences in infrastructure. For example, locations with higher average temperature, such 

as the Southeast, will generally 

have greater prevalence and use 

of air conditioning. However, 

there is also evidence that there 

is a physiological acclimatization 

(the ability to gradually adapt 

to heat), with changes in sweat 

volume and timing, blood flow and heat transfer to the skin, 

and kidney function and water conservation occurring over 

the course of weeks to months of exposure to a hot climate.53

For example, as a result of this type of adaptation, heat events 

later in the summer have less of an impact on deaths than 

those earlier in the summer, all else being equal,15 although 

some of this effect is also due to the deaths of some of the 

most vulnerable earlier in the season. However, children and 

older adults remain vulnerable given their reduced ability to 

regulate their internal temperature and limited acclimatization 

capacities.53

An increased tolerance to extreme temperatures has also 

been observed over multiyear and multidecadal periods.9, 10, 

54, 55, 56 This improvement is likely due to some combination of 

physiological acclimatization, increased prevalence and use of 

air conditioning,10 and general improvements in public health 

over time,9, 54 but the relative importance of each is not yet 

clear.56

The impact on human health of a given 

temperature event (for example, a 95°day) can 

depend on where and when it occurs.

The relationship between mortality and an additional day of 

extreme heat is generally much larger than the relationship 

between mortality and an additional day of extreme cold.
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U.S. Global Change Research Program 50 Impacts of Climate Change on Human Health in the United States

Recent changes in urban planning and development programs 

reflect an adaptive trend implemented partially in response 

to the anticipated temperature health risks of climate change. 

For example, because urban areas tend to be warmer than 

surrounding rural areas (the “urban heat island” effect), there 

is an increased emphasis on incorporating green space and 

other technologies, such as cool roofs, in new development 

or redevelopment projects.57 Similarly, programs that provide 

advice and services in preparation for or response to extreme 

temperatures continue to increase in number and expand the 

scope of their activity (see for example guidance documents 

on responses to extreme temperature developed by the 

Centers for Disease Control and Prevention and the Environmental Protection Agency).58, 59 Continued changes in personal 

behavior as a result of these efforts, for example, seeking 

access to air-conditioned areas during extreme heat events 

or limiting outside activity, may continue to change future 

exposure to extreme temperatures and other climate-sensitive 

health stressors, such as outdoor air pollutants and vectors for 

disease such as ticks or mosquitoes.

Observed Trends in Heat Deaths

As discussed in Chapter 1, U.S. average temperature has 

increased by 1.3°F to 1.9°F since 1895, with much of that increase occurring since 1970, though this temperature increase 

has not been uniform geographically and some regions, such 

as the Southeast, have seen little increase in temperature and 

extreme heat over time.1, 15 This warming is attributable to 

elevated concentrations of greenhouse gases and it has been 

estimated that three-quarters of moderately hot extremes 

are already a result of this historical warming.60 As discussed 

in the previous section, there have also been changes in the 

tolerance of populations within the United States to extreme 

temperatures. Changes in mortality due to high temperatures 

are therefore a result of the combination of higher temperatures and higher heat tolerance. Use of the direct attribution 

approach, based on diagnosis codes in medical records, to examine national trends in heat mortality over time is challenging because of changes in classification methods over time.34

The few studies using statistical methods that have presented 

total mortality estimates over time suggest that, over the 

last several decades, reductions in mortality due to increases 

in tolerance have outweighed increases in mortality due to 

increased temperatures.15, 61

2.6 Observed Impact of Temperature on Illness

Temperature extremes are linked to a range of illnesses reported at emergency rooms and hospitals. However, estimates 

for the national burden of illness associated with extreme 

temperatures are limited. 

Using a direct attribution approach, an analysis of a nationally 

representative database from the Healthcare Utilization Project (HCUP) produced an annual average estimate of 65,299 

emergency visits for acute heat illness during the summer 

months (May through September)—an average rate of 21.5 

visits for every 100,000 people each year.62 This result was 

based only on recorded diagnosis codes for hyperthermia and 

probably underestimates the true number of heat-related 

healthcare visits, as a wider range of health outcomes is potentially affected by extreme heat. For example, hyperthermia 

is not the only complication from extreme heat, and not every 

individual that suffers from a heat illness visits an emergency 

department. In a national study of Medicare patients from 

2004 to 2005, an annual average of 5,004 hyperthermia cases 

and 4,381 hypothermia cases were reported for inpatient and 

outpatient visits.63 None of these studies link health episodes 

to observed temperature data, thus limiting the opportunity 

to attribute these adverse outcomes to specific heat events or 

conditions.

High ambient heat has been associated with adverse impacts 

for a wide range of illnesses.25 Examples of illnesses associated with extreme heat include cardiovascular, respiratory, and 

renal illnesses; diabetes; hyperthermia; mental health issues; 

and preterm births. Children spend more time outdoors and 

have insufficient ability for physiologic adaptation, and thus 

may be particularly vulnerable during heat waves.64 Respiratory illness among the elderly population was most commonly 

reported during extreme heat.65

Statistical studies examine the association between extreme 

heat and illness using data from various healthcare access 

points (such as hospital admissions, emergency department 

visits, and ambulance dispatches). The majority of these 

studies examine the association of extreme heat with cardiovascular and respiratory illnesses. For these particular health 

outcomes, the evidence is mixed, as many studies observed 

elevated risks of illness during periods of extreme heat but 

others found no evidence of elevated levels of illness.24, 51, 66, 67, 

68, 69, 70 The evidence on some of the other health outcomes is 

more robust. Across emergency department visits and hospital 

admissions, high temperature have been associated with 

renal diseases, electrolyte imbalance, and hyperthermia.24, 67, 

Certain occupational groups such as agricultural workers, 

construction workers, and electricity and pipeline utility 

workers are at increased risk for heat- and cold-related illness, 

especially where jobs involve heavy exertion.
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U.S. Global Change Research Program 51 Impacts of Climate Change on Human Health in the United States

71, 72 These health risks vary not only across types of illness but 

also for the same illness across different healthcare settings. In 

general, evidence for associations with morbidity outcomes, 

other than cardiovascular impacts, is strong. 

While there is still uncertainty about how levels of heat-related illnesses are expected to change with projected increases 

in summer temperature from climate change,41 advances have 

been made in surveillance of heat-related illness. For example, monitoring of emergency ambulance calls during heat 

waves can be used to establish real-time surveillance systems 

to identify extreme heat events.73 The increase in emergency 

visits for a wide range of illnesses during the 2006 heat wave 

in California points to the potential for using this type of information in real-time health surveillance systems.24

2.7 Projected Deaths and Illness from Temperature 

Exposure

Climate change will increase the frequency and severity of 

future extreme heat events while also resulting in generally 

warmer summers and milder winters,1 with implications for 

human health. Absent further adaptation, these changes 

are expected to lead to an increase in illness and death from 

increases in heat, and reductions in illness and death resulting 

from decreases in cold, due to changes in outcomes such as 

heat stroke, cardiovascular disease, respiratory disease, cerebrovascular disease, and kidney disorders.41, 74

A warmer future is projected to lead to increases in future 

mortality on the order of thousands to tens of thousands of 

additional premature deaths per year across the United States 

by the end of this century.22, 38, 54, 75, 76, 77, 78, 79 Studies differ in 

which regions of the United States are examined and in how 

they account for factors such as adaptation, mortality displacement, demographic changes, definitions of heat waves 

and extreme cold, and air quality factors, and some studies 

examine only extreme events while others take into account 

the health effects of smaller deviations from average seasonal 

temperatures. Despite these differences there is reasonable 

agreement on the magnitude of the projected changes. 

Additionally, studies have projected an increase in premature 

deaths due to increases in temperature for Chicago, IL,39, 80

Dallas, TX,18 the Northeast corridor cities of Boston, MA, New 

York, NY, and Philadelphia, PA,18, 26, 81, 82 Washington State,83, 

84 California,85 or a group of cities including Portland, OR; 

Minneapolis and St. Paul, MN; Chicago, IL; Detroit, MI; Toledo, 

Cleveland, Columbus, and Cincinnati, OH; Pittsburgh and Philadelphia, PA; and Washington, DC.86 However, these regional 

projections use a variety of modeling strategies and therefore 

show more variability in mortality estimates than studies that 

are national in scope. 

Less is known about how non-fatal illnesses will change in 

response to projected increases in heat. However, hospital admissions related to respiratory, hormonal, urinary, genital, and 

renal problems are generally projected to increase.72, 87 Kidney 

stone prevalence has been linked to high temperatures, possibly due to dehydration leading to concentration of the salts 

that form kidney stones. In the United States, an increased 

rate of kidney stones is observed in southern regions of the 

country, especially the Southeast. An expansion of the regions 

where the risk of kidney stones is higher is therefore plausible 

in a warmer future.88, 89, 90 

The decrease in deaths and illness due to reductions in winter 

cold have not been as well studied as the health impacts of 

increased heat, but the reduction in premature deaths from 

cold are expected to be smaller than the increase in deaths 

from heat in the United States.22, 26, 38, 41, 75, 77 While this is true 

nationally (with the exception of Barreca 2012),75 it may not 

hold for all regions within the country.27 Similarly, international 

studies have generally projected a net increase in deaths from 

a warming climate, though in some regions, decreases in cold 

mortality may outweigh increases in heat mortality.91 The 

projected net increase in deaths is based in part on historical 

studies that show that an additional extreme hot day leads to 

more deaths than an additional extreme cold day, and in part 

on the fact that the decrease in extreme cold deaths is limited 

as the total number of cold deaths approaches zero in a given 

location. 

It is important to distinguish between generally higher wintertime mortality rates that are not strongly associated with daily 

temperatures—such as respiratory infections and some cardiovascular disease 12, 92—from mortality that is more directly 

related to the magnitude of the cold temperatures. Some 

recent studies have suggested that factors leading to higher 

wintertime mortality rates may not be sensitive to climate 

warming, and that deaths due to these factors are expected to 

occur with or without climate change. Considering this, some 

estimates of wintertime mortality may overstate the benefit of 

climate change in reducing wintertime deaths.49, 93, 94

The U.S. population has become less sensitive to heat over 

time. Factors that have contributed to this change include 

infrastructure improvements, including increased access and 

use of air conditioning in homes and businesses, and improved 

societal responses, including increased access to public health 

programs and healthcare.15, 54, 61, 95, 96, 97 Projecting these trends 

into the future is challenging, but this trend of increasing tolerance is projected to continue, with future changes in adaptive 

capacity expected to reduce the future increase in mortality.56

However, there are limits to adaptation, whether physiological53

or sociotechnical (for example, air conditioning, awareness 

programs, or cooling centers). While historically adaptation has 

outpaced warming, most studies project a future increase in 

mortality even when including assumptions regarding adaptation.18, 22, 81, 85, 91 Additionally, the occurrence of events such 

as power outages simultaneous with a heat wave may reduce 

some of these adaptive benefits. Such simultaneous events can 
2–TEMPERATURE-RELATED DEATH AND ILLNESS

U.S. Global Change Research Program 52 Impacts of Climate Change on Human Health in the United States

Research Highlight: Modeling the Effect of Warming on U.S. Deaths

Importance: A warming climate is expected to result in more days that are warmer than today’s usual temperature 

in the summer, leading to an increase in heat-related deaths. A warming climate is also expected to result in fewer 

days that are colder than today’s usual temperatures in the winter, leading to a decrease in cold-related deaths. 

Understanding these changes is an important factor in understanding the human health response to climate 

change. 

Objective: A quantitative projection of future deaths from heat and cold for 209 U.S. cities with a total population 

of over 160 million inhabitants.

Method: A relationship between average daily temperature and deaths by city and month was developed using 

historical data on deaths and temperatures from 1996–2006, generating results for both same-day temperature 

and the average of the previous five-day temperatures to account for delayed responses to temperature. Cities, 

which are defined using county borders, were allocated to nine different clusters based on similarity of climates. 

Temperature–death relationships were refined for cities within a given cluster based on the other cities in that 

cluster. Projections of temperature in future time periods were based on the RCP6.0 scenario from two climate 

models: the Geophysical Fluid Dynamic Laboratory–Coupled Physical Model 3 (GFDL–CM3) and the Model for 

Interdisciplinary Research on Climate (MIROC5). These projections were adjusted to match the historical data 

from the same weather stations that were used in the statistical analysis. Further details can be found in Schwartz 

et al. 2015.27

Figure 3: This figure shows the projected decrease in death rates due to warming in colder months (October–March, top left), 

the projected increase in death rates due to warming in the warmer months (April–September, top right), and the projected net 

change in death rates (combined map, bottom), comparing results for 2100 to those for a 1990 baseline period in 209 U.S. cities. 

These results are from one of the two climate models (GFDL–CM3 scenario RCP6.0) studied in Schwartz et al. (2015). In the 

study, mortality data for a city is based on county-level records, so the borders presented reflect counties corresponding to the 

study cities. Geographic variation in the death rates are due to a combination of differences in the amount of projected warming 

and variation in the relationship between deaths and temperatures derived from the historical health and temperature data. 

These results are based on holding the 2010 population constant in the analyses, with no explicit assumptions or adjustment for 

potential future adaptation. Therefore, these results reflect only the effect of the anticipated change in climate over time. (Figure 

source: Schwartzet al. 2015)27

Projected Changes in Temperature-Related Death Rates
2–TEMPERATURE-RELATED DEATH AND ILLNESS

U.S. Global Change Research Program 53 Impacts of Climate Change on Human Health in the United States

Research Highlight: Modeling the Effect of Warming on U.S. Deaths, continued

Results: The modeling done for this study projects that future warming, without any adjustments for future 

adaptation, will lead to an increase in deaths during hotter months, defined as April–September, and a decrease in 

deaths during colder months, defined as October–March. Overall, this leads to a total net increase of about 2,000 

to 10,000 deaths per year in the 209 cities by the end of the century compared to a 1990 baseline (Figure 4). Net 

effects vary from city to city, and a small number of cities are projected to experience a decrease in deaths (Figures 3 

and 4). 

Conclusions: This study is an improvement on previous studies because it examines a greater proportion of the 

U.S. population, uses more recent data on deaths, takes advantage of similar relationships between deaths and 

temperature between nearby cities to generate more statistically robust results, and addresses the difference in 

these relationships by month of the year. The results are consistent with most of the previous studies in projecting 

that climate change will lead to an increase in heat deaths on the order of thousands to tens of thousands of annual 

deaths by the end of the century compared to the 1990 baseline, and that the increase in deaths from heat will 

be larger than the reduction in deaths from cold. In contrast to some previous similar studies,22 some individual 

cities show a net reduction in future deaths due to future warming, mainly in locations where the population is 

already well-adapted to heat but poorly prepared for cold (like Florida). Barreca 201275 also shows net mortality 

benefits in some counties, though with a different spatial pattern due to humidity effects. Some other studies also 

have different spatial patterns, projecting high excess mortality in Southern states despite a lower risk per degree 

change, due to larger increases in frequency and duration of heat waves in that region.79 Like most previous studies, 

this analysis does not account for the effects of further adaptation on future mortality. Results are based on the 

temperature–death relationships observed for the period from 1996 to 2006, which reflect historical adaptation to 

extreme temperatures. However, future adaptation would, all else equal, mean that these results may overestimate 

the potential impact of climate change on changes in both heat- and cold-related deaths.

This study increases the confidence in the key finding that the number of heat deaths will increase in the future 

compared to a future with no climate change, and that the increase in heat deaths will be larger than the reduction 

in cold deaths.

Figure 4: This figure shows the projected increase 

in deaths due to warming in the summer months (hot 

season, April–September), the projected decrease 

in deaths due to warming in the winter months (cold 

season, October–March), and the projected net 

change in deaths for the 209 U.S. cities examined. 

These results compare projected deaths for future 

reporting years to results for the year 1990 while 

holding the population constant at 2010 levels and 

without any quantitative adjustment for potential future 

adaptation, so that temperature–death relationships 

observed in the last decade of the available data 

(1997–2006) are assumed to remain unchanged in 

projections over the 21st century. 

With these assumptions, the figure shows an 

increasing health benefit in terms of reduced deaths 

during the cold season (October–March) over the 

21st century from warming temperatures, while 

deaths during the hot season (April–September) 

increase. Overall, the additional deaths from the warming in the hot season exceed the reduction in deaths during the cold season, 

resulting in a net increase in deaths attributable to temperature over time as a result of climate change. The baseline and future 

reporting years are based on 30-year periods where possible, with the exception of 2100: 1990 (1976–2005), 2030 (2016–2045), 

2050 (2036–2065), and 2100 (2086–2100). (Figure source: adapted from Schwartz et al. 2015)27

Projected Changes in Deaths in U.S. Cities by Season
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U.S. Global Change Research Program 54 Impacts of Climate Change on Human Health in the United States

be more common because of the additional demand on the 

electricity grid due to high air-conditioning usage.98 Another 

potential effect is that if current trends of population growth 

and migration into large urban areas continue, there may be an 

increasing urban heat island effect which will magnify the rate 

of warming locally, possibly leading to more heat-related deaths 

and fewer cold-related deaths.

Projected changes in future health outcomes associated with 

extreme temperatures can be difficult to quantify. Projections 

can depend on 1) the characterization of population sensitivity to temperature event characteristics such as magnitude, 

duration, and humidity; 2) differences in population sensitivity 

depending on the timing and location of an extreme event; 3) 

future changes in baseline rates of death and illness as well 

as human tolerance and adaptive capacity; 4) the changing 

proportions of vulnerable populations, including the elderly, in 

the future; and 5) uncertainty in climate projections.

2.8 Populations of Concern for Death and Illness 

from Extreme Temperatures

Impacts of temperature extremes are geographically varied 

and disproportionally affect certain populations of concern 

(see also Ch. 9: Populations of Concern).41 Certain populations 

are more at risk for experiencing detrimental consequences of 

exposure to extreme temperatures due to their sensitivity to 

hot and cold temperatures and limitations to their capacity for 

adapting to new climate conditions. 

Older adults are a rapidly growing population in the United 

States, and heat impacts are projected to occur in places 

where older adults are heavily concentrated and therefore 

most exposed.99 Older adults are at higher risk for temperature-related mortality and morbidity, particularly those who 

have preexisting diseases, those who take certain medications 

that affect thermoregulation or block nerve impulses (for example, beta-blockers, major tranquilizers, and diuretics), those 

who are living alone, or those with limited mobility (see also 

Ch. 9: Populations of Concern).17, 24, 42, 45, 100 The relationship 

between increased temperatures and death in older adults is 

well-understood with strong evidence of heat-related vulnerability for adults over 65 and 75 years old.101 An increased risk 

for respiratory and cardiovascular death is observed in older 

adults during temperature extremes due to reduced thermoregulation.17, 42, 45, 65 Morbidity studies have also identified links 

between increased temperatures and respiratory and cardiovascular hospitalizations in older adults.65

Children are particularly vulnerable because they must rely 

on others to help keep them safe. This is especially true in 

environments that may lack air conditioning, including homes, 

schools, or cars (see also Ch. 9: Populations of Concern).102 The 

primary health complications observed in children exposed 

to extreme heat include dehydration, electrolyte imbalance, 

fever, renal disease, heat stress, and hyperthermia.64 Infectious and respiratory diseases in children are affected by both 

hot and cold temperatures.64 Inefficient thermoregulation, 

reduced cardiovascular output, and heightened metabolic 

rate are physiological factors driving vulnerability in children 

to extreme heat. Children also spend a considerable amount 

of time outdoors and participating in vigorous physical 

activities.17, 42, 64, 103 High-school football players are especially vulnerable to heat illness (see also Ch. 9: Populations of 

Concern).104 A limited number of studies show evidence of 

cold-related mortality in children. However, no study has 

examined the relationship between cold temperature and 

cause-specific mortality.64 Pregnant women are also vulnerable to temperature extremes as preterm birth has been associated with extreme heat.42, 105, 106 Elevated heat exposure can 

increase dehydration, leading to the release of labor-inducing 

hormones.107 Extreme heat events are also associated with 

adverse birth outcomes, such as low birth weight and infant 

mortality (see Ch. 9: Populations of Concern).

Where a person lives, works, or goes to school can also make 

them more vulnerable to health impacts from extreme temperatures. Of particular concern for densely populated cities is 

the urban heat island effect, where manmade surfaces absorb 

sunlight during the day and then radiate the stored energy 

at night as heat. This process will exacerbate any warming 

from climate change and limit the potential relief of cooler 

nighttime temperatures in urban areas.81 In addition to the 

urban heat island effect, land cover characteristics and poor 

air quality combine to increase the impacts of high ambient 

temperatures for city dwellers and further increase the burden 

on populations of concern within the urban area.12, 17, 45, 108 The 

homeless are often more exposed to heat and cold extremes, 

while also sharing many risk factors with other populations of 

concern such as social isolation, psychiatric illness, and other 

health issues.109

Race, ethnicity, and socioeconomic status can affect vulnerability to temperature extremes. Non-Hispanic Black persons 

FPO

Physiological factors and participation in vigorous outdoor 

activities make children particularly vulnerable to extreme heat.
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U.S. Global Change Research Program 55 Impacts of Climate Change on Human Health in the United States

have been identified as being more vulnerable than other 

racial and ethnic groups to detrimental consequences of 

exposure to temperature extremes.17, 42, 45, 103, 110, 111 One study 

found that non-Hispanic Blacks were 2.5 times more likely to 

experience heat-related mortality compared to non-Hispanic 

Whites, and non-Hispanic Blacks had a two-fold risk of dying 

from a heat-related event compared to Hispanics.17 Evidence 

of racial differences in heat tolerance due to genetic differences is inconclusive.110 However, other factors may contribute to 

increased vulnerability of Black populations, including comorbidities (co-existing chronic conditions) that increase susceptibility to higher temperatures, disparities in the availability 

and use of air conditioning and in heat risk-related land cover 

characteristics (for example, living in urban areas prone to 

heat-island effects), and environmental justice issues.17, 42, 108, 

110, 112 Overall, the link between temperature extremes, race, 

ethnicity, and socioeconomic status is multidimensional and 

dependent on the outcome being studied. Education level, 

income, safe housing, occupational risks, access to health care, 

and baseline health and nutrition status can further distort 

the association between temperature extremes, race, and 

ethnicity.45, 110

Outdoor workers spend a great deal of time exposed to temperature extremes, often while performing vigorous activities. 

Certain occupational groups such as agricultural workers, construction workers, and electricity and pipeline utility workers 

are at increased risk for heat- and cold-related illness, especially where jobs involve heavy exertion.100, 113, 114 One study 

found failure of employers to provide for acclimatization to be 

the factor most clearly associated with heat-related death in 

workers.113

Mental, behavioral, and cognitive disorders can be triggered 

or exacerbated by heat waves. Specific illnesses impacted by 

heat include dementia, mood disorders, neurosis and stress, 

and substance abuse.100, 115, 116, 117 Some medications interfere 

with thermoregulation, thereby increasing vulnerability to 

heat.116 One study in Australia found that hospital admissions 

for mental and behavioral disorders increased by 7.3% during 

heat waves above 80°F.115 Studies have also linked extreme 

heat and increased aggressive behavior. (See also Ch. 8: Mental Health).

2.9 Emerging and Cross-Cutting Issues

Emerging and cross-cutting issues include 1) disparate ways 

that extreme temperature and health are related, 2) urban 

and rural differences, 3) interactions between impacts and 

future changes in adaptation, and 4) projections of extreme 

temperature events. 

The health effects addressed in this chapter are not the only 

ways in which heat and health are related. For example, 

research indicates that hotter temperatures may lead to an increase in violent crime118 and could negatively affect the labor 

force, especially occupational health for outdoor sectors.119, 120

Extreme temperatures also interact with air quality, which can 

complicate estimating how extreme temperature events impact human health in the absence of air quality changes (see 

"Confounding Factors and Effect Modifiers" on page 49). 

In addition, increased heat may also increase vulnerability to 

poor air quality and allergens, leading to potential non-linear 

health outcome responses. Extreme temperature events, as 

well as other impacts from climate change, can also be associated with changes in electricity supply and distribution that 

can have important implications for the availability of heating 

and air conditioning, which are key adaptive measures. 

Though the estimates of the health impact from extreme heat 

discussed in the “Research Highlight” were produced only for 

urban areas (which provided a large sample size for statistical 

validity), there is also emerging evidence regarding high rates 

of heat-related illness in rural areas.6, 62 Occupational exposure 

and a lack of access to air conditioning are some of the factors 

that may make rural populations particularly susceptible to 

extreme heat. There are quantitative challenges to using 

statistical methods to estimate mortality impacts of temperatures in rural areas due to lower population density and 

more dispersed weather stations, but rural residents have also 

demonstrated vulnerability to heat events.121

Other changes in human behavior will also have implications 

for the linkage between climate and heat-related illness. 

Changes in building infrastructure as a response to changes in 

temperature can have impacts on indoor air quality. Similarly, changes in behavior as a result of temperature changes, 

for example, seeking access to air conditioning, can change 

exposure to indoor and outdoor pollution and vectorborne 

diseases (see Ch. 3: Air Quality Impacts; Ch. 5: Vectorborne 

Diseases). 

Finally, projecting climate variability and the most extreme 

temperature events can be more challenging than projecting average warming. Extreme temperatures may rise faster 

than average temperatures,122 with the coldest days warming 

faster than average for much of the twentieth century, and the 

warmest days warming faster than average temperatures in 

the past 30 years.123 Extremely high temperatures in the future 

may also reach levels outside of past experience, in which case 

statistically based relationships may no longer hold for those 

events. There have been suggestive links between rapid recent 

Arctic sea ice loss124 and an increased frequency of cold125 and 

warm extremes,126 but this is an active area of research with 

conflicting results.127, 128 In regions where temperature variability 

increases, mortality will be expected to increase; mortality is 

expected to decrease in regions where variability decreases.129
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U.S. Global Change Research Program 56 Impacts of Climate Change on Human Health in the United States

2.10 Research Needs

In addition to the emerging issues identified above, the 

authors highlight the following potential areas for additional 

scientific and research activity on temperature-related illness 

and death based on their review of the literature. Improved 

modeling and more robust projections of climate variability 

and extreme temperatures will enhance the modeling of 

health impacts associated with extremes of heat and cold. 

While the surveillance for temperature-related deaths is relatively robust, understanding the impacts of future changes in 

heat waves and extreme temperatures can be improved with 

better surveillance and documentation of non-fatal illnesses, 

including hospitalizations and emergency room visits, for temperature-associated reasons. With growing implementation of 

heat early warning systems around the country, there is also a 

need for the development of evaluation methods and associated collection of data to be able to assess effectiveness of 

such systems and other means of health adaptation.

Future assessments can benefit from research activities that:

• further explore the associations between exposure to a 

range of high and low temperatures and exacerbation of 

illnesses across locations and healthcare settings;

• improve understanding of how genetic factors and social 

determinants contribute to vulnerability to illness and death 

from extreme temperature exposures;

• analyze the combined health effects of temperature and 

other discrete climate-sensitive stressors, such as changing 

air quality, smoke from wildfires, or impacts of extreme 

weather events; 

• attribute changes in observed mortality to a changing 

climate; 

• develop effective adaptive responses to reduce the potential 

adverse health outcomes attributable to changing temperatures; and

• explore how future adaptive measures and behaviors can be 

included in quantitative models of health impacts associated with extreme temperatures
U.S. Global Change Research Program 57 Impacts of Climate Change on Human Health in the United States

Supporting Evidence

The chapter was developed through technical discussions 

of relevant evidence and expert deliberation by the report 

authors at several workshops, teleconferences, and email 

exchanges. The authors considered inputs and comments 

submitted by the public, the National Academies of Sciences, 

and Federal agencies. For additional information on the overall 

report process, see Appendices 2 and 3.

The content of this chapter was determined after reviewing 

the collected literature. The authors determined that there was 

substantial literature available to characterize both observed 

and projected mortality from elevated temperatures, with 

sufficient literature available to also characterize mortality 

from cold as well as cold-related hospitalizations and illness. 

Populations of concern were also considered to be a high 

priority for this chapter. As discussed in the chapter, there 

were limitations in terms of the state of the literature on 

understanding how future adaptation will influence climaterelated changes in temperature-related mortality, addressing 

the impact of temperature on rural populations, and examining 

health-related endpoints other than mortality and morbidity.

KEY FINDING TRACEABLE ACCOUNTS

Future Increases in Temperature-Related Deaths 

Key Finding 1: Based on present-day sensitivity to heat, an 

increase of thousands to tens of thousands of premature 

heat-related deaths in the summer [Very Likely, High 

Confidence] and a decrease of premature cold-related deaths 

in the winter [Very Likely, Medium Confidence] are projected 

each year as a result of climate change by the end of the 

century. Future adaptation will very likely reduce these 

impacts (see Changing Tolerance to Extreme Heat Finding). 

The reduction in cold-related deaths is projected to be smaller 

than the increase in heat-related deaths in most regions 

[Likely, Medium Confidence].

Description of evidence base

An extensive literature examines projections of mortality due 

to increasing temperatures. In particular, nine studies were 

identified that provide heat mortality projections in the United 

States for at least 10% of the U.S. population.22, 27, 38, 54, 75, 76, 

77, 78, 79 Each of these studies projected an increase in heatrelated mortality due to projections of future warming, though 

several noted the potential modification effect of adaptation 

(discussed in Key Finding #3). In general, the magnitude of 

projected increases in annual premature deaths in these 

studies was in the hundreds to thousands by mid-century, and 

thousands to tens of thousands by the end of the century, 

when scaled to the total U.S. population. These conclusions 

are further supported by studies at the city, county, and state 

level.18, 26, 39, 80, 81, 82, 83, 84, 85

The Third National Climate Assessment (2014 NCA) found that 

“While deaths and injuries related to cold events are projected 

to decline due to climate change, these reductions are not 

expected to compensate for the increase in heat-related 

deaths,”41 and studies published since that time have further 

supported this finding. Of those studies that examine both 

heat and cold at the national scale, only Barreca found that the 

reductions in cold deaths would more than compensate for the 

increase in heat deaths.22, 27, 38, 75, 77 Barreca’s study was novel in 

terms of its treatment of humidity, finding that weather that 

was both cold and dry, or both hot and humid, was associated 

with higher mortality. However, this treatment of humidity 

was not the cause of the difference with other studies, as 

leaving out humidity actually showed a greater benefit from 

future climate change. Instead, the author stated that the 

reduction in net deaths was a result of relying on counties 

with over 100,000 inhabitants, and that using a state-level 

model covering all U.S. deaths would lead to a prediction of 

an increase of 1.7% in mortality rates rather than a decrease 

of 0.1%. The finding by the majority of studies at a national 

scale that heat deaths will increase more than cold deaths will 

decrease is consistent with studies at smaller spatial scales.26

Moreover, several studies provide rationales for why heat 

mortality is expected to outpace cold mortality,12, 22, 27 and some 

studies suggest that cold mortality may not be responsive to 

warming.49, 93, 94 Barnett et al. (2012) showed that cold waves 

were not generally associated with an increase in deaths 

beyond the mortality already associated with cold weather, in 

contrast to heat waves.2

Major uncertainties

The largest remaining uncertainties concern questions 

of future adaptation, which are discussed in Key Finding 

#3. A related uncertainty involves the link between the 

temperatures measured at weather stations and the 

temperatures experienced by individuals. As long as 

the relationship between the weather station and the 

microclimate or indoor/outdoor difference remains constant, 

this should not impair projections. However, as microclimates, 

building construction, or behavior change, the relationship 

between recorded weather station temperature and actual 

temperature exposure will change. This is related to, but 

broader than, the question of adaptation. Additionally, 

there are uncertainties regarding the non-linearities of heat 

response with increasing temperatures. 

Assessment of confidence and likelihood based on evidence

There is high confidence that heat deaths will very likely 

increase in the future compared to a future without climate 

change, based on high agreement and a large number of 

studies as well as consistency across scenarios and regions. 

Because there are fewer studies on winter mortality, and 

because studies exist that suggest that winter mortality is not 

strongly linked to temperatures, there is medium confidence

that deaths due to extreme cold will very likely decrease. 

The majority of the studies that examine both heat and cold 

deaths find that the increase in heat deaths due to climate 

change will likely be larger than the decrease in cold deaths in 

most regions, but there are a limited number of such studies, 

leading to an assessment of medium confidence. 
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U.S. Global Change Research Program 58 Impacts of Climate Change on Human Health in the United States

Even Small Differences from Seasonal Average 

Temperatures Result in Illness and Death

Key Finding 2: Days that are hotter than usual in the summer 

or colder than usual in the winter are both associated with 

increased illness and death [Very High Confidence]. Mortality 

effects are observed even for small differences from seasonal 

average temperatures [High Confidence]. Because small 

temperature differences occur much more frequently than 

large temperature differences, not accounting for the effect 

of these small differences would lead to underestimating the 

future impact of climate change [Likely, High Confidence].

Description of evidence base

Two well-recognized conclusions from the literature are that 

extreme temperatures lead to illness and premature death 

and that these extreme temperatures are best described 

in relation to local average seasonal temperatures rather 

than absolute temperature values. Epidemiological studies 

find an increase in mortality at temperatures that are high 

related to the local average.9, 10, 12, 15, 17, 38 Based on absolute 

temperatures, Anderson and Bell 2011 found that cities in 

the South and Southeast were the least sensitive to heat, 

demonstrating acclimatization.9

Illness has been linked with hot daily average temperature4, 

6, 51, 69, 71 and apparent temperature, among other metrics.3, 

66, 68, 87 Across studies, adverse health episodes were most 

strongly associated with exposures to high temperatures that 

occurred on the same day or the previous day.3, 51 However, a 

cumulative effect of heat was also observed at periods of up 

to one week after exposure, tapering off beyond seven days.69, 

105 Cardiovascular and respiratory illness has been most 

commonly examined in relation to extreme heat, but the 

association is more varied for illness than for mortality due 

to effects across age groups69, 70 and differences in morbidity 

risk associated with emergency room records versus hospital 

admissions.4, 6, 24, 51, 66, 67, 68, 69, 70

The evidence for mortality is clearest for extreme 

temperatures, as addressed in threshold-based studies,12

but studies that account for smaller changes in temperature 

found mortality changes even for small deviations of 

temperature.11, 27 This is consistent with studies showing a 

U-shaped relationship of temperature and mortality—while 

there may be some plateau near the optimal temperature, 

the plateau is often small, and not always coincident with 

the seasonal average temperature.11, 40 However, some of the 

individuals who die in response to elevated temperatures 

were already near death, and so the temperature event is 

sometimes considered to have “displaced” the death by 

a matter of days rather than created an additional death. 

Studies have found that this effect is generally below 50% of 

the total deaths, and is much smaller than that (10% or less) 

for the most extreme events, such as the 2003 European 

heat wave.12, 48, 49, 50 In contrast, one recent study found that in 

seven U.S. cities mortality displacement was greater than 80% 

for small temperature deviations and around 50% even for 

the 3% of warmest events in the study sample.7

Major uncertainties

This finding reflects consideration of a number of recent 

studies17, 54 not referenced in the recent 2014 NCA.41 There 

is a consensus of studies linking extreme temperatures and 

mortality, and a growing body of literature demonstrating 

that smaller differences in temperature are also linked with 

mortality. However, the mortality displacement effect, and 

the fact that deaths that do not occur during an identified 

heat wave are less likely to be directly attributed to extreme 

heat, contribute to continuing uncertainty about the 

magnitude of the effect of temperature on mortality. 

Assessment of confidence and likelihood based on evidence

There is very high confidence in the relationship between 

extreme temperatures and premature deaths due to the 

consistency and strength of the literature, particularly given 

the different study designs that produce this result. There 

is high confidence that small temperature deviations from 

normal temperatures contribute to premature mortality due 

to high agreement among those studies that have examined 

the issue. Though some studies indicate that for these small 

temperature differences, mortality displacement may play 

a larger role than for more extreme temperatures. Fewer 

studies have examined the role of these smaller temperature 

differences in projections, but the directionality of the effect 

is clear, so the determination of the authors was that not 

including this effect would likely lead to an underestimate of 

future mortality, with high confidence. 

Changing Tolerance to Extreme Heat

Key Finding 3: An increase in population tolerance to extreme 

heat has been observed over time [Very High Confidence]. 

Changes in this tolerance have been associated with increased 

use of air conditioning, improved social responses, and/or 

physiological acclimatization, among other factors [Medium 

Confidence]. Expected future increases in this tolerance will 

reduce the projected increase in deaths from heat [Very 

Likely, Very High Confidence].

Description of evidence base

The increasing tolerance of the U.S. population to extreme 

heat has been shown by a number of studies.9, 10, 54 However, 

there is less confidence in attributing this increase in 

tolerance: increased prevalence and use of air conditioning, 

physiological adaptation, available green space, and improved 

social responses have all been proposed as explanatory 

factors. There have been some indications (Sheridan et al. 

2009)97 that tolerance improvements in the United States 

might have plateaued, but Bobb et al. 2014 found continuing 

improvements through 2005.54

Several approaches to including adaptation have been used 

in temperature mortality projection studies. For example, 

two studies used an “analog city” approach, where the 

response of the population to future temperatures in a 

given city is assumed to be equal to that of a city with a 

hotter present-day climate.22, 81 Another approach is to 

assume that critical temperature thresholds change by 
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U.S. Global Change Research Program 59 Impacts of Climate Change on Human Health in the United States

some quantity over time.18, 91 A third approach is to calculate 

sensitivity to air conditioning prevalence in the present, and 

make assumptions about air conditioning in the future.85 In 

general, inclusion of adaptation limits the projected increase 

in deaths, sometimes modestly, other times dramatically. 

However, approaches used to account for adaptation may 

be optimistic. Historically, adaptive measures have occurred 

as a response to extreme events, and therefore could be 

expected to lag warming.39, 96 While the increase in mortality 

projected in these studies is reduced, the studies generally 

found that mortality increases compared to present day even 

under optimistic adaptation assumptions.18, 22, 81, 85 A limit 

to adaptation may be seen in that even in cities with nearly 

100% air conditioning penetration, heat deaths are observed 

today.

Major uncertainties

While studies have been published in recent years that 

include adaptation in sensitivity analyses,22 this remains 

a challenging area of research. Difficulties in attributing 

observed increases in tolerance make it challenging to project 

future changes in tolerance, whether due to autonomous 

adaptation by individuals or planned adjustments by 

governments. Extrapolation of acclimatization is limited as 

there must be an increase in temperature beyond which 

acclimatization will not be possible. 

Assessment of confidence and likelihood based on evidence

There is very high confidence that a decrease in sensitivity to 

heat events has occurred based on high agreement between 

studies, but only medium confidence that this decrease is due 

to some specific combination of air conditioning prevalence, 

physiological adaptation, presence of green space, and 

improved social responses because of the challenges involved 

in attribution. There is very high confidence that mortality 

due to heat will very likely be reduced compared to a noadaptation scenario when adaptation is included, because 

all studies examined were in agreement with this conclusion, 

though the magnitude of this reduction is poorly constrained. 

Some Populations at Greater Risk

Key Finding 4: Older adults and children have a higher risk 

of dying or becoming ill due to extreme heat [Very High 

Confidence]. People working outdoors, the socially isolated 

and economically disadvantaged, those with chronic illnesses, 

as well as some communities of color, are also especially 

vulnerable to death or illness [Very High Confidence].

Description of evidence base

The relationship between increased temperatures and deaths 

in elderly populations is well-understood. An increased risk 

of respiratory and cardiovascular death is observed in elderly 

populations during temperature extremes due to reduced 

thermoregulation.17, 42, 45, 65

Studies cite dehydration, electrolyte imbalance, fever, heat 

stress, hyperthermia, and renal disease as the primary health 

conditions in children exposed to heat waves. Causes of heatrelated illness in children include inefficient thermoregulation, 

reduced cardiovascular output, and heightened metabolic 

rate. Children also spend a considerable amount of time 

outdoors and participating in vigorous activities.17, 42, 64, 103 A 

limited number of studies found evidence of cold-related 

mortality in children; however, no study has examined the 

relationship between cold temperature and cause-specific 

mortality.64

Certain occupational groups that spend a great deal of time 

exposed to extreme temperatures, such as agricultural 

workers, construction workers, and electricity and pipeline 

utility workers, are at increased risk for heat- and cold-related 

illness, especially where jobs involve heavy exertion.100, 113, 114

Lack of heat-illness-prevention programs in the workplace 

that include provisions for acclimatization was found to be a 

factor strongly associated with extreme temperature-related 

death.113

Race, ethnicity, and socioeconomic status have been shown 

to impact vulnerability to temperature extremes. Several 

studies have identified non-Hispanic Black populations to 

be more vulnerable than other racial and ethnic groups 

for experiencing detrimental consequences of exposure 

to temperature extremes.17, 42, 45, 103, 110 Studies suggest 

comorbidities that enhance susceptibility to higher 

temperatures, availability and use of air conditioning, 

disparities in heat risk-related land cover characteristics, and 

other environmental justice issues contribute to increased 

vulnerability of non-Hispanic Blacks.17, 42, 108, 110, 112

Dementia, mood disorders, neurosis and stress-related 

illnesses, and substance abuse are shown to be impacted by 

extreme heat.100, 115, 116, 117 Some medications interfere with 

thermoregulation, increasing vulnerability to heat.116

Major uncertainties

The literature available at the time of the development of the 

2014 NCA had identified a number of vulnerable populations 

that were disproportionately at risk during heat waves, 

and literature since that time has only strengthened the 

understanding of the elevated risks for these populations. 

There continues to be a need for better understanding of the 

relative importance of genetics and environmental justice 

issues with regards to the observed higher risk for nonHispanic Blacks, more work on understanding the risks to 

pregnant women from extreme temperature events, and a 

better understanding of the relationship between extreme 

cold vulnerabilities in populations of concern. 

Assessment of confidence and likelihood based on evidence

Although some details regarding causation and identifying the 

most vulnerable subpopulations still require research, there 

is a large body of literature that demonstrates the increased 

vulnerability to extreme heat of a number of groups, and 

therefore there is very high confidence that the listed 

populations of concern are at greater risk of temperaturerelated death and illness. 
2–TEMPERATURE-RELATED DEATH AND ILLNESS

U.S. Global Change Research Program 60 Impacts of Climate Change on Human Health in the United States

DOCUMENTING UNCERTAINTY

This assessment relies on two metrics to communicate 

the degree of certainty in Key Findings. See Appendix 4: 

Documenting Uncertainty for more on assessments of 

likelihood and confidence.

PHOTO CREDITS

Pg. 43–Construction worker: © Fotosearch

Pg. 44–Large Crowd: © iStockImages.com/Ints Vikmanis

Pg. 49–Snowstorm: © iStockImages.com/Dreef

Pg. 50–Construction worker: © Fotosearch

Pg. 54–Young baseball catcher: © iStockImages.com/jpbcpa

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few sources, limited consistency, models 

incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts

Likelihood

Very Likely

≥ 9 in 10

Likely

≥ 2 in 3

As Likely As Not

≈ 1 in 2

Unlikely

≤ 1 in 3

Very Unlikely

≤ 1 in 10
U.S. Global Change Research Program 61 Impacts of Climate Change on Human Health in the United States

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End
3 AIR QUALITY IMPACTS

On the web: health2016.globalchange.gov

U.S. Global Change Research Program

THE IMPACTS OF CLIMATE CHANGE ON HUMAN HEALTH IN THE UNITED STATES

A Scientific Assessment

Lead Author

Neal Fann

U.S. Environmental Protection Agency

Contributing Authors

Terry Brennan

Camroden Associates, Inc.

Patrick Dolwick 

U.S. Environmental Protection Agency

Janet L. Gamble 

U.S. Environmental Protection Agency

Vito Ilacqua 

U.S. Environmental Protection Agency

Laura Kolb

U.S. Environmental Protection Agency

Christopher G. Nolte

U.S. Environmental Protection Agency

Tanya L. Spero

U.S. Environmental Protection Agency

Lewis Ziska

U.S. Department of Agriculture

Recommended Citation: Fann, N., T. Brennan, P. Dolwick, J.L. Gamble, V. Ilacqua, L. Kolb, C.G. Nolte, T.L. Spero, and L. Ziska, 2016: Ch. 

3: Air Quality Impacts. The Impacts of Climate Change on Human Health in the United States: A Scientific Assessment. U.S. Global 

Change Research Program, Washington, DC, 69–98. http://dx.doi.org/10.10.7930/J0GQ6VP6 

Acknowledgements: Susan Anenberg, U.S. Chemical Safety Board; Amanda Curry Brown, U.S. Environmental Protection Agency; William 

Fisk, Lawrence Berkeley National Laboratory; Patrick Kinney, Columbia University; Daniel Malashock,* U.S. Department of Health and 

Human Services, Public Health Service; David Mudarri, CADMUS; Sharon Phillips, U.S. Environmental Protection Agency; Marcus C. 

Sarofim,* U.S. Environmental Protection Agency;

*Chapter Coordinators
70

3 AIR QUALITY IMPACTS

Key Findings 

Exacerbated Ozone Health Impacts 

Key Finding 1: Climate change will make it harder for any given regulatory approach to reduce ground-level 

ozone pollution in the future as meteorological conditions become increasingly conducive to forming ozone 

over most of the United States [Likely, High Confidence]. Unless offset by additional emissions reductions of 

ozone precursors, these climate-driven increases in ozone will cause premature deaths, hospital visits, lost 

school days, and acute respiratory symptoms [Likely, High Confidence].

Increased Health Impacts from Wildfires

Key Finding 2: Wildfires emit fine particles and ozone precursors that in turn increase the risk of premature 

death and adverse chronic and acute cardiovascular and respiratory health outcomes [Likely, High 

Confidence]. Climate change is projected to increase the number and severity of naturally occurring wildfires 

in parts of the United States, increasing emissions of particulate matter and ozone precursors and resulting 

in additional adverse health outcomes [Likely, High Confidence].

Worsened Allergy and Asthma Conditions 

Key Finding 3: Changes in climate, specifically rising temperatures, altered precipitation patterns, and 

increasing concentrations of atmospheric carbon dioxide, are expected to contribute to increases in the 

levels of some airborne allergens and associated increases in asthma episodes and other allergic illnesses 

[High Confidence].
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U.S. Global Change Research Program 71 Impacts of Climate Change on Human Health in the United States

3.1 Introduction 

Changes in the climate affect the air we breathe, both indoors 

and outdoors. Taken together, changes in the climate affect 

air quality through three pathways—via outdoor air pollution, 

aeroallergens, and indoor air pollution. The changing climate 

has modified weather patterns, which in turn have influenced 

the levels and location of outdoor air pollutants such as 

ground-level ozone (O3) and fine particulate matter.1, 2, 3, 4 Increasing carbon dioxide (CO2) levels also promote the growth 

of plants that release airborne allergens (aeroallergens). Finally, these changes to outdoor air quality and aeroallergens also 

affect indoor air quality as both pollutants and aeroallergens 

infiltrate homes, schools, and other buildings. 

Climate change influences outdoor air pollutant concentrations in many ways (Figure 1). The climate influences temperatures, cloudiness, humidity, the frequency and intensity of 

precipitation, and wind patterns,5 each of which can influence air quality. At the same time, climate-driven changes in 

meteorology can also lead to changes in naturally occurring 

emissions that influence air quality (for example, wildfires, 

wind-blown dust, and emissions from vegetation). Over longer 

time scales, human responses to climate change may also 

affect the amount of energy that humans use, as well as how 

land is used and where people live. These changes would in 

turn modify emissions (depending on the fuel source) and 

thus further influence air quality.6, 7 Some air pollutants such 

as ozone, sulfates, and black carbon also cause changes in 

Figure 1: This conceptual diagram for an outdoor air quality example illustrates the key pathways by which humans are exposed to 

health threats from climate drivers, and potential resulting health outcomes (center boxes). These exposure pathways exist within the 

context of other factors that positively or negatively influence health outcomes (gray side boxes). Key factors that influence vulnerability 

for individuals are shown in the right box, and include social determinants of health and behavioral choices. Key factors that influence 

vulnerability at larger scales, such as natural and built environments, governance and management, and institutions, are shown in the 

left box. All of these influencing factors can affect an individual’s or a community’s vulnerability through changes in exposure, sensitivity, 

and adaptive capacity and may also be affected by climate change. See Chapter 1: Introduction for more information.

Climate Change and Health—Outdoor Air Quality
3-AIR QUALITY IMPACTS

U.S. Global Change Research Program 72 Impacts of Climate Change on Human Health in the United States

climate.8 However, this chapter does not consider the climate 

effects of air pollutants, remaining focused on the health 

effects resulting from climate-related changes in air pollution 

exposure.

Poor air quality, whether outdoors or indoors, can negatively affect the human respiratory and cardiovascular systems. 

Outdoor ground-level ozone and particle pollution can have 

a range of adverse effects on human health. Current levels of 

ground-level ozone have been estimated to be responsible 

for tens of thousands of hospital and emergency room visits, 

millions of cases of acute respiratory symptoms and school 

absences, and thousands of premature deaths each year 

in the United States.9, 10 Fine 

particle pollution has also been 

linked to even greater health 

consequences through harmful 

cardiovascular and respiratory 

effects.11

A changing climate can also 

influence the level of aeroallergens such as pollen, which 

in turn adversely affect human 

health. Rising levels of CO2 and resulting climate changes alter 

the production, allergenicity (a measure of how much particular allergens, such as ragweed, affect people), distribution, and 

seasonal timing of aeroallergens. These changes increase the 

severity and prevalence of allergic diseases in humans. Higher 

pollen concentrations and longer pollen seasons can increase 

allergic sensitization and asthma episodes and thereby limit 

productivity at work and school.

Finally, climate change may alter the indoor concentrations of 

pollutants generated outdoors (such as ground-level ozone), 

particulate matter, and aeroallergens (such as pollen). Changes 

in the climate may also increase pollutants generated indoors, 

such as mold and volatile organic compounds. Most of the 

air people breathe over their lifetimes will be indoors, since 

people spend the vast majority of their time in indoor environments. Thus, alterations in indoor air pollutant concentrations 

from climate change have important health implications.

3.2 Climate Impacts on Outdoor Air Pollutants and 

Health

Changes in the climate affect air pollution levels.8, 12, 13, 14, 15, 16, 

17, 18, 19, 20, 21, 22 Human-caused climate change has the potential 

to increase ozone levels,1, 4 may have already increased ozone 

pollution in some regions of the United States,3 and has the 

potential to affect future concentrations of ozone and fine particles (particulate matter smaller 

than 2.5 microns in diameter, 

referred to as PM2.5).2, 7 Climate 

change and air quality are both 

affected by, and influence, several factors; these include the 

levels and types of pollutants 

emitted, how land is used, the 

chemistry governing how these 

pollutants form in the atmosphere, and weather conditions. 

Ground-Level Ozone

Ozone levels and subsequent ozone-related health impacts depend on 1) the amount of pollutants emitted that form ozone, 

and 2) the meteorological conditions that help determine 

the amount of ozone produced from those emissions. Both 

of these factors are expected to change in the future. The 

emissions of pollutants from anthropogenic (of human origin) 

sources that form ozone (that is, ozone “precursors”) are 

expected to decrease over the next few decades in the United 

States.23 However, irrespective of these changes in emissions, 

climate change will result in meteorological conditions more 

favorable to forming ozone. Consequently, attaining national 

air quality standards for ground-level ozone will also be more 

difficult, as climate changes offset some of the improvements 

that would otherwise be expected from emissions reductions. 

This effect is referred to as the “climate penalty.”7, 24

Meteorological conditions influencing ozone levels include air 

temperatures, humidity, cloud cover, precipitation, wind trajectories, and the amount of vertical mixing in the atmosphere.1, 

2, 25, 26 Higher temperatures can increase the chemical rates at 

which ozone is formed and increase ozone precursor emissions 

from anthropogenic sources and biogenic (vegetative) sources. 

Lower relative humidity reduces cloud cover and rainfall, promoting the formation of ozone and extending ozone lifetime in 

the atmosphere. A changing climate will also modify wind patterns across the United States, which will influence local ozone 

levels. Over much of the country, the worst ozone episodes tend 

to occur when the local air mass does not change over a period 

of several days, allowing ozone and ozone precursor emissions 

Higher pollen concentrations and longer pollen seasons can 

increase allergic sensitization and asthma episodes.

Human-caused climate change has the 

potential to increase ozone levels, may have 

already increased ozone pollution in some 

regions of the United States, and has the 

potential to affect future concentrations of 

ozone and fine particles.
3-AIR QUALITY IMPACTS

U.S. Global Change Research Program 73 Impacts of Climate Change on Human Health in the United States

to accumulate over time.27, 28 Climate change is already increasing the frequency of these types of stagnation events over parts 

of the United States,3 and further increases are projected.29

Ozone concentrations near the ground are strongly influenced 

by upward and downward movement of air (“vertical mixing”). 

For example, high concentrations of ozone near the ground often occur in urban areas when there is downward movement of 

air associated with high pressure (“subsidence”), reducing the 

extent to which locally emitted pollutants are diluted in the atmosphere.30 In addition, high concentrations of ozone can occur 

in some rural areas resulting from downward transport of ozone 

from the stratosphere or upper troposphere to the ground.31

Aside from the direct meteorological influences, there are also 

indirect impacts on U.S. ozone levels from other climate-influenced factors. For instance, higher water vapor concentrations due to increased temperatures will increase the natural 

rate of ozone depletion, particularly in remote areas,32 thus 

decreasing the baseline level of ozone. Additionally, potential 

climate-driven increases in nitrogen oxides (NOx) created by 

lightning or increased exchange of naturally produced ozone 

in the stratosphere to the troposphere could also affect ozone 

in those areas of the country most influenced by background 

ozone concentrations.33 Increased occurrences of wildfires due 

to climate change can also lead to increased ozone concentrations near the ground.34 

There is natural year-to-year variability in temperature and 

other meteorological factors that influence ozone levels.7

While global average temperature over 30-year climatic timescales is expected to increase, natural interannual variability 

will continue to play a significant role in year-to-year changes 

in temperature.35 Over the next several decades, the influence 

of climate change on meteorological parameters affecting 

average levels of ozone is expected to be smaller than the 

natural interannual variability.36

To address these issues, most assessments of climate impacts 

on meteorology and associated ozone formation concurrently 

simulate global and regional chemical transport over multiple 

years using “coupled” models. This approach can isolate the 

influence of meteorology in forming ozone from the effect of 

changes in emissions. The consensus of these model-based 

assessments is that accelerated rates of photochemical reaction, increased occurrence of stagnation events, and other 

direct meteorological influences are likely to lead to higher 

levels of ozone over large portions of the United States.8, 14, 16, 17

At the same time, ozone levels in certain regions are projected 

to decrease as a result of climate change, likely due to localized 

increases in cloud cover, precipitation, and/or increased dilution 

resulting from deeper mixed layers. These climate-driven changes in projected ozone vary by season and location, with climate 

and air quality models showing the most consistency in ozone 

increases due to climate change in the northeastern United 

States.8, 37

Generally, ozone levels will likely increase across the United 

States if ozone precursors are unchanged (see "Research Highlight: Ozone-Related Health Effects" on page 74).4, 7, 8 This climate penalty for ozone will offset some of the expected health 

benefits that would otherwise result from the expected ongoing 

reductions of ozone precursor emissions, and could prompt the 

need for adaptive measures (for example, additional ozone precursor emissions reductions) to meet national air quality goals. 

Ozone (O3) is a compound that occurs naturally in Earth’s atmosphere but is also formed by human activities. In 

the stratosphere (10–50 kilometers above the Earth’s surface), O3 prevents harmful solar ultraviolet radiation from 

reaching the Earth’s surface. Near the surface, however, O3 irritates the respiratory system. Ground-level O3, a key 

component of smog, is formed by chemical interactions between sunlight and pollutants including nitrogen oxides 

(NOx) and volatile organic compounds (VOCs). The emissions leading to O3 formation can result from both human 

sources (for example, motor vehicles and electric power generation) and natural sources (for example, vegetation 

and wildfires). Occasionally, O3 that is created naturally in the stratosphere can be mixed downward and contribute 

to O3 levels near the surface. Once formed, O3 can be transported by the wind before eventually being removed from 

the atmosphere via chemical reactions or by depositing on the surface. 

At any given location, O3 levels are influenced by complex interactions between emissions and meteorological 

conditions. Generally, higher temperatures, sunnier skies, and lighter winds lead to higher O3 concentrations by 

increasing the rate of chemical reactions and by decreasing the extent to which pollutants are mixed with “clean” 

(less polluted) background air. 

For a given level of emissions of O3 precursors, climate change is generally expected to increase O3 pollution in the 

future throughout much of the United States, in part due to higher temperatures and more frequent stagnant air 

conditions.7 Unless offset by additional emissions reductions of ozone precursors, these climate-driven increases in 

O3 will cause premature deaths, hospital visits, lost school days, and acute respiratory symptoms.14

What is Ozone?
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U.S. Global Change Research Program 74 Impacts of Climate Change on Human Health in the United States

Research Highlight: Ozone-Related Health Effects

Importance: Ozone is formed in the atmosphere by photochemical reactions of volatile organic compounds (VOCs) 

and nitrogen oxides (NOx) in the presence of sunlight. Although U.S. air quality policies are projected to reduce 

VOC and NOx emissions,56 climate change will increase the frequency of regional weather patterns conducive to 

increasing ground-level ozone, partially offsetting the expected improvements in air quality. 

Objective: Project the number and geographic distribution of additional ozone-related illnesses and premature 

deaths in the contiguous United States due to climate change between 2000 and 2030 under projected U.S. air 

quality policies.

Method: Climate scenarios from two global climate models (GCMs) using two different emissions pathways 

(RCP8.5 and RCP6.0) were dynamically downscaled following Otte et al. (2012)57 and used with emissions 

projections for 2030 and a regional chemical transport model to simulate air quality in the contiguous United 

States. The resulting changes in ozone in each scenario were then used to compute regional ozone-related health 

effects attributable to climate change. Ozone-related health impacts were estimated using the environmental 

Benefits Mapping and Analysis Program–Community Edition (BenMAP–CE). Population exposure was estimated 

using projected population data from the Integrated Climate and Land Use Scenarios (ICLUS). Further details can 

be found in Fann et al. (2015).14

Results: The two downscaled GCM projections result in 1°C to 4°C (1.8°F to 7.2°F) increases in average daily 

maximum temperatures and 1 to 5 parts per billion increases in daily 8-hour maximum ozone in 2030 throughout 

the contiguous United States. As seen in previous modeling analyses of climate impacts on ozone, the air quality 

response to climate change can vary substantially by region and across scenarios.22, 58 Unless reductions in ozone 

precursor emissions offset the influence of climate change, this climate penalty of increased ozone concentrations 

due to climate change would result in tens to thousands of additional ozone-related illnesses and premature 

deaths per year. 

Los Angeles, California, May 22, 2012. Unless offset by additional emissions reductions of ozone precursors, climate-driven

increases in ozone will cause premature deaths, hospital visits, lost school days, and acute respiratory symptoms.
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U.S. Global Change Research Program 75 Impacts of Climate Change on Human Health in the United States

Research Highlight: Ozone-Related Health Effects, continued

Conclusions: Future climate change will result in higher ozone levels in polluted regions of the contiguous 

United States. This study isolates the effect of climate change on ozone by using the same emissions of ozone 

precursors for both 2000-era and 2030-era climate. In addition, this study uses the latest generation of GCM 

scenarios and represents the most comprehensive analysis of climate-related, ozone-attributable health effects 

in 2030, and includes not only deaths but also emergency department admissions for asthma, hospital visits for 

respiratory causes, acute respiratory symptoms, and missed days of school. These results are subject to important 

uncertainties and limitations. The ozone-climate modeling reflects two scenarios (based on two separate GCMs) 

considered. Several emissions categories that are important in the formation of ozone and that could be affected 

by climate, such as motor vehicles, electrical generating units, and wildfires, were left unchanged between the 

current and future periods. The analysis applied concentration–response relationships from epidemiology studies 

of historical air pollution episodes; this both implies that the relationship between air pollution and risk will 

remain constant into the future and that populations will not attempt to reduce their exposure to ozone. 

Projected Change in Temperature, Ozone, and Ozone-Related Premature Deaths in 2030

Figure 2. Projected changes in average daily maximum temperature (degrees Fahrenheit), summer average maximum daily 

8-hour ozone (parts per billion), and excess ozone-related deaths (incidences per year by county) in the year 2030 relative to 

the year 2000, following two global climate models and two greenhouse gas concentration pathways, known as Representative 

Concentration Pathways, or RCPs (see van Vuuren et al. 201149). Each year (2000 and 2030) is represented by 11 years of 

modeled data for May through September, the traditional ozone season in the United States. 

The top panels are based on the National Center for Atmospheric Research/Department of Energy (NCAR/DOE) Community 

Earth System Model (CESM) following RCP8.5 (a higher greenhouse gas concentration pathway), and the bottom panels are 

based on the National Aeronautics and Space Administration (NASA) Goddard Institute for Space Studies (GISS) ModelE2-R 

following RCP6.0 (a moderate greenhouse gas concentration pathway).

The leftmost panels are based on dynamically downscaled regional climate using the NCAR Weather Research and Forecasting 

(WRF) model, the center panels are based on air quality simulations from the U.S. Environmental Protection Agency (EPA) 

Community Multiscale Air Quality (CMAQ) model, and the rightmost panels are based on the U.S. EPA Environmental Benefits 

and Mapping Program (BenMAP). 

Fann et al. 2015 reports a range of mortality outcomes based on different methods of computing the mortality effects of ozone 

changes—the changes in the number of deaths shown in the rightmost panels were computed using the method described in 

Bell et al. 2004.14, 38 (Figure source: adapted from Fann et al. 2015)14
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U.S. Global Change Research Program 76 Impacts of Climate Change on Human Health in the United States

Air pollution epidemiology studies describe the relationship 

between a population’s historical exposure to air pollutants 

and the risk of adverse health outcomes. Populations exposed 

to ozone air pollution are at greater risk of dying prematurely, 

being admitted to the hospital for respiratory hospital admissions, being admitted to the emergency department, and 

suffering from aggravated asthma, among other impacts.38, 39, 40

Air pollution health impact assessments combine risk estimates 

from these epidemiology studies with modeled changes in 

future or historical air quality changes to estimate the number 

of air-pollution-related premature deaths and illness.41 Future 

ozone-related human health impacts attributable to climate 

change are projected to lead to hundreds to thousands of premature deaths, hospital admissions, and cases of acute respiratory illnesses per year in the United States in 2030.14, 42, 43, 44, 45, 46

Health outcomes that can be attributed to climate change 

impacts on air pollution are sensitive to a number of factors 

noted above—including the climate models used to describe 

meteorological changes (including precipitation and cloud 

cover), the models simulating air quality levels (including 

wildfire incidence), the size and distribution of the population 

exposed, and the health status of that population (which influences their susceptibility to air pollution; see Ch. 1: Introduction).42, 47, 48, 49 Moreover, there is emerging evidence that 

air pollution can interact with climate-related stressors such as 

temperature to affect the human physiological response to air 

pollution.39, 42, 50, 51, 52, 53, 54, 55 For example, the risk of dying from 

exposure to a given level of ozone may increase on warmer 

days.51

Particulate Matter

Particulate matter (PM) is a complex mixture of solid- or 

liquid-phase substances in the atmosphere that arise from 

both natural and human sources. Principal constituents of PM 

include sulfate, nitrate, ammonium, organic carbon, elemental carbon, sea salt, and dust. These particles (also known as 

aerosols) can either be directly emitted or can be formed in 

the atmosphere from gas-phase precursors. PM smaller than 

2.5 microns in diameter (PM2.5) is associated with serious 

chronic and acute health effects, including lung cancer, chronic 

obstructive pulmonary disease (COPD), cardiovascular disease, 

and asthma development and exacerbation.11 The elderly are 

particularly sensitive to short-term particle exposure, with a 

higher risk of hospitalization and death.59, 60

As is the case for ozone, atmospheric PM2.5 concentrations 

depend on emissions and on meteorology. Emissions of sulfur 

dioxide (SO2), NOx, and black carbon are projected to decline 

substantially in the United States over the next few decades 

due to regulatory controls,56, 61, 62, 63 which will lead to reductions in sulfate and nitrate aerosols. 

Projected Change in Ozone-Related Premature Deaths

Figure 3. Projected change in ozone-related premature deaths from 2000 to 2030 by U.S. region and based on CESM/

RCP8.5. Each year (2000 and 2030) is represented by 11 years of modeled data. Ozone-related premature deaths were 

calculated using the risk coefficient from Bell et al. (2004).38 Boxes indicate 25th, 50th, and 75th percentile change over 11-

year sample periods, and vertical lines extend to 1.5 times the interquartile range. U.S. regions follow geopolitical boundaries 

shown in Figure 2. (Figure source: Fann et al. 2015)14

Research Highlight: Ozone-Related Health Effects, continued
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U.S. Global Change Research Program 77 Impacts of Climate Change on Human Health in the United States

Climate change is expected to alter several meteorological 

factors that affect PM2.5, including precipitation patterns and 

humidity, although there is greater consensus regarding the 

effects of meteorological changes on ozone than on PM2.5.

2

Several factors, such as increased humidity, increased stagnation events, and increased biogenic emissions are likely to 

increase PM2.5 levels, while increases in precipitation, enhanced atmospheric mixing, and other factors could decrease 

PM2.5 levels.2, 8, 37, 64 Because of the strong influence of changes 

in precipitation and atmospheric mixing on PM2.5 levels, and 

because there is more variability in projected changes to those 

variables, there is no consensus yet on whether meteorological changes will lead to a net increase or decrease in PM2.5

levels in the United States.2, 8, 17, 21, 22, 64, 65

As a result, while it is clear that PM2.5 accounts for most of the 

health burden of outdoor air pollution in the United States,10

the health effects of climate-induced changes in PM2.5 are 

poorly quantified. Some studies have found that changes in 

PM2.5 will be the dominant driver of air quality-related health 

effects due to climate change,44 while others have suggested 

a potentially more significant health burden from changes in 

ozone.50

PM resulting from natural sources (such as plants, wildfires, 

and dust) is sensitive to daily weather patterns, and those fluctuations can affect the intensity of extreme PM episodes (see 

also Ch. 4: Extreme Events, Section 4.6).8 Wildfires are a major 

source of PM, especially in the western United States during 

summer.66, 67, 68 Because winds carry PM2.5 and ozone precursor 

gases, air pollution from wildfires can affect people even far 

downwind from the fire location.35, 69 PM2.5 from wildfires affects human health by increasing the risk of premature death 

and hospital and emergency department visits.70, 71, 72

Climate change has already led to an increased frequency 

of large wildfires, as well as longer durations of individual 

wildfires and longer wildfire seasons in the western United 

States.73 Future climate change is projected to increase wildfire risks74, 75 and associated emissions, with harmful impacts 

on health.76 The area burned by wildfires in North America is 

expected to increase dramatically over the 21st century due to 

climate change.77, 78 By 2050, changes in wildfires in the western United States are projected to result in 40% increases of 

organic carbon and 20% increases in elemental carbon aerosol 

concentrations.79 Wildfires may dominate summertime PM2.5

concentrations, offsetting even large reductions in anthropogenic PM2.5 emissions.22

Likewise, dust can be an important constituent of PM, especially in the southwest United States. The severity and spatial 

extent of drought has been projected to increase as a result 

of climate change,80 though the impact of increased aridity on 

airborne dust PM has not been quantified (see Ch. 4. Extreme 

Events).2

3.3 Climate Impacts on Aeroallergens and 

Respiratory Diseases

Climate change may alter the production, allergenicity, distribution, and timing of airborne allergens (aeroallergens). These 

changes contribute to the severity and prevalence of allergic 

disease in humans. The very young, those with compromised 

immune systems, and the medically uninsured bear the brunt 

of asthma and other allergic illnesses. While aeroallergen 

exposure is not the sole, or even necessarily the most significant factor associated with allergic illnesses, that relationship 

is part of a complex pathway that links aeroallergen exposure to the prevalence of allergic illnesses, including asthma 

episodes.81, 82 On the other hand, climate change may reduce 

adverse allergic and asthmatic responses in some areas. For 

example, as some areas become drier, there is the potential 

for a shortening of the pollen season due to plant stress. 

Aeroallergens and Rates of Allergic Diseases in the United 

States 

Aeroallergens are substances present in the air that, once 

inhaled, stimulate an allergic response in sensitized individuals. Aeroallergens include tree, grass, and weed pollen; indoor 

and outdoor molds; and other allergenic proteins associated 

with animal dander, dust mites, and cockroaches.83 Ragweed 

is the aeroallergen that most commonly affects persons in the 

United States.84

Allergic diseases develop in response to complex and multiple interactions among both genetic and non-genetic factors, 

including a developing immune system, environmental exposures (such as ambient air pollution or weather conditions), 

and socioeconomic and demographic factors.85, 86, 87 Aeroallergen exposure contributes to the occurrence of asthma 

episodes, allergic rhinitis or hay fever, sinusitis, conjunctivitis, 

urticaria (hives), atopic dermatitis or eczema, and anaphylaxis 

(a severe, whole-body allergic reaction that can be life-threatNearly 6.8 million children in the United States are affected by 

asthma, making it a major chronic disease of childhood.
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U.S. Global Change Research Program 78 Impacts of Climate Change on Human Health in the United States

ening).84, 88 Allergic illnesses, including hay fever, affect about 

one-third of the U.S. population, and more than 34 million 

Americans have been diagnosed with asthma.81 These diseases 

have increased in the United States over the past 30 years (see 

Ch. 1 Introduction). The prevalence of hay fever has increased 

from 10% of the population in 1970 to 30% in 2000.84 Asthma 

rates have increased from approximately 8 to 55 cases per 

1,000 persons to approximately 55 to 90 cases per 1,000 persons over that same time period;89 however, there is variation 

in reports of active cases of asthma as a function of geography 

and demographics.90

Climate Impacts on Aeroallergen Characteristics 

Climate change contributes to changes in allergic illnesses as 

greater concentrations of CO2, together with higher temperatures and changes in precipitation, extend the start or duration 

of the growing season, increase the quantity and allergenicity of 

pollen, and expand the spatial distribution of pollens.84, 91, 92, 93, 94

Historical trends show that climate change has led to changes in the length of the growing season for certain allergenic 

pollens. For instance, the duration of pollen release for common 

ragweed (Ambrosia artemisiifolia) has been increasing as a 

function of latitude in recent decades in the midwestern region 

of North America (see Figure 4). Latitudinal effects on increasing 

season length were associated primarily with a delay in first 

frost during the fall season and lengthening of the frost-free 

period.95 Studies in controlled indoor environments find that 

increases in temperature and CO2 result in earlier flowering, 

greater floral numbers, greater pollen production, and increased allergenicity in common ragweed.96, 97 In addition, studies using urban areas as proxies for both higher CO2 and higher 

temperatures demonstrate earlier flowering of pollen species, 

which may lead to a longer total pollen season.98, 99, 100

For trees, earlier flowering associated with higher winter and 

spring temperatures has been observed over a 50-year period 

Figure 4: Ragweed pollen season length has increased in central North America between 1995 and 2011 by as much as 11 to 27 

days in parts of the United States and Canada, in response to rising temperatures. Increases in the length of this allergenic pollen 

season are correlated with increases in the number of days before the first frost. The largest increases have been observed in 

northern cities. (Figure source: Melillo et al. 2014. Photo credit: Lewis Ziska, USDA).35

Ragweed Pollen Season Lengthens
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U.S. Global Change Research Program 79 Impacts of Climate Change on Human Health in the United States

for oak.101 Research on loblolly pine (Pinus taeda) also demonstrates that elevated CO2 could induce earlier and greater 

seasonal pollen production.102 Annual birch (Betula) pollen production and peak values from 2020 to 2100 are projected to be 

1.3 to 2.3 times higher, relative to average values for 2000, with 

the start and peak dates of pollen release advancing by two to 

four weeks.103

Climate Variability and Effects on Allergic Diseases

Climate change related alterations in local weather patterns, 

including changes in minimum and maximum temperatures 

and rainfall, affect the burden of allergic diseases.104, 105, 106 The 

role of weather on the initiation or exacerbation of allergic 

symptoms in sensitive persons is not well understood.86, 107

So-called “thunderstorm asthma” results as allergenic particles are dispersed through osmotic rupture, a phenomenon 

where cell membranes burst. Pollen grains may, after contact 

with rain, release part of their cellular contents, including 

allergen-laced fine particles. Increases in the intensity and 

frequency of heavy rainfall and storminess over the coming 

decades is likely to be associated with spikes in aeroallergen 

concentrations and the potential for related increases in the 

number and severity of allergic illnesses.108, 109

Potential non-linear interactions between aeroallergens and 

ambient air pollutants (including ozone, nitrogen dioxide, sulfur dioxide, and fine particulate matter) may increase health 

risks for people who are simultaneously exposed.87, 88, 106, 108, 110, 

111, 112, 113, 114 In particular, pre-exposure to air pollution (especially ozone or fine particulate matter) may magnify the effects 

of aeroallergens, as prior damage to airways may increase 

the permeability of mucous membranes to the penetration of 

allergens, although existing evidence suggests greater sensitivity but not necessarily a direct link with ozone exposure.115 A 

recent report noted remaining uncertainties across the epidemiologic, controlled human exposure, and toxicology studies 

on this emerging topic.39

3.4 Climate Impacts on Indoor Air Quality and 

Health: An Emerging Issue 

Climate change may worsen existing indoor air problems and 

create new problems by altering outdoor conditions that 

affect indoor conditions and by creating more favorable conditions for the growth and spread of pests, infectious agents, 

and disease vectors that can migrate indoors.116 Climate 

change can also lead to changes in the mixing of outdoor and 

indoor air. Reduced mixing of outdoor and indoor air limits 

penetration of outdoor pollutants into the indoors, but also 

leads to higher concentrations of pollutants generated indoors 

since their dilution by outdoor air is decreased.

Indoor air contains a complex mixture of chemical and biological pollutants or contaminants. Contaminants that can be 

found indoors include carbon monoxide (CO), fine particles 

(PM2.5), nitrogen dioxide, formaldehyde, radon, mold, and 

pollen. Indoor air quality varies from building to building and 

over the course of a day in an individual building.

Public and environmental health professionals have known for 

decades that poor indoor air quality is associated with adverse 

respiratory and other health effects.116, 117, 118, 119, 120, 121 Since 

most people spend about 90% of their time indoors,122, 123, 124, 

125, 126 much of their exposures to airborne pollutants (both 

those influenced by climate change and those driven by other 

factors) happen indoors.

Outdoor Air Changes Reflected in Indoor Air 

Indoor air pollutants may come from indoor sources or may 

be transported into the building with outdoor air.127, 128 Indoor 

pollutants of outdoor origin may include ozone, dust, pollen, 

and fine PM (PM2.5). Even if a building has an outdoor air intake, 

some air will enter the building through other openings, such as 

open windows or under doors, or through cracks in the buildings, bypassing any filters and bringing outdoor air pollutants 

inside.129 If there are changes in airborne pollutants of outdoor 

origin, such as pollen and mold (see Section 3.3) and fine PM 

from wildfires (see "Particulate Matter" on page 76), there 

will be changes in indoor exposures to these contaminants. 

Although indoor fine PMlevels from wildfires are typically lower 

than outdoors (about 50%), because people spend most of their 

time indoors, most of their exposure to and health effects from 

wildfire particles (about 80%) will come from particles inhaled 

Dampness and mold in U.S. homes are linked to approximately 

4.6 million cases of worsened asthma.
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U.S. Global Change Research Program 80 Impacts of Climate Change on Human Health in the United States

indoors.130 Climate-induced changes in indoor-outdoor temperature differences may somewhat reduce the overall intake 

of outdoor pollutants into buildings for certain regions and 

seasons (see “Research Highlight: Residential Infiltration and 

Indoor Air”).131

Most exposures to high levels of ozone occur outdoors; however, indoor exposures, while lower, occur for much longer time 

periods. Indoors, ozone concentrations are usually about 10% 

to 50% of outdoor concentrations; however, since people spend 

most of their time indoors, most of their exposure to ozone is 

from indoor air.130 Thus, about 45% to 75% of a person’s overall 

exposure to ozone will occur indoors.132 About half of the health 

effects resulting from any outdoor increases in ozone (see Section "Ground-Level Ozone" on page 72) will be due to indoor 

ozone exposures.130 The elderly and children are particularly 

sensitive to short-term ozone exposure; however, they may 

spend even more time indoors than the general population 

and consequently their exposure to ozone is at lower levels for 

longer periods than the general public.133, 134 In addition, ozone 

Research Highlight: Residential Infiltration and Indoor Air 

Importance: Indoor and outdoor air are constantly mixing as air flows through small cracks and openings in buildings 

(infiltration) in addition to any open doors, windows, and vents. Infiltration or air exchange is driven by differences 

in barometric pressure, as a result of wind, and of the temperature difference between indoor and outdoor air. The 

greater this air exchange, the more similar the composition of indoor and outdoor air. Lower air exchange rates 

accentuate the impact of indoor sources while reducing that of some outdoor pollution. As climate change increases 

the average temperature of outdoor air, while indoor air continues to be maintained at the same comfortable 

temperatures, infiltration driven by temperature differences will change as well, modifying exposure to indoor and 

outdoor air pollution sources.

Objective: Project the relative change in infiltration and its effects on exposure to indoor and outdoor air pollution 

sources for different climates in the United States, between a late-20th century reference and the middle of the 

current century, in typical detached homes. 

Method: The infiltration change projected for 2040–2070 compared to 1970–2000 was modeled for typical 

single-family residences in urban areas, using temperatures and wind speeds from eight global–regional model 

combinations for nine U.S. cities (Atlanta, Boston, Chicago, Houston, Los Angeles, Minneapolis, New York, Phoenix, 

and Seattle). This analysis compares a building to itself, removing the effects of individual building characteristics 

on infiltration. Indoor temperatures were assumed unchanged between these two periods. Further details can be 

found in Ilacqua et al. 2015.131

Results: Because current average yearly temperatures across the contiguous United States are generally below 

comfortable indoor temperatures, model results indicate that, under future warmer temperatures, infiltration 

is projected to decrease by about 5%, averaged across cities, seasons, and climate models. Exposure to some 

pollutants emitted indoors would correspondingly increase, while exposure to some outdoor air pollutants would 

decrease to some extent. Projections vary, however, among location, seasons, and climate models. In the warmer 

cities, infiltration during summer months would rise by up to 25% in some models, raising peak exposures to ozone 

and other related pollutants just when their concentrations are typically highest. Predictions of different models are 

less consistent for summer months, however, displaying more uncertainty (average modeling relative range of 14%) 

for summer than for the rest of the year, and in fact not all models predict summer infiltration increases. Modeling 

uncertainty for the rest of the year is lower than in the summer (relative range less than 6%). 

Conclusions: This study shows the potential shifts in residential exposure to indoor and outdoor air pollution sources 

driven by a changing climate.131 These conclusions can be applied to small buildings, including single-family 

homes, row houses, and small offices. Potential adaptations intended to promote energy efficiency by reducing the 

leakage area of buildings will enhance the effect of decreasing infiltration and increasing exposure to indoor sources. 

Because of its novelty and lack of additional evidence, the study results should be considered as suggestive of an 

emerging issue. If replicated by other studies, these findings would add to the evidence on the potential for climate 

change to alter indoor air quality and further emphasize the impact of indoor air sources on human health. The 

overall implications of these findings for exposure to ambient and indoor air pollution remain uncertain at present, as 

they need to be considered along with other determinants of air exchange, such as window-opening behavior, whose 

relationship with climate change remains poorly characterized.
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U.S. Global Change Research Program 81 Impacts of Climate Change on Human Health in the United States

entering a building reacts with some organic compounds to 

produce secondary indoor air pollutants. These reactions lower 

indoor ozone concentrations but introduce new indoor air contaminants, including other respiratory irritants.135

Climate-related increases in droughts and dust storms may 

result in increases in indoor transmission of dust-borne pathogens, as the dust penetrates the indoor environment. Dust contains particles of biologic origin, including pollen and bacterial 

and fungal spores. Some of the particles are allergenic.136 Pathogenic fungi and bacteria can be found in dust both indoors and 

outdoors.137 For example, in the southwestern United States, 

spores from the fungi Coccidiodes, which can cause valley fever, 

are found indoors.138 The geographic range where Coccidiodes

is commonly found is increasing. Climate changes, including 

increases in droughts and temperatures, may be contributing 

to this spread and to a rise in valley fever (see Ch. 4: Extreme 

Events).

Legionnaires’ disease is primarily contracted from aerosolized 

water contaminated with Legionella bacteria.139 Legionella

bacteria are naturally found outdoors in water and soil; they are 

also known to contaminate treated water systems in buildings,140 as well as building cooling systems such as swamp coolers or cooling towers.141 Legionella can also be found indoors 

inside plumbing fixtures such as showerheads, faucets, and 

humidifiers.142, 143 Legionella can cause outbreaks of a pneumonia known as Legionnaire’s disease, which is a potentially fatal 

infection.144 Exposure can occur indoors when a spray or mist 

of contaminated water is inhaled, including mist or spray from 

showers and swamp coolers.145 The spread of Legionella bacteria can be affected by regional environmental factors.116 Legionnaires’ disease is known to follow a seasonal pattern, with more 

cases in late summer and autumn, potentially due to warmer 

and damper conditions.146, 147 Cases of Legionnaires’ disease are 

rising in the United States, with an increase of 192% from 2000 

to 2009.148, 149 If climate change results in sustained higher temperatures and damper conditions in some areas, there could be 

increases in the spread and transmission of Legionella. 

Contaminants Generated Indoors

Although research directly linking indoor dampness and climate 

change is not available, information on building science, climate 

change, and outdoor environmental factors that affect indoor 

air quality can be used to project how climate change may 

influence indoor environments.130 Climate change could result 

in increased indoor dampness in at least two ways: 1) if there 

are more frequent heavy precipitation events and other severe 

weather events (including high winds, flooding, and winter 

storms) that result in damage to buildings, allowing water or 

moisture entry; and 2) if outdoor humidity rises with climate 

change, indoor humidity and the potential for condensation 

and dampness will likely rise. Outdoor humidity is usually the 

largest contributor to indoor dampness on a yearly basis.127

Increased indoor dampness and humidity will in turn increase 

indoor mold, dust mites, bacteria, and other bio-contamination 

indoors, as well as increase levels of volatile organic compounds 

(VOCs) and other chemicals resulting from the off-gassing of 

damp or wet building materials.116, 119, 150 Dampness and mold 

in U.S. homes are linked to approximately 4.6 million cases of 

worsened asthma and between 8% and 20% of several common 

respiratory infections, such as acute bronchitis.151, 152 If there are 

climate-induced rises in indoor dampness, there could be increases in adverse health effects related to dampness and mold, 

such as asthma exacerbation. 

Additionally, power outages due to more frequent extreme 

weather events such as flooding could lead to a number of 

health effects (see Ch 4: Extreme Events). Heating, ventilation, 

and air conditioning (HVAC) systems will not function without 

power; therefore, many buildings could have difficulty maintaining indoor temperatures or humidity. Loss of ventilation, 

filtration, air circulation, and humidity control can lead to indoor 

mold growth and increased levels of indoor contaminants,153

including VOCs such as formaldehyde.119, 154, 155, 156 Power outages 

are also associated with increases in hospital visits from carbon 

monoxide (CO) poisoning, primarily due to the incorrect use 

of backup and portable generators that contaminate indoor 

air with carbon monoxide.135 Following floods, CO poisoning is 

also associated with the improper indoor use of wood-burning 

appliances and other combustion appliances designed for use 

outdoors.157 There were at least nine deaths from carbon monoxide poisoning related to power outages from 2000 to 2009.158

Climate factors can influence populations of rodents that produce allergens and can harbor pathogens such as hantaviruses, 

which can cause Hantavirus Pulmonary Syndrome. Hantaviruses can be spread to people by rodents that infest buildings,159

and limiting indoor exposure is a key strategy to prevent the 

spread of hantavirus.160 Climate change may increase rodent 

populations in some areas, including indoors, particularly when 

droughts are followed by periods of heavy rain (see Ch. 4: Extreme Events) and with increases in temperature and rainfall.161

Also, extreme weather events such as heavy rains and flooding 

may drive some rodents to relocate indoors.162 Increases in 

rodent populations may result in increased indoor exposures to 

rodent allergens and related health effects.159, 163, 164 In addition, 

climate factors may also influence the prevalence of hantaviruses in rodents.163, 164 This is a complex dynamic, because climate 

change may influence rodent populations, ranges, and infection 

rates.
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U.S. Global Change Research Program 82 Impacts of Climate Change on Human Health in the United States

3.5 Populations of Concern

Certain groups of people may be more susceptible to 

harm from air pollution due to factors including age, 

access to healthcare, baseline health status, or other 

characteristics.60 In the contiguous United States, Blacks 

or African-Americans, women, and the elderly experience the greatest baseline risk from air pollution.165

The young, older adults, asthmatics, and people whose 

immune systems are compromised are more vulnerable 

to indoor air pollutants than the general population.166

Lower 

socioeconomic status and housing disrepair have been 

associated with higher indoor allergen exposures, 

though higher-income populations may be more exposed to certain allergens such as dust mites.167, 168

Nearly 6.8 million children in the United States are 

affected by asthma, making it a major chronic disease 

of childhood.169 It is also the main cause of school 

absenteeism and hospital admissions among children.83

In 2008, 9.3% of American children age 2 to 17 years 

were reported to have asthma.169 The onset of asthma 

in children has been linked to early allergen exposure 

and viral infections, which act in concert with genetic 

susceptibility.170 Children can be particularly susceptible to allergens due to their immature respiratory and 

immune systems, as well as indoor or outdoor activities 

that contribute to aeroallergen exposure (see Table 

1).170, 171, 172, 173

Minority adults and children also bear a disproportionate burden associated with asthma as measured 

by emergency department visits, lost work and school 

days, and overall poorer health status (see Table 1).175, 176

Twice as many Black children had asthma-related emergency department visits and hospitalizations compared 

with White children. Fewer Black and Hispanic children 

reported using preventative medication like inhaled corticosteroids (ICS) as compared to White children. Black 

and Hispanic children also had more poorly controlled 

asthma symptoms, leading to increased emergency 

department visits and greater use of rescue medications 

rather than routine daily use of ICS, regardless of symptom control.173, 177

Children living in poverty were 1.75 times more likely to 

be hospitalized for asthma than their non-poor counterparts. 

When income is accounted for, no significant difference was 

observed in the rate of hospital admissions by race or ethnicity. 

This income effect may be related to access and use of health 

care and appropriate use of preventive medications such as 

ICS.178

Percentage of population with active asthma, by year and 

selected characteristics: United States, 2001 and 2010.

Characteristic Year 2001 % Year 2010 %

Total 7.3 8.4

Gender

Male 6.3 7.0

Female 8.3 9.8

Race

White 7.2 7.8

Black 8.4 11.9

Other 7.2 8.1

Ethnicity

Hispanic 5.8 7.2

Non-Hispanic 7.6 8.7

Age

Children (0-17) 8.7 9.3

Adults (18 and older) 6.9 8.2

Age Group

0-4 years 5.7 6.0

5-14 years 9.9 10.7

15-34 years 8.0 8.6

35-64 years 6.7 8.1

65 years and older 6.0 8.1

Region

Northeast 8.3 8.8

Midwest 7.5 8.6

South 7.1 8.3

West 6.7 8.3

Federal Poverty Threshold

Below 100% 9.9 11.2

100% to < 250% 7.7 8.7

250% to < 450% 6.8 8.2

450% or higher 6.6 7.1

Source: Moorman et al. 2012174

Table 1: A recent study of children in California found that racial 

and ethnic minorities are more affected by asthma.175 Among 

minority children, the prevalence of asthma varies with the highest rates among Blacks and American Indians/Alaska Natives 

(17%), followed by non-Hispanic or non-Latino Whites (10%), 

Hispanics (7%), and Asian Americans (7%).

People with preexisting medical conditions—including hypertension, diabetes, and chronic obstructive pulmonary 

disorder—are at greater risk for outdoor air pollution-related 

health effects than the general population.179 Populations with 

irregular heartbeats (atrial fibrillation) who were exposed to air 

pollution and high temperatures experience increased risk.165

People who live or work in buildings without air conditioning 

and other ventilation controls or in buildings that are unable to 

withstand extreme precipitation or flooding events are at great-
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U.S. Global Change Research Program 83 Impacts of Climate Change on Human Health in the United States

er risk of adverse health effects. Other health risks are related to 

exposures to poor indoor air quality from mold and other biological contaminants and chemical pollutants emitted from wet 

building materials. While the presence of air conditioning has 

been found to greatly reduce the risk of ozone-related deaths, 

communities with a higher percentage of unemployment and a 

greater population of Blacks are at greater risk.59

3.6 Research Needs

In addition to the emerging issues identified above, the 

authors highlight the following potential areas for additional 

scientific and research activity on air quality. Understanding of 

future air quality and the ability to model future health impacts associated with air quality changes—particularly PM2.5

impacts—will be enhanced by improved modeling and projections of climate-dependent variables like wildfires and land-use 

patterns, as well as improved modeling of ecosystem responses 

to climate change. Improved collection of data on aeroallergen 

concentrations in association with other ecosystem variables 

will facilitate research and modeling of related health impacts. 

Future assessments can benefit from research activities that:

• enhance understanding of how interactions among climate-related factors, such as temperature or relative humidity, aeroallergens, and air pollution, affect human health, and 

how to attribute health impacts to changes in these different 

risk factors;

• improve the ability to model and project climate change 

impacts on the formation and fate of air contaminants and 

quantify the compounded uncertainty in the projections; and

• identify the impacts of changes in indoor dampness, such as 

mold, other biological contaminants, volatile organic compounds, and indoor air chemistry on indoor air pollutants and 

health. 
U.S. Global Change Research Program 84 Impacts of Climate Change on Human Health in the United States

Supporting Evidence

PROCESS FOR DEVELOPING CHAPTER

The chapter was developed through technical discussions 

of relevant evidence and expert deliberation by the report 

authors at several workshops, teleconferences, and email 

exchanges. The authors considered inputs and comments 

submitted by the public, the National Academies of Sciences, 

and Federal agencies. For additional information on the 

overall report process, see Appendices 2 and 3.

In addition, the author team held an all-day meeting at 

the U.S. Environmental Protection Agency National Center 

for Environmental Assessment in Crystal City, Virginia, 

on October 15, 2014, to discuss the chapter and develop 

initial drafts of the Key Findings. A quorum of the authors 

participated and represented each of the three sections of 

the chapter—outdoor air quality, aeroallergens, and indoor 

air quality. These discussions were informed by the results 

of the literature review as well as the research highlights 

focused on outdoor air quality and indoor air quality. The 

team developed Key Finding 2 in response to comments from 

the National Research Council review panel and the general 

public.

The Key Findings for outdoor ozone, wildfires, and 

aeroallergen impacts reflect strong empirical evidence linking 

changes in climate to these outcomes. When characterizing 

the human health impacts from outdoor ozone, the team 

considered the strength of the toxicological, clinical, and 

epidemiological evidence evaluated in the Ozone Integrated 

Science Assessment.39 Because there is increasing evidence 

that climate change will increase the frequency and intensity 

of wildfire events, this outcome was included as a key finding, 

despite the inability to quantify this impact with the available 

tools and data. Because altered patterns of precipitation 

and increasing levels of CO2 are anticipated to promote the 

level of aeroallergens, this outcome is also included as a 

Key Finding. Finally, because the empirical evidence linking 

climate change to indoor air quality was more equivocal, we 

identified this topic as an emerging issue. 

KEY FINDING TRACEABLE ACCOUNTS

Exacerbated Ozone Health Impacts

Key Finding 1: Climate change will make it harder for any 

given regulatory approach to reduce ground-level ozone 

pollution in the future as meteorological conditions become 

increasingly conducive to forming ozone over most of the 

United States [Likely, High Confidence]. Unless offset by 

additional emissions reductions of ozone precursors, these 

climate-driven increases in ozone will cause premature 

deaths, hospital visits, lost school days, and acute respiratory 

symptoms [Likely, High Confidence].

Description of evidence base

The Intergovernmental Panel on Climate Change (IPCC) 

has concluded that warming of the global climate system 

is unequivocal and that continued increases in greenhouse 

gas emissions will cause further temperature increases.5, 

35 At the same time, there is a well-established relationship 

between measured temperature and monitored peak ozone 

levels in the United States.1, 25 Numerous climate and air 

quality modeling studies have also confirmed that increasing 

temperatures, along with other changes in meteorological 

variables, are likely to lead to higher peak ozone levels in the 

future over the United States,7, 37 if ozone precursor emissions 

remain unchanged.

Risk assessments using concentration–response relationships 

from the epidemiological literature and modeled air quality 

data have projected substantial health impacts associated 

with climate-induced changes in air quality.14, 42, 43, 44, 46, 50 This 

literature reports a range of potential changes in ozonerelated, non-accidental mortality due to modeled climate 

change between the present and 2030 or 2050, depending 

upon the scenario modeled, the climate and air quality 

models used, and assumptions about the concentration–

response function and future populations. Many of the 

studies suggest that tens to thousands of premature deaths 

could occur in the future due to climate change impacts on 

air quality.14, 42 At the same time, hundreds of thousands of 

days of missed school and hundreds of thousands to millions 

of cases of acute respiratory symptoms also result from the 

climate-driven ozone increases in the United States.14

Major uncertainties

Climate projections are driven by greenhouse gas emission 

scenarios, which vary substantially depending on assumptions 

for economic growth and climate change mitigation policies. 

There is significant internal variability in the climate system, 

which leads to additional uncertainties in climate projections, 

particularly on a regional basis. Ozone concentrations also 

depend on emissions that are influenced indirectly by climate 

change (for example, incidence of wildfires, changes in energy 

use, energy technology choices), which further compounds 

the uncertainty. Studies projecting human health impacts 

apply concentration–response relationships from existing 

epidemiological studies characterizing historical air quality 

changes; it is unclear how future changes in the relationship 

between air quality, population exposure, and baseline 

health may affect the concentration–response relationship. 

Finally, these studies do not account for the possibility 

of a physiological interaction between air pollutants and 

temperature, which could lead to increases or decreases in air 

pollution-related deaths and illnesses.
3-AIR QUALITY IMPACTS

U.S. Global Change Research Program 85 Impacts of Climate Change on Human Health in the United States

Assessment of confidence and likelihood based on evidence

Given the known relationship between temperature and 

ozone, as well as the numerous air quality modeling studies 

that suggest climate-driven meteorological changes will 

yield conditions more favorable for ozone formation in the 

future, there is high confidence that ozone levels will likely

increase due to climate change, unless offset by reductions in 

ozone precursor emissions. Based on observed relationships 

between ozone concentrations and human health responses, 

there is high confidence that any climate-driven increases 

in ozone will likely cause additional cases of premature 

mortality, as well as increasingly frequent cases of hospital 

visits and lost school days due to respiratory impacts. 

Increased Health Impacts from Wildfires

Key Finding 2: Wildfires emit fine particles and ozone 

precursors that in turn increase the risk of premature death 

and adverse chronic and acute cardiovascular and respiratory 

health outcomes [Likely, High Confidence]. Climate change 

is projected to increase the number and severity of naturally 

occurring wildfires in parts of the United States, increasing 

emissions of particulate matter and ozone precursors and 

resulting in additional adverse health outcomes [Likely, High 

Confidence]. 

Description of evidence base

The harmful effects of PM concentrations on human health 

have been well-documented, and there is equally strong 

evidence linking wildfires to higher PM concentrations 

regionally. Recent studies have established linkages between 

wildfire incidence and adverse health outcomes in the nearby 

population.70, 71 Though projections of climate change impacts 

on precipitation patterns are less certain than those on 

temperature, there is greater agreement across models that 

precipitation will decrease in the western United States.74

Rising temperatures, decreasing precipitation, and earlier 

springtime onset of snowmelt are projected to lead to 

increased frequency and severity of wildfires.22, 75, 77

Major uncertainties

Future climate projections, especially projections of 

precipitation, are subject to considerable uncertainty. Land 

management practices, including possible adaptive measures 

taken to mitigate risk, could alter the frequency and severity 

of wildfires, the emissions from wildfires, and the associated 

human exposure to smoke.

Assessment of confidence and likelihood based on evidence

Given the known association between PM and health 

outcomes and between wildfires and PM concentrations, 

there is high confidence that an increase in wildfire frequency 

and severity will likely lead to an increase in adverse 

respiratory and cardiac health outcomes. Based on the 

robustness of the projection by global climate models that 

precipitation amounts will decrease in parts of the United 

States, and that summer temperatures will increase, there is 

high confidence that the frequency and severity of wildfire 

occurrence will likely increase, particularly in the western 

United States.

Worsened Allergy and Asthma Conditions 

Key Finding 3: Changes in climate, specifically rising 

temperatures, altered precipitation patterns, and increasing 

concentrations of atmospheric carbon dioxide, are expected 

to contribute to increases in the levels of some airborne 

allergens and associated increases in asthma episodes and 

other allergic illnesses [High Confidence]. 

Description of evidence base

There is a large body of evidence supporting the observation 

that climate change will alter the production, allergenicity, 

distribution, and timing of aeroallergens. Historical trends 

show that climate change has led to changes in the length 

of the growing season for certain allergenic pollens. Climate 

change also contributes to changes in allergic illnesses 

as greater concentrations of CO2, together with higher 

temperatures and changes in precipitation, extend the start 

or duration of the growing season, increase the quantity and 

allergenicity of pollen, and expand the spatial distribution of 

pollens.84, 91, 92, 93, 94 While the role of weather on the initiation 

or exacerbation of allergic symptoms in sensitive persons 

is not entirely understood,86, 107 increases in intensity and 

frequency of rainfall and storminess over the coming decades 

is expected to be associated with spikes in aeroallergen 

concentrations and the potential for related increases in the 

number and severity of allergic illnesses.108, 109

These changes in exposure to aeroallergens contribute to 

the severity and prevalence of allergic disease in humans. 

Given that aeroallergen exposure is not the sole, or even 

necessarily the most significant, factor associated with 

allergic illnesses, that relationship is part of a complex 

pathway that links exposure to aeroallergens to the 

prevalence of allergic illnesses.81 There is consistent and 

robust evidence that aeroallergen exposure contributes 

significantly to the occurrence of asthma episodes, hay fever, 

sinusitis, conjunctivitis, hives, and anaphylaxis.84, 88 There is 

also compelling evidence that allergic diseases develop in 

response to complex and multiple interactions among both 

genetic and non-genetic factors, including a developing 

immune system, environmental exposures (such as ambient 

air pollution or weather conditions), and socioeconomic and 

demographic factors.85, 86, 87 Finally, there is evidence that 

potential non-linear interactions between aeroallergens and 

ambient air pollutants is likely to increase health risks for 

people who are simultaneously exposed.87, 88, 106, 108, 110, 111, 112, 

113, 114

Major uncertainties

The interrelationships between climate variability and 

change and exposure to aeroallergens are complex. Where 
3-AIR QUALITY IMPACTS

U.S. Global Change Research Program 86 Impacts of Climate Change on Human Health in the United States

PHOTO CREDITS

Pg. 69 –Girl with inhaler : © Stephen Welstead/LWA/Corbis

Pg. 70–Firefighters walking in smoke: © Ted Soqui/Corbis

Pg. 72–Ragweed pollen: Courtesy of Roy Morsch/Corbis 

Pg. 74–L.A. smog: © Ringo Chiu/ZUMA Press/Corbis

Pg. 77 –Girl with inhaler : © Stephen Welstead/LWA/Corbis

Pg. 79–Moldy archway: Courtesy of Bart Everson/flickr

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few sources, limited consistency, models 

incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts

Likelihood

Very Likely

≥ 9 in 10

Likely

≥ 2 in 3

As Likely As Not

≈ 1 in 2

Unlikely

≤ 1 in 3

Very Unlikely

≤ 1 in 10

they exist, differences in findings from across the relevant 

scientific literature may be due to study designs, references 

to certain species of pollen, geographic characteristics, 

climate variables, and degree of allergy sensitization.104

There are also uncertainties with respect to the role of 

climate change and the extent and nature of its effects as 

they contribute to aeroallergen-related diseases, especially 

asthma.91 Existing uncertainties can be addressed through 

the development of standardized approaches for measuring 

exposures and tracking outcomes across a range of allergic 

illnesses, vulnerable populations, and geographic proximity to 

exposures.82

Assessment of confidence and likelihood based on evidence 

The scientific literature suggests that there is high confidence

that changes in climate, including rising temperatures and 

altered precipitation patterns, will affect the concentration, 

allergenicity, season length, and spatial distribution of a 

number of aeroallergens, and these changes are expected 

to impact the prevalence of some allergic diseases, including 

asthma attacks.

DOCUMENTING UNCERTAINTY

This assessment relies on two metrics to communicate 

the degree of certainty in Key Findings. See Appendix 4: 

Documenting Uncertainty for more on assessments of 

likelihood and confidence.
U.S. Global Change Research Program 87 Impacts of Climate Change on Human Health in the United States

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© Ted Soqui/Corbis 
IMPACTS OF EXTREME EVENTS 4 ON HUMAN HEALTH

On the web: health2016.globalchange.gov

U.S. Global Change Research Program

Lead Authors

Jesse E. Bell

Cooperative Institute for Climate and Satellites–North Carolina

Stephanie C. Herring

National Oceanic and Atmospheric Administration

Lesley Jantarasami*

U.S. Environmental Protection Agency

Contributing Authors

Carl Adrianopoli

U.S. Department of Health and Human Services

Kaitlin Benedict

Centers for Disease Control and Prevention

Kathryn Conlon

Centers for Disease Control and Prevention

Vanessa Escobar

National Aeronautics and Space Administration

Jeremy Hess

University of Washington

Jeffrey Luvall

National Aeronautics and Space Administration

Carlos Perez Garcia-Pando

Columbia University

Dale Quattrochi

National Aeronautics and Space Administration

Jennifer Runkle*

Cooperative Institute for Climate and Satellites–North Carolina

Carl J. Schreck, III

Cooperative Institute for Climate and Satellites–North Carolina

Recommended Citation: Bell, J.E., S.C. Herring, L. Jantarasami, C. Adrianopoli, K. Benedict, K. Conlon, V. Escobar, J. Hess, J. Luvall, 

C.P. Garcia-Pando, D. Quattrochi, J. Runkle, and C.J. Schreck, III, 2016: Ch. 4: Impacts of Extreme Events on Human Health. 

The Impacts of Climate Change on Human Health in the United States: A Scientific Assessment. U.S. Global Change Research 

Program, Washington, DC, 99–128. http://dx.doi.org/10.7930/J0BZ63ZV

*Chapter Coordinators

Acknowledgements: Mark Keim, formerly of the Centers for Disease Control and Prevention; Andrea Maguire*, U.S. Environmental 

Protection Agency

THE IMPACTS OF CLIMATE CHANGE ON HUMAN HEALTH IN THE UNITED STATES

A Scientific Assessment
100

IMPACTS OF EXTREME EVENTS ON 

HUMAN HEALTH

FPO

4

Key Findings

Increased Exposure to Extreme Events

Key Finding 1: Health impacts associated with climate-related changes in exposure to extreme events include 

death, injury, or illness; exacerbation of underlying medical conditions; and adverse effects on mental health 

[High Confidence]. Climate change will increase exposure risk in some regions of the United States due to 

projected increases in the frequency and/or intensity of drought, wildfires, and flooding related to extreme 

precipitation and hurricanes [Medium Confidence].

Disruption of Essential Infrastructure

Key Finding 2: Many types of extreme events related to climate change cause disruption of infrastructure, 

including power, water, transportation, and communication systems, that are essential to maintaining access 

to health care and emergency response services and safeguarding human health [High Confidence].

Vulnerability to Coastal Flooding

Key Finding 3: Coastal populations with greater vulnerability to health impacts from coastal flooding include 

persons with disabilities or other access and functional needs, certain populations of color, older adults, 

pregnant women and children, low-income populations, and some occupational groups [High Confidence]. 

Climate change will increase exposure risk to coastal flooding due to increases in extreme precipitation and 

in hurricane intensity and rainfall rates, as well as sea level rise and the resulting increases in storm surge 

[High Confidence].
4–IMPACTS OF EXTREME EVENTS ON HUMAN HEALTH

U.S. Global Change Research Program 101 Impacts of Climate Change on Human Health in the United States

4.1 Introduction

Some regions of the United States have already experienced 

costly impacts—in terms of both lives lost and economic damages—from observed changes in the frequency, intensity, or 

duration of certain extreme events (Figure 1). Climate change 

projections show that there will be continuing increases in the 

occurrence and severity of some extreme events by the end 

of the century, while for other extremes the links to climate 

change are more uncertain (Table 1). (See also Ch. 1: Introduction)

Four categories of extreme events with important health 

impacts in the United States are addressed in this chapter: 1) 

flooding related to extreme precipitation, hurricanes, and coastal storms, 2) droughts, 3) wildfires, and 4) winter storms and 

severe thunderstorms. The health impacts of extreme heat and 

extreme cold are discussed in Chapter 2: Temperature-Related 

Death and Illness. For each event type, the chapter integrates 

discussion of populations of concern that have greater vulnerability to adverse health outcomes. The air quality impacts of 

wildfires are discussed below and also in Chapter 3: Air Quality 

Impacts. Although mental health effects are noted briefly here 

and in later sections of this chapter, in-depth discussion of the 

impacts of extreme events on mental health is presented in 

Chapter 8: Mental Health.

While it is intuitive that extremes can have health impacts 

such as death or injury during an event (for example, drowning 

during floods), health impacts can also occur before or after 

an extreme event as individuals may be involved in activities 

that put their health at risk, such as disaster preparation and 

post-event cleanup.1 Health risks may also arise long after the 

event, or in places outside the area where the event took place, 

as a result of damage to property, destruction of assets, loss of 

infrastructure and public services, social and economic impacts, 

environmental degradation, and other factors. Extreme events 

also pose unique health risks if multiple events occur simultaneously or in succession in a given location, but these issues of 

cumulative or compounding impacts are still emerging in the 

literature (see Front Matter and Ch. 1: Introduction).

Dynamic interactions between extreme events, their physical 

impacts, and population vulnerability and response can make 

it difficult to quantitatively measure all the health impacts 

that may be associated with an extreme event type, particularly those that are distributed over longer periods of time 

(See “Emerging Issues,” Section 4.8). These complexities make 

it difficult to integrate human health outcomes into climate 

impact models, and thus projections of future health burdens 

due to extreme events under climate change are not available 

Figure 1: This figure provides 10-year estimates of fatalities related to extreme events from 2004 to 2013,204 as well as estimated 

economic damages from 58 weather and climate disaster events with losses exceeding $1 billion (see Smith and Katz 2013 to 

understand how total losses were calculated).205 These statistics are indicative of the human and economic costs of extreme weather 

events over this time period. Climate change will alter the frequency, intensity, and geographic distribution of some of these extremes,2

which has consequences for exposure to health risks from extreme events. Trends and future projections for some extremes, 

including tornadoes, lightning, and wind storms are still uncertain.

Estimated Deaths and Billion Dollar Losses from Extreme Events in the United States 2004–2013
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U.S. Global Change Research Program 102 Impacts of Climate Change on Human Health in the United States

Table 1: Health Impacts of Extreme Events

Event Type

Example Health Risks

and Impacts

(not a comprehensive list)

Observed and Projected Impacts of 

Climate Change on Extreme Events from 2014 

NCA2

Flooding Related to 

Extreme Precipitation, 

Hurricanes, Coastal 

Storms 

• Traumatic injury and death 

(drowning)

• Mental health impacts 

• Preterm birth and low birth weight

• Infrastructure disruptions and 

post-event disease spread

• Carbon monoxide poisoning 

related to power outages

Heavy downpours are increasing nationally, especially over 

the last three to five decades, with the largest increases in 

the Midwest and Northeast. Increases in the frequency and 

intensity of extreme precipitation events are projected for all 

U.S. regions. [High Confidence].

The intensity, frequency, and duration of North Atlantic 

hurricanes, as well as the frequency of the strongest 

hurricanes, have all increased since the 1980s [High 

Confidence]. Hurricane intensity and rainfall are projected 

to increase as the climate continues to warm [Medium 

Confidence].

Increasing severity and frequency of flooding have been 

observed throughout much of the Mississippi and Missouri 

River Basins. Increased flood frequency and severity 

are projected in the Northeast and Midwest regions [Low 

Confidence]. In the Western United States, increasing 

snowmelt and rain-on-snow events (increased runoff when 

rain falls onto existing snowpack) will increase flooding in 

some mountain watersheds [Medium Confidence].

In the next several decades, storm surges and high tides 

could combine with sea level rise and land subsidence to 

further increase coastal flooding in many regions. The U.S. 

East and Gulf Coasts, Hawaii, and the U.S.-affiliated Pacific 

Islands are particularly at risk. 

Droughts

• Reduced water quality and 

quantity

• Respiratory impacts related to 

reduced air quality

• Mental health impacts

Over the last several decades, drought patterns and trends 

have been changing, but patterns vary regionally across the 

United States. Droughts in the Southwest are projected to 

become more intense [High Confidence].

Wildfires

• Smoke inhalation

• Burns and other traumatic injury 

• Asthma exacerbations 

• Mental health impacts 

Increased warming, drought, and insect outbreaks, all 

caused by or linked to climate change, have increased 

wildfires and impacts to people and ecosystems in the 

Southwest [High Confidence]. 

Rising temperatures and hotter, drier summers are projected 

to increase the frequency and intensity of large wildfires, 

particularly in the western United States and Alaska. 

Winter Storms & 

Severe Thunderstorms 

• Traumatic injury and death

• Carbon monoxide poisoning 

related to power outages

• Hypothermia and frostbite 

• Mental health impacts

Winter storms have increased in frequency and intensity 

since the 1950s, and their tracks have shifted northward 

[Medium Confidence]. Future trends in severe storms, 

including the intensity and frequency of tornadoes, hail, and 

damaging thunderstorm winds, are uncertain and are being 

studied intensively [Low Confidence]. 
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in the literature. Instead, this chapter focuses on explaining the 

physical processes and pathways that scientists know contribute 

to human exposure and identifying overarching conclusions regarding the risk of adverse health impacts as a result of changing extreme weather and climate.

4.2 Complex Factors Determine Health Impacts 

The severity and extent of health effects associated with extreme events depend on the physical impacts of the extreme 

events themselves as well as the unique human, societal, and 

environmental circumstances at the time and place where 

events occur. This complex set of factors can moderate or 

exacerbate health outcomes and vulnerability in the affected 

people and communities (Figure 2). Vulnerability is the tendency or predisposition to be adversely affected by climate-related health effects. It encompasses three elements—exposure, sensitivity, and adaptive capacity—that also interact 

with and are influenced by the social determinants of health 

(See Ch. 1: Introduction and Ch. 9: Populations of Concern for 

additional discussion and definitions of these terms.) 

Exposure is contact between a person and one or more biological, psychosocial, chemical, or physical stressors, including 

stressors affected by climate change. Contact may occur in a 

single instance or repeatedly over time, and may occur in one 

location or over a wider geographic area. Demographic shifts 

and population migration may change exposure to public 

health impacts. For example, since 1970, coastal population 

growth (39%) has substantially increased compared to population growth for the United States as a whole (about 13%).3

 In 

the future, this coastal migration in conjunction with rising sea 

levels has the potential to result in increased vulnerability to 

storm surge events for a greater proportion of the U.S. population concentrated in these coastal areas. Choices by individuals and governments can reduce or increase some exposure 

risk to extreme events.4 As shown in Figure 2, such choices 

can include whether to build or allow development in floodplains and coastal areas subject to extreme high tides and 

sea level rise. Individuals’ responses to evacuation orders and 

other emergency warnings also affect their exposure to health 

threats. Factors such as income have been linked to how people perceive the risks to which they are exposed and choose 

Figure 2: This conceptual diagram for a flooding event illustrates the key pathways by which humans are exposed to health threats 

from climate drivers, and potential resulting health outcomes (center boxes). These exposure pathways exist within the context of 

other factors that positively or negatively influence health outcomes (gray side boxes). Key factors that influence health outcomes and 

vulnerability for individuals are shown in the right box, and include social determinants of health and behavioral choices. Key factors 

that influence health outcomes and vulnerability at larger community or societal scales, such as natural and built environments, 

governance and management, and institutions, are shown in the left box. All of these influencing factors may also be affected by 

climate change. See Chapter 1: Introduction for more information.

Climate Change and Health—Flooding
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to respond, as well as their ability to evacuate or relocate to 

a less risk-prone location.5 The condition of the built environment also affects exposure to extreme events, and those living 

in low-quality, poorly maintained, or high-density housing may 

have greater risks of health impacts.6

Sensitivity is the degree to which people or communities are 

affected, either adversely or beneficially, by climate variability 

and change. It is determined, at least in part, by biologically 

based traits such as age. For example, older adults (generally 

defined as age 65 and older) are physiologically more sensitive 

to health impacts from extreme events because of normal 

aging processes; they are generally more frail, more likely to 

have chronic medical conditions that make them more dependent on medications, and require more assistance in activities 

of daily living.7, 8 In addition, social determinants of health 

affect disparities in the prevalence of medical conditions that 

contribute to biological sensitivity.9, 10 Health disparities are 

more prevalent in low-income populations, as well as in some 

communities of color, and are frequently exacerbated during 

extreme events.11 For example, Black or African American 

populations have higher rates of chronic conditions such as 

asthma, decreased lung function, and cardiovascular issues, all 

of which are known to increase sensitivity to health effects of 

smoke from wildfires (Ch. 3: Air Quality Impacts).12

Adaptive capacity is the ability of communities, institutions, 

or people to adjust to potential hazards, to take advantage of 

opportunities, or to respond to 

consequences. Having strong 

adaptive capacity contributes to 

resilience—the ability to prepare 

and plan for, absorb, recover 

from, and more successfully 

adapt to adverse events.13 In 

the context of extreme events, 

people with low adaptive capacity have difficulty responding, evacuating, or relocating when 

necessary, and recovering from event-related health impacts. 

For individuals, health outcomes are strongly influenced by the 

social determinants of health that affect a person’s adaptive 

capacity. Poverty is a key risk factor, and the poor are disproportionately affected by extreme events.4, 9, 14 Low-income individuals may have fewer financial resources and social capital (such 

as human networks and relationships) to help them prepare for, 

respond to, and recover from an extreme event.15, 16 In many 

urban, low-income neighborhoods, adaptive capacity is reduced 

where physical and social constructs, such as community infrastructure, neighborhood cohesion, and social patterns, promote 

social isolation.17, 18, 19 Those with higher income possess a much 

higher level of resilience and availability of resources to increase 

their adaptive capacity.20, 21 Other attributes of individuals that 

contribute to lower adaptive capacity include their age (very 

young or very old) and associated dependency on caregivers, 

disabilities such as mobility or cognitive impairments, having 

specific access and functional needs, medical or chemical dependence, limited English proficiency, social or cultural isolation, homelessness, and institutionalization (prisons, psychiatric 

facilities, nursing homes).1, 8, 22

At a larger community or societal level, adaptive capacity is 

heavily influenced by governance, management, and institutions.23 Governments and non-governmental organizations 

provide essential extreme-event preparedness, coordination, 

emergency response, and recovery functions that increase 

adaptive capacity at the local, state, tribal, and federal levels—

for example, in providing early warning systems where possible, evacuation assistance, and disaster relief.13, 24 Risk sharing, 

management, and recovery schemes such as insurance can 

also play a significant role in 

building resilience in the context 

of extreme events and climate 

change.25, 26 For instance, lack of 

health insurance has been associated with greater risk of hospital admission after exposure to 

certain weather events.27 Public 

health actions or interventions 

that maintain or strengthen the adaptive capacity of communities, institutions, or people could help mediate certain 

health impacts due to extreme events.28 On the other hand, 

climate change—particularly its effect on extreme events—has 

the potential to create unanticipated public health stressors 

that could overwhelm the U.S. public health system’s adaptive 

capacity and could require new approaches.28

4.3 Disruption of Essential Infrastructure

When essential infrastructure and related services are disrupted during and after an extreme event, a population’s exposure 

to health hazards can increase, and losses related to the event 

can reduce adaptive capacity.4 Disruptions can include reduced 

functionality, such as poor road conditions that limit travel, or 

complete loss of infrastructure, such as roads and bridges being 

washed away. Serious health risks can arise from infrastructure 

and housing damage and disruption or loss of access to electricity, sanitation, safe food and water supplies, health care, comFamily affected by Hurricane Sandy prepares to take shelter in 

Morristown, New Jersey, October 31, 2012. 

Having strong adaptive capacity contributes 

to resilience—the ability to prepare and plan 

for, absorb, recover from, and more successfully 

adapt to adverse events.
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munication, and transportation.1, 29, 30, 31, 32 Identifying vulnerable 

infrastructure and investing in strategies to reduce vulnerability, 

including redundancy (having additional or alternate systems in 

place as backup) and ensuring a certain standard of condition 

and performance can reduce the likelihood of significant adverse impacts to infrastructure from extreme weather events.33

Health Risks Related to Infrastructure

Existing infrastructure is generally designed to perform at its 

engineered capacity assuming historical weather patterns, and 

these systems could be more vulnerable to failure in response 

to weather-related stressors under future climate scenarios.4, 

34, 35 Shifts in the frequency or intensity of extreme events outside their historical range pose infrastructure risks, which may 

be compounded by the fact that much of the existing critical 

infrastructure in the United States, like water and sewage systems, roads, bridges, and power plants, are aging and in need 

of repair or replacement.4, 36 For example, the 2013 American 

Society of Civil Engineer’s Report Card assigned a letter grade 

of D+ to the condition and performance of the Nation’s infrastructure.37

In addition, recurrent weather-related stressors, such as “nuisance flooding” (frequent coastal flooding that is increasing in 

frequency due to sea level rise), contribute to overall deterioration of infrastructure like stormwater drainage systems 

and roads (see Ch. 6: Water-Related Illness).38 These systems 

are important in the context of health because drainage helps 

to avoid sewage overflows and maintain water quality,39 and 

roads are vital for evacuations and emergency response during 

and after extreme events.40

Energy infrastructure that relies on environmental inputs, such 

as water for cooling in power generation or for hydroelectric 

dams, is also vulnerable to changes in extreme events due 

to climate change.34, 41 Power generation accounts for one of 

the largest withdrawals of freshwater in the United States.42

Longer or more intense droughts that are projected for some 

regions of the United States (see Table 1) will contribute to 

reduced energy production in those regions, which may lead 

to supply interruptions of varying lengths and magnitudes 

and adverse impacts to other infrastructure that depends on 

energy supply.34

Power Outages

Electricity is fundamental to much modern infrastructure, 

and power outages are commonly associated with the types 

of extreme events highlighted in this chapter.43 During power 

outages, observed health impacts include increased deaths 

from accidental and natural causes,44 increased cases of foodborne diarrheal illness from consuming food spoiled by lack 

of refrigeration (see Ch. 7: Food Safety),1 and increased rates 

of hospitalization.45 In addition, extreme-event-related power 

outages are associated with increased injuries and deaths 

from carbon monoxide poisoning after floods, hurricanes, 

severe winter storms, and ice storms.1, 31, 46, 47, 48, 49 This is due to 

increased use of gasoline-powered generators, charcoal grills, 

and kerosene and propane heaters or stoves inside the home 

or other areas without proper ventilation (see also Ch. 3: Air 

Quality Impacts). Populations considered especially vulnerable 

to the health impacts of power outages include older adults, 

young children, those reliant on electrically powered medical 

equipment like ventilators and oxygen, those with preexisting 

health conditions, and those with disabilities (see Ch. 9: Populations of Concern).1, 43, 44 In rural communities, power and 

communications can take longer to restore after damage from 

an extreme event.50

Transportation, Communication, and Access 

Damage to transportation infrastructure or difficult road 

conditions may delay first responders, potentially delaying 

treatment of acute injuries and requiring more serious intervention or hospitalization.40 Extreme events can disrupt access 

to health care services via damage to or loss of transportation 

infrastructure, evacuation, and population displacement.32

For chronically ill people, treatment interruptions and lack of 

access to medication can exacerbate health conditions both 

during and after the extreme event.1, 51 Surveys of patients 

after Hurricane Katrina showed that those with cancer, hypertension, kidney disease requiring dialysis, cardiovascular 

disease, and respiratory illnesses were particularly affected.51, 

52, 53 Evacuations also pose health risks to older adults—especially those who are frail, medically incapacitated, or residing 

in nursing or assisted living facilities—and may be complicated 

by the need for concurrent transfer of medical records, medications, and medical equipment.1, 54 Some individuals with 

disabilities may also be disproportionally affected during evacuations if they are unable to access evacuation routes, have 

difficulty in understanding or receiving warnings of impending 

danger, or have limited ability to communicate their needs.55

Power lines damaged by Hurricane Isaac's wind and surge in 

Plaquemines Parish, Louisiana, September 3, 2012.
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In addition, persons with limited English proficiency are less 

likely to understand or have timely access to emergency 

information, which may lead to delayed evacuation.56, 57 Health 

risks increase if evacuation is delayed until after a storm hits; 

loss of power and damage to communications and transportation infrastructure can hinder health system operations.1

Water Infrastructure

Extreme precipitation events and storms can overwhelm or 

damage stormwater and wastewater treatment infrastructure, 

increasing the risk of exposure to contaminated water (see Ch. 

6: Water-Related Illness). Risk of post-flood gastrointestinal illness outbreaks are considered to be low in the United States, 

but risk increases for displaced populations—especially young 

children and infants with immature immune systems—where 

shelter conditions are crowded or have poor sanitation.1, 29

There is potential for post-flood mold and fungi growth inside 

houses to worsen allergic and asthmatic symptoms, but these 

types of health impacts have not been documented following 

floods or storms.1, 29, 58, 59 In general, however, adverse health 

effects from dampness and mold in homes are well known and 

studied.60, 61, 62

Cascading Failures

Many infrastructure systems are reliant on one another, 

and disruption or failure of one system or at any place in 

the system can lead to the disruption of interconnected 

systems—a phenomenon referred to as a cascading failure. 

For example, electricity is essential to multiple systems, and 

a failure in the electrical grid can have cascading effects on 

water and sewage treatment, transportation, and health care 

systems.36, 43 Extreme events can simultaneously strain single 

or multiple components of interconnected infrastructure and 

related facilities and equipment, which increases the risk of 

cascading infrastructure failure.63, 64 This risk to interconnected 

systems has been particularly notable in the context of urban 

areas (especially cities for which the design or maintenance 

of critical infrastructure needs improvement) and industrial 

sites containing chemicals or hazardous materials that rely 

on specific equipment—such as holding tanks, pipelines, and 

electricity-dependent safety mechanisms like automatic shutoff valves—to prevent releases.4, 65, 66 Dramatic infrastructure 

system failures are rare, but such cascading failures can lead 

to public health consequences when they do occur, including 

shifts in disease incidence.67

The 2003 blackout in the northeastern United States, caused 

indirectly by surging electrical demand during a heat wave, is 

an illustrative example of how climate change could introduce 

or exacerbate health threats from cascading infrastructure 

failures related to extreme weather. During this 31-hour event, 

lack of electricity compromised traffic control, health care and 

emergency services, wastewater treatment, solid waste collection, and a host of other critical infrastructure operations.68, 

69, 70, 71 New York City health officials responded to failure of 

hospital emergency generators and interruptions in electrically 

powered medical equipment, contamination of recreational 

water and beaches with untreated sewage, pest control issues, 

and loss of refrigeration leading to potential impacts on food 

and vaccine spoilage.72 Increased incidence of gastrointestinal 

illness from contaminated food or water, and a large increase 

in accidental and non-accidental deaths and hospitalizations 

in New York City were attributed to the blackout.44, 45, 72 See 

Chapter 6: Water-Related Illness for other examples of health 

impacts when interconnected wastewater, stormwater, and 

drinking water infrastructure fails, such as during the 1993 

Milwaukee Cryptosporidium outbreak. 

4.4 Flooding Related to Extreme Precipitation,

Hurricanes, and Coastal Storms

Floods are the primary health hazard associated with extreme 

precipitation events, hurricanes, and coastal storms. Risk of 

exposure to floods varies by region in the United States and 

by type of flooding that occurs in that location (see Table 1 

and “Flood Terminology”). People in flood-prone regions are 

expected to be at greater risk of exposure to flood hazards due 

to climate change (Table 1),9, 73, 74 which may result in various 

types of health impacts described below.

Most flood deaths in the United States are due to drowning 

associated with flash flooding.1, 29, 58 The majority of these 

deaths are associated with becoming stranded or swept away 

when driving or walking near or through floodwaters.58, 76, 77, 

78 Flash floods in the United States occurred more frequently 

from 2006 to 2012 and were associated with more deaths and 

injuries in rural areas compared to urban areas.78 Contributing 

factors include the following: 1) small, rural basins develop 

flash flood conditions much more quickly, providing less time 

to notify rural residents with emergency procedures like 

Coastal floods – predominately caused by storm 

surges that are exacerbated by sea level rise. Coastal 

floods can destroy buildings and infrastructure, 

cause severe coastal erosion, and submerge large 

areas of the coast. 

Flash and urban floods – occur in smaller inland 

natural or urban watersheds and are closely tied to 

heavy rainfall. Flash floods develop within minutes or 

hours after a rainfall event, and can result in severe 

damage and loss of life due to high water velocity, 

heavy debris load, and limited warning.

River floods – occur in large watersheds like the 

Mississippi and Missouri River Basins. River floods 

depend on many factors including precipitation, 

preexisting soil moisture conditions, river basin 

topography, and human factors like land-use change 

and flood control infrastructure (dams, levees). 

Adapted from Georgakakos et al. (2014).75

Flood Terminology
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warnings, road closures, and evacuations; 2) more rural roads 

intersect low-water crossings without bridge infrastructure 

and rural areas have fewer alternative transportation options 

when roads are closed; and 3) rural areas have fewer emergency response units and slower response times.78 Although 

flash floods are less frequent in urban areas, a single urban 

event is likely to result in more deaths and injuries than a rural 

event.78

Drowning in floodwaters was the leading cause of death 

(estimated 2,544 persons) among people directly exposed 

to hazards associated with hurricanes and coastal storms 

from 1963 to 2012.79 Hurricanes are typically associated with 

coastal flooding, but they can also cause substantial inland 

flooding before, during, and after landfall, even when far from 

the storm’s center (Figure 3).80, 81, 82 The deadliest U.S. storms of this century to date were Hurricane Katrina and Superstorm Sandy. Katrina was a very 

large and powerful Category 3 storm that hit the Gulf Coast 

region in 2005. Hurricane Katrina was responsible for almost 

half of the hurricane-related deaths over the past 50 years,79

with the majority of deaths directly related to the storm in 

Louisiana (an estimated 971 to 1,300 deaths) due to drowning or flood-related physical trauma due to the failure of the 

levees in New Orleans.83, 84 Sandy was a historically rare storm 

that affected a large portion of the country in October 2012, 

with particularly significant human health and infrastructure 

impacts in New Jersey and the greater New York City area. Superstorm Sandy is estimated to have caused between 117 and 

147 direct deaths across the Atlantic basin, also with drowning and flood-related physical trauma as the leading cause of 

death.85, 86

Both fatal and non-fatal flood-related injuries can occur in any 

phase of the event: before (preparation or evacuation), during, 

and after (cleanup and recovery). Common flood-related injuries include blunt trauma from falling debris or objects moving 

quickly in floodwater, electrocution, falls, and motor vehicle 

accidents from wet, damaged, or obstructed roads.1, 29, 58 Other 

common, generally non-fatal injuries include cuts, puncture 

wounds, sprains/strains, burns, hypothermia, and animal bites.1, 

29, 58 Exposure to floodwaters or to contaminated drinking water 

can cause gastrointestinal illness; wound infections; skin irritations and infections; and eye, ear, nose, and throat infections.1, 

29 Many of these injuries have been observed in occupational 

settings 31 and in rural areas.78

In the United States, populations with greater vulnerability to 

flood-related injuries and illnesses include older adults, the 

immunocompromised and others with existing illness (especially if dependent on routine medical treatments or drug 

prescriptions), certain racial/ethnic groups (Black and Hispanic 

or Latino), people with limited English proficiency, and people with lower socioeconomic status (especially if uninsured, 

unemployed, or living in poor-quality housing).1, 73 Differences in 

Figure 3: Composite map of floods associated with landfalling 

hurricanes over the past 31 years, based on stream gauge data. 

The Flood Ratio (Q) refers to maximum hurricane-related flood 

peaks compared to 10-year flood peaks (expected to occur, 

on average, once every 10 years and corresponds to the 90th 

percentile of the flood peak distribution) calculated for the same 

area. See Villarini et al. 2014 for a detailed description of how Q 

values are calculated.80

Q values between 0.6 and 1 (light blue and yellow) generally 

indicate minor to moderate flooding, while values above 1 

(orange and red) generally indicate major flooding larger than the 

10-year flood peak. The dark gray areas of the map represent 

the extent of the 500-km buffer around the center of circulation 

of the hurricanes included during the study period (the light gray 

areas of the map fall outside of the study area). 

Figure 3 shows that hurricanes are important contributors to 

flooding in the eastern United States, as well as large areas of 

the central United States. Land use/land cover properties and 

soil moisture conditions are also important factors for flooding. 

(Figure source: adapted from Villarini et al. 2014)80

Hurricane-Induced Flood Effects

in Eastern and Central United States

A truck gets stuck in the storm surge covering Highway 90 in 

Gulfport, Mississippi, during Hurricane Isaac, August 29, 2012.
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exposure, sensitivity, and adaptive capacity lead to a disproportionate number of flood-related fatalities among older adults, 

males, and some low-income communities of color.29 For example, almost half of deaths from Hurricane Katrina were people 

over age 75, while for Superstorm Sandy almost half were over 

age 65.1, 29 The Black adult mortality rate from Hurricane Katrina 

was 1.7 to 4 times higher than that of whites.29, 84 Floods and 

storms also create numerous occupational health risks, with 

most storm-related fatalities associated with cleanup activities 

(44%), construction (26%), public utilities restoration (8%), and 

security/policing (6%).1 First responders and other emergency 

workers face greater health and safety risks when working in 

conditions with infrastructure disruptions, communication 

interruptions, and social unrest or violence following floods and 

storms.73, 87, 88

Pregnant women and newborns are uniquely vulnerable to 

flood health hazards. Flood exposure was associated with 

adverse birth outcomes (preterm birth, low birth weight) after 

Hurricane Katrina and the 1997 floods in North Dakota.89, 90

Floods and storms can also create conditions in which children can become separated from their parents or caregivers, 

which—particularly for children with disabilities or special 

health care needs—increases their vulnerability to a range of 

health threats, including death, injury, disease, psychological 

trauma, and abuse.91, 92, 93 Flood-related mental health impacts 

are associated with direct and longer-term losses, social impacts, stress, and economic hardship.1, 29, 58 Women, children, 

older adults, low-income populations, and those in poor 

health, with prior mental health issues, or with weak social 

networks may be especially vulnerable to the mental health 

impacts of floods (Ch. 8: Mental Health).

4.5 Droughts 

Drought may be linked to a broad set of health hazards, 

including wildfires, dust storms, extreme heat events, flash 

flooding, degraded air and water quality, and reduced water 

quantity.74 Exposure risk to potential drought health hazards 

is expected to vary widely across the nation, depending on 

several localized variables, such as characteristics of the built 

environment, loss of livelihoods, local demand for water, and 

changes in ecosystems.94, 95 Researching the health effects of 

drought poses unique challenges given multiple definitions 

of the beginning and end of a drought, and because health 

effects tend to accumulate over time. In addition, health impacts do not occur in isolation. For example, droughts intensify 

heat waves by reducing evaporative cooling,2 further complicating efforts to attribute specific health outcomes to specific 

drought conditions.

A primary health implication of drought arises from the 

contamination and depletion of water sources,95 but there 

are few studies documenting specific health consequences 

in the United States.96 Drought in coastal areas can increase 

saltwater intrusion (the movement of ocean water into fresh 

groundwater), reducing the supply and quality of potable water.97, 98, 99 In addition to reducing water quantity, drought can 

decrease water quality when low flow or stagnant conditions 

increase concentrations of pollutants or contaminants (such as 

chemicals and heavy metals) and when higher temperatures 

encourage pathogen growth.95, 96, 100, 101, 102, 103 Heavy rain following drought can flush accumulated pathogens or contaminants 

into water bodies.104, 105 Reduced surface and groundwater 

quality can increase risk of water-related illness as well as 

foodborne illness if pathogens or contaminants enter the food 

chain (see Ch. 6: Water-Related Illness and Ch. 7: Food Safety).

In some regions of the United States, drought has been associated with increased incidence of West Nile virus disease.106, 107, 

108, 109, 110 Human exposure risk to West Nile virus may increase 

during drought conditions due to a higher prevalence of the 

virus in mosquito and bird populations as a result of closer 

contact between birds (virus hosts) and mosquitoes (vectors) 

as they congregate around remaining water sources (see Ch. 

5: Vector-Borne Diseases) .111 Primarily in the Southwest, 

droughts followed by periods of heavy rainfall have been 

associated with an increase in rodent populations.112, 113, 114

This could lead to increased exposures to rodent allergens and 

rodent-borne diseases, such as hantavirus.115, 116, 117

Farmer in drought-stressed peanut field in Georgia. Health 

implications of drought include contamination and depletion of 

water sources.
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Fungi growth and dispersal are sensitive to changes in temperature, moisture, and wind.136 Illnesses or allergic 

reactions related to fungal toxins and superficial or invasive fungal infections can cause serious illness, permanent 

disability, or death. People generally become infected by breathing in fungal spores directly from the environment 

or having spores enter the skin at sites of injury. Coccidioidomycosis, also called “Valley Fever,” is an infection 

caused by Coccidioides, a fungus found mainly in the southwestern United States. Reports of these infections 

are on the rise.137 The fungus appears to grow best in soil after heavy rainfall and then becomes airborne most 

effectively during hot, dry conditions.138 Several studies in Arizona and California, where most reported cases in 

the United States occur, suggest that climate likely plays a role in seasonal and yearly infection patterns.139, 140

Recently, Coccidioides was found in soil in south-central Washington, far north of where it was previously known 

to exist.141 Climate factors such as drought and increased temperature may be contributing to Coccidioides’ 

expanded geographic range.142 Thus, more prolonged or intense droughts resulting from climate change could lead 

to improved conditions for the spread of Coccidioides.143 Understanding the impact of climate change on fungal 

infections (such as Coccidioidomycosis, Crypotcoccos gattii, and Mucormycosis) would require comprehensive 

epidemiologic surveillance, better methods to detect disease-causing fungi in the environment, and ongoing 

multidisciplinary collaboration. 

Fungal Diseases and Climate Change 

FPO

Drought may increase the potential for 

wind erosion to cause soil dust to become 

airborne, and there is evidence from past 

trends showing regional increases in dust 

activity due to drought cycles, but there is 

large uncertainty about future projections 

of climate impacts on frequency or 

intensity of dust storms.119, 128, 129 Wind 

erosion can also be exacerbated by 

human activities that disturb the soil, 

including growing crops, livestock grazing, 

recreation and suburbanization, and 

water diversion for irrigation.119, 128, 130

Major dust activity in the United States is 

centered in the Southwest, where sources 

are mostly natural, and the Great Plains, 

extending from Montana to southern Texas, where sources are mainly from human activities associated with land 

use, such as agriculture.131 These are also regions where climate change is expected to affect drought patterns.2

In the United States, dust exposure has been linked to increased incidence in respiratory disease, including 

asthma, acute bronchitis, and pneumonia.27, 132, 133 However, the dust characteristics (such as composition and 

particle size), exposure levels, and biological mechanisms responsible for the observed health effects of dust are not 

completely understood. In part, this is because observations are generally unavailable in areas where dust exposure 

is greatest, including drylands and agricultural areas.122 Apart from illness, intense dust storms are also associated 

with impaired visibility, which can cause road traffic accidents resulting in injury and death.134, 135

Wind Erosion and Dust Storms

April 14, 2013. Dust storm on Interstate Highway 10 California USA.

"In the United States, dust exposure has been linked to increased incidence in 

respiratory disease, including asthma, acute bronchitis, and pneumonia."
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U.S. Global Change Research Program 110 Impacts of Climate Change on Human Health in the United States

Drought conditions also tend to reduce air quality and exacerbate respiratory illness by way of several mechanisms associated with soil drying, loss of vegetation, airborne particulate 

matter, and the creation of conditions conducive for dust 

storms and wildfires.

118, 119 In addition, air pollutants such as 

soluble trace gases and particles remain suspended in the air 

when there is a lack of precipitation (see Ch. 3: Air Quality 

Impacts).120 Inhalation of particles can irritate bronchial passages and lungs, resulting in exacerbated chronic respiratory 

illnesses.95 The size of particles is directly linked to their potential health effects. Exposure to fine particles is associated with 

cardiovascular illness (for example, heart attacks and strokes) 

and premature death, and is likely associated with adverse 

respiratory effects.121 There is greater uncertainty regarding 

the health effects of inhaling coarse particles (often found in 

soil dust), but some evidence indicates an association with 

premature death and cardiovascular and respiratory effects.121, 

122, 123

Mental health issues have also been observed during drought 

periods through research primarily conducted in Australia 

(see also Ch. 8: Mental Health).94 Rural areas, in particular, can 

experience a rise in mental health issues related to economic 

insecurity from drought.94, 124, 125, 126, 127

4.6 Wildfires

Climate change is projected to increase the frequency and 

intensity of large wildfires (Figure 4), with associated health 

risks projected to increase in many regions.74, 144 Wildfire can 

have health impacts well beyond the perimeter of the fire. 

Populations near the fire or even thousands of miles downwind may be exposed to a complex smoke mixture containing 

various substances including carbon monoxide, ozone, toxic 

chemicals, and both fine and coarse particles,145, 146 presenting 

a serious health risk for the exposed populations (see Ch. 3: 

Air Quality Impacts).147, 148 For example, the 2002 forest fires 

in Quebec resulted in up to a 30-fold increase in airborne fine 

particulate concentrations in Baltimore, Maryland, a city nearly 1,000 miles downwind.74 Exposure times can range from a 

few days to several weeks.145, 149, 150

Exposure to smoke-related air pollutants from wildfires has 

been associated with a wide range of human health effects, 

including early deaths and low infant birth weight, with the 

strongest evidence for acute respiratory illness.145, 146, 151, 152, 153, 

154, 155 Inhalation of smoke from wildfire has been linked to exacerbated respiratory problems, such as shortness of breath, 

asthma, and chronic obstructive pulmonary disease (COPD).154, 

156, 157, 158 While the association between smoke exposure and 

cardiovascular outcomes is uncertain,154 exposure to fine particles contributes to risk of cardiovascular disease and premature death.159, 160, 161, 162

Wildfires can also affect indoor air quality for those living near 

affected areas by increasing particulate matter concentrations 

within homes, leading to many of the adverse health impacts 

already discussed.149, 163 For example, during the 2007 San 

Diego wildfires, health monitoring showed excess emergency 

room visits for asthma, respiratory problems, chest pain, and 

COPD. During times of peak fire particulate matter concentrations, the odds of a person seeking emergency care increased 

by 50% when compared to non-fire conditions.164 Smoke from 

wildfires can also impair driving visibility, increasing risks of 

motor vehicle deaths and injuries.134, 165, 166, 167

Exposure to smoke-related air pollutants from wildfires has 

been associated with a wide range of human health effects.

Figure 4: Based on 17 climate model simulations for the 

continental United States using a higher emissions pathway 

(RCP8.5), the map shows projected percentage increases in 

weeks with risk of very large fires by mid-century (2041–2070) 

compared to the recent past (1971–2000). The darkest shades 

of red indicated that up to a 6-fold increase in risk is projected for 

parts of the West. This area includes the Great Basin, Northern 

Rockies, and parts of Northern California. Gray represents areas 

within the continental United States where there is either no data 

or insufficient historical observations on very large fires to build 

robust models. The potential for very large fire events is also 

expected to increase along the southern coastline and in areas 

around the Great Lakes. (Figure source: adapted from Barbero 

et al. 2015 by NOAA)206

Projected Increases in Very Large Fires
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U.S. Global Change Research Program 111 Impacts of Climate Change on Human Health in the United States

Pregnant women, children, and the elderly are more sensitive 

to the harmful health effects of wildfire smoke exposure (see 

also Ch. 9: Populations of Concern).12, 156, 168, 169 Firefighters are 

exposed to significantly higher levels and longer periods of 

exposure to combustion products from fires, leading to health 

risks that include decreased lung function, inflammation, and 

respiratory system problems, as well as injuries from burns 

and falling trees.145, 168, 170, 171, 172, 173

Wildfires can also create an increased burden on the health 

care system and public health infrastructure. For example, 

wildfires near populated areas often necessitate large evacuations, requiring extensive public health resources, including 

shelter, and treatment of individuals for injuries, smoke inhalation, and mental health impacts.67, 166, 174, 175 Housing development in or near the wildland–urban interface has expanded 

over the last several decades and is expected to continue to 

expand.176 These changing development patterns in combination with a changing climate are increasing the vulnerability of 

these areas to wildfires.177, 178, 179

Following wildfire, increased soil erosion rates and changes 

to runoff generation may contaminate water-supply reservoirs and disrupt downstream drinking water supplies.180, 181

Post-wildfire erosion and runoff has been linked to increased 

flooding and debris flow hazards, depending on the severity 

of the fire, seasonal rainfall patterns, watershed characteristics, and the size of the burn area.182, 183, 184, 185 Wildfires have a 

range of short- and long-term effects on watersheds that have 

the potential to change water quality, quantity, availability, 

and treatability downstream from the burned area.186, 187, 188

4.7 Winter Storms and Severe Thunderstorms

The primary health hazards of severe thunderstorms are from 

lightning and high winds, while the principal winter storm hazards include extreme cold temperatures (see Ch. 2: Temperature-related Deaths and Illness), frozen precipitation, and associated dangerous road and other conditions. Future health 

impacts associated with these types of storms are uncertain 

and will depend on how climate change affects storm trends. 

During the period 1956 to 2006, lightning caused an estimated 

101.2 deaths per year,189 while thunderstorm winds are estimated to have caused approximately 26 deaths per year from 

1977 to 2007.190 Thunderstorm precipitation and winds can 

damage structures, fell trees, and create hazardous road conditions and impair driving visibility, increasing risks of motor 

vehicle deaths and injuries.134, 191, 192 Thunderstorm winds can 

cause blunt trauma or injuries, such as from being struck by 

falling trees and other flying debris,46 and were responsible for 

an estimated 4,366 injuries during the period 1993 to 2003.192

Winter storms can be accompanied by freezing winds and frigid temperatures that can cause frostbite and hypothermia (see 

also Ch. 2: Temperature-Related Deaths and Illness).193, 194 Individuals that lack proper clothing and shelter (for example, the 

homeless) are more at risk of injuries from direct exposure to 

weather conditions associated with winter storms and severe 

thunderstorms.195 Low-income populations have increased exposure risk to severe winter weather conditions because they 

are more likely to live in low-quality, poorly insulated housing; 

be unable to afford sufficient domestic heating; or need to 

make tradeoffs between food and heating expenditures.196, 197

Freezing rain, snow, and ice have been linked to increased injuries associated with falling198 as well as motor vehicle deaths 

and injuries due to treacherous road conditions and impaired 

driving visibility.134, 199

After severe thunderstorms, individuals can suffer injuries 

during debris removal and cleanup activities192, 200 as well as 

exposure to hazards if flooding occurs (see Section 4.4 of this 

chapter). Mental health issues and stress are also possible 

after storms (see Ch. 8: Mental Health). This is especially true 

of thunderstorms associated with tornadoes, as the aftermath 

of the storm can involve dealing with the loss of property, displacement, or loss of life.201 After winter storms, snow removal 

can be strenuous work and can increase the likelihood of illness and death for individuals with preexisting cardiovascular 

or pulmonary conditions.202

4.8 Emerging Issues

Climate change and changing patterns of extreme weather 

have the potential to strain the capacity of public health systems. However, few comprehensive or systematic studies have 

examined the human health impacts of such health-system 

strain.203 Particularly in the context of floods and hurricanes, 

the impacts on health systems from short- and long-term population displacement are not fully understood or well quantified.67 In addition, the role of future population migration 

and demographic changes is just beginning to be elucidated 

in assessments of local adaptive capacity or resilience to the 

effects of future extreme events. Methodological challenges 

remain for accurately quantifying and attributing delayed 

Freezing rain, snow, and ice have been linked to increased 

injuries associated with treacherous road conditions and 

impaired driving visibility.
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U.S. Global Change Research Program 112 Impacts of Climate Change on Human Health in the United States

mortality associated with, but not caused directly by, extreme 

event exposure—for example, elevated mortality associated 

with heart disease, cancer, diabetes, and infections and other 

complications from injuries in populations exposed to hurricanes.30, 31

4.9 Research Needs 

In addition the emerging issues identified above, the authors 

highlight the following potential areas for additional scientific 

and research activity on extreme events based on their review 

of the literature. Current understanding is limited by a lack 

of systematic surveillance for the range of health impacts, 

both short and long term, associated with a wider range of 

extreme events, including prolonged events like droughts and 

other extremes that do not currently trigger post-event health 

surveillance. 

Future assessments can benefit from multidisciplinary research activities that:

• better define the health implications associated with particular extreme events where longer-term impacts, as well as 

regional differences in health outcomes, are currently not 

well understood, such as droughts and floods;

• enhance understanding of how specific attributes that 

contribute to individual and community level vulnerability to 

health impacts after extreme events, including social and behavioral characteristics, interact and contribute to or mitigate 

risks of adverse health outcomes; and

• examine how health outcomes can be impacted by other 

cumulative, compounding, or secondary effects of extreme 

events, such as access to or disruption of healthcare services 

and damages to and cascading failures of infrastructure.
U.S. Global Change Research Program 113 Impacts of Climate Change on Human Health in the United States

Supporting Evidence

PROCESS FOR DEVELOPING CHAPTER

The chapter was developed through technical discussions 

of relevant evidence and expert deliberation by the report 

authors at several workshops, teleconferences, and email 

exchanges. Authors considered inputs and comments 

submitted by the public, the National Academies of Sciences, 

and Federal agencies. For additional information on the 

overall report process, please see Appendices 2 and 3.

The health outcomes selected and prioritized for the chapter 

were based primarily on those that had substantial peerreviewed literature to support statements. While many 

connections between changes in extreme events due to 

climate change and human health impacts appear intuitive, in 

some cases there may not be a robust body of peer-reviewed 

literature to support statements about direct effects. For 

example, while it is believed that droughts have the ability to 

impact water quality, which could in turn impact health, there 

are few studies documenting specific health consequences in 

the United States.96

In addition, due to space constraints, the authors did not 

intend to exhaustively identify all possible health impacts 

from every type of extreme event addressed in this chapter. 

Instead, the authors have provided an overview of possible 

impacts from different types of extreme events and provided 

a framework for understanding what additional factors 

(for example, population vulnerability, existing quality of 

infrastructure, etc.) can exacerbate or reduce adverse health 

outcomes.

Due to limited space and the uncertainty around future 

projections of tornadoes, we do not include detailed 

discussion of this topic in this chapter. We recognize that 

tornadoes can cause significant infrastructure damage 

and significant health impacts, and understanding how 

climate change will impact tornado intensity, frequency, 

and geographic distribution is an area of active scientific 

investigation. 

KEY FINDING TRACEABLE ACCOUNTS

Increased Exposure to Extreme Events

Key Finding 1: Health impacts associated with climate-related 

changes in exposure to extreme events include death, injury, 

or illness; exacerbation of underlying medical conditions; and 

adverse effects on mental health [High Confidence]. Climate 

change will increase exposure risk in some regions of the 

United States due to projected increases in the frequency 

and/or intensity of drought, wildfires, and flooding related to 

extreme precipitation and hurricanes [Medium Confidence].

Description of evidence base

The Third National Climate Assessment (2014 NCA) provides 

the most recent, peer-reviewed assessment conclusions 

for projected increases in the frequency and/or intensity 

of extreme precipitation, hurricanes, coastal inundation, 

drought, and wildfires in the United States.2 To the extent 

that these extreme events are projected to increase in some 

regions of the United States, people are expected to be at 

greater risk of exposure to health hazards. 

Flooding associated with extreme precipitation, hurricanes, 

and coastal storms is expected to increase in some regions of 

the United States due to climate change, thereby increasing 

exposure to a variety of health hazards.9, 73, 74 The health 

impacts of floods and storms include death, injury, and illness; 

exacerbation of underlying medical conditions; and adverse 

effects on mental health.1, 29, 31, 46, 51, 52, 53, 58

Climate change is projected to lengthen or intensify droughts, 

especially in the Southwest,2, 144 which may increase exposure 

to a broad set of health hazards.9, 74 The potential health 

impacts of drought include: illness associated with reduced 

water quality and quantity 96, 100, 101, 102, 103 and reduced air 

quality,95, 118, 119 associations with increased rates of some 

infectious diseases,106, 107, 108, 109, 110 and adverse mental health 

impacts.94, 124, 125, 126, 127

Large, intense wildfires will occur more frequently in some 

regions of the United States, particularly in the western 

United States and Alaska,2 and this is expected to increase 

exposure to wildfire-related health risks.74, 144 The health 

impacts of wildfire include death, injury, and illness,134, 145, 146, 

151, 152, 153, 154, 155, 165, 166, 167, 168, 170, 172, 173 including exacerbation of 

underlying medical conditions.154, 156, 157

Major uncertainties 

The role of climate change in observed shifts in and 

future projections of the frequency, intensity, geographic 

distribution, and duration of certain extreme events is an 

ongoing, active area of research. For example, although the 

2014 NCA2 concluded that extreme events will increase in 

some regions of the United States, uncertainties remain 

with respect to projections of climate impacts at smaller, 

more local scales and the timing of such impacts (see Table 

1). Climate change related projections of winter storms and 

severe storms, including tornadoes, hail, and thunderstorms, 

are still uncertain. 
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U.S. Global Change Research Program 114 Impacts of Climate Change on Human Health in the United States

The human health implications of the changes in extreme 

events have not received as much research attention to 

date, and there are currently no published, national-scale, 

quantitative projections of changes in exposure risks for 

the four categories of extreme events addressed in this 

chapter. Relevant health surveillance and epidemiological 

data for extreme events are limited by underreporting, 

underestimation, and lack of a common definition of what 

constitutes an adverse health impact from an extreme 

event.30, 31 For drought in particular, there are few studies 

documenting specific health consequences in the United 

States.96 Challenges to quantitatively estimating future 

human health risks for the four types of extreme events 

addressed in this chapter include limited data availability 

and lack of comprehensive modeling methods. For winter 

storms and severe storms especially, scientists need a better 

understanding of how climate change will affect future storm 

trends before they can make projections of future health 

impacts.

Assessment of confidence and likelihood based on evidence

There is high confidence that the types of health impacts 

associated with climate-related changes in extremes include 

death, injury, or illness; exacerbation of underlying medical 

conditions; and adverse effects on mental health (see Table 

1). Based on the evidence presented in the peer-reviewed 

literature, there is medium confidence regarding increases 

in exposure to health hazards associated with projected 

increases in the frequency and/or intensity of extreme 

precipitation, hurricanes, coastal inundation, drought, 

and wildfires in some regions of the United States. Many 

qualitative studies have been published about the potential 

or expected health hazards from these events, but few draw 

strong or definitive conclusions that exposure to health 

hazards will increase due to climate change. Thus, the 

evidence is suggestive and supports a medium confidence

level that, to the extent that these extreme events are 

projected to increase in some regions of the United States, 

people are expected to be at greater risk of exposure to 

health hazards. There is no quantitative information on which 

to base probability estimates of the likelihood of increasing 

exposure to health hazards associated with extreme 

precipitation, hurricanes, coastal inundation, drought, and 

wildfires.

Disruption of Essential Infrastructure

Key Finding 2: Many types of extreme events related to 

climate change cause disruption of infrastructure, including 

power, water, transportation, and communication systems, 

that are essential to maintaining access to health care and 

emergency response services and safeguarding human health 

[High Confidence].

Description of evidence base

The frequency, intensity, and duration of extreme events 

determines their physical impacts and the extent to which 

essential infrastructure is disrupted. There is strong, 

consistent evidence from multiple studies that infrastructure 

can either exacerbate or moderate the physical impacts of 

extreme events, influencing the ultimate nature and severity 

of health impacts. Projections of increasing frequency and/

or intensity of some extreme events suggest that they 

pose threats to essential infrastructure, such as water, 

transportation, and power systems.4, 34, 36, 43 Disruption of 

essential infrastructure and services after extreme events 

can increase population exposure to health hazards and 

reduce their adaptive capacity.4 There is substantial, highquality literature supporting a finding that serious health risks 

can arise from utility outages; infrastructure and housing 

damage; and disruption or loss of access to sanitation, 

safe food and water supplies, health care, communication, 

and transportation.1, 29, 30, 31, 40, 43, 44, 45, 46, 47, 48, 49, 51, 52, 53, 54, 58, 87

Infrastructure disruptions can have more or less impact on 

human health depending on the underlying vulnerability of 

the affected people and communities.4 Urban populations 

face unique exposure risks due to their dependence on 

complex, often interdependent infrastructure systems that 

can be severely disrupted during extreme events.2, 65 Rural 

communities also have vulnerabilities that are different from 

those faced by urban communities. For example, power and 

communications can take longer to restore after an outage.50

Existing infrastructure is generally designed to perform at its 

engineered capacity assuming historical weather patterns, 

and these systems could be more vulnerable to failure in 

response to weather-related stressors under future climate 

scenarios.4, 34, 35 Shifts in the frequency or intensity of extreme 

events outside their historical range pose infrastructure 

risks that may be compounded by the fact that much of the 

existing critical infrastructure in the United States, including 

water and sewage systems, roads, bridges, and power plants, 

are aging and in need of repair or replacement.4, 36

Major uncertainties

Many of the uncertainties are similar to those of the 

previous key finding. There are few studies directly linking 

infrastructure impacts to health outcomes, and most are not 

longitudinal. Health impacts may occur after the event as 

a result of loss of infrastructure and public services. These 

impacts can be distributed over longer periods of time, 

making them harder to observe and quantify. Thus, the actual 

impact is likely underreported. 

Uncertainties remain with respect to projecting how climate 

change will affect the severity of the physical impacts, 

including on infrastructure, of extreme events at smaller, 

more local scales and the timing of such impacts. Therefore, 

the subsequent impact on infrastructure also has a great 
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U.S. Global Change Research Program 115 Impacts of Climate Change on Human Health in the United States

There is strong, consistent evidence in the literature that 

coastal flooding will increase exposure to a variety of health 

hazards—for example, direct physical impacts and impacts 

associated with disruption of essential infrastructure—

which can result in death, injury, or illness; exacerbation of 

underlying medical conditions; and adverse effects on mental 

health.1, 29, 31, 46, 51, 52, 53, 58 Multiple studies also consistently 

identify certain populations as especially vulnerable to 

the health impacts of coastal flooding. These populations 

include older adults (especially those who are frail, medically 

incapacitated, or residing in nursing or assisted living 

facilities), children, those reliant on electrically powered 

medical equipment like ventilators and oxygen supplies, 

those with preexisting health conditions, and people with 

disabilities.1, 8, 22, 43, 44, 53, 54, 195, 196, 197 In addition, differences 

in exposure, sensitivity, and adaptive capacity lead to a 

disproportionate number of flood-related fatalities among 

older adults, males, and some low-income communities of 

color.29 Floods and storms also create occupational health 

risks to first responders and other emergency workers and 

to people involved in cleanup activities, construction, public 

utilities restoration, and security/policing.1, 73, 87, 88,

Major uncertainties

It is nearly certain that coastal flooding will increase in the 

United States. There are varying estimates regarding the 

exact degree of flooding at any particular location along the 

coast. Modeling does provide estimated ranges with varying 

levels of confidence depending on the location. There is 

greater uncertainty about how coastal flooding will impact 

the health of specific populations. There are various ways 

in which these key risk factors interact with and contribute 

to the vulnerability (comprised of exposure, sensitivity, and 

adaptive capacity) of a population. Some uncertainties exist 

regarding the relative importance of each of these factors in 

determining a population’s vulnerability to health impacts 

from extreme events. In addition, there is some uncertainty 

regarding how future demographic and population changes 

may affect the relative importance of each of these factors.

Assessment of confidence based on evidence

Based on the evidence presented in the peer-reviewed 

literature, there is high confidence that coastal flooding will 

increase in the United States, and that age, health status, 

socioeconomic status, race/ethnicity, and occupation are 

key risk factors that individually and collectively affect a 

population’s vulnerability to health impacts from coastal 

flooding. Many qualitative studies have been published 

regarding how these key risk factors interact with and 

contribute to the exposure, sensitivity, and adaptive capacity 

of a population, and this evidence is of good quality and 

consistent.

deal of uncertainty. Thus, the key finding does not make 

any statements about future impacts. Instead the focus 

is on impacts that have occurred to date because there is 

supporting peer-reviewed literature. The extent to which 

infrastructure is exposed to extreme events, and the adaptive 

capacity of a community to repair infrastructure in a timely 

manner both influence the extent of the health outcomes. 

Thus, while the chapter makes general statements about 

trends in impacts due to extremes, there are uncertainties 

in the extent to which any specific location or infrastructure 

system could be impacted and the resulting health outcomes. 

Assessment of confidence and likelihood based on evidence

There is high confidence that many types of extreme events 

can cause disruption of essential infrastructure (such as water, 

transportation, and power systems), and that such disruption 

can adversely affect human health. Many qualitative studies 

have been published about the effects of these factors on 

health impacts from an extreme event (noted above), and the 

evidence is of good quality and consistent.

Vulnerability to Coastal Flooding

Key Finding 3: Coastal populations with greater vulnerability 

to health impacts from coastal flooding include persons with 

disabilities or other access and functional needs, certain 

populations of color, older adults, pregnant women and 

children, low-income populations, and some occupational 

groups [High Confidence]. Climate change will increase 

exposure risk to coastal flooding due to increases in extreme 

precipitation and in hurricane intensity and rainfall rates, 

as well as sea level rise and the resulting increases in storm 

surge [High Confidence].

Description of evidence base

The evidence in the peer-reviewed literature that climate 

change will increase coastal flooding in the future is very 

robust.2, 4 Global sea level has risen by about 8 inches since 

reliable record keeping began in 1880 and it is projected to 

rise another 1 to 4 feet by 2100.2 Rates of sea level rise are 

not uniform along U.S. coasts and can be exacerbated locally 

by land subsidence or reduced by uplift. In the next several 

decades, storm surges and high tides could combine with 

sea level rise and land subsidence to further increase coastal 

flooding in many regions. The U.S. East and Gulf coasts, Hawaii, 

and the U.S.-affiliated Pacific Islands are particularly at risk. 

In addition, recurrent weather-related stressors, such as 

“nuisance flooding” (frequent coastal flooding causing 

public inconveniences), contribute to overall deterioration 

of infrastructure like stormwater drainage systems and 

roads (see Ch. 6: Water-Related Illness).38 These systems are 

important in the context of health because drainage helps 

to avoid sewage overflows and maintain water quality,39

and roads are vital for evacuations and emergency response 

during and after extreme events.40
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U.S. Global Change Research Program 116 Impacts of Climate Change on Human Health in the United States

DOCUMENTING UNCERTAINTY

This assessment relies on two metrics to communicate 

the degree of certainty in Key Findings. See Appendix 4: 

Documenting Uncertainty for more on assessments of 

likelihood and confidence.

PHOTO CREDITS

Pg. 99–Firefighters battling fire: © Erich Schlegel/Corbis

Pg. 100–Family escaping flood waters: © Greg Vote/Corbis

Pg. 104–Young family in shelter: © © Robert Sciarrino/The 

Star-Ledger/Corbis

Pg. 105–Damaged power lines: © Julie Dermansky/Corbis

Pg. 107–Truck stuck in flood waters: © Mike Theiss/National

Geographic Creative/Corbis 

Pg. 108–Farmer in drought-stressed peanut field: © ERIK S.

LESSER/epa/Corbis

Pg. 109–Dust storm: © Martyn Goddard/Corbis

Pg. 110–Firefighters battling fire: © Erich Schlegel/Corbis

Pg. 111–Gridlocked traffic: © Robin Nelson/ZUMA Press/

Corbis

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few sources, limited consistency, models 

incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts

Likelihood

Very Likely

≥ 9 in 10

Likely

≥ 2 in 3

As Likely As Not

≈ 1 in 2

Unlikely

≤ 1 in 3

Very Unlikely

≤ 1 in 10
U.S. Global Change Research Program 117 Impacts of Climate Change on Human Health in the United States

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181. Emelko, M.B., U. Silins, K.D. Bladon, and M. Stone, 

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188. Rhoades, C.C., D. Entwistle, and D. Butler, 2012: Water 

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190. Black, A.W. and W.S. Ashley, 2010: Nontornadic convective wind fatalities in the United States. Natural Hazards, 54,

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192. Ashley, W.S. and T.L. Mote, 2005: Derecho hazards in the 

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193. Jahromi, A.H., R. Wigle, and A.M. Youssef, 2011: Are we 

prepared yet for the extremes of weather changes? Emergence 

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194. Lim, C. and J. Duflou, 2008: Hypothermia fatalities in a 

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195. Ramin, B. and T. Svoboda, 2009: Health of the homeless 

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198. Dey, A.N., P. Hicks, S. Benoit, and J.I. Tokars, 2010: Automated monitoring of clusters of falls associated with severe 

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206. Barbero, R., J.T. Abatzoglou, N.K. Larkin, C.A. Kolden, and 

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WF15083

End
5 VECTOR-BORNE DISEASES

On the web: health2016.globalchange.gov

U.S. Global Change Research Program

*Chapter Coordinator

Recommended Citation: Beard, C.B., R.J. Eisen, C.M. Barker, J.F. Garofalo, M. Hahn, M. Hayden, A.J. Monaghan, N.H. Ogden, and P.J. 

Schramm, 2016: Ch. 5: Vectorborne Diseases. The Impacts of Climate Change on Human Health in the United States: A Scientific 

Assessment. U.S. Global Change Research Program, Washington, DC, 129–156. http://dx.doi.org/10.7930/J0765C7V 

THE IMPACTS OF CLIMATE CHANGE ON HUMAN HEALTH IN THE UNITED STATES

A Scientific Assessment

Lead Authors

Charles B. Beard

Centers for Disease Control and Prevention

Rebecca J. Eisen

Centers for Disease Control and Prevention

Contributing Authors

Christopher M. Barker

University of California, Davis 

Jada F. Garofalo*

Centers for Disease Control and Prevention

Micah Hahn

Centers for Disease Control and Prevention

Mary Hayden

National Center for Atmospheric Research 

Andrew J. Monaghan

National Center for Atmospheric Research

Nicholas H. Ogden

Public Health Agency of Canada

Paul J. Schramm

Centers for Disease Control and Prevention
130

5 VECTOR-BORNE DISEASES

Key Findings 

Changing Distributions of Vectors and Vector-Borne Diseases

Key Finding 1: Climate change is expected to alter the geographic and seasonal distributions of existing 

vectors and vector-borne diseases [Likely, High Confidence].

Earlier Tick Activity and Northward Range Expansion

Key Finding 2: Ticks capable of carrying the bacteria that cause Lyme disease and other pathogens will 

show earlier seasonal activity and a generally northward expansion in response to increasing temperatures 

associated with climate change [Likely, High Confidence]. Longer seasonal activity and expanding 

geographic range of these ticks will increase the risk of human exposure to ticks [Likely, Medium 

Confidence].

Changing Mosquito-Borne Disease Dynamics

Key Finding 3: Rising temperatures, changing precipitation patterns, and a higher frequency of some extreme 

weather events associated with climate change will influence the distribution, abundance, and prevalence of 

infection in the mosquitoes that transmit West Nile virus and other pathogens by altering habitat availability 

and mosquito and viral reproduction rates [Very Likely, High Confidence]. Alterations in the distribution, 

abundance, and infection rate of mosquitoes will influence human exposure to bites from infected 

mosquitoes, which is expected to alter risk for human disease [Very Likely, Medium Confidence].

Emergence of New Vector-Borne Pathogens 

Key Finding 4: Vector-borne pathogens are expected to emerge or reemerge due to the interactions of climate 

factors with many other drivers, such as changing land-use patterns [Likely, High Confidence]. The impacts 

to human disease, however, will be limited by the adaptive capacity of human populations, such as vector 

control practices or personal protective measures [Likely, High Confidence].
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U.S. Global Change Research Program 131 Impacts of Climate Change on Human Health in the United States

5.1 Introduction 

Vector-borne diseases are illnesses that are transmitted by vectors, which include mosquitoes, ticks, and fleas. These vectors 

can carry infective pathogens such as viruses, bacteria, and 

protozoa, which can be transferred from one host (carrier) to another. In the United States, there are currently 14 vector-borne 

diseases that are of national public health concern. These 

diseases account for a significant number of human illnesses 

and deaths each year and are required to be reported to the National Notifiable Diseases Surveillance System at the Centers for 

Disease Control and Prevention (CDC). In 2013, state and local 

health departments reported 51,258 vector-borne disease cases 

to the CDC (Table 1).

The seasonality, distribution, and prevalence of vector-borne 

diseases are influenced significantly by climate factors, primarily high and low temperature extremes and precipitation 

patterns.11 Climate change can result in modified weather patterns and an increase in extreme events (see Ch. 1: Introduction) that can affect disease outbreaks by altering biological 

variables such as vector population size and density, vector 

survival rates, the relative abundance of disease-carrying 

animal (zoonotic) reservoir hosts, and pathogen reproduction rates. Collectively, these changes may contribute to an 

increase in the risk of the pathogen being carried to humans.

Climate change is likely to have both short- and long-term 

effects on vector-borne disease transmission and infection 

patterns, affecting both seasonal risk and broad geographic 

changes in disease occurrence over decades. However, models 

for predicting the effects of climate change on vector-borne 

diseases are subject to a high degree of uncertainty, largely 

due to two factors: 1) vector-borne diseases are maintained 

in nature in complex transmission cycles that involve vectors, 

other intermediate zoonotic hosts, and humans; and 2) there 

are a number of other significant social and environmental 

drivers of vector-borne disease transmission in addition to cliSummary of Reported Case Counts of Notifiablea

 Vector-Borne Diseases in the 

United States. 

Diseases 2013 Reported Cases Median (range) 2004–2013b

Tick-Borne 

 Lyme disease 36,307 30,495 (19,804–38,468)

 Spotted Fever Rickettsia 3,359 2,255 (1,713–4,470)

 Anaplasmosis/Ehrlichiosis 4,551 2,187 (875–4,551)

 Babesiosisb 1,792 1,128 (940–1,792)

 Tularemia 203 136 (93–203)

 Powassan 15 7 (1–16)

Mosquito-Borne

 West Nile virus 2,469 1,913 (712–5,673)

 Malariac 1,594 1,484 (1,255–1,773)

 Dengueb,c 843 624 (254–843)

 California serogroup viruses 112 78 (55–137)

 Eastern equine encephalitis 8 7 (4–21)

 St. Louis encephalitis 1 10 (1–13)

Flea-Borne 

 Plague 4 4 (2–17)

a State Health Departments are required by law to report regular, frequent, and timely information about individual 

cases to the CDC in order to assist in the prevention and control of diseases. Case counts are summarized based 

on annual reports of nationally notifiable infectious diseases.1, 2, 3, 4, 5, 6, 7, 8, 9, 10

b

 Babesiosis and dengue were added to the list of nationally notifiable diseases in 2011 and 2009, respectively. 

Median and range values encompass cases reported from 2011 to 2013 for babesiosis and from 2010 to 2013 

for dengue.

c

 Primarily acquired outside of the United States and based on travel-related exposures.

Table 1: Vectors and hosts involved in the transmission of these infective pathogens are sensitive to climate change and other 

environmental factors which, together, affect vector-borne diseases by influencing one or more of the following: vector and host 

survival, reproduction, development, activity, distribution, and abundance; pathogen development, replication, maintenance, and 

transmission; geographic range of pathogens, vectors, and hosts; human behavior; and disease outbreak frequency, onset, and 

distribution.11
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U.S. Global Change Research Program 132 Impacts of Climate Change on Human Health in the United States

mate change. For example, while climate variability and climate 

change both alter the transmission of vector-borne diseases, 

they will likely interact with many other factors, including how 

pathogens adapt and change, the availability of hosts, changing ecosystems and land use, demographics, human behavior, 

and adaptive capacity.12, 13 These complex interactions make it 

difficult to predict the effects of climate change on vector-borne 

diseases.

The risk of introducing exotic pathogens and vectors not currently present in the United States, while likely to occur, is similarly difficult to project quantitatively.14, 15, 16 In recent years, several important vector-borne pathogens have been introduced 

or reintroduced into the United States. These include West 

Nile virus, dengue virus, and chikungunya virus. In the case of 

the 2009 dengue outbreak in southern Florida, climate change 

was not responsible for the reintroduction of the virus in this 

area, which arrived via infected travelers from disease-endemic 

regions of the Caribbean.17 In fact, vector populations capable of 

transmitting dengue have been present for many years throughout much of the southern United States, including Florida.18 

Climate change has the potential to increase human exposure 

risk or disease transmission following shifts in extended spring 

and summer seasons as dengue becomes more established in 

the United States. Climate change effects, however, are difficult 

to quantify due to the adaptive capacity of a population that 

may reduce exposure to vector-borne pathogens through such 

means as air conditioning, screens on windows, vector control 

and public health practices. 

This chapter presents case studies of Lyme disease and West 

Nile virus infection in relation to weather and climate. Although 

ticks and mosquitoes transmit multiple infectious pathogens 

to humans in the United States, Lyme disease and West Nile 

virus infection are the most commonly reported tick-borne and 

mosquito-borne diseases in this country (Table 1). In addition, a 

substantial number of studies have been conducted to elucidate the role of climate in the transmission of these infectious 

pathogens. These broad findings, together with the areas of 

uncertainty from these case studies, are generalizable to other 

vector-borne diseases.11 

5.2 Lyme Disease

State of the Science

Lyme disease is a tick-borne bacterial disease that is endemic 

(commonly found) in parts of North America, Europe, and Asia. 

In the United States, Lyme disease is caused by the bacterium 

Borrelia burgdorferi sensu stricto (B. burgdorferi; one of the 

spiral-shaped bacteria known as spirochetes) and is the most 

commonly reported vector-borne illness. It is primarily transmitted to humans in the eastern United States by the tick species 

Ixodes scapularis (formerly I. dammini), known as blacklegged 

ticks or deer ticks, and in the far western United States by I. 

pacificus, commonly known as western blacklegged ticks.19 Illness in humans typically presents with fever, headache, fatigue, 

and a characteristic skin rash called erythema migrans. If left 

untreated, infection can spread to joints, the heart, and the 

nervous system.20 Since 1991, when standardized surveillance 

and reporting of Lyme disease began in the United States, case 

counts have increased steadily.21 Since 2007, more than 25,000 

Lyme disease cases have been reported annually.22 The geographic distribution of the disease is limited to specific regions 

in the United States (Figure 2), transmission occurs seasonally, 

and year-to-year variation in case counts and in seasonal onset 

is considerable.20, 21, 23 Each of these observations suggest that 

geographic location and seasonal climate variability may play a 

significant role in determining when and where Lyme disease 

cases are most likely to occur.

Although the reported incidence of Lyme disease is greater 

in the eastern United States compared with the westernmost 

United States,20, 21 in both geographical regions, nymphs (small 

immature ticks) are believed to be the life stage that is most 

significant in pathogen transmission from infected hosts (primarily rodents) to humans (Figure 2, Figure 3).24, 25 Throughout 

the United States, the majority of human cases report onset of 

clinical signs of infection during the months of June, July, and 

August. The summer is a period of parallel increased activity for 

both blacklegged and western blacklegged ticks in the nymphal 

life stage (the more infectious stage) and for human recreational 

activity outdoors.21, 25 

Infection rates in humans vary significantly from year to year. 

From 1992 to 2006, variation in case counts of Lyme disease 

was as high as 57% from one year to the next.21 Likewise, the 

precise week of onset of Lyme disease cases across states in the 

eastern United States, where Lyme disease is endemic, differed 

by as much as 10 weeks from 1992 to 2007. Much of this variation in timing of disease onset can be explained by geographic 

region (cases occurred earlier in warmer states in the mid-Atlantic region compared with cooler states in the North); however, 

the annual variation of disease onset within regions was notable 

and linked to winter and spring climate variability (see "Annual 

and Seasonal Variation in Lyme Disease" on page 136).23 

The geographic and seasonal distributions of Lyme disease case 

occurrence are driven, in part, by the life cycle of vector ticks 

(Figure 3). Humans are only exposed to Lyme disease spirochetes (B. burgdorferi) in locations where both the vector tick 

populations and the infection-causing spirochetes are present.27 

Within these locations, the potential for contracting Lyme 

disease depends on three key factors: 1) tick vector abundance 

(the density of host-seeking nymphs being particularly important), 2) prevalence of B. burgdorferi infection in ticks (the prevalence in nymphs being particularly important), and 3) contact 

frequency between infected ticks and humans.28 To varying 

degrees, climate change can affect all three of these factors.
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U.S. Global Change Research Program 133 Impacts of Climate Change on Human Health in the United States

Figure 1: This conceptual diagram illustrates the key pathways by which climate change influences human exposure to Lyme 

disease and the potential resulting health outcomes (center boxes). These exposure pathways exist within the context of other factors 

that positively or negatively influence health outcomes (gray side boxes). Key factors that influence vulnerability for individuals are 

shown in the right box, and include social determinants of health and behavioral choices. Key factors that influence vulnerability at 

larger scales, such as natural and built environments, governance and management, and institutions, are shown in the left box. All 

of these influencing factors can affect an individual’s or a community’s vulnerability through changes in exposure, sensitivity, and 

adaptive capacity and may also be affected by climate change. See Ch. 1: Introduction for more information.

Climate Change and Health—Lyme Disease

Figure 2: Maps show the reported cases of Lyme disease in 2001 and 2014 for the areas of the country where Lyme disease is 

most common (the Northeast and Upper Midwest). Both the distribution and the numbers of cases have increased. (Figure source: 

adapted from CDC 2015)26

Changes in Lyme Disease Case Report Distribution

2001 2014
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U.S. Global Change Research Program 134 Impacts of Climate Change on Human Health in the United States

The seasonal occurrence of Lyme disease cases is related, partially, to the timing of a blood meal (host-seeking 

activity) of ticks and the three-stage life cycle (larvae, nymph, and adult) of ticks.48 Increasing temperatures and the 

accompanying changes in seasonal patterns are expected to result in earlier seasonal tick activity and an expansion 

in tick habitat range, increasing the risk of human exposure to ticks.

For blacklegged ticks and western blacklegged ticks, spirochete transmission from adult ticks to eggs is rare or 

does not occur.49 Instead, immature ticks (larvae and nymphs) acquire infection-causing B. burgdorferi spirochetes 

by feeding on rodents, other small mammals, and birds during the spring and summer months. The spirochetes 

are maintained throughout the tick life cycle from larva to nymph and from nymph to adult. The spirochetes are 

primarily passed to humans from nymphs and less frequently by adults. 

Prevalence of B. burgdorferi infection in nymphal ticks depends in part on the structure of the host community.50, 

51 Larval ticks are more likely to be infected in areas where they feed mostly on animals that can carry and transmit 

the disease-causing bacteria (such as white-footed mice), compared with areas where they feed mostly on hosts 

that cannot become infected and thus do not pass on the bacteria (such as certain lizards). 

Natural variation in potential for rodents, birds, and reptiles to carry B. burgdorferi in the wild leads to large 

differences in infection rates in nymphal ticks, resulting in considerable geographic variation in the transmission 

cycles and in the opportunity for humans to contract Lyme disease.52 Unlike nymphal or larval ticks, adult ticks 

feed mainly during the cooler months of the year, and primarily on deer, which are resistant to B. burgdorferi

infection and thus play little role in increasing the abundance of infected ticks in the population. However, deer are 

important for tick reproduction and therefore influence the abundance of nymphs in subsequent generations.19

Life Cycle of Blacklegged Ticks, Ixodes scapularis

Life Cycle of Blacklegged Ticks, Ixodes scapularis

Figure 3: Figure depicts the life cycle of blacklegged ticks, including the phases in which humans can be exposed to Lyme 

disease, and some of the changes in seasonality expected with climate change. (Figure source: adapted from CDC 2015)47
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U.S. Global Change Research Program 135 Impacts of Climate Change on Human Health in the United States

Aside from short periods of time when they are feeding on 

hosts (less than three weeks of their two- to three-year life 

cycle), ticks spend most of their lives off of hosts in various 

natural landscapes (such as woodlands or grasslands) where 

weather factors including temperature, precipitation, and 

humidity affect their survival and host-seeking behavior. In 

general, both low and high temperatures increase tick mortality rates, although increasing humidity can increase their 

ability to tolerate higher temperatures.29, 30, 31, 32, 33, 34, 35, 36, 37, 38 

Within areas where tick vector populations are present, some 

studies have demonstrated an association among temperature, humidity, and tick abundance.39, 40, 41 Factors that are less 

immediately dependent on climate (for example, landscape 

and the relative proportions within a community of zoonotic 

hosts that carry or do not carry Lyme disease-causing bacteria) may be more important in smaller geographic areas.42, 

43 Temperature and humidity also influence the timing of 

host-seeking activity,32, 35, 36, 44 and can influence which seasons 

are of highest risk to the public. 

In summary, weather-related variables can determine geographic distributions of ticks and seasonal activity patterns. 

However, the importance of these weather variables in 

Lyme disease transmission to humans compared with other 

important predictors is likely scale-dependent. In general, 

across the entire country, climate-related variables often play 

a significant role in determining the occurrence of tick vectors 

and Lyme disease incidence in the United States (for example, Lyme disease vectors are absent in the arid Intermountain West where climate conditions are not suitable for tick 

survival). However, within areas where conditions are suitable 

for tick survival, other variables (for example, landscape and 

the relative proportions within a community of zoonotic hosts 

that carry or do not carry Lyme disease-causing bacteria) are 

more important for determining tick abundance, infection 

rates in ticks, and ultimately human infection rates.39, 45, 46 

Observed Trends and Measures of Human Risk

Geographic Distribution of Ticks

Because the presence of tick vectors is required for B. burgdorferi transmission to humans, information on where vector 

tick species live provides basic information on where Lyme 

disease risk occurs. Minimum temperature appears to be a 

key variable in defining the geographic distribution of blacklegged ticks.39, 45, 53 Low minimum temperatures in winter may 

lead to environmental conditions that are unsuitable for tick 

population survival. The probability of a given geographic area 

being suitable for tick populations increases as minimum temperature rises.45 In the case of the observed northward range 

expansion of blacklegged ticks into Canada, higher temperatures appear to be a key factor affecting where, and how fast, 

ticks are colonizing new localities.54, 55, 56, 57, 58 

Maximum temperatures also significantly affect where 

blacklegged ticks live.39, 45 Higher temperatures increase tick 

development and hatching rates, but reduce tick survival and 

egg-laying (reproduction) success.30 

Declines in rainfall amount and humidity are also important in 

limiting the geographic distribution of blacklegged ticks. Ticks 

are more likely to reside in moister areas because increased 

humidity can increase tick survival.35, 38, 39, 45, 53, 55 

Geographic Distribution of Infected Ticks

Climate variables have been shown to be strong predictors 

of geographic locations in which blacklegged ticks reside, but 

less important for determining how many nymphs live in a 

given area or what proportion of those ticks is infected.39, 40 

The presence of uninfected nymphs and infected nymphs can 

vary widely over small geographic areas experiencing similar 

temperature and humidity conditions, which supports the hypothesis that factors other than weather play a significant role 

in determining nymph survival and infection rates.37, 39, 40, 41, 44 

Additional studies that modeled nymphal density within small 

portions of the blacklegged tick range (north-central states 

and Hudson River Valley, NY), and modeling studies that include climate and other non-biological variables indicate only 

a weak relationship to nymphal density.59, 60 Nonetheless, climate variables can be used to model nymphal density in some 

instances. For example, in a single county in northern coastal 

California with strong climate gradients, warmer areas with 

less variation between maximum and minimum monthly water vapor in the air were characteristic of areas with elevated 

concentrations of infected nymphs.41 However, it is likely that 

differences in animal host community structure, which vary 

with climatic conditions (for example, relative abundances of 

hosts that carry or do not carry Lyme disease-causing bacteria), influenced the concentration of infected nymphs.37, 61 

In the eastern United States, Lyme disease is transmitted to

humans primarily by blacklegged (deer) ticks.
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U.S. Global Change Research Program 136 Impacts of Climate Change on Human Health in the United States

Geographic Distribution of Lyme Disease 

Though there are links between climate and tick distribution, 

studies that look for links between weather and geographical 

differences in human infection rates do not show a clear or consistent link between temperature and Lyme disease incidence.46, 

62, 63 

Annual and Seasonal Variation in Lyme Disease 

Temperature and precipitation both influence the host-seeking 

activity of ticks, which may result in year-to-year variation in the 

number of new Lyme disease cases and the timing of the season in which Lyme disease infections occur. However, identified 

associations between precipitation and Lyme disease incidence, 

or temperature and Lyme disease incidence, are limited or 

weak.64, 65 Overall, the association between summer moisture 

and Lyme disease infection rates in humans remains inconsistent across studies. 

The peak period when ticks are seeking hosts starts earlier in 

the warmer, more southern, states than in northern states.44 

Correspondingly, the onset of human Lyme disease cases occurs 

earlier as the growing degree days (a measurement of temperature thresholds that must be met for biological processes to 

occur) increases, yet, the timing of the end of the Lyme disease 

season does not appear to be determined by weather-related 

variables.23 Rather, the number of potential carriers (for example, deer, birds, and humans) likely influences the timing of the 

end of the Lyme disease season.

The effects of temperature and humidity or precipitation on the 

seasonal activity patterns of nymphal western blacklegged ticks 

is more certain than the impacts of these factors on the timing 

of Lyme disease case occurrence.36, 37 Peak nymphal activity is 

generally reached earlier in hotter and drier areas, but lasts for 

shorter durations. Host-seeking activity ceases earlier in the 

season in cooler and more humid conditions. The density of 

nymphal western blacklegged ticks in north-coastal California 

consistently begins to decline when average daily maximum 

temperatures are between 70°F (21°C) and 73.5°F (23°C), and 

when average maximum daily relative humidity decreases 

below 83%–85%.36, 37

Projected Impacts

Warmer winter and spring temperatures are projected to lead 

to earlier annual onset of Lyme disease cases in the eastern 

United States (see “Research Highlight” below) and in an earlier 

onset of nymphal host-seeking behavior.66 Limited research 

shows that the geographic distribution of blacklegged ticks is 

expected to expand to higher latitudes and elevations in the 

future and retract in the southern United States.67 Declines in 

subfreezing temperatures at higher latitudes may be responsible for improved survival of ticks. In many woodlands, ticks can 

find refuge from far-subzero winter air temperatures in the surface layers of the soil.68, 69 However, a possibly important impact 

of climate change will be acceleration of the tick life cycles due 

to higher temperatures during the spring, summer, and autumn, 

which would increase the likelihood that ticks survive to reproduce.58, 70 This prediction is consistent with recent observations 

of the spread of I. scapularisin Canada.55, 71 

Importance: Lyme disease occurrence is highly seasonal. The annual springtime onset of Lyme disease cases is 

regulated by climate variability in preceding months. Until now, the possible effects of climate change on the 

timing of Lyme disease infection in humans early and late in the 21st century have not been addressed for the 

United States, where Lyme disease is the most commonly reported vector-borne disease. 

Objectives: Examine the potential impacts of 21st century climate change on the timing of the beginning of the 

annual Lyme disease season (annual onset week) in the eastern United States.

Methods: Downscaled future climate projections for four greenhouse gas (GHG) concentration trajectories from five 

atmosphere–ocean global climate models (AOGCMs) are input to the national-level empirical model of Moore et al. 

(2014)23 to simulate the potential impact of 21st century climate change on the annual onset week of Lyme disease 

in the United States.23 The four GHG trajectories in order of lowest to highest concentrations are RCP2.6, RCP4.5, 

RCP6.0, and RCP8.5 (see Appendix 1: Technical Support Document).

Results: Historical and future projections for the beginning of the Lyme disease season are shown in Figure 4. 

Historical results are for the period 1992–2007, where the national-average peak onset date for Lyme disease 

occurs on week 21.2 of the calendar year (mid-May). Future projections are for two time periods: 1) 2025–2040 

and 2) 2065–2080. On average, the start of the Lyme disease season is projected to arrive a few days earlier 

for 2025–2040 (0.4–0.5 weeks), and approximately one to two weeks earlier for 2065–2080 (0.7–1.9 weeks) 

depending on the GHG trajectory. Winter and spring temperature increases are primarily responsible for the earlier 

peak onset of Lyme disease infections. 

Research Highlight: Lyme Disease
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U.S. Global Change Research Program 137 Impacts of Climate Change on Human Health in the United States

To project accurately the changes in Lyme disease risk in 

humans based on climate variability, long-term data collection 

on tick vector abundance and human infection case counts 

are needed to better understand the relationships between 

changing climate conditions, tick vector abundance, and Lyme 

disease case occurrence.

5.3 West Nile Virus

State of the Science 

West Nile virus (WNV) is the leading cause of mosquito-borne 

disease in the United States. From 1999 to 2013, a total of 

39,557 cases of WNV disease were reported in the United 

States.73 Annual variation is substantial, both in terms of case 

counts and the geographic distribution of cases of human 

infection (Figure 5).73 Since the late summer of 1999, when 

an outbreak of WNV first occurred in New York City,74 human 

WNV cases have occurred in the United States every year. 

After the introduction of the virus to the United States, WNV 

spread westward, and by 2004 WNV activity was reported 

throughout the contiguous United States.75, 76 Annual human 

WNV incidence remained stable through 2007, decreased 

substantially through 2011, and increased again in 2012, raising questions about the factors driving year-to-year variation 

in disease transmission.75 The locations of annual WNV outbreaks vary, but several states have reported consistently high 

rates of disease over the years, including Arizona, California, 

Colorado, Idaho, Illinois, Louisiana, New York, North Dakota, 

South Dakota, and Texas.73, 75

Research Highlight: Lyme Disease, continued

Projected Change in Lyme Disease Onset Week

Figure 4: Box plots comparing the distributions of the national-level historical observed data for annual Lyme disease onset 

week (1992–2007 in green) with the distributions of AOGCM multi-model mean projections of Lyme onset week for each of 

four Representative Concentration Pathways (RCP2.6, 4.5, 6.0, and 8.5) and two future time periods (2025–2040 in blue, 

2065–2080 in red). Each box plot shows the values of Lyme disease onset week for the maximum (top of dashed line), 75th 

percentile (top of box), average (line through box), 25th percentile (bottom of box), and minimum (bottom of dashed line) of the 

distribution. All distributions are comprised of values for 12 eastern states and 16 years (N = 192). Additional details can be 

found in Monaghan et al. (2015). (Figure source: adapted from Monaghan et al. 2015).72

Conclusions: Results demonstrate that 21st century climate change will lead to environmental conditions suitable 

for earlier annual onset of Lyme disease cases in the United States, with possible implications for the timing of 

public health interventions. The end of the Lyme disease season is not strongly affected by climate variables; 

therefore, conclusions about the duration of the transmission season or changes in the annual number of new 

Lyme disease cases cannot be drawn from this study.
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U.S. Global Change Research Program 138 Impacts of Climate Change on Human Health in the United States

The majority (70% to 80%) of people infected with WNV do 

not show symptoms of the disease. Of those infected, 20% to 

30% develop acute systemic febrile illness, which may include 

headache, myalgias (muscle pains), rash, or gastrointestinal 

symptoms; fewer than 1% experience neuroinvasive disease, 

which may include meningitis (inflammation around the brain 

and spinal cord), encephalitis (inflammation of the brain), 

or myelitis (inflammation of the spinal cord) (see "5.4 Populations of Concern" on page 142).77 Because most infected 

persons are asymptomatic (showing no symptoms), there is 

significant under-reporting of cases.78, 79, 80 More than three 

million people were estimated to be infected with WNV in the 

United States from 1999 to 2010, resulting in about 780,000 

illnesses.77 However, only about 30,700 cases were reported 

during the same time span.73

West Nile virus is maintained in transmission cycles between 

birds (the natural hosts of the virus) and mosquitoes (Figure 

6). The number of birds and mosquitoes infected with WNV increases as mosquitoes pass the virus from bird to bird starting 

in late winter or spring. Human infections can occur from a 

bite of a mosquito that has previously bitten an infected bird.81 

Humans do not pass on the virus to biting mosquitoes because 

they do not have sufficient concentrations of the virus in their 

bloodstreams.82, 83 In rare instances, WNV can be transmitted 

through blood transfusions or organ transplants.82, 84 Peak 

transmission of WNV to humans in the United States typically 

occurs between June and September, coinciding with the summer season when mosquitoes are most active and temperatures are highest.85 

Incidence of West Nile Neuroinvasive Disease by County in the United States 

Figure 5: Maps show the incidence of West Nile neuroinvasive disease in the United States for 2010 through 2013. Shown as 

cases per 100,000 people. (Data source: CDC 2014)73
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U.S. Global Change Research Program 139 Impacts of Climate Change on Human Health in the United States

Observed Impacts and Indicators

Mosquito vectors and bird hosts are required for WNV to 

persist, and the dynamics of both are strongly affected by climate in a number of ways. Geographical variation in average 

climate constrains the ranges of both vectors and hosts, while 

shorter-term climate variability affects many aspects of vector 

and host population dynamics. Unlike ticks, mosquitoes have 

short life cycles and respond more quickly to climate drivers 

over relatively short timescales of days to weeks. Impacts on 

bird abundance are often realized over longer timescales of 

months to years due to impacts on annual reproduction and 

migration cycles. 

Climate Impacts on West Nile Virus Transmission

Figure 6: Climate Impacts on West Nile Virus Transmission
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U.S. Global Change Research Program 140 Impacts of Climate Change on Human Health in the United States

WNV has been detected in 65 mosquito species and more 

than 300 bird species in the United States,85 although only a 

relatively small number of these species contribute substantively to human infections. Three Culex (Cx.) mosquito species

are the primary vectors of the virus in different regions of the 

continental United States, and differences in their preferred 

breeding habitats mean that climate change will likely impact 

human WNV disease risk differently across these regions (Figure 5). Bird species that contribute to WNV transmission include those that develop sufficient viral concentrations in their 

blood to transmit the virus to feeding mosquitoes.86, 87 As with 

mosquitoes, the bird species involved in the transmission cycle 

are likely to respond differently to climate change, increasing 

the complexity of projecting future WNV risk. 

Impacts of Climate and Weather

Climate, or the long-term average weather, is important for 

defining WNV’s transmission range limits because extreme 

conditions—too cold, hot, wet, 

or dry—can alter mosquito and 

bird habitat availability, increase 

mortality in mosquitoes or birds, 

and/or disrupt viral transmission. 

WNV is an invasive pathogen that 

was first detected in the United 

States just over 15 years ago, 

which is long enough to observe 

responses of WNV to key weather variables, but not long 

enough to observe responses to climate change trends.

Climate change may influence mosquito survival rates through 

changes in season length, although mosquitoes are also able 

to adapt to changing conditions. For example, mosquitoes that 

transmit WNV are limited to latitudes and altitudes where 

winters are short enough for them to survive.88 However, 

newly emerged adult female mosquitoes have some ability to 

survive cold temperatures by entering a reproductive arrest 

called diapause as temperatures begin to cool and days grow 

shorter in late summer.89, 90 These females will not seek a 

blood meal until temperatures begin to warm the following 

year. Even during diapause, very harsh winters may reduce 

mosquito populations, as temperatures near freezing have 

been shown to kill diapausing Cx. tarsalis.91 

During the warmer parts of the year, Culex mosquitoes must 

have aquatic habitat available on a nearly continuous basis 

because their eggs hatch within a few days after they are laid 

and need moisture to remain viable. The breeding habitats of 

WNV vectors vary by species, ranging from fresh, sunlit water 

found in irrigated crops and wetlands preferred by Cx. tarsalis 

to stagnant, organically enriched water sources, such as urban 

storm drains, unmaintained swimming pools, or backyard containers, used by Cx. pipiens and Cx. quinquefasciatus.

92, 93, 94

WNV has become endemic within a wide range of climates in 

the United States, but there is substantial geographic variation 

in the intensity of virus transmission. Part of this geographic 

variation can be attributed to the 

abundance and distributions of 

suitable bird hosts.95 Important 

hosts, such as robins, migrate 

annually between summer breeding grounds and winter foraging 

areas.86, 96 Migrating birds have 

shown potential as a vehicle for 

long-range virus movement.97, 

98 Although the timing of migration is driven by climate, the 

impact of climate change-driven migration changes on WNV 

transmission have not yet been documented by scientists. Climate change has already begun to cause shifts in bird breeding and migration patterns,99 but it is unknown how these 

changes may affect WNV transmission.

Temperature is the most studied climate driver of the dynamics of WNV transmission. It is clear that warm temperatures 

accelerate virtually all of the biological processes that affect 

transmission: accelerating the mosquito life cycle,100, 101, 102, 

103, 104 increasing the mosquito biting rates that determine the 

frequency of contact between mosquitoes and hosts,105, 106

and increasing viral replication rates inside the mosquito that 

decrease the time needed for a blood-fed mosquito to be able 

to pass on the virus.107, 108, 109 These relationships between increasing temperatures and the biological processes that affect 

WNV transmission suggest a subsequent increase in risk of 

human disease.110, 111, 112, 113 However, results from models have 

suggested that extreme high temperatures combined with decreased precipitation may decrease mosquito populations.114

Precipitation can create aquatic breeding sites for WNV 

vectors,115, 116 and in some areas snowpack increases the 

amount of stored water available for urban or agricultural 

systems, which provide important habitat for WNV vectors,117, 

118 although effects depend on human water management Birds such as the house finch are the natural host of West Nile 

virus.

Climate change has already begun to cause 

shifts in bird breeding and migration 

patterns,but it is unknown how these changes 

may affect West Nile virus transmission.
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U.S. Global Change Research Program 141 Impacts of Climate Change on Human Health in the United States

Humans can be infected from a bite of a mosquito that has previously bitten an infected bird.

decisions and vary spatially.101 Droughts have been associated 

with increased WNV activity, but the association between 

decreased precipitation and WNV depends on location and 

the particular sequence of drought and wetting that precedes 

the WNV transmission season.119, 120, 121, 122

The impact of year-to-year changes in precipitation on mosquito populations varies among the regions of the United 

States and is affected by the typical climate of the area as well 

as other non-climate factors, such as land use or water infrastructure and management practices. In the northern Great 

Plains—a hotspot for WNV activity—increased precipitation 

has been shown to lead to higher Cx. tarsalis abundance a 

few weeks later.116 In contrast, in the typically wet Pacific 

Northwest, weekly precipitation was found to be unrelated 

to subsequent mosquito abundance.123 In urban areas, larvae 

(aquatic immature mosquitoes) may be washed out of their 

underground breeding habitats by heavy rainfall events, making drier conditions more favorable for WNV transmission.110, 

124, 125 In rural areas or drier regions, increased precipitation or 

agricultural irrigation may provide the moisture necessary for 

the development of breeding habitats.121 

Impacts of Long-Term Climate Trends

The relatively short period of WNV’s transmission in the United States prevents direct observation of the impacts of longterm climate trends on WNV incidence. However, despite the 

short history of WNV in the United States, there are some 

lessons to be learned from other mosquito-borne diseases 

with longer histories in the United States. 

Western equine encephalomyelitis virus (WEEV) and St. Louis 

encephalitis virus (SLEV) were first identified in the 1930s and 

have been circulating in the United States since that time. 

Like WNV, both viruses are transmitted primarily by Culex 

mosquitoes and are climate-sensitive. WEEV outbreaks were 

associated with wet springs followed by warm summers.118, 

126 Outbreaks of SLEV were associated with hot, dry periods 

when urban mosquito production increased due to stagnation 

of water in underground systems or when cycles of drought 

and wetting set up more complex transmission dynamics.127, 128 

Despite climatic warming that would be expected to favor 

increased WEEV and SLEV transmission, both viruses have had 

sharply diminished incidence during the past 30 to 40 years.129, 

130 Although the exact reason for this decline is unknown, it is 

likely a result of non-climate factors, such as changes in human behavior or undetected aspects of viral evolution. Several 

other mosquito-borne pathogens, such as chikungunya and 

dengue, have grown in importance as global health threats 

during recent decades; however, a link to climate change induced disease expansion in the United States has not yet been 

confirmed. These examples demonstrate the variable impact 

that climate change can have on different mosquito-borne 

diseases and help to explain why the direction of future trends 

in risk for WNV remain unclear. 

Projected Impacts

Given WNV’s relatively short history in the United States, the 

described geographic variation in climate responses, and the 

complexity of transmission cycles, projecting the future distribution of WNV under climate change remains a challenge. 

Despite the growing body of work examining the connections 

between WNV and weather, climate-based seasonal forecasts 

of WNV outbreak risk are not yet available at a national scale. 

Forecasting the annual presence of WNV disease on the basis 

of climate and other ecological factors has been attempted 

for U.S. counties, with general agreement between modeled 

expectations and observed data, but more quantitative predictions of disease incidence or the risk for human exposure are 

needed.131 

Longer-term projections of WNV under climate change scenarios are also rare. WNV is projected to increase in much of the 

northern and southeastern United States due to rising temperatures and declining precipitation, respectively, with the poten-
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U.S. Global Change Research Program 142 Impacts of Climate Change on Human Health in the United States

tial for decreased occurrence across the central United States.132 

Future projections show that the season when mosquitoes are 

most abundant will begin earlier and end later, possibly resulting in fewer mosquitoes in mid-summer in southern locations 

where extreme heat is predicted to coincide with decreased 

summer precipitation.114

5.4 Populations of Concern 

Climate change will influence human vulnerability to vector-borne disease by influencing the seasonality and the 

location of exposures to pathogens and vectors. These impacts 

may influence future disease patterns; certain vector-borne 

diseases may emerge in areas where they had previously not 

been observed and other diseases may become less common in 

areas where they had previously been very common. As such, 

some segments of the U.S. population may be disproportionately affected by, or exposed to, vector-borne diseases in response 

to climate change (see also Ch. 9: Populations of Concern).

In addition to climate factors, 

multiple non-climate factors also 

influence human exposure to 

vector-borne pathogens.17, 133, 134, 

135, 136, 137 Some of these include 

factors from an environmental or 

institutional context (Figure 1), 

such as pathogen adaptation and 

change, changes in vector and host population and composition, changes in pathogen infection rates, and vector control or 

other public health practices (pesticide applications, integrated 

vector management, vaccines, and other disease interventions). Other non-climate factors that influence vulnerability to 

vector-borne disease include those from a social and behavioral 

context, such as outdoor activity, occupation, landscape design, 

proximity to vector habitat, and personal protective behaviors (applying repellents before spending time in tick habitat, 

performing tick checks, and bathing after being outdoors).138 

For Lyme disease, behavioral factors, especially the number of 

hours spent working or playing outdoors in tick habitat as well 

as proximity to dense shrubbery, can increase exposure to the 

ticks that transmit the bacteria that causes Lyme disease.139 For 

example, outdoor workers in the northeastern United States are 

at higher risk for contact with blacklegged ticks and, therefore, 

are at a greater risk for contracting Lyme disease.140, 141, 142 If 

outdoor workers are working in areas where there are infected 

mosquitoes, occupational exposures can also occur for WNV.143

Individual characteristics, such as age, gender, and immune 

function, may also affect vulnerability by influencing susceptibility to infection.21, 80, 140, 143, 144, 145 Lyme disease is more 

frequently reported in children between 5 and 9 years of age 

and in adults between the ages of 55 and 59,21 and advanced 

age and being male contribute to a higher risk for severe WNV 

infections.79, 144, 145

The impacts of climate change on human vulnerability to 

vector-borne disease may be minimized by individual- or 

community-level adaptive capacity, or the ability to reduce the 

potential exposures that may be caused by climate change. 

For example, socioeconomic status and domestic protective 

barriers, such as screens on windows and doors, can limit 

exposures to vector-borne pathogens.17, 134, 135, 136, 137 From 1980 

to 1999, the infected mosquito counts in Laredo, Texas, were 

significantly higher than in three adjoining Mexican states—

yet, while there were only 64 cases of dengue fever reported 

in Texas, more than 62,000 dengue fever cases were reported 

in the Mexican states.137 In Texas, socioeconomic factors and 

adaptive measures, including houses with air conditioning and 

intact screens, contributed to the significantly lower dengue 

incidence by reducing human–mosquito contact.137 The adaptive capacity of a population may augment or limit the impacts 

of climate change to human vulnerability for vector-borne 

disease.137 

Climate factors are useful benchmarks to indicate seasonal risk 

and broad geographic changes in 

disease occurrence over decades. However, human vulnerability to vector-borne disease 

is more holistically evaluated by 

examining climate factors with 

non-climate factors (environmental or institutional context, 

social and behavioral context, and individual characteristics). 

Ultimately, a community’s capacity to adapt to both the 

climate and non-climate factors will affect population vulnerability to vector-borne disease. 

5.5 Emerging Issues 

Some vector-borne diseases may be introduced or become 

re-established in the United States by a variety of mechanisms. In conjunction with trade and travel, climate change 

may contribute by creating habitats suitable for the establishment of disease-carrying vectors or for locally sustained transmission of vector-borne pathogens. Examples of emerging vector-borne diseases in the United States include the West Nile 

virus introduction described above, recent outbreaks of locally 

acquired dengue in Florida17, 146 and southern Texas,147 and chikungunya cases in the Caribbean and southern Florida,148 all of 

which have raised public health concern about emergence and 

re-emergence of these mosquito-borne diseases in the United 

States. Collecting data on the spread of disease-causing insect 

vectors and the viruses that cause dengue and chikungunya is 

critical to understanding and predicting the threat of emergence or reemergence of these diseases. Understanding the 

role of climate change in disease emergence and reemergence 

would also require additional research. 

Some segments of the U.S. population may 

be disproportionately affected by, or exposed 

to, vector-borne diseases in response to 

climate change.
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U.S. Global Change Research Program 143 Impacts of Climate Change on Human Health in the United States

5.6 Research Needs

In addition to the emerging issues identified above, and based 

on their review of the literature, the authors highlight the 

following areas for potential scientific research activity on 

vector-borne disease. Climate and non-climate factors interact 

to determine the burden of vector-borne diseases on humans, but the mechanisms of these processes are still poorly 

understood.149 Evidence-based models that include vector–

host interaction, host immunity, pathogen evolution, and land 

use, as well as socioeconomic drivers of transmission, human 

behavior, and adaptive capacity are needed to facilitate a 

better understanding of the mechanisms by which climate and 

non-climate factors drive vector-borne disease emergence. 

Socioeconomic and human behavioral factors, in particular, 

appear to limit vector-borne diseases, even in neighboring cities.136, 137 This is a fertile area for future research, and one that 

is particularly relevant for increasing our adaptive capacity to 

address future vector-borne disease threats.

Numerous studies have identified associations between 

vector-borne diseases and weather or climate, but most have 

focused on risk mapping or estimating associations of broad 

aggregates of temperature and precipitation with disease-related outcomes. A move beyond correlative associations to 

a more mechanistic understanding of climate’s impacts on 

the discrete events that give rise to transmission is needed. 

Models must also be accompanied by empirical research to 

inform their parameters. Climate effects are complex, and 

models frequently borrow information across vector species 

and pathogens or make simplifying assumptions that can lead 

to incorrect conclusions.150

The risk for vector-borne diseases is highly variable geographically and over time. Monitoring responses of pathogens to 

climate change at a continental scale requires coordinated, 

systematically collected long-term surveillance datasets to 

document changes in vector occurrence, abundance, and 

infection rates. Collecting these data will provide a clearer 

understanding of how external drivers work in conjunction 

with climate change to determine the risk for human exposure 

to vector-borne disease.

Future assessments can benefit from research activities that:

• evaluate how climatic variables, socioeconomic factors, and 

human behavior influence vector-borne disease occurrence 

and are expected to affect human adaptive capacity and the 

ability to respond to future disease threats;

• enhance long-term, systematic data collection on vector and 

pathogen distributions to detect changes over time. Such 

datasets must span a range of land-use types, including 

urban areas, and should be coupled with data on human 

disease;

• utilize mechanistic models that provide an evidence-based 

view of climate’s impacts on vector-borne diseases by explicitly accounting for the series of discrete but intertwined 

events that give rise to transmission. Models should be supported and validated by data specific to the disease system 

and include a realistic assessment of parameter uncertainty 

and variability;

• study the natural maintenance cycles of vector-borne pathogen evolution, emergence, and transmission as well as how 

climatic variables influence these cycles.
U.S. Global Change Research Program 144 Impacts of Climate Change on Human Health in the United States

Supporting Evidence

PROCESS FOR DEVELOPING CHAPTER

The chapter was developed through technical discussions 

of relevant evidence and expert deliberation by the report 

authors at several workshops, teleconferences, and email 

exchanges. The authors considered inputs and comments 

submitted by the public, the National Academies of Sciences, 

and Federal agencies. For additional information on the 

overall report process, see Appendices 2 and 3.

The approach and organization of this chapter was decided 

after conducting a comprehensive literature review. Two 

case studies, Lyme disease and West Nile virus, were chosen 

as representative examples of vector-borne diseases in 

the United States for this chapter because of their high 

incidence rates and the body of literature available on the 

association between climatic and meteorological variables 

and occurrence of these diseases. 

Regarding human outcomes related to vector-borne diseases, 

there is a much greater volume of published literature 

available on meteorological and climatic influences on 

vectors. As a result, our certainty in how climate change is 

likely to influence the vectors far exceeds our certainty in how 

changing climatic conditions are likely to affect when, where, 

and how many cases of vector-borne diseases are likely to 

occur.

Although the topic of zoonotic diseases was included in 

the original prospectus, it was later removed due to space 

constraints. Additionally, since both West Nile virus infection 

and Lyme disease are zoonotic diseases, these case studies 

address concepts that are common to both vector-borne and 

zoonotic diseases. 

KEY FINDING TRACEABLE ACCOUNTS

Changing Distributions of Vectors and Vector-Borne 

Diseases

Key Finding 1: Climate change is expected to alter the 

geographic and seasonal distributions of existing vectors and 

vector-borne diseases [Likely, High Confidence].

Description of evidence base

Vector-borne diseases result from complex interactions 

involving vectors, reservoirs, humans, and both climate and 

non-climate factors. Numerous studies explain how climate 

variables influence the relationships between vectors, 

animal reservoirs, humans, and other non-climate factors to 

ultimately influence the spatial and temporal distribution of 

vector-borne disease.11, 39, 45, 53, 101, 104, 114, 116, 123, 135

Major uncertainties

It is certain that climate change will alter the geographic 

and seasonal distribution of existing vectors, pathogens, and 

reservoirs; the influence of climate change on the timing, 

prevalence, and location of specific vector-borne disease 

outbreaks is likely to vary depending on the influence of other 

significant non-climate drivers of disease occurrence.

Assessment of confidence and likelihood based on evidence

Based on the evidence that climate change will influence 

the temporal and spatial distributions of vectors, pathogens, 

and animal reservoirs, there is high confidence that climate 

change is likely to alter the geographic and seasonal 

distributions of vectors and vector-borne diseases.

Earlier Tick Activity and Northward Range Expansion

Key Finding 2: Ticks capable of carrying the bacteria that 

cause Lyme disease and other pathogens will show earlier 

seasonal activity and a generally northward expansion in 

response to increasing temperatures associated with climate 

change [Likely, High Confidence]. Longer seasonal activity and 

expanding geographic range of these ticks will increase the 

risk of human exposure to ticks [Likely, Medium Confidence].

Description of evidence base

There is strong evidence that temperature affects the 

geographical distribution of ticks,39, 45, 53, 67 the timing of hostseeking activity of ticks,36, 37, 44 and even the timing of Lyme 

disease case occurrence.23 However, the abundance of ticks 

infected with Lyme disease spirochetes, which is considered 

a better predictor of human risk for Lyme disease compared 

with nymphal density alone, has rarely been found to be 

strongly associated with meteorological variables.41 Studies 

aimed at identifying meteorological variables associated 

with the geographical distribution of human Lyme disease 

vary in their support for demonstrating positive associations 

between temperature and Lyme disease.46, 62, 63

Major uncertainties

While the effects of temperature, precipitation, and humidity 

on the spatial distribution of ticks and the timing of their 

host-seeking activity have been clearly established in both 

the eastern and western regions of the United States, where 

Lyme disease is common, the degree to which climate change 

will alter Lyme disease incidence remains uncertain. The 

observation that meteorological variables play a lesser role 

than other variables in predicting the density of nymphs 

infected with Lyme disease bacteria raises uncertainty in how 

climate change will affect the distribution and magnitude of 

Lyme disease incidence. This uncertainty is reflected in results 

from models aiming to associate meteorological variables with 

Lyme disease incidence that yielded inconsistent findings.46, 62, 63
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U.S. Global Change Research Program 145 Impacts of Climate Change on Human Health in the United States

Assessment of confidence and likelihood based on evidence

Based on the evidence, there is high confidence that climate 

change, especially temperature change, is likely to cause 

shifts in the geographical distribution of ticks capable of 

carrying B. burgdorferi to more northern latitudes, the timing 

of host-seeking activity of ticks, and the timing of Lyme 

disease case occurrence. While these changes are likely to 

influence human disease, due to the few sources with limited 

consistency, incomplete models with methods still emerging, 

and some competing schools of thought, there is medium 

confidence surrounding how, and how much, climate change 

will influence the risk of human exposure to ticks carrying B. 

burgdorferi. 

Changing Mosquito-Borne Disease Dynamics

Key Finding 3: Rising temperatures, changing precipitation 

patterns, and a higher frequency of some extreme weather 

events associated with climate change will influence the 

distribution, abundance, and prevalence of infection in 

the mosquitoes that transmit West Nile virus and other 

pathogens by altering habitat availability and mosquito 

and viral reproduction rates [Very Likely, High Confidence]. 

Alterations in the distribution, abundance, and infection 

rate of mosquitoes will influence human exposure to bites 

from infected mosquitoes, which is expected to alter risk for 

human disease [Very Likely, Medium Confidence].

Description of evidence base

Higher temperatures affect the West Nile virus (WNV) 

system by accelerating mosquito development102, 104 and virus 

reproduction rates,101, 107, 108, 109 increasing egg-laying and biting 

frequency,106 and affecting mosquito survival.102, 126 Increased 

WNV activity has been associated with warm temperatures, 

mild winters, and drought.101, 110, 116 Very few studies have 

used climate variables to predict the occurrence of human 

WNV cases in the United States in response to climate change 

(for example, Harrigan et al. 2014),132 but available results 

suggest that areas of WNV transmission will expand in the 

northern latitudes and higher elevations driven by increasing 

temperature, while WNV transmission may decrease in the 

South if increasing temperatures reduce mosquito survival 

or limit availability of surface water, such as that provided by 

agricultural irrigation.

Major uncertainties

While the influence of temperature and precipitation on 

mosquito and WNV biology are fairly well-understood, these 

relationships vary across the United States depending on 

the local mosquito vector species, land use, and human 

activity.112, 121 For mosquitoes in urban areas, droughts 

may lead to stagnation of water and increased mosquito 

populations that enhance WNV transmission,110, 125 while in 

rural or agricultural areas, droughts may reduce mosquito 

populations by reducing available mosquito habitat for 

breeding,101 except when irrigation compensates for drought 

conditions.121 Long-term projections of human WNV risk 

under climate change scenarios are still in the early stages 

of development and are impeded by the complexities 

of the disease transmission cycle. Evolution of the virus, 

improvements in mosquito control, and the potential for longterm changes in human behavior that may affect exposure to 

WNV are key sources of uncertainty. For this reason, shortterm, seasonal forecasts of WNV may be more fruitful in the 

near term and may provide information for seasonal resource 

allocation and public health planning. 

Assessment of confidence and likelihood based on evidence

Based on the evidence, there is high confidence that climate 

change is very likely to influence mosquito distribution, 

abundance, and infection prevalence by altering habitat 

availability and mosquito and viral reproduction rates. While 

this is very likely to influence human disease, due to the few 

sources with limited consistency, incomplete models with 

methods still emerging, and some competing schools of 

thought, there is medium confidence surrounding how, and 

how much, climate change will influence human incidence of 

disease.

Emergence of New Vector-Borne Pathogens

Key Finding 4: Vector-borne pathogens are expected to 

emerge or reemerge due to the interactions of climate factors 

with many other drivers, such as changing land-use patterns 

[Likely, High Confidence]. The impacts to human disease, 

however, will be limited by the adaptive capacity of human 

populations, such as vector control practices or personal 

protective measures [Likely, High Confidence].

Description of evidence base

The literature shows that climate change must be considered 

together with the many other non-climate factors of disease 

emergence11, 12 and the availability of other mitigating factors, 

such as air conditioning, screens on windows, and vector 

control practices,17, 134, 136, 137 in order to appropriately quantify 

the impact climate has on the risk of emerging or reemerging 

exotic pathogens and vectors.

Major uncertainties

It remains uncertain how climate interacts as a driver with 

travel-related exposures and evolutionary adaptation of 

invasive vectors and pathogens to affect human disease. 

Improved longitudinal datasets and empirical models that 

include vector–host interaction, host immunity, and pathogen 

evolution as well as socioeconomic drivers of transmission 

are needed to address these knowledge gaps in research on 

climate sensitive diseases.

Assessment of confidence and likelihood based on evidence

Based on the evidence, there is high confidence that a 

multitude of interacting factors, one of which being climate 

change, will likely influence the emergence or reemergence 

of vector-borne pathogens to the United States. Additionally, 
5-VECTOR-BORNE DISEASES

U.S. Global Change Research Program 146 Impacts of Climate Change on Human Health in the United States

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few sources, limited consistency, models 

incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts

Likelihood

Very Likely

≥ 9 in 10

Likely

≥ 2 in 3

As Likely As Not

≈ 1 in 2

Unlikely

≤ 1 in 3

Very Unlikely

≤ 1 in 10

there is high confidence that the influence of climate change 

on human disease is likely to be limited by the adaptive 

capacity of a population. 

DOCUMENTING UNCERTAINTY

See Appendix 4: Documenting Uncertainty for more 

information on assessments of confidence and likelihood used 

in this report. 

PHOTO CREDITS

Pg. 129–Mosquito: ©CDC/Science Faction/Corbis

Pg. 130–Woman applying repellent: © iStockPhoto.com/

powerofforever

Pg. 135–Blacklegged tick: ©Science Stills/ARS/Visuals 

Unlimited/Corbis

Pg. 140–House finch: ©Pete Oxford/Minden Pictures/Corbis

Pg. 141–Mosquito warning sign: © iStockPhotos.com/leekris

Pg. 141–Mosquito: ©CDC/Science Faction/Corbis
U.S. Global Change Research Program 147 Impacts of Climate Change on Human Health in the United States

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Geological Survey. http://diseasemaps.usgs.gov/2013/del_

us_human.html

148. PAHO, 2014: Chikungunya. Pan American Health Organization, Washington, D.C. http://www.paho.org/hq/index.

php?Itemid=40931

149. Parham, P.E., J. Waldock, G.K. Christophides, D. Hemming, F. Agusto, K.J. Evans, N. Fefferman, H. Gaff, A. 

Gumel, S. LaDeau, S. Lenhart, R.E. Mickens, E.N. Naumova, R.S. Ostfeld, P.D. Ready, M.B. Thomas, J. Velasco-Hernandez, and E. Michael, 2015: Climate, environmental and 

socio-economic change: Weighing up the balance in vector-borne disease transmission. Philosophical Transactions of 

the Royal Society B: Biological Sciences, 370. http://dx.doi.

org/10.1098/rstb.2013.0551

150. Reiner, R.C., T.A. Perkins, C.M. Barker, T. Niu, L.F. Chaves, 

A.M. Ellis, D.B. George, A. Le Menach, J.R.C. Pulliam, 

D. Bisanzio, C. Buckee, C. Chiyaka, D.A.T. Cummings, 

A.J. Garcia, M.L. Gatton, P.W. Gething, D.M. Hartley, G. 

Johnston, E.Y. Klein, E. Michael, S.W. Lindsay, A.L. Lloyd, 

D.M. Pigott, W.K. Keisen, N. Ruktanonchai, B.K. Singh, 

A.J. Tatem, U. Kitron, S.I. Hay, T.W. Scott, and D.L. Smith, 

2013: A systematic review of mathematical models of mosquito-borne pathogen transmission: 1970-2010. Journal of 

the Royal Society Interface, 10. http://dx.doi.org/10.1098/

rsif.2012.0921
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© Pete Oxford/Minden Pictures/Corbis
CLIMATE IMPACTS ON WATER- 6 RELATED ILLNESS

U.S. Global Change Research Program

Lead Authors

Juli Trtanj

National Oceanic and Atmospheric Administration

Lesley Jantarasami* 

U.S. Environmental Protection Agency

Contributing Authors

Joan Brunkard

Centers for Disease Control and Prevention

Tracy Collier

National Oceanic and Atmospheric Administration and

University Corporation for Atmospheric Research

John Jacobs

National Oceanic and Atmospheric Administration

Erin Lipp

The University of Georgia

Sandra McLellan

University of Wisconsin-Milwaukee

Stephanie Moore

National Oceanic and Atmospheric Administration and 

University Corporation for Atmospheric Research

Hans Paerl

The University of North Carolina at Chapel Hill

John Ravenscroft

U.S. Environmental Protection Agency

Mario Sengco

U.S. Environmental Protection Agency

Jeanette Thurston

U.S. Department of Agriculture

Recommended Citation: Trtanj, J., L. Jantarasami, J. Brunkard, T. Collier, J. Jacobs, E. Lipp, S. McLellan, S. Moore, H. Paerl, J. 

Ravenscroft, M. Sengco, and J. Thurston, 2016: Ch. 6: Climate Impacts on Water-Related Illness. The Impacts of Climate Change on 

Human Health in the United States: A Scientific Assessment. U.S. Global Change Research Program, Washington, DC, 157–188. 

http://dx.doi.org/10.7930/J03F4MH4

THE IMPACTS OF CLIMATE CHANGE ON HUMAN HEALTH IN THE UNITED STATES

A Scientific Assessment

On the web: health2016.globalchange.gov *Chapter Coordinator

Acknowledgements: Sharon Nappier, U.S. Environmental Protection Agency
158

CLIMATE IMPACTS ON WATERRELATED ILLNESS

FPO

6

Key Findings

Seasonal and Geographic Changes in Waterborne Illness Risk

Key Finding 1: Increases in water temperatures associated with climate change will alter the seasonal windows 

of growth and the geographic range of suitable habitat for freshwater toxin-producing harmful algae [Very 

Likely, High Confidence], certain naturally occurring Vibrio bacteria [Very Likely, Medium Confidence], and 

marine toxin-producing harmful algae [Likely, Medium Confidence]. These changes will increase the risk of 

exposure to waterborne pathogens and algal toxins that can cause a variety of illnesses [Medium Confidence].

Runoff from Extreme Precipitation Increases Exposure Risk

Key Finding 2: Runoff from more frequent and intense extreme precipitation events will increasingly compromise 

recreational waters, shellfish harvesting waters, and sources of drinking water through increased introduction 

of pathogens and prevalence of toxic algal blooms [High Confidence]. As a result, the risk of human exposure 

to agents of water-related illness will increase [Medium Confidence]. 

Water Infrastructure Failure

Key Finding 3: Increases in some extreme weather events and storm surges will increase the risk that 

infrastructure for drinking water, wastewater, and stormwater will fail due to either damage or exceedance 

of system capacity, especially in areas with aging infrastructure [High Confidence]. As a result, the risk of 

exposure to water-related pathogens, chemicals, and algal toxins will increase in recreational and shellfish 

harvesting waters and in drinking water where treatment barriers break down [Medium Confidence].
6–CLIMATE IMPACTS ON WATER-RELATED ILLNESS

U.S. Global Change Research Program 159 Impacts of Climate Change on Human Health in the United States

6.1 Introduction

Across most of the United States, climate change is expected 

to affect fresh and marine water resources in ways that will 

increase people’s exposure to water-related contaminants 

that cause illness. Water-related illnesses include waterborne 

diseases caused by pathogens, such as bacteria, viruses, and 

protozoa. Water-related illnesses are also caused by toxins 

produced by certain harmful algae and cyanobacteria (also 

known as blue-green algae) and by chemicals introduced 

into the environment by human activities. Exposure occurs 

through ingestion, inhalation, or direct contact with contaminated drinking or recreational water and through consumption 

of fish and shellfish. 

Factors related to climate change—including temperature, 

precipitation and related runoff, hurricanes, and storm surge—

affect the growth, survival, spread, and virulence or toxicity of 

agents (causes) of water-related illness. Heavy downpours are 

already on the rise and increases in the frequency and intensity of extreme precipitation events are projected for all U.S. 

regions.1 Projections of temperature, precipitation, extreme 

events such as flooding and drought, and other climate factors 

vary by region of the United States, and thus the extent of 

climate health impacts will also vary by region. 

Figure 1: This conceptual diagram for an example of infection by Vibrio species (V. vulnificus, V. parahaemolyticus, or V. alginolyticus) 

illustrates the key pathways by which humans are exposed to health threats from climate drivers. These climate drivers create more 

favorable growing conditions for these naturally occurring pathogens in coastal environments through their effects on coastal salinity, 

turbidity (water clarity), or plankton abundance and composition. Longer seasons for growth and expanding geographic range of 

occurrence increase the risk of exposure to Vibrio, which can result in various potential health outcomes (center boxes). These 

exposure pathways exist within the context of other factors that positively or negatively influence health outcomes (gray side boxes). 

Key factors that influence vulnerability for individuals are shown in the right box and include social determinants of health and 

behavioral choices. Key factors that influence vulnerability at larger scales, such as natural and built environments, governance and 

management, and institutions, are shown in the left box. All of these influencing factors can affect an individual’s or a community’s 

vulnerability through changes in exposure, sensitivity, and adaptive capacity and may also be affected by climate change. See Ch. 

1: Introduction for more information.

Climate Change and Health—Vibrio
6–CLIMATE IMPACTS ON WATER-RELATED ILLNESS

U.S. Global Change Research Program 160 Impacts of Climate Change on Human Health in the United States

Waterborne pathogens are estimated to cause 8.5% to 12% 

of acute gastrointestinal illness cases in the United States, 

affecting between 12 million and 19 million people annually.2, 

3, 4 Eight pathogens, which are all affected to some degree 

by climate, account for approximately 97% of all suspected 

waterborne illnesses in the United States: the enteric viruses 

norovirus, rotavirus, and adenovirus; the bacteria Campylobacter jejuni, E. coli O157:H7, and Salmonella enterica; and 

the protozoa Cryptosporidium and Giardia.

5

Specific health outcomes are determined by different exposure 

pathways and multiple other social and behavioral factors, 

some of which are also affected by climate (Figure 1). Most 

research to date has focused on understanding how climate 

drivers affect physical and ecological processes that act as key 

exposure pathways for pathogens and toxins, as shown by the 

arrow moving from the top to the middle box in Figure 1. There 

is currently less information and fewer methods with which to 

measure actual human exposure and incidence of illness based 

on those physical and ecological metrics (arrow moving from 

middle to bottom box in Figure 1). Thus, it is often not possible 

to quantitatively project future health outcomes from water-related illnesses under climate change (bottom box in Figure 1).

This chapter covers health risks associated with changes in 

natural marine, coastal, and freshwater systems and water 

infrastructure for drinking water, wastewater, and stormwater 

(Legionella in aerosolized water is covered in Ch. 3: Air Quality 

Impacts). This chapter also includes fish and shellfish illnesses 

associated with the waters in which they grow and which are 

affected by the same climate factors that affect drinking and 

recreational waters (impacts related to handling and post-harvest processing of seafood are covered in Ch. 7: Food Safety). 

The framing of this chapter addresses sources of contaminations, exposure pathways, and health outcomes when available. Based on the available data and research, many of the 

examples are regionally focused and make evident that the 

impact of climate change on water-related illness is inherently 

regional. Table 1 lists various health outcomes that can result 

from exposure to agents of water-related illness as well as key 

climate-related changes affecting their occurrence. 

Whether or not illness results from exposure to contaminated water, fish, or shellfish is dependent on a complex set of 

factors, including human behavior and social determinants 

of health that may affect a person’s exposure, sensitivity, and 

adaptive capacity (Figure 1; see also Ch. 1: Introduction and 

Table 1. Climate Sensitive Agents of Water Related Illness

Pathogen or Toxin 

Producer

Exposure 

Pathway

Selected Health Outcomes & 

Symptoms 

Major Climate Correlation or 

Driver (strongest driver(s) 

listed first) 

Algae: Toxigenic marine 

species of Alexandrium, 

Pseudo-nitzschia, Dinophysis, 

Gambierdiscus; Karenia 

brevis

Shellfish

Fish

Recreational 

waters 

(aerosolized 

toxins)

Gastrointestinal and neurologic illness 

caused by shellfish poisoning (paralytic, 

amnesic, diarrhetic, neurotoxic) or fish 

poisoning (ciguatera). 

Asthma exacerbations, eye irritations 

caused by contact with aerosolized 

toxins (K. brevis).

Temperature (increased water 

temperature), ocean surface 

currents, ocean acidification, 

hurricanes (Gambierdiscus spp. 

and K. brevis)

Cyanobacteria (multiple 

freshwater species producing 

toxins including microcystin)

Drinking water 

Recreational 

waters

Liver and kidney damage, 

gastroenteritis (diarrhea and vomiting), 

neurological disorders, and respiratory 

arrest.

Temperature, precipitation patterns

Enteric bacteria & protozoan 

parasites: Salmonella 

enterica; Campylobacter 

species; Toxigenic Escherichia 

coli; Cryptosporidium; Giardia

Drinking water 

Recreational 

waters 

Shellfish

Enteric pathogens generally cause 

gastroenteritis. Some cases may be 

severe and may be associated with 

long-term and recurring effects.

Temperature (air and water; both 

increase and decrease), heavy 

precipitation, and flooding

Enteric viruses: enteroviruses; 

rotaviruses; noroviruses; 

hepatitis A and E

Drinking water 

Recreational 

waters

Shellfish

Most cases result in gastrointestinal 

illness. Severe outcomes may include 

paralysis and infection of the heart or 

other organs.

Heavy precipitation, flooding, and 

temperature (air and water; both 

increase and decrease)

Leptospira and Leptonema

bacteria

Recreational 

waters

Mild to severe flu-like illness (with 

or without fever) to severe cases of 

meningitis, kidney, and liver failure.

Flooding, temperature (increased 

water temperature), heavy 

precipitation

Vibrio bacteria species Recreational 

waters 

Shellfish

Varies by species but include 

gastroenteritis (V. parahaemolyticus,

V. cholerae), septicemia (bloodstream 

infection) through ingestion or wounds 

(V. vulnificus), skin, eye, and ear 

infections (V. alginolyticus).

Temperature (increased water 

temperature), sea level rise, 

precipitation patterns (as it affects 

coastal salinity)
6–CLIMATE IMPACTS ON WATER-RELATED ILLNESS

U.S. Global Change Research Program 161 Impacts of Climate Change on Human Health in the United States

Ch. 9: Populations of Concern). Water resource, public health, 

and environmental agencies in the United States provide many 

public health safeguards to reduce risk of exposure and illness 

even if water becomes contaminated. These include water 

quality monitoring, drinking water treatment standards and 

practices, beach closures, and issuing advisories for boiling 

drinking water and harvesting shellfish.

Many water-related illnesses are either undiagnosed or unreported, and therefore the total incidence of waterborne disease 

is underestimated (see Ch. 1: Introduction for discussion of public health surveillance data limitations related to “reportable” 

and “nationally notifiable” diseases).6, 7 On average, illnesses 

from pathogens associated with water are thought to be underestimated by as much as 43-fold, and may be underestimated 

by up to 143 times for certain Vibrio species.7

6.2 Sources of Water-Related Contaminants 

The primary sources of water contamination are human and 

animal waste and agricultural activities, including the use of 

fertilizers. Runoff and flooding resulting from expected increases in extreme precipitation, hurricane rainfall, and storm surge 

(see Ch. 4: Extreme Events) may increase risks of contamination. 

Contamination occurs when agents of water-related illness and 

nutrients, such as nitrogen and phosphorus, are carried from 

urban, residential, and agricultural areas into surface waters, 

groundwater, and coastal waters (Figure 2). The nutrient loading 

can promote growth of naturally occurring pathogens and 

algae. Human exposure occurs via contamination of drinking 

water sources (page 163), recreational waters (page 164), and 

fish and shellfish (page 165).

Water contamination by human waste is tied to failure of local 

urban or rural water infrastructure, including municipal wastewater, septic, and stormwater conveyance systems. Failure can 

occur either when rainfall and subsequent runoff overwhelm 

the capacity of these systems—causing, for example, sewer 

overflows, basement backups, or localized flooding—or when 

extreme events like flooding and storm surges damage water 

conveyance or treatment infrastructure and result in reduction 

or loss of performance and functionality. Many older cities 

in the Northeast and around the Great Lakes region of the 

United States have combined sewer systems (with stormwater and sewage sharing the same pipes), which are prone 

to discharging raw sewage directly into surface waters after 

moderate to heavy rainfall.8 The amount of rain that causes 

combined sewer overflows is highly variable between cities 

because of differences in infrastructure capacity and design, 

and ranges from 5 mm (about 0.2 inches) to 2.5 cm (about 1 

inch).9, 10 Overall, combined sewer overflows are expected to 

increase,11 but site-specific analysis is needed 

to predict the extent of these increases (see 

Case Study on page 164). Extreme precipitation events will exacerbate existing problems 

with inadequate, aging, or deteriorating wastewater infrastructure throughout the country.12, 

13 These problems include broken or leaking 

sewer pipes and failing septic systems that 

leach sewage into the ground. Runoff or contaminated groundwater discharge also carries 

pathogens and nutrients into surface water, 

including freshwater and marine coastal areas 

and beaches.14, 15, 16, 17, 18, 19, 20, 21

Water contamination from agricultural activities 

is related to the release of microbial pathogens 

or nutrients in livestock manure and inorganic 

fertilizers that can stimulate rapid and excessive 

growth or blooms of harmful algae. Agricultural 

land covers about 900 million acres across the 

United States,22 comprising over 2 million farms, 

with livestock sectors concentrated in certain 

regions of the United States (Figure 3). Depending on the type and number of animals, a large 

livestock operation can produce between 2,800 

and 1,600,000 tons of manure each year.23, 24

With the projected increases in heavy precipitation for all U.S. regions,1

 agricultural sources of 

contamination can affect water quality across 

Links between Climate Change, Water Quantity and Quality, 

and Human Exposure to Water-Related Illness. 

Figure 2: Precipitation and temperature changes affect fresh and marine water 

quantity and quality primarily through urban, rural, and agricultural runoff. This 

runoff in turn affects human exposure to water-related illnesses primarily through 

contamination of drinking water, recreational water, and fish and shellfish.
6–CLIMATE IMPACTS ON WATER-RELATED ILLNESS

U.S. Global Change Research Program 162 Impacts of Climate Change on Human Health in the United States

the Nation. Runoff from lands where manure has been used as 

fertilizer or where flooding has caused wastewater lagoons to 

overflow can carry contamination agents directly from the land 

into water bodies.23, 24, 25

Management practices and technologies, such as better timing 

of manure application and improved animal feeds, help reduce 

or eliminate the risks of manure-borne contaminant transport 

to public water supplies and shellfish harvesting waters and 

reduce nutrients that stimulate harmful algal blooms.23, 25, 28, 29

Drinking water treatment and monitoring practices also help 

to decrease or eliminate exposure to waterborne illness agents 

originating from agricultural environments. 

Water contamination from wildlife (for example, rodents, birds, 

deer, and wild pigs) occurs via feces and urine of infected animals, which are reservoirs of enteric and other pathogens.29, 30, 

31 Warmer winters and earlier springs are expected to increase 

animal activity and alter the ecology and habitat of animals that 

may carry pathogens.1 This may lengthen the exposure period 

for humans and expand the geographic ranges in which pathogens are transmitted.1, 32

6.3 Exposure Pathways and Health Risks

Humans are exposed to agents of water-related illness through 

several pathways, including drinking water (treated and untreated), recreational waters (freshwater, coastal, and marine), and 

fish and shellfish. 

Locations of Livestock and Projections of Heavy Precipitation

Figure 3: This figure compares the geographic distribution of chicken, cattle, and hog and pig densities to the projected change in 

annual maximum 5-day precipitation totals (2046–2065 compared to 1981–2000, multi-model average using RCP8.5) across the 

continental United States. Increasing frequency and intensity of precipitation and subsequent increases in runoff are key climate 

factors that increase the potential for pathogens associated with livestock waste to contaminate water bodies. (Figure sources: 

adapted from USDA 2014 and Sun et al. 2015).26, 27
6–CLIMATE IMPACTS ON WATER-RELATED ILLNESS

U.S. Global Change Research Program 163 Impacts of Climate Change on Human Health in the United States

Drinking Water

Although the United States has one of the safest municipal 

drinking water supplies in the world, water-related outbreaks 

(more than one illness case linked to the same source) still 

occur.33 Public drinking water systems provide treated water to 

approximately 90% of Americans at their places of residence, 

work, or schools.34 However, about 15% of the population 

relies fully or in part on untreated private wells or other private 

sources for their drinking water.35 These private sources are not 

regulated under the Safe Drinking Water Act.36 The majority of 

drinking water outbreaks in the United States are associated 

with untreated or inadequately treated groundwater and distribution system deficiencies.33, 37

Pathogen and Algal Toxin Contamination 

Between 1948 and 1994, 68% of waterborne disease outbreaks 

in the United States were preceded by extreme precipitation 

events,38 and heavy rainfall and flooding continue to be cited 

as contributing factors in more recent outbreaks in multiple 

regions of the United States.39 Extreme precipitation events 

have been statistically linked to increased levels of pathogens in 

treated drinking water supplies40 and to an increased incidence 

of gastrointestinal illness in children.21, 41 This established 

relationship suggests that extreme precipitation is a key climate 

factor for waterborne disease.42, 43, 44, 45 The Milwaukee Cryptosporidium outbreak in 1993—the largest documented waterborne disease outbreak in U.S. history, causing an estimated 

403,000 illnesses and more than 50 deaths46—was preceded by 

the heaviest rainfall event in 50 years in the adjacent watersheds.10 Various treatment plant operational problems were 

also key contributing factors.47 (See future projections in the 

Case Study on page 164). Observations in England and Wales 

also show waterborne disease outbreaks were preceded by 

weeks of low cumulative rainfall and then heavy precipitation 

events, suggesting that drought or periods of low rainfall may 

also be important climate-related factors.48

Small community or private groundwater wells or other 

drinking water systems where water is untreated or minimally 

treated are especially susceptible to contamination following 

extreme precipitation events.49 For example, in May 2000, following heavy rains, livestock waste containing E. coli O157:H7 

and Campylobacter was carried in runoff to a well that served 

as the primary drinking water source for the town of Walkerton, Ontario, Canada, resulting in 2,300 illnesses and 7 

deaths.43, 44, 50 High rainfall amounts were an important catalyst 

for the outbreak, although non-climate factors, such as well infrastructure, operational and maintenance problems, and lack 

of communication between public utilities staff and local health 

officials were also key factors.44, 51

Likewise, extreme precipitation events and subsequent increases in runoff are key climate factors that increase nutrient 

loading in drinking water sources, which in turn increases the 

likelihood of harmful cyanobacterial blooms that produce algal 

toxins.52 The U.S. Environmental Protection Agency has established health advisories for two algal toxins (microcystins and 

cylindrospermopsin) in drinking water.53 Lakes and reservoirs 

that serve as sources of drinking water for between 30 million 

and 48 million Americans may be periodically contaminated 

by algal toxins.54 Certain drinking water treatment processes 

can remove cyanobacterial toxins; however, efficacy of the 

treatment processes may vary from 60% to 99.9%. Ineffective 

treatment could compromise water quality and may lead to 

severe treatment disruption or treatment plant shutdown.53, 

54, 55, 56 Such an event occurred in Toledo, Ohio, in August 2014, 

when nearly 500,000 residents of the state’s fourth-largest city 

lost access to their drinking water after tests revealed the presence of toxins from a cyanobacterial bloom in Lake Erie near 

the water plant’s intake.57 

Water Supply

Climate-related hydrologic changes such as those related to 

flooding, drought, runoff, snowpack and snowmelt, and saltwater intrusion (the movement of ocean water into fresh groundwater) have implications for freshwater management and 

supply (see also Ch. 4: Extreme Events).58 Adequate freshwater 

supply is essential to many aspects of public health, including 

provision of drinking water and proper sanitation and personal 

hygiene. For example, following floods or storms, short-term 

loss of access to potable water has been linked to increased 

incidence of illnesses including gastroenteritis and respiratory 

tract and skin infections.59 Changes in precipitation and runoff, 

combined with changes in consumption and withdrawal, have 

reduced surface and groundwater supplies in many areas, 

primarily in the western United States.58 These trends are expected to continue under future climate change, increasing the 

likelihood of water shortages for many uses.58

Future climate-related water shortages may result in more 

municipalities and individuals relying on alternative sources for 

drinking water, including reclaimed water and roof-harvested 

rainwater.60, 61, 62, 63 Water reclamation refers to the treatment of 

stormwater, industrial wastewater, and municipal wastewater 

Extreme precipitation events have been statistically linked to 

increased levels of pathogens in treated drinking water supplies.
6–CLIMATE IMPACTS ON WATER-RELATED ILLNESS

U.S. Global Change Research Program 164 Impacts of Climate Change on Human Health in the United States

for beneficial reuse.64 States like California, Arizona, New Mexico, Texas, and Florida are already implementing wastewater 

reclamation and reuse practices as a means of conserving and 

adding to freshwater supplies.65 However, no federal regulations 

or criteria for public health protection have been developed 

or proposed specifically for potable water reuse in the United 

States.66 Increasing household rainwater collection has also 

been seen in some areas of the country (primarily Arizona, Colorado, and Texas), although in some cases, exposure to untreated 

rainwater has been found to pose health risks from bacterial or 

protozoan pathogens, such as Salmonella enterica and Giardia 

lamblia.

67, 68, 69

Projected Changes

Runoff from more frequent and intense extreme precipitation 

events will contribute to contamination of drinking water sources with pathogens and algal toxins and place additional stresses on the capacity of drinking water treatment facilities and 

distribution systems.10, 52, 59, 70, 71, 72, 73 Contamination of drinking 

water sources may be exacerbated or insufficiently addressed 

by treatment processes at the treatment plant or by breaches 

in the distribution system, such as during water main breaks 

or low-pressure events.13 Untreated groundwater drawn from 

municipal and private wells is of particular concern. 

Climate change is not expected to substantially increase the risk 

of contracting illness from drinking water for those people who 

are served by treated drinking water systems, if appropriate 

treatment and distribution is maintained. However, projections 

of more frequent or severe extreme precipitation events, flooding, and storm surge suggest that drinking water infrastructure 

may be at greater risk of disruption or failure due to damage or 

exceedance of system capacity.6, 58, 70, 74, 75 Aging drinking water 

infrastructure is one longstanding limitation in controlling waterborne disease, and may be especially susceptible to failure.6, 

13, 74 For example, there are more than 50,000 systems providing 

treated drinking water to communities in the United States, 

and most water distribution pipes in these systems are already 

failing or will reach their expected lifespan and require replacement within 30 years.6

 Breakdowns in drinking water treatment 

and distribution systems, compounded by aging infrastructure, 

could lead to more serious and frequent health consequences 

than those we experience now.

Recreational Waters

Humans are exposed to agents of water-related illness through 

recreation (such as swimming, fishing, and boating) in freshwater and marine or coastal waters. Exposure may occur directly 

(ingestion and contact with water) or incidentally (inhalation of 

aerosolized water droplets). 

Pathogen and Algal Toxin Contamination

Enteric viruses, especially noroviruses, from human waste are 

a primary cause of gastrointestinal illness from exposure to 

contaminated recreational fresh and marine water (Table 1).77

Although there are comparatively few reported illnesses and 

outbreaks of gastrointestinal illness from recreating in marine 

waters compared to freshwater, marine contamination still presents a significant health risk.39, 78, 79, 80, 81 Illnesses from marine 

sources are less likely to be reported than those from freshwater beaches in part because the geographical residences of 

beachgoers are more widely distributed (for example, tourists 

may travel to marine beaches for vacation) and illnesses are less 

often attributed to marine exposure as a common source.39, 77

Key climate factors associated with risks of exposure to enteric 

pathogens in both freshwater and marine recreational waters 

include extreme precipitation events, flooding, and temperature. For example, Salmonella and Campylobacter concentrations in freshwater streams in the southeastern United States 

increase significantly in the summer months and following 

heavy rainfall.82, 83, 84 In the Great Lakes—a freshwater system—

changes in rainfall, higher lake temperatures, and low lake levels 

have been linked to increases in fecal bacteria levels.10 The 

zoonotic bacteria Leptospira are introduced into water from the 

urine of animals,85, 86 and increased illness rates in humans are 

linked to warm temperatures and flooding events.87, 88, 89, 90, 91

In marine waters, recreational exposure to naturally occurring 

bacterial pathogens (such as Vibrio species) may result in eye, 

ear, and wound infections, diarrheal illness, or death (Table 1).92, 

93, 94 Reported rates of illness for all Vibrio infections have tripled 

since 1996, with V. alginolyticus infections having increased by 

40-fold.92 Vibrio growth rates are highly responsive to rising sea 

Case Study: Modeling Future Extreme 

Precipitation and Combined Sewer Overflows in 

Great Lakes Urban Coastal Areas

The Great Lakes contain 20% of the Earth’s surface 

freshwater and provide drinking water to 40 million 

people. Milwaukee, WI, is typical of urban areas in the 

Great Lakes in that it has a combined sewer system 

that overflows during moderate or heavy rainfall. 

In 1994, unrelated to but shortly after the 1993 

Cryptosporidium outbreak, the city completed a project 

to increase sewer capacity; reducing combined sewage 

overflows from 50 to 60 per year, to 2 to 3 per year.10

In order to assess how changing rainfall patterns 

might affect sewer capacity in the future, Milwaukee 

was one of the first cities to integrate regional climate 

projections into its detailed engineering models. Under 

a future climate scenario (for 2050) that had one of 

the largest projected increases in spring rain, a 37% 

increase in the number of combined sewage overflows 

in spring was projected, resulting in an overall 20% 

increase from the baseline in the volume of discharge 

each year.76 
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U.S. Global Change Research Program 165 Impacts of Climate Change on Human Health in the United States

surface temperatures, particularly in coastal waters, which generally have high levels of the dissolved organic carbon required 

for Vibrio growth. The distribution of species changes with salinity patterns related to sea level rise and to changes in delivery of 

freshwater to coastal waters, which is affected by flooding and 

drought. For instance, V. parahaeomolyticus and V. alginolyticus 

favor higher salinities while V. vulnificus favors more moderate 

salinities.95, 96, 97, 98, 99, 100

Harmful algal blooms caused by cyanobacteria were responsible 

for nearly half of all reported outbreaks in untreated recreational freshwater in 2009 and 2010, resulting in approximately 61 

illnesses (health effects included dermatologic, gastrointestinal, 

respiratory, and neurologic symptoms), primarily reported in 

children/young adults age 1–19.101 Cyanobacterial blooms are 

strongly influenced by rising temperatures, altered precipitation 

patterns, and changes in freshwater discharge or flushing rates 

of water bodies (Table 1).102, 103, 104, 105, 106, 107, 108 Higher temperatures (77°F and greater) favor surface-bloom-forming cyanobacteria over less harmful types of algae.109 In marine water, 

the toxins associated with harmful “red tide” blooms of Karenia 

brevis can aerosolize in water droplets through wind and wave 

action and cause acute respiratory illness and eye irritation in 

recreational beachgoers.110, 111 People with preexisting respiratory diseases, specifically asthma, are at increased risk of 

illness.112, 113 Prevailing winds and storms are important climate 

factors influencing the accumulation of K. brevis cells in the water.78, 114 For example, in 1996, Tropical Storm Josephine transported a Florida panhandle bloom as far west as Louisiana,115

the first documented occurrence of K. brevis in that state. 

Projected Changes

Overall, climate change will contribute to contamination 

of recreational waters and increased exposure to agents of 

water-related illness.10, 82, 116, 117, 118, 119, 120 Increases in flooding, 

coastal inundation, and nuisance flooding (linked to sea level 

rise and storm surge from changing patterns of coastal storms 

and hurricanes) will negatively affect coastal infrastructure and 

increase chances for pathogen contamination, especially in 

populated areas (see also Ch. 4: Extreme Events).70, 121 In areas 

where increasing temperatures lengthen the seasons for recreational swimming and other water activities, exposure risks are 

expected to increase.122, 123

As average temperatures rise, the seasonal and geographic 

range of suitable habitat for cyanobacterial species is projected 

to expand.124, 125, 126, 127, 128 For example, tropical and subtropical 

species like Cylindrospermopsis raciborskii, Anabaena spp., and 

Aphanizomenon spp. have already shown poleward expansion into mid-latitudes of Europe, North America, and South 

America.107, 129, 130 Increasing variability in precipitation patterns 

and more frequent and intense extreme precipitation events 

(which will increase nutrient loading) will also affect cyanobacterial communities. If such events are followed by extended 

drought periods, the stagnant, low-flow conditions accompanying droughts will favor cyanobacterial dominance and bloom 

formation.103, 131

In recreational waters, projected increases in sea surface 

temperatures are expected to lengthen the seasonal window 

of growth and expand geographic range of Vibrio species,96, 

132 although the certainty of regional projections is affected by 

underlying model structure.133 While the specific response of 

Vibrio and degree of growth may vary by species and locale, 

in general, longer seasons and expansion of Vibrio into areas 

where it had not previously been will increase the likelihood 

of exposure to Vibrio in recreational waters. Regional climate 

changes that affect coastal salinity (such as flooding, drought, 

and sea level rise) can also affect the population dynamics of 

these agents,97, 99, 134 with implications for human exposure risk. 

Increases in hurricane intensity and rainfall are projected as the 

climate continues to warm (see Ch 4: Extreme Events). Such 

increases may redistribute toxic blooms of K. brevis (“red tide” 

blooms) into new geographic locations, which would change 

human exposure risk in newly affected areas. 

Fish and Shellfish

Water-related contaminants as well as naturally occurring 

harmful bacteria and algae can be accumulated by fish or 

shellfish, providing a route of human exposure through 

consumption (see also Ch. 7: Food Safety).135, 136, 137 Shellfish, 

including oysters, are often consumed raw or very lightly 

cooked, which increases the potential for ingestion of an infectious pathogen.138

Pathogens Associated with Fish and Shellfish

Enteric viruses (for example, noroviruses and hepatitis A virus) 

found in sewage are the primary causes of gastrointestinal 

illness due to shellfish consumption.139, 140 Rainfall increases 

the load of contaminants associated with sewage delivered to 

shellfish harvesting waters and may also temporarily reduce 

salinity, which can increase persistence of many enteric bacteria and viruses.141, 142, 143, 144 Many enteric viruses also exhibit 

In areas where increasing temperatures lengthen recreational 

swimming seasons, exposure risks are expected to increase.
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seasonal patterns in infection rates and detection rates in the 

environment, which may be related to temperature.145, 146, 147

Among naturally occurring water-related pathogens, Vibrio 

vulnificus and V. parahaemolyticus are the species most often 

implicated in foodborne illness in the United States, accounting for more than 50% of reported shellfish-related illnesses 

annually.140, 148, 149, 150, 151 Cases have increased significantly since 

1996.92, 148 Rising sea surface temperatures have contributed 

to an expanded geographic and seasonal range in outbreaks 

associated with shellfish.96, 152, 153, 154, 155

Precipitation is expected to be the primary climate driver 

affecting enteric pathogen loading to shellfish harvesting areas, 

although temperature also affects bioaccumulation rates of 

enteric viruses in shellfish. There are currently no national projections for the associated risk of illness from shellfish consumption. Many local and state agencies have developed plans for 

closing shellfish beds in the event of threshold-exceeding rain 

events that lead to loading of these contaminants and deterioration of water quality.156

Importance: Vibrio species are naturally occurring pathogens in coastal environments that cause illnesses 

ranging from gastroenteritis to septicemia (bloodstream infection) and death from both water contact and 

ingestion of raw or undercooked seafood, especially shellfish.93 Vibrio are highly responsive to environmental 

conditions. For example, local 

nutrient availability can affect Vibrio

abundance, though coastal waters 

generally have sufficient levels of the 

dissolved organic carbon required for 

Vibrio growth.159

Over longer timescales and larger 

geographic areas, key climate-related 

factors that increase Vibrio growth 

and abundance include rising sea 

surface temperatures and changes 

in precipitation, freshwater runoff, 

drought, sea level rise, coastal 

flooding, and storm surge, with 

resulting changes to coastal salinity 

patterns, turbidity (water clarity), and 

plankton abundance and composition 

(see Figure 1).95, 96, 97, 98, 99, 100, 134, 160, 

161, 162, 163

Water temperature is a major 

contributor to Vibrio growth potential 

and, in turn, human exposure risk. 

The minimum water temperature 

threshold for the growth of most 

Vibrio species that cause illness in 

humans is 15°C (59°F), with growth 

rates increasing as temperature 

increases.132, 152, 154, 157 Thus, it is 

projected that global ocean warming 

will increase risk of exposure by 

extending seasonal windows of 

growth and geographical range of 

occurrence.132

Projections of Vibrio Occurrence and Abundance in 

Chesapeake Bay

Figure 4: Seasonal and decadal projections of abundance of V. parahaemolyticus

in oysters of Chesapeake Bay (top) and probability of occurrence of V. vulnificus

in Chesapeake Bay surface waters (bottom). The circles show average values in 

the baseline period (1985–2000) and future years averaged by decadal period: 

2030 (2025–2034), 2050 (2045–2054), and 2095 (2090–2099). (Figure source: 

adapted from Jacobs et al. 2015).132

Research Highlight: The Effect of Warming on Seasonal Vibrio Abundance and Distribution
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Research Highlight: The Effect of Warming on Seasonal Vibrio Abundance and Distribution, continued 

Objective: A quantitative projection of future shifts in Vibrio seasonal abundance and geographic range.

Method: Monthly average sea surface temperatures were projected for the 2030s, 2050s, and 2090s based 

on statistical downscaling of up to 21 global climate models for the Chesapeake Bay and Alaskan coastline. 

Previously published empirical models relating sea surface temperature and salinity to Vibrio vulnificus and 

V. parahaemolyticus were used to project probability of occurrence and abundance in Chesapeake Bay waters 

and oysters. Geographic information system (GIS) mapping of Alaskan coastal waters was used to project the 

distribution of monthly 

average water temperatures 

exceeding 15°C (59°F), 

considered to be the 

minimum temperature 

favorable for growth.132

Results and Conclusions: 

Modeling results find 

increases in abundance, 

geographical range, and 

seasonal extent of available 

habitat for Vibrio. In the 

Chesapeake Bay, the 

probability of occurrence of 

V. vulnificus is projected to 

increase by nearly 16% in 

the shoulder months of the 

growing season (May and 

September), with a similar 

increase in abundance of V. 

parahaemolyticus in oysters 

(Figure 4). 

Analysis of temperature 

projections for Alaskan coastal 

waters based on an average of 

four climate models showed 

that habitat availability for 

Vibrio growth will increase to 

nearly 60% of the Alaskan 

shoreline in August by the 

2090s (Figure 5). 

Sources of uncertainty include 

different rates of warming 

associated with each model 

ensemble and other factors 

that affect growth and 

abundance, but all models 

used in this study project 

warming of coastal waters.

Changes in Suitable Coastal Vibrio Habitat in Alaska 

Figure 5: Vibrio growth increases in temperatures above 15°C (59°F). These maps show 

the low and high end of the ranges for projected area of Alaskan coastline with water 

temperature averages in August that are greater than this threshold. The projections were 

made for the following future time periods: 2030 (2026–2035), 2050 (2046–2055), and 

2090 (2086–2095). On average, the models project that by 2090, nearly 60% of the 

Alaskan shoreline in August will become suitable Vibrio habitat. (Figure source: adapted 

from Jacobs et al. 2015)132
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Increases in sea surface temperatures, changes in precipitation 

and freshwater delivery to coastal waters, and sea level rise will 

continue to affect Vibrio growth and are expected to increase 

human exposure risk.96, 134, 152, 157 Regional models project increased abundance and extended seasonal windows of growth 

of Vibrio pathogens (see Research Highlight on page 166).132

The magnitude of health impacts depends on the use of intervention strategies and on public and physician awareness.158

Harmful Algal Toxins

Harmful algal blooms (HABs) that contaminate seafood with 

toxins are becoming increasingly frequent and persistent in 

coastal marine waters, and some have expanded into new 

geographic locations.164, 165, 166, 167, 168 Attribution of this trend 

has been complicated for some species, with evidence to 

suggest that human-induced changes (such as ballast water 

exchange, aquaculture, nutrient loading to coastal waters, and 

climate change) have contributed to this expansion.167, 169

Among HABs associated with 

seafood, ciguatera fish poisoning 

(CFP) is most strongly influenced by climate.170, 171, 172 CFP 

is caused by toxins produced by 

the benthic algae Gambierdiscus

(Table 1) and is the most frequently reported fish poisoning 

in humans.173 There is a well-established link between warm 

sea surface temperatures and increased occurrences of CFP,170, 

171, 172 and in some cases, increases have also been linked to El 

Niño–Southern Oscillation events.174 The frequency of tropical 

cyclones in the United States has also been associated with 

CFP, but with an 18-month lag period associated with the time 

required for a new Gambierdiscus habitat to develop.170, 171

Paralytic shellfish poisoning (PSP) is the most globally widespread shellfish poisoning associated with algal toxins,175

and records of PSP toxins in shellfish tissues (an indicator of 

toxin-producing species of Alexandrium) provide the longest 

time series in the United States for evaluating climate impacts. 

Warm phases of the naturally occurring climate pattern known 

as the Pacific Decadal Oscillation co-occur with increased 

PSP toxins in Puget Sound shellfish on decadal timescales.176

Further, it is very likely that the 20th century warming trend 

also contributed to the observed increase in shellfish toxicity 

since the 1950s.177, 178 Warm spring temperatures also contributed to a bloom of Alexandrium in a coastal New York estuary 

in 2008.179 Decadal patterns in PSP toxins in Gulf of Maine 

shellfish show no clear relationships with long-term trends in 

climate,180, 181, 182 but ocean–climate interactions and changing 

oceanographic conditions are important factors for understanding Alexandrium bloom dynamics in this region.183

There is less agreement on the extent of climate impacts 

on other marine HAB-related diseases in the United States. 

Increased abundances of Pseudo-nitzschia species, which 

can cause amnesic shellfish poisoning, have been attributed 

to nutrient enrichment in the Gulf of Mexico.184 On the U.S. 

West Coast, increased abundances of at least some species of 

Pseudo-nitzschia occur during warm phases associated with El 

Niño events.185 For Dinophysis species that can cause diarrhetic shellfish poisoning, data records are too short to evaluate 

potential relationships with climate in the United States,164, 186

but studies in Sweden have found relationships with natural 

climate oscillations.187

The projected impacts of climate change on toxic marine 

harmful algae include geographic range changes in both 

warm- and cold-water species, changes in abundance and 

toxicity, and changes in the timing of the seasonal window 

of growth.188, 189, 190, 191 These impacts will likely result from 

climate change related impacts on one or more of 1) water 

temperatures, 2) salinities, 3) 

enhanced surface stratification, 

4) nutrient availability and supply to coastal waters (upwelling 

and freshwater runoff), and 

5) altered winds and ocean 

currents.188, 190, 191, 192, 193

Limited understanding of the 

interactions among climate and non-climate stressors and, in 

some cases, limitations in the design of experiments for investigating decadal- or century-scale trends in phytoplankton 

communities, makes forecasting the direction and magnitude 

of change in toxic marine HABs challenging.189, 191 Still, changes 

to the community composition of marine microalgae, including harmful species, will occur.188, 194 Conditions for the growth 

of dinoflagellates—the algal group containing numerous toxic 

species—could potentially be increasingly favorable with 

climate change because these species possess certain physiological characteristics that allow them to take advantage of 

climatically-driven changes in the structure of the ocean (for 

example, stronger vertical stratification and reduced turbulence).190, 193, 195, 196, 197

Climate change, especially continued warming, will dramatically increase the burden of some marine HAB-related diseases in some parts of the United States, with strong implications 

for disease surveillance and public health preparedness. For 

example, the projected 4.5°F to 6.3°F increase in sea surface 

temperature in the Caribbean over the coming century is 

expected to increase the incidence of ciguatera fish poisoning 

by 200% to 400%.171 In Puget Sound, warming is projected 

to increase the seasonal window of growth for Alexandrium 

by approximately 30 days by 2040, allowing blooms to begin 

earlier in the year and persist for longer.177, 190, 198

Climate change, especially continued warming, 

will dramatically increase the burden of some 

marine HAB-related diseases in some parts 

of the United States
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U.S. Global Change Research Program 169 Impacts of Climate Change on Human Health in the United States

Research Highlight: Increased Risk of Ciguatera Fish Poisoning (CFP)

Importance: Ciguatera fish poisoning is caused by consumption of fish contaminated with toxins produced by 

dinoflagellates, such as those of the genus Gambierdiscus. There is a well-established link between warm sea 

surface temperatures and increased occurrence of CFP,171 and thus concern that global ocean warming will 

affect the risk of illness.

Objective: A quantitative projection of future shifts in species of Gambierdiscus. 

Method: Growth models developed for three Caribbean species of Gambierdiscus were run using 11 global 

climate model projections for specific buoy locations in the western Gulf of Mexico, Yucatan channel, and 

eastern Caribbean Sea through 2099. For further detail, see Kibler et al. 2015.199

Results and Conclusions: Modeling results suggest substantial changes in dominant species composition (Figure 

6). Lower thermal tolerances of some species may result in geographic range shifts to more northern latitudes, 

particularly from the Yucatan and eastern Caribbean Sea. The projected shift in distribution is likely to mean 

that dominant CFP toxins enter the marine food web through different species, with increases of toxins in new 

areas where waters are warming and potential decreases in existing areas where waters are warming less rapidly.

Figure 6: Water temperature data from 1990–2013 were collected or reconstructed for buoy sites in the western 

Gulf of Mexico, Yucatan channel, and eastern Caribbean Sea. These data were then used in calculations to 

project average annual water temperature and average growth rates for three Caribbean Gambierdiscus species 

(G. caribaeus, G. belizeanus, G. carolinianus) for the period 2014–2099. (Figure source: adapted from Kibler et 

al. 2015).199

Projected Changes in Caribbean Gambierdiscus Species
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infrastructure, and various environmental, political, economic, 

and social factors jointly create these disparities.201 

Children, older adults (primarily age 65 and older), pregnant 

women, and immunocompromised individuals have higher risk 

of gastrointestinal illness and severe health outcomes from contact with contaminated water.4, 209, 210, 211, 212, 213 Pregnant women 

who develop severe gastrointestinal illness are at high risk for 

adverse pregnancy outcomes (pregnancy loss and preterm 

birth).214 Because children swallow roughly twice as much 

water as adults while swimming, they have higher recreational 

exposure risk for both pathogens and freshwater HABs.101, 120

Recent cryptosporidiosis and giardiasis cases have frequently 

been reported in children aged one to nine years, with onset of 

illness peaking during the summer months.215 In addition, 40% 

Research Highlight: Expanded Seasonal Windows for Harmful Algal Blooms

Importance: When some harmful algae in the 

genus Alexandrium bloom, toxins that can 

accumulate in shellfish are produced. When 

these shellfish are consumed, gastrointestinal 

illness and neurological symptoms, known 

as paralytic shellfish poisoning (PSP), can 

occur. Death can result in extreme cases. 

Because growth of Alexandrium is regulated 

in part by water temperature, warm water 

conditions appropriate for bloom formation 

may expand seasonally, increasing the risk of 

illness.

Objective: A quantitative projection of future 

conditions appropriate for Alexandrium bloom 

formation in Puget Sound. 

Method: Monthly average sea surface 

temperature was projected for Quartermaster 

Harbor, Puget Sound, for the 2030s, 2050s, 

and 2090s based on statistical downscaling 

of 21 global climate models. The projections 

were applied to previously published 

empirical models relating temperature and 

salinity to Alexandrium growth. For more 

detail, see Jacobs et al. 2015.132

Results and Conclusions: Modeling results indicate that Alexandrium blooms could develop up to two months earlier 

in the year and persist for up to two months longer by 2100 compared to the present day (Figure 7). All model 

projections indicate that the bloom season will expand by at least one month on either side of the present-day bloom 

season by 2100. Therefore, it is likely that the risk of Alexandrium blooms that can contaminate shellfish with potent 

toxins will increase. This may increase the risk of human exposure to the toxins, which can cause paralytic shellfish 

poisoning. Sources of uncertainty include different rates of warming associated with each model ensemble and other 

factors that affect growth and abundance, but all models used project warming of coastal waters.

Figure 7: Seasonal and decadal projections of growth of Alexandrium

in Puget Sound. The circles show average values in the baseline period 

(2006–2013) and future years averaged by decadal period: 2030 (2025–

2035), 2050 (2045–2055), and 2095 (2090–2099). Growth rate values 

above 0.25μd-1 constitute a bloom of Alexandrium (Figure source: adapted 

from Jacobs et al. 2015)132

Projections of Growth of Alexandrium in Puget Sound

6.4 Populations of Concern

Climate change impacts on the drinking water exposure pathway (see page 163) will act as an additional stressor on top of 

existing exposure disparities in the United States. Lack of consistent access to potable drinking water and inequities in exposure 

to contaminated water disproportionately affects the following 

populations: tribes and Alaska Natives, especially those in 

remote reservations or villages; residents of low-income rural 

subdivisions known as colonias along the U.S.–Mexico border; 

migrant farm workers; the homeless; and low-income communities not served by public water utilities—which can be urban, 

suburban, or rural, and some of which are predominately Hispanic or Latino and Black or African American communities in 

certain regions of the country.200, 201, 202, 203, 204, 205, 206, 207, 208 In general, the heightened vulnerability of these populations primarily results from unequal access to adequate water and sewer 
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of swimming-related eye and ear infections from Vibrio alginolyticus during the period 1997−2006 were reported in children 

(median age of 15).93

Traditional tribal consumption of fish and shellfish in the Pacific Northwest and Alaska can be on average 3 to 10 times higher than that of average U.S. consumers, or even up to 20 times 

higher.216 Climate change will contribute to increased seafood 

contamination by toxins and potentially by chemical contaminants (see "6.5 Emerging Issues" below), with potential health 

risks and cultural implications for tribal communities. Those 

who continue to consume traditional diets may face increased 

health risks from contamination.217 Alternatively, replacing 

these traditional nutrition sources may involve consuming less 

nutritious processed foods and the loss of cultural practices 

tied to fish and shellfish harvest.218, 219

6.5 Emerging Issues

A key emerging issue is the impact of climate on new and 

re-emerging pathogens. While cases of nearly-always-fatal 

primary amoebic meningoencephalitis due to the amoeba 

Naegleria fowleri and other related species remain relatively 

uncommon, a northward expansion of cases has been observed in the last five years.220, 221 Evidence suggests that in 

addition to detection in source water (ground and surface 

waters), these amoebae may be harbored in biofilms associated with water distribution systems, where increased temperatures decrease efficacy of chlorine disinfection and support 

survival and potentially growth.222, 223, 224

Climate change may also alter the patterns or magnitude of 

chemical contamination of seafood, leading to altered effects 

on human health—most of which are chronic conditions. 

Rising temperatures and reduced ice cover are already linked 

to increasing burdens of mercury and organohalogens in arctic 

fish,225 a sign of increasing contamination of the arctic food 

chain. Changes in hydrology resulting from climate change 

are expected to alter releases of chemical contaminants into 

the Nation’s surface waters,226 with as-yet-unknown effects on 

seafood contamination. 

6.6 Research Needs

In addition to those identified in the emerging issues discussion above, the authors highlight the following potential areas 

for additional scientific and research activity on water-related illness, based on their review of the literature. Enhanced 

understanding of climate change impacts will be facilitated by 

improved public health surveillance for water-related infectious diseases and expanded monitoring and surveillance 

of surface and coastal water quality. In addition, improved 

understanding of how human behaviors affect the risk of waterborne diseases can facilitate the development of predictive 

models and effective adaptation measures. Predictive models 

can also help identify major areas of uncertainty and refine 

key research questions.

Future assessments can benefit from research activities that

• assess the interactions among climate drivers, ecosystem 

changes, water quality and infectious pathogens, including 

Vibrio spp., N. fowlerii, chemical contaminants, and harmful 

algal blooms;

• increase understanding of how marine and terrestrial wildlife, including waterfowl, contribute to the distribution of 

pathogens and transmission of infectious disease and assess 

the role of climate;

• explore how ocean acidification affects toxin production and 

distribution of marine HABs and pathogens;

• analyze the hydrologic (discharge, flow-residence time, and 

mixing) thresholds for predicting HAB occurrences; and

• increase understanding of how the impacts of climate 

change on drinking water infrastructure, including the need 

for development of new and emerging technologies for 

provision of drinking water, affect the risks of waterborne 

diseases.

Water-related contamination of shellfish may reduce 

consumption and contribute to loss of tribal cultural practices 

tied to shellfish harvest.
U.S. Global Change Research Program 172 Impacts of Climate Change on Human Health in the United States

Supporting Evidence

PROCESS FOR DEVELOPING CHAPTER

The chapter was developed through technical discussions 

of relevant evidence and expert deliberation by the report 

authors at several workshops, teleconferences, and email 

exchanges. Authors considered inputs and comments 

submitted by the public, the National Academies of Sciences, 

and Federal agencies. For additional information on the 

overall report process, see Appendices 2 and 3.

Many water-related illnesses are of critical importance 

globally, such as cholera and hepatitis E virus, and they affect 

U.S. interests abroad, but the focus of this chapter is to 

address climate impacts on water-related illnesses of primary 

importance to human health within the United States. In 

addition, although climate change has the potential to impact 

national as well as global seafood supplies, this chapter does 

not cover these types of impacts because the peer-reviewed 

literature is not yet robust enough to make connections to 

human health outcomes in the United States. Even with those 

constraints, the impacts of climate on water-related illness are 

regionally or locally specific and may include increased risks 

as well as benefits. For example, the projected geographic 

range shifts of some Gambieridiscus species to more northern 

latitudes may mean that dominant ciguatera fish poisoning 

toxins enter the marine food web through different species, 

with increases of toxins in new areas where waters are 

warming and potential decreases in areas such as the Yucatan 

and eastern Caribbean Sea.199 

KEY FINDING TRACEABLE ACCOUNTS

Seasonal and Geographic Changes in Waterborne 

Illness Risk

Key Finding 1: Increases in water temperatures associated 

with climate change will alter the seasonal windows of 

growth and the geographic range of suitable habitat for 

freshwater toxin-producing harmful algae [Very Likely, High 

Confidence], certain naturally occurring Vibrio bacteria [Very 

Likely, Medium Confidence], and marine toxin-producing 

harmful algae [Likely, Medium Confidence]. These changes 

will increase the risk of exposure to waterborne pathogens 

and algal toxins that can cause a variety of illnesses [Medium 

Confidence].

Description of evidence base

Vibrio, a genus of naturally occurring waterborne pathogens, 

thrives in water temperatures above a 15°C/59°F threshold.132, 

152, 154, 157 Rising sea surface temperatures have contributed to 

an expanded geographic and seasonal range in outbreaks of 

human illness associated with Vibrio in shellfish.96, 152, 153, 154, 

155 In recreational waters, projected increases in sea surface 

temperatures are expected to lengthen the seasonal window 

of growth and expand geographic range of Vibrio.

96, 132 Like 

other heterotrophic bacteria, growth of Vibrio is ultimately 

limited by availability of carbon substrate, though the coastal 

areas where Vibrio exposure is most likely, either through 

recreation or consumption of shellfish, generally have 

sufficient dissolved organic carbon.159 Reported rates of all 

Vibrio infections have tripled since 1996 in the United States, 

with V. alginolyticus infections having increased by 40-fold.92

Increasing sea surface temperatures, changes in precipitation 

and freshwater delivery to coastal waters, and sea level rise 

will continue to affect Vibrio growth and are expected to 

increase human exposure.96, 134, 152, 157

Most harmful algae, including freshwater cyanobacteria that 

can contaminate drinking water and marine dinoflagellate 

species that can contaminate fish and shellfish with natural 

toxins, thrive during the warm summer season or when water 

temperatures are higher than usual. As the climate continues 

to warm, water temperatures will rise above thresholds that 

promote bloom development earlier in the spring and will 

persist longer into the fall and expand into higher latitudes. 

This will result in a longer seasonal window and expanded 

geographic range for human exposure into higher latitudes.124, 

125, 126, 127, 128, 188, 189, 190, 191, 192, 193 Climate change, especially 

continued warming, will increase the burden of some marine 

HAB-related diseases, particularly ciguatera fish poisoning, in 

some regions of the United States. 

Major uncertainties

Uncertainty remains regarding the relative importance of 

additional factors that may also act on naturally occurring 

pathogens and harmful algae at local or regional levels to 

influence their growth, distribution, and toxicity. In many 

cases, it is uncertain how these multiple factors may interact 

with each other to influence the seasonal windows and 

geographic range for pathogens and harmful algae, especially 

in dynamic coastal marine environments. For example, 

changes in salinity, competition with other plankton, and 

presence of viruses or other organisms that consume 

plankton or bacteria can affect abundance.162, 163 Changing 

distribution patterns for some marine species of harmful 

algae is not well understood and some regions may become 

too warm for certain species of harmful algae to grow, 

shifting (without changing in total size) or even shrinking their 

geographic range.

Additionally, there are limited studies on projections for 

changes in illness rates due to naturally occurring waterborne 

pathogens and harmful algae. Uncertainty remains regarding 

appropriate methods for projecting changes in illness rates, 

including how to integrate considerations of human behavior 

into modeling (current methods to assess exposure risk 
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U.S. Global Change Research Program 173 Impacts of Climate Change on Human Health in the United States

assume similar human behavior across time scales and 

geography). Methodological challenges are related to 1) 

underreporting and underdiagnosis of cases that affect the 

accuracy of baseline estimates of illness, 2) ability to project 

changes in strain virulence, 3) accounting for the effects of 

potential adaptation strategies/public health interventions 

(for example, public service announcements on how to avoid 

exposure), and 4) accounting for changes in public healthcare 

infrastructure and access that can reduce the risk of exposure 

or illness/death if exposed.

Assessment of confidence and likelihood based on evidence

Based on the evidence, there is medium confidence 

that, with changing climate, the annual seasonal and the 

geographic range for Vibrio and certain marine harmful algae 

will expand. The assessment of medium confidence is due to 

less certainty from modeling results regarding the magnitude 

of projected changes in abundance. The conclusions were 

deemed very likely to occur for Vibrio and likely for marine 

harmful algae based on good levels of agreement found in the 

published quantitative modeling projections for both Vibrio

and marine harmful algae (Alexandrium and Gambieridiscus) 

cited above. This conclusion takes into consideration that 

for some marine algae (for example, Gambieridiscus), lower 

latitudes may become too warm and risk may decline in 

those areas as it increases at higher latitudes. For freshwater 

harmful algae, there is high confidence that annual season 

and geographic range will expand with changing climate, 

which will also prolong the time for exposure and the 

potential for public health impacts. Consistent and highquality evidence from a limited number of laboratory 

studies, modeling efforts, field surveys, and comparisons 

of historic and contemporary conditions support this 

assessment. The conclusion was deemed very likely to occur 

for freshwater harmful algae with high confidence based 

on laboratory studies and field observations, as well as a 

greater fundamental understanding of inland hydrodynamics 

and bloom ecology as indicated in the literature cited in the 

chapter. There is medium confidence regarding increased risk 

to human health from a longer potential time for exposure 

to waterborne pathogens and algal toxins and potential 

exposure for a wider (or novel) population. This confidence 

level was chosen due to less certainty stemming from a 

relative lack of quantitative data and projections for future 

illness rates in the peer-reviewed literature.

Runoff from Extreme Precipitation Increases Exposure 

Risk

Key Finding 2: Runoff from more frequent and intense 

extreme precipitation events will increasingly compromise 

recreational waters, shellfish harvesting waters, and

sources of drinking water through increased introduction 

of pathogens and prevalence of toxic algal blooms [High 

Confidence]. As a result, the risk of human exposure to agents 

of water-related illness will increase [Medium Confidence].

Description of evidence base

Extreme precipitation can mobilize pathogens, nutrients, 

and chemical contaminants from agricultural, wildlife, and 

urban sources. Waterborne illness and outbreaks from 

pathogens following heavy precipitation events have been 

well documented in multiple studies using both passive 

and active surveillance on a local and regional level.38, 39, 

40, 42, 43, 44, 45, 46, 47 Likewise, extreme precipitation events and 

subsequent increases in runoff are key climate factors 

that increase nutrient loading in freshwater and marine 

recreational waters, shellfish harvesting waters, and sources 

of drinking water, which in turn increases the likelihood 

of harmful cyanobacterial blooms that produce algal 

toxins.56 The drinking water treatment process can remove 

cyanobacterial blooms; however, efficacy of the treatment 

processes may vary from 60% to 99.9%. Ineffective treatment 

could compromise water quality and may lead to severe 

treatment disruption or treatment plant shutdown.53, 54, 55, 56

More frequent and intense extreme precipitation events are 

projected for many regions in the United States as climate 

changes. Consistent, high-quality evidence from multiple 

studies supports a finding that increased runoff and flooding 

events are expected to increase contamination of source 

waters (for drinking water supply) and surface waters used 

for recreation, which may increase people’s exposure to 

pathogens and algal toxins that cause illness.10, 52, 59, 70, 71, 72, 73, 

76, 82, 116, 117, 118, 119, 120 Other factors may modify these risks, such 

as increased air or water temperatures, residence time in the 

environment, lower water levels, or dilution. 

Major uncertainties

Changes in exposure and risk are attributable to many factors 

in addition to climate. While extreme precipitation and 

flooding events introduce contaminants and pathogens to 

water to varying degrees depending on the characteristics 

of each individual event, they may not always result in 

increases in exposure due to planning and adaptive actions. 

There are limited studies on actual projections for changes 

in illness rates due to increasing frequency or intensity 

of extreme precipitation events. Uncertainty remains 

regarding appropriate methods for projecting changes in 

illness rates, including how to integrate considerations of 

human behavior into modeling (current methods to assess 

exposure risk assume similar human behavior across time 

scales and geography). Methodological challenges are related 

to 1) baseline case reporting issues (underreporting and 

underdiagnosis), 2) accounting for the effects of potential 

adaptation strategies/public health interventions (for 

example, public service announcements about how to avoid 

exposure), and 3) accounting for changes in public healthcare 

infrastructure and access that can reduce the risk of exposure 

or of illness/death if exposed.
6–CLIMATE IMPACTS ON WATER-RELATED ILLNESS

U.S. Global Change Research Program 174 Impacts of Climate Change on Human Health in the United States

Assessment of confidence and likelihood based on evidence

Based on the evidence, there is high confidence that 

increasing frequency or intensity of extreme precipitation 

events will compromise recreational waters and sources 

of drinking water with pathogens, nutrients, and chemical 

contaminants from agricultural, wildlife, and urban sources.

There is consistent qualitative evidence that flooding 

associated with extreme precipitation events and storm 

surge results in loading of pathogens and nutrients to surface 

and groundwater (and drinking water distribution systems) 

through stormwater runoff and sewage overflows. However, 

other human and social factors modify risk, and there are 

no national-level studies upon which to draw conclusions 

regarding quantitative projections of increased exposure. 

Thus, the limited number of studies supports a medium

confidence level that human exposure risk will increase due 

to changes in extreme events.

Water Infrastructure Failure 

Key Finding 3: Increases in some extreme weather events 

and storm surges will increase the risk that infrastructure for 

drinking water, wastewater, and stormwater will fail due to 

either damage or exceedance of system capacity, especially in 

areas with aging infrastructure [High Confidence]. As a result, 

the risk of exposure to water-related pathogens, chemicals, 

and algal toxins will increase in recreational and shellfish 

harvesting waters and in drinking water where treatment 

barriers break down [Medium Confidence]. 

Description of evidence base

Water infrastructure in the United States is aging and may be 

inadequate or deteriorating. Combined sewers in many older 

cities were not designed to handle extreme precipitation 

events that are becoming more frequent with climate 

change. Multiple studies provide consistent, high-quality 

evidence that these systems are at risk of being overwhelmed 

during flood events or may be further damaged during 

other extreme weather events (e.g., storm surge), allowing 

contaminated surface water to run off into drinking water 

and recreational water sources.10, 52, 59, 70, 76, 116 Drinking water 

source contamination may be exacerbated or insufficiently 

addressed by treatment processes at the plant or the 

distribution system. Drinking water treatment plants may be 

challenged by high pathogen loads and toxic cyanobacterial 

bloom events.52, 55, 56 Multiple studies support a finding that 

climate change will place additional stresses on the capacity 

of drinking water treatment facilities and may increase the 

risk that water infrastructure, especially aging infrastructure, 

will fail through either damage or exceedance of system 

capacity.6, 70, 74, 75 

Major uncertainties

The human health consequences of aging water infrastructure 

failure depend not only on the local and regional climate 

factors that contribute to damage or capacity challenges 

but also the nature of the system and the pressures on it, 

the population affected, and the timeliness and adequacy of 

the response—all of which are inherently local or regional 

factors. Due to the complicated local and regional specificity, 

there are no national projections of the human health 

impact of water infrastructure failure. Uncertainty remains 

regarding appropriate methods for projecting changes in 

illness rates, including how to integrate considerations of 

human behavior into modeling (current methods to assess 

exposure risk assume similar human behavior across time 

scales and geography). Methodological challenges are related 

to 1) baseline case reporting issues (underreporting and 

underdiagnosis), 2) accounting for the effects of potential 

adaptation strategies/public health interventions (for 

example, mitigating risk with improvements to current water 

and sewerage systems), and 3) accounting for changes in 

public healthcare infrastructure and access that can reduce 

the risk of exposure or of illness/death if exposed.

Assessment of confidence based on evidence

Based on the evidence found in the peer-reviewed literature, 

there is high confidence that the anticipated climate change 

related increases in some extreme weather events and in 

storm surge will increase the risk that water infrastructure for 

drinking water, wastewater, and stormwater will fail through 

either damage or exceedance of system capacity, with aging 

infrastructure being particularly vulnerable. Evidence shows 

contamination to or from these systems occurs with heavy 

precipitation and other extreme weather events. There is 

consistent qualitative evidence suggesting that projected 

climate change effects on extreme weather patterns—

particularly extreme precipitation and storm surge—can 

adversely affect water infrastructure and lead to increased 

loading of pathogens, algal toxins, and contaminants. 

However, there are no national-level studies upon which 

to draw conclusions regarding quantitative projections of 

increased exposure. Thus, the limited number of studies 

supports a medium confidence level regarding risk of 

exposure.
6–CLIMATE IMPACTS ON WATER-RELATED ILLNESS

U.S. Global Change Research Program 175 Impacts of Climate Change on Human Health in the United States

DOCUMENTING UNCERTAINTY

This assessment relies on two metrics to communicate 

the degree of certainty in Key Findings. See Appendix 4: 

Documenting Uncertainty for more on assessments of 

likelihood and confidence.

PHOTO CREDITS

Pg. 157–Hands cupping water: © iStockPhotos.com/

jacky9946 

Pg. 158–Young women walking through floodwater: 

© Richard Ellis/Corbis

Pg. 163–Heavy rain: © iStockPhoto.com/AndreasWeber 

Pg. 165–Family jumping in lake: © Juice Images/Corbis

Pg. 171–Razor clam dig: Courtesy of Vera Trainer/NOAA 

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few sources, limited consistency, models 

incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts

Likelihood

Very Likely

≥ 9 in 10

Likely

≥ 2 in 3

As Likely As Not

≈ 1 in 2

Unlikely

≤ 1 in 3

Very Unlikely

≤ 1 in 10
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Future Generations, T.A. Mitchell, J.L. Donatuto, G.W. 

Burns, T.M. Burbacher, and E.M. Faustman, 2005: Framing scientific analyses for risk management of environmental 

hazards by communities: Case studies with seafood safety 

issues. Environmental Health Perspectives, 113, 1502-1508. 

http://dx.doi.org/10.1289/ehp.7655

217. Donatuto, J.L., T.A. Satterfield, and R. Gregory, 2011: Poisoning the body to nourish the soul: Prioritising health risks 

and impacts in a Native American community. Health, Risk 

& Society, 13, 103-127. http://dx.doi.org/10.1080/136985

75.2011.556186

218. Lefebvre, K.A. and A. Robertson, 2010: Domoic acid and 

human exposure risks: A review. Toxicon, 56, 218-230. 

http://dx.doi.org/10.1016/j.toxicon.2009.05.034

219. Lewitus, A.J., R.A. Horner, D.A. Caron, E. Garcia-Mendoza, B.M. Hickey, M. Hunter, D.D. Huppert, R.M. Kudela, 

G.W. Langlois, J.L. Largier, E.J. Lessard, R. RaLonde, J.E.J. 

Rensel, P.G. Strutton, V.L. Trainer, and J.F. Tweddle, 2012: 

Harmful algal blooms along the North American west coast 

region: History, trends, causes, and impacts. Harmful Algae,

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Visvesvara, R.C. Ratard, V.R. Hill, J.D. Wilson, A.J. Linscott, R. Crager, N.A. Kozak, R. Sriram, J. Narayanan, 

B. Mull, A.M. Kahler, C. Schneeberger, A.J. da Silva, M. 

Poudel, K.L. Baumgarten, L. Xiao, and M.J. Beach, 2012: 

Primary amebic meningoencephalitis deaths associated with 

sinus irrigation using contaminated tap water. Clinical Infectious Diseases, 55, e79-e85. http://dx.doi.org/10.1093/cid/

cis626

221. Kemble, S.K., R. Lynfield, A.S. DeVries, D.M. Drehner, 

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V.R. Hill, J.S. Yoder, L. Xiao, K.E. Smith, and R. Danila, 

2012: Fatal Naegleria fowleri infection acquired in Minnesota: Possible expanded range of a deadly thermophilic organism. Clinical Infectious Diseases, 54, 805-809. http://dx.doi.

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222. Goudot, S., P. Herbelin, L. Mathieu, S. Soreau, S. Banas, 

and F. Jorand, 2012: Growth dynamic of Naegleria fowleri 

in a microbial freshwater biofilm. Water Research, 46, 3958-

3966. http://dx.doi.org/10.1016/j.watres.2012.05.030

223. Puzon, G.J., J.A. Lancaster, J.T. Wylie, and J.J. Plumb, 

2009: Rapid detection of Naegleria fowleri in water distribution pipeline biofilms and drinking water supplies. Environmental Science & Technology, 43, 6691-6696. http://dx.

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224. Cope, J.R., R.C. Ratard, V.R. Hill, T. Sokol, J.J. Causey, J.S. 

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M. Doucet, Y. Qvarnstrom, C.N. Poole, O.A. Akingbola, J. 

Ritter, Z. Xiong, A. da Silva, D. Roellig, R. Van Dyke, H. 

Stern, L. Xiao, and M.J. Beach, 2015: First association of 

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225. Carrie, J., F. Wang, H. Sanei, R.W. Macdonald, P.M. Outridge, and G.A. Stern, 2010: Increasing contaminant burdens in an arctic fish, Burbot (Lota lota), in a warming 

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226. Balbus, J.M., A.B. Boxall, R.A. Fenske, T.E. McKone, and 

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etc.2046
End

© Richard Ellis/Corbis
FOOD SAFETY, NUTRITION, 7 AND DISTRIBUTION

THE IMPACTS OF CLIMATE CHANGE ON HUMAN HEALTH IN THE UNITED STATES

A Scientific Assessment

U.S. Global Change Research Program

Lead Authors

Lewis Ziska

U.S. Department of Agriculture 

Allison Crimmins*

U.S. Environmental Protection Agency

Contributing Authors

Allan Auclair

U.S. Department of Agriculture

Stacey DeGrasse

U.S. Food and Drug Administration

Jada F. Garofalo

Centers for Disease Control and Prevention

Ali S. Khan

University of Nebraska Medical Center

Irakli Loladze

Bryan College of Health Sciences

Adalberto A. Pérez de León

U.S. Department of Agriculture

Allan Showler

U.S. Department of Agriculture

Jeanette Thurston

U.S. Department of Agriculture

Isabel Walls

U.S. Department of Agriculture

Recommended Citation: Ziska, L., A. Crimmins, A. Auclair, S. DeGrasse, J.F. Garofalo, A.S. Khan, I. Loladze, A.A. Pérez de León, A. 

Showler, J. Thurston, and I. Walls, 2016: Ch. 7: Food Safety, Nutrition, and Distribution. The Impacts of Climate Change on Human 

Health in the United States: A Scientific Assessment. U.S. Global Change Research Program, Washington, DC, 189–216. http://

dx.doi.org/10.7930/J0ZP4417

On the web: health2016.globalchange.gov *Chapter Coordinators

Acknowledgements: Steve Gendel, Formerly of the U.S. Food and Drug Administration
U.S. Global Change Research Program 190 Impacts of Climate Change on Human Health in the United States

FOOD SAFETY, NUTRITION, 7 AND DISTRIBUTION

Key Findings

Increased Risk of Foodborne Illness

Key Finding 1: Climate change, including rising temperatures and changes in weather extremes, is expected 

to increase the exposure of food to certain pathogens and toxins [Likely, High Confidence]. This will increase 

the risk of negative health impacts [Likely, Medium Confidence], but actual incidence of foodborne illness 

will depend on the efficacy of practices that safeguard food in the United States [High Confidence]. 

Chemical Contaminants in the Food Chain

Key Finding 2: Climate change will increase human exposure to chemical contaminants in food through 

several pathways [Likely, Medium Confidence]. Elevated sea surface temperatures will lead to greater 

accumulation of mercury in seafood [Likely, Medium Confidence], while increases in extreme weather 

events will introduce contaminants into the food chain [Likely, Medium Confidence]. Rising carbon dioxide 

concentrations and climate change will alter incidence and distribution of pests, parasites, and microbes 

[Very Likely, High Confidence], leading to increases in the use of pesticides and veterinary drugs [Likely, 

Medium Confidence].

Rising Carbon Dioxide Lowers Nutritional Value of Food

Key Finding 3: The nutritional value of agriculturally important food crops, such as wheat and rice, will 

decrease as rising levels of atmospheric carbon dioxide continue to reduce the concentrations of protein and 

essential minerals in most plant species [Very Likely, High Confidence].

Extreme Weather Limits Access to Safe Foods

Key Finding 4: Increases in the frequency or intensity of some extreme weather events associated with 

climate change will increase disruptions of food distribution by damaging existing infrastructure or slowing 

food shipments [Likely, High Confidence]. These impediments lead to increased risk for food damage, 

spoilage, or contamination, which will limit availability of and access to safe and nutritious food depending 

on the extent of disruption and the resilience of food distribution infrastructure [Medium Confidence].
7–FOOD SAFETY, NUTRITION, AND DISTRIBUTION

U.S. Global Change Research Program 191 Impacts of Climate Change on Human Health in the United States

7.1 Introduction 

A safe and nutritious food supply is a vital component of food 

security. Food security, in a public health context, can be summarized as permanent access to a sufficient, safe, and nutritious food supply needed to maintain an active and healthy 

lifestyle.1

The impacts of climate change on food production, prices, and 

trade for the United States and globally have been widely examined, including in the U.S. Global Change Research Program 

(USGCRP) report, “Climate Change, Global Food Security, and 

the U.S. Food System,” in the most recent Intergovernmental 

Panel on Climate Change report, and elsewhere.1, 2, 3, 4, 5, 6, 7 An 

overall finding of the USGCRP report was that “climate change 

is very likely to affect global, regional, and local food security 

by disrupting food availability, decreasing access to food, and 

making utilization more difficult.”1

Farm to Table

The Potential Interactions of Rising CO2 and Climate Change on Food Safety and Nutrition

Figure 1: The food system involves a network of interactions with our physical and biological environments as 

food moves from production to consumption, or from “farm to table.” Rising CO2 and climate change will affect 

the quality and distribution of food, with subsequent effects on food safety and nutrition. 

This chapter focuses on some of the less reported aspects of 

food security, specifically, the impacts of climate change on 

food safety, nutrition, and distribution in the context of human 

health in the United States. While ingestion of contaminated 

seafood is discussed in this chapter, details on the exposure 

pathways of water related pathogens (for example, through 

recreational or drinking water) are discussed in Chapter 6: 

Water-Related Illness.

Systems and processes related to food safety, nutrition, and 

production are inextricably linked to their physical and biological environment.5, 8 Although production is important, for most 

developed countries such as the United States, food shortages 

are uncommon; rather, nutritional quality and food safety are 

the primary health concerns.5, 9 Certain populations, such as 

the poor, children, and Indigenous populations, may be more 

vulnerable to climate impacts on food safety, nutrition, and 

distribution (see also Ch. 9: Populations of Concern). 
7–FOOD SAFETY, NUTRITION, AND DISTRIBUTION

U.S. Global Change Research Program 192 Impacts of Climate Change on Human Health in the United States

There are two overarching means by which increasing carbon 

dioxide (CO2) and climate change alter safety, nutrition, and 

distribution of food. The first is associated with rising global 

temperatures and the subsequent changes in weather patterns 

and extreme climate events.13, 14, 15 Current and anticipated 

changes in climate and the physical environment have consequences for contamination, spoilage, and the disruption of 

food distribution. 

The second pathway is through the direct CO2 “fertilization” 

effect on plant photosynthesis. Higher concentrations of CO2

stimulate growth and carbohydrate production in some plants, 

but can lower the levels of protein and essential minerals in a 

number of widely consumed crops, including wheat, rice, and 

potatoes, with potentially negative implications for human 

nutrition.16

Figure 2: This conceptual diagram for a Salmonella example illustrates the key pathways by which humans are exposed to health 

threats from climate drivers, and potential resulting health outcomes (center boxes). These exposure pathways exist within the context 

of other factors that positively or negatively influence health outcomes (gray side boxes). Key factors that influence vulnerability for 

individuals are shown in the right box, and include social determinants of health and behavioral choices. Key factors that influence 

vulnerability at larger scales, such as natural and built environments, governance and management, and institutions, are shown in 

the left box. All of these influencing factors can affect an individual’s or a community’s vulnerability through changes in exposure, 

sensitivity, and adaptive capacity and may also be affected by climate change. See Ch. 1: Introduction for more information.

Climate Change and Health—Salmonella

Food Safety – Those conditions and measures 

necessary for food production, processing, 

storage, and distribution in order to ensure a safe, 

sound, wholesome product that is fit for human 

consumption.10

Foodborne Illness or Disease – Foodborne illness 

(sometimes called “food poisoning”) is a common 

public health problem. Each year, one in six 

Americans reports getting sick by consuming 

contaminated foods or beverages.11 Foodborne 

disease is caused by ingestion of contaminated 

food. Many different disease-causing microbes, or 

pathogens, can contaminate foods, so there are 

many different foodborne infections. In addition, 

food contaminated by toxins or chemicals can also 

result in foodborne illness.12

Terminology
7–FOOD SAFETY, NUTRITION, AND DISTRIBUTION

U.S. Global Change Research Program 193 Impacts of Climate Change on Human Health in the United States

7.2 Food Safety

Although the United States has one of the safest food supplies 

in the world,17 food safety remains an important public health 

issue. In the United States, the Centers for Disease Control 

and Prevention (CDC) estimate that there are 48 million cases 

of foodborne illnesses per year, with approximately 3,000 

deaths.12 As climate change drives changes in environmental 

variables such as ambient temperature, precipitation, and 

weather extremes (particularly flooding and drought), increases in foodborne illnesses are expected.18, 19

Most acute illnesses are caused by foodborne viruses (specifically noroviruses), followed by bacterial pathogens (such as 

Salmonella; see Table 1). Of the common foodborne illnesses 

in the United States, most deaths are caused by Salmonella, 

followed by the parasite Toxoplasma gondii.

20, 21, 22, 23 In addition, 

climate change impacts on the transport of chemical contaminants or accumulation of pesticides or heavy metals (such as 

mercury) in food, can also represent significant health threats 

in the food chain.22, 24, 25, 26, 27, 28

How Climate Affects Food Safety

Climate already influences food safety within an agricultural 

system—prior to, during, and after the harvest, and during 

transport, storage, preparation, and consumption. Changes 

in climate factors, such as temperature, precipitation, and extreme weather are key drivers of pathogen introduction, food 

contamination, and foodborne disease, as well as changes in 

the level of exposure to specific contaminants and chemical 

residues for crops and livestock.29, 30, 31

The impact of climate on food safety occurs through multiple 

pathways. Changes in air and water temperatures, weather-related changes, and extreme events can shift the seasonal and geographic occurrence of bacteria, viruses, pests, 

parasites, fungi, and other chemical contaminants.23, 30, 31, 32, 33

For example:

• Higher temperatures can increase the number of pathogens 

already present on produce34 and seafood.35, 36

Figure 3: A review of the published literature from 1960 to 2010 indicates a summertime peak in the incidence of illnesses associated 

with infection from a) Campylobacter, b) Salmonella, and c) Escherichia coli (E. coli). For these three pathogens, the monthly 

seasonality index shown here on the y-axis indicates the global disease incidence above or below the yearly average, which is 

denoted as 100. For example, a value of 145 for the month of July for Salmonellosis would mean that the proportion of cases for 

that month was 45% higher than the 12 month average. Unlike these three pathogens, incidence of norovirus, which can be attained 

through food, has a wintertime peak. The y-axis of the norovirus incidence graph (d) uses a different metric than (a–c): the monthly 

proportion of the annual sum of norovirus cases in the northern hemisphere between 1997 and 2011. For example, a value of 0.12 

for March would indicate that 12% of the annual cases occurred during that month). Solid line represents the average; confidence 

intervals (dashed lines) are plus and minus one standard deviation. (Figure sources: a, b, and c: adapted from Lal et al. 2012; d: 

Ahmed et al. 2013)49, 183

Seasonality of Human Illnesses Associated With Foodborne Pathogens 
7–FOOD SAFETY, NUTRITION, AND DISTRIBUTION

U.S. Global Change Research Program 194 Impacts of Climate Change on Human Health in the United States

Table 1. Foodborne Illness and Climate Change 

Foodborne Hazard Symptoms Estimated Annual 

Illness and Disease

Other Climate 

Drivers

Temperature/ 

Humidity 

Relationship

Norovirus Vomiting, non-bloody diarrhea with 

abdominal pain, nausea, aches, 

low grade fever

• 5,500,000 illnesses

• 15,000 hospitalizations 

• 150 deaths 

Extreme 

weather events 

(such as heavy 

precipitation and 

flooding)

Pathogens 

Favoring Colder/

Dryer 

Conditions

Pathogens 

Favoring Warmer/

Wetter Conditions

Listeria 

monocytogene

Fever, muscle aches, and rarely 

diarrhea. Intensive infection can 

lead to miscarriage, stillbirth, 

premature delivery, or lifethreatening infections (meningitis).

• 1,600 illnesses

• 1,500 hospitalizations

• 260 deaths 

Toxoplasma Minimal to mild illness with fever, 

serious illness in rare cases. 

Inflammation of the brain and 

infection of other organs, birth 

defects.

• 87,000 illnesses

• 4,400 hospitalizations 

• 330 deaths

Campylobacter Diarrhea, cramping, abdominal 

pain, nausea, and vomiting. 

In serious cases can be lifethreatening.

• 850,000 illnesses

• 8,500 hospitalizations

• 76 deaths

Changes in the 

timing or length 

of seasons, 

precipitation and 

flooding

Salmonella spp. 

(non typhoidal)

Diarrhea, fever, and abdominal 

cramps; in severe cases death.

• 1,000,000 illnesses 

• 19,000 hospitalizations

• 380 deaths 

Extreme weather 

events, changes 

in the timing or 

length of seasons

Vibrio vulnificus 

and 

parahaemolyticus

When ingested: watery diarrhea 

often with abdominal cramping, 

nausea, vomiting, fever and chills. 

Can cause liver disease. When 

exposed to an open wound: 

infection of the skin.

• 35,000 illnesses

• 190 hospitalizations 

• 40 deaths

Sea surface 

temperature, 

extreme weather 

events

Escherichia coli 

(E coli)

E. coli usually causes mild 

diarrhea. More severe pathogenic 

types, such as enterohemorrhagic 

E. Coli (EHEC), are associated with 

hemolytic uremic syndrome (a toxin 

causing destruction of red blood 

cells, leading to kidney failure).

• 200,000 illnesses 

• 2,400 hospitalizations

• 20 deaths

Extreme weather 

events, changes 

in the timing or 

length of seasons

Estimated annual number of foodborne illnesses and deaths in the United States. (Adapted from Scallan et al. 

2011; Akil et al. 2014; Kim et al. 2015; Lal et al. 2012)20, 48, 49, 80
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U.S. Global Change Research Program 195 Impacts of Climate Change on Human Health in the United States

• Bacterial populations can increase during food storage 

which, depending on time and temperature, can also increase food spoilage rates.37

• Sea surface temperature is directly related to seafood exposure to pathogens (see Ch. 6: Water-Related Illness).38, 39, 40

• Precipitation has been identified as a factor in the contamination of irrigation water and produce,30, 31, 33, 41 which has 

been linked to foodborne illness outbreaks.42, 43

• Extreme weather events like dust storms or flooding can 

introduce toxins to crops during development (see Ch. 4: 

Extreme Events).44

• Changing environmental conditions and soil properties may 

result in increases in the incidence of heavy metals in the 

food supply.45, 46, 47

Climate Impacts on Pathogen Prevalence

While climate change affects the prevalence of pathogens 

harmful to human health, the extent of exposure and resulting illness will depend on individual and institutional sensitivity and adaptive capacity, including human behavior and the 

effectiveness of food safety regulatory, surveillance, monitoring, and communication systems.

Rising Temperature and Humidity

Climate change will influence the fate, transport, transmission, 

viability, and multiplication rate of pathogens in the food chain. 

For example, increases in average global temperatures and 

humidity will lead to changes in the geographic range, seasonal 

occurrence, and survivability of certain pathogens.9, 48, 49, 50

Ongoing changes in temperature 

and humidity will not affect all 

foodborne pathogens equally (Table 1). The occurrence 

of some pathogens, such as 

Salmonella, Escherichia coli (E. 

coli), and Campylobacter, could 

increase with climate change because these pathogens thrive in warm, humid conditions. For 

example, Salmonella on raw chicken will double in number 

approximately every hour at 70°F, every 30 minutes at 80°F, 

and every 22 minutes at 90°F.51, 52

There is a summertime peak in the incidence of illnesses 

associated with these specific pathogens (see Figure 3).18, 48, 53, 

54 This peak may be related not only to warmer temperatures 

favoring pathogen growth but also to an increase in outdoor 

activities, such as barbecues and picnics. Risk for foodborne 

illness is higher when food is prepared outdoors where the 

safety controls that a kitchen provides—thermostat-controlled cooking, refrigeration, and washing facilities—are 

usually not available.5, 18, 19, 48, 55, 56

Norovirus, the most common cause of stomach flu, can be 

transmitted by consumption of contaminated food. Although 

norovirus generally has a winter seasonal peak (see Figure 

3), changing climate parameters, particularly temperature 

and rainfall, may influence its incidence and spread. Overall, 

localized climate impacts could improve health outcomes 

(fewer cases during warmer winters) or worsen them (elevated transmission during floods), such that projected trends in 

overall health outcomes for norovirus remain unclear.48, 57

Rising ocean temperatures can increase the risk of pathogen 

exposure from ingestion of contaminated seafood. For example, significantly warmer coastal waters in Alaska from 1997 

to 2004 were associated with an outbreak in 2004 of Vibrio

parahaemolyticus, a bacterium that causes gastrointestinal 

illnesses when contaminated seafood is ingested.58 Vibrio parahaemolyticus is one of the leading causes of seafood-related 

gastroenteritis in the United States and is associated with the 

consumption of raw oysters harvested from warm-water estuaries.59 Similarly, the emergence of a related bacterium, Vibrio 

vulnificus, may also be associated with high water temperatures.40 While increasing average water temperatures were 

implicated in a 2004 outbreak,58 ambient air temperature also 

affects pathogen levels of multiple species of Vibrio in shellfish.35, 36 For example, Vibrio vulnificus may increase 10- to 

100-fold when oysters are stored at ambient temperatures for 

ten hours before refrigeration.60 Increases in ambient ocean 

water and air temperatures would accelerate Vibrio growth 

in shellfish, potentially necessitating changes in post-harvest 

controls to minimize the increased risk of exposure. (For more 

information on Vibrio and other 

water-related pathogens, including contamination of recreational and drinking water, see Ch. 6: 

Water-Related Illness). 

Finally, climate change is 

projected to result in warmer 

winters, earlier springs, and an 

increase in the overall growing season in many regions.61, 62

While there are potential food production benefits from such 

changes, warmer and longer growing seasons could also alter 

the timing and occurrence of pathogen transmissions in food 

and the chance of human exposure.63, 64, 65

Extreme Events

In addition to the effects of increasing average temperature 

and humidity on pathogen survival and growth, increases in 

temperature and precipitation extremes can contribute to 

changes in pathogen transmission, multiplication, and survivability. More frequent and severe heavy rainfall events can inClimate change will influence the fate, 

transport, transmission, viability, and 

multiplication rate of pathogens in the

 food chain.
7–FOOD SAFETY, NUTRITION, AND DISTRIBUTION

U.S. Global Change Research Program 196 Impacts of Climate Change on Human Health in the United States

crease infection risk from most pathogens, particularly when it 

leads to flooding.66 Flooding, and other weather extremes, can 

increase the incidence and levels of pathogens in food production, harvesting, and processing environments. Groundwater 

and surface water used for irrigation, harvesting, and washing 

can be contaminated with runoff or flood waters that carry partially or untreated sewage, manure, or other wastes containing 

foodborne contaminants.55, 67, 68, 69, 70, 71 The level of Salmonella in 

water is elevated during times of monthly maximum precipitation in the summer and fall months;56, 72 consequently the 

likelihood of Salmonella in water may increase in regions experiencing increased total or heavy precipitation events. 

Water is also an important factor in food processing. Climate 

and weather extremes, such as flooding or drought, can reduce 

water quality and increase the risk of pathogen transfer during 

the handling and storage of food following harvest.9

The direct effect of drought on food safety is less clear. Dry 

conditions can pose a risk for pathogen transmission due to 

reduced water quality, increased risk of runoff when rains 

do occur, and increased pathogen concentration in reduced 

water supplies if such water is used for irrigation, food processing, or livestock management.29, 31, 55, 73 Increasing drought 

generally leads to an elevated risk of exposure to pathogens 

such as norovirus and Cryptosporidium.

66 However, drought 

and extreme heat events could also decrease the survivability 

of certain foodborne pathogens, affecting establishment and 

transmission, and thus reducing human exposure.66, 74

Mycotoxins and Phycotoxins

Mycotoxins are toxic chemicals produced by molds that grow 

on crops prior to harvest and during storage. Prior to harvest, 

increasing temperatures and drought can stress plants, making them more susceptible to mold growth.75 Warm and moist 

conditions favor mold growth directly and affect the biology 

of insect vectors that transmit molds to crops. Post-harvest 

contamination is also affected by environmental parameters, 

including extreme temperatures and moisture. If crops are 

not dried and stored at low humidity, mold growth and mycotoxin production can increase to very high levels.76, 77

Phycotoxins are toxic chemicals produced by certain harmful 

freshwater and marine algae that may affect the safety of 

drinking water and shellfish or other seafood. For example, 

the alga responsible for producing ciguatoxin (the toxin that 

causes the illness known as ciguatera fish poisoning) thrives 

in warm water (see also Ch. 6: Water-Related Illness). Projected increases in sea surface temperatures may expand the 

endemic range of ciguatoxin-producing algae and increase 

ciguatera fish poisoning incidence following ingestion.78 Predicted increases in sea surface temperature of 4.5° to 6.3°F 

(2.5° to 3.5°C) could yield increases in ciguatera fish poisoning 

cases of 200% to 400%.79

Crop dusting of a corn field in Iowa.

Climate change will expand the geographical range 

where mold growth and mycotoxin production 

occur.9, 32, 37, 75 Corn, a major U.S. crop, is especially 

susceptible to mold growth and mycotoxin 

production.81 Human dietary exposure to these toxins 

has resulted in illness and death in tropical regions, 

or where their presence remains unregulated.82 In the 

United States, regulations are designed to prevent 

mycotoxins entering the food supply. 

Aflatoxins (naturally occurring mycotoxins found 

in corn) are known carcinogens and can also 

cause impaired development in children, immune 

suppression, and, with severe exposure, death.82, 83, 84

Recent models show that aflatoxin contamination in 

corn may increase with climate change in Europe.85

Other commodities susceptible to contamination by 

mycotoxins include peanuts, cereal grains, and fruit.37

Crops Susceptible to Mycotoxin Infections
7–FOOD SAFETY, NUTRITION, AND DISTRIBUTION

U.S. Global Change Research Program 197 Impacts of Climate Change on Human Health in the United States

Once introduced into the food chain, these poisonous toxins 

can result in adverse health outcomes, with both acute and 

chronic effects. Current regulatory laws and management 

strategies safeguard the food supply from mycotoxins and 

phycotoxins; however, increases in frequency and range of 

their prevalence may increase the vulnerability of the food 

safety system. 

Climate Impacts on Chemical Contaminants 

Climate change will affect human exposure to metals, pesticides, pesticide residues, and other chemical contaminants. 

However, resulting incidence of illness will depend on the 

genetic predisposition of the person exposed, type of contaminant, and extent of exposure over time.86

Metals and Other Chemical Contaminants

There are a number of environmental contaminants, such as 

polychlorinated biphenyls, persistent organic pollutants, dioxins, pesticides, and heavy metals, which pose a human health 

risk when they enter the food chain. Extreme events may 

facilitate the entry of such contaminants into the food chain, 

particularly during heavy precipitation and flooding.45, 46, 47 For 

example, chemical contaminants in floodwater following Hurricane Katrina included spilled oil, pesticides, heavy metals, 

and hazardous waste.47, 87

Methylmercury is a form of mercury that can be absorbed 

into the bodies of animals, including humans, where it can 

have adverse neurological effects. Elevated water temperatures may lead to higher concentrations of methylmercury in 

fish and mammals.88, 89 This is related to an increase in metabolic rates and increased mercury uptake at higher water 

temperatures.28, 90, 91 Human exposure to dietary mercury is 

influenced by the amount of mercury ingested, which can 

vary with the species, age, and size of the fish. If future fish 

consumption patterns are unaltered, increasing ocean temperature would likely increase mercury exposure in human 

diets. Methylmercury exposure can affect the development of 

children, particularly if exposed in utero.92

Pesticides

Climate change is likely to exhibit a wide range of effects on 

the biology of plant and livestock pests (weeds, insects, and 

microbes). Rising minimum winter temperatures and longer 

growing seasons are very likely to alter pest distribution and 

populations.93, 94, 95 In addition, rising average temperature and 

CO2 concentration are also likely to increase the range and 

distribution of pests, their impact, and the vulnerability of host 

plants and animals.3, 96, 97

Pesticides are chemicals generally regulated for use in agriculture to protect plants and animals from pests; chemical 

management is the primary means for agricultural pest control 

in the United States and most developed countries. Because 

climate and CO2 will intensify pest distribution and populations,98, 99 increases in pesticide use are expected.100, 101 In addition, the efficacy of chemical management may be reduced 

in the context of climate change. This decline in efficacy can 

reflect CO2-induced increases in the herbicide tolerance of certain weeds or climate-induced shifts in invasive weed, insect, 

Protein. Protein content of major food crops is very 

likely to decline significantly as atmospheric CO2

concentrations increase to between 540 and 960 

parts per million (ppm),129, 134, 135, 137 the range 

projected by the end of this century (see description of 

Representative Concentration Pathways in Appendix 1: 

Technical Support Document).14 Current atmospheric 

concentrations of CO2 are approximately 400 ppm.138

Minerals and trace elements. Rising CO2 levels are very 

likely to lower the concentrations of essential micro- and 

macroelements such as iron, zinc, calcium, magnesium, 

copper, sulfur, phosphorus, and nitrogen in most plants 

(including major cereals and staple crops).16, 128, 132, 133, 

139, 140

Ratio of major macronutrients (carbohydrates to protein). It is very likely that rising CO2 will alter the relative proportions 

of major macronutrients in many crops by increasing carbohydrate content (starch and sugars) while at the same 

time decreasing protein content.16 An increase in dietary carbohydrates-to-protein ratio can have unhealthy effects on 

human metabolism and body mass.136, 141, 142, 143

Impacts of Rising CO2 on the Nutritional Value of Crops

Wheat grown in southeast Washington state, August, 2008.
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U.S. Global Change Research Program 198 Impacts of Climate Change on Human Health in the United States

and plant pathogen populations100, 102, 103, 104, 105, 106, 107, 108 as well as 

climate-induced changes that enhance pesticide degradation or 

affect coverage.108, 109

Increased pest pressures and reductions in the efficacy of pesticides are likely to lead to increased pesticide use, contamination 

in the field, and exposure within the food chain.110 Increased 

exposure to pesticides could have implications for human health.5, 

29, 44 However, the extent of pesticide use and potential exposure 

may also reflect climate change induced choices for crop selection 

and land use.

Pesticide Residues 

Climate change, especially increases in temperature, may be 

important in altering the transmission of vector-borne diseases 

in livestock by influencing the life cycle, range, 

and reproductive success of disease vectors.8, 65

Potential changes in veterinary practices, including 

an increase in the use of parasiticides and other 

animal health treatments, are likely to be adopted to maintain livestock health in response to 

climate-induced changes in pests, parasites, and 

microbes.5, 23, 110 This could increase the risk of pesticides entering the food chain or lead to evolution 

of pesticide resistance, with subsequent implications for the safety, distribution, and consumption 

of livestock and aquaculture products.111, 112, 113

Climate change may affect aquatic animal health 

through temperature-driven increases in disease.114 The occurrence of increased infections 

in aquaculture with rising temperature has been 

observed for some diseases (such as Ichthyophthirius multifiliis and Flavobacterium columnare)

115

and is likely to result in greater use of aquaculture 

drugs.76

7.3 Nutrition

While sufficient quantity of food is an obvious 

requirement for food security, food quality is 

essential to fulfill basic nutritional needs. Globally, 

chronic dietary deficiencies of micronutrients such 

as vitamin A, iron, iodine, and zinc contribute to 

“hidden hunger,” in which the consequences of the 

micronutrient insufficiency may not be immediately visible or easily observed. This type of micronutrient deficiency constitutes one of the world’s 

leading health risk factors and adversely affects 

metabolism, the immune system, cognitive development and maturation—particularly in children. 

In addition, micronutrient deficiency can exacerbate the effects of diseases and can be a factor in 

prevalence of obesity.116, 117, 118, 119, 120, 121

In developed countries with abundant food supplies, like 

the United States, the health burden of malnutrition may 

not be intuitive and is often underappreciated. In the 

United States, although a number of foods are supplemented with nutrients, it is estimated that the diets of 

38% and 45% of the population fall below the estimated average requirements for calcium and magnesium, 

respectively.122 Approximately 12% of the population is at 

risk for zinc deficiency, including perhaps as much as 40% 

of the elderly.123 In addition, nutritional deficiencies of 

magnesium, iron, selenium, and other essential micronutrients can occur in overweight and obese individuals, 

whose diets might reflect excessive intake of calories and 

refined carbohydrates but insufficient intake of vitamins 

and essential minerals.119, 124, 125, 126

Figure 4: Direct effect of rising atmospheric carbon dioxide (CO2) 

on the concentrations of protein and minerals in crops. The top 

figure shows that the rise in CO2 concentration from 293 ppm (at the 

beginning of the last century) to 385 ppm (global average in 2008) 

to 715 ppm (projected to occur by 2100 under the RCP8.5 and 

RCP6.0 pathways),184 progressively lowers protein concentrations in 

wheat flour (the average of four varieties of spring wheat). The lower 

figure—the average effect on 125 plant species and cultivars—

shows that a doubling of CO2 concentration from preindustrial 

levels diminishes the concentration of essential minerals in wild and 

crop plants, including ionome (all the inorganic ions present in an 

organism) levels, and also lowers protein concentrations in barley, 

rice, wheat and potato. (Figure source: Experimental data from Ziska 

et al. 2004 (top figure), Taub et al. 2008, and Loladze 2014 (bottom 

figure)).16, 129, 134

Effects of Carbon Dioxide on Protein and Minerals
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U.S. Global Change Research Program 199 Impacts of Climate Change on Human Health in the United States

7.4 Distribution and Access

A reliable and resilient food distribution system is essential 

for access to a safe and nutritious food supply. Access to food 

is characterized by transportation and availability, which are 

defined by infrastructure, trade management, storage requirements, government regulation, and other socioeconomic 

factors.146

The shift in recent decades to a more global food market 

has resulted in a greater dependency on food transport and 

distribution, particularly for growing urban populations. 

Consequently, any climate-related disturbance to food distribution and transport may have significant impacts not only 

on safety and quality but also on food access. The effects of 

climate change on each of these interfaces will differ based 

on geographic, social, and economic factors.4 Ultimately, the 

outcome of climate-related disruptions and damages to the 

food transportation system will be strongly influenced by the 

resilience of the system, as well as the adaptive capacity of 

individuals, populations, and institutions. 

How Extreme Events Affect Food 

Distribution and Access

Projected increases in the frequency or severity of some 

extreme events will interrupt food delivery, particularly for vulnerable transport routes.13, 15, 147, 148 The degree of disruption is 

related to three factors: a) popularity of the transport pathway, 

b) availability of alternate routes, and c) timing or seasonality 

of the extreme event.149 As an example, the food transportation 

system in the United States frequently moves large volumes 

of grain by water. In the case of an extreme weather event 

affecting a waterway, there are few, if any, alternate pathways 

for transport.150 This presents an especially relevant risk to food 

access if an extreme event, like flooding or drought, coincides 

with times of agricultural distribution, such as the fall harvest. 

Immediately following an extreme event, food supply and safety can be compromised.150, 151, 152 Hurricanes or other storms can 

disrupt food distribution infrastructure, damage food supplies,7

and limit access to safe and nutritious food, even in areas 

not directly affected by such events (see also Ch. 4: Extreme 

Events).153 For example, the Gulf Coast transportation network 

is vulnerable to storm surges of 23 feet.154 Following Hurricane 

Katrina in 2005, where storm surges of 25 to 28 feet were 

recorded along parts of the Gulf Coast, grain transportation by 

rail or barge was severely slowed due to physical damage to 

infrastructure and the displacement of employees.151, 155 Barriers to food transport may also affect food markets, reaching 

consumers in the form of increased food costs.156

The risk for food spoilage and contamination in storage facilities, supermarkets, and homes is likely to increase due to the 

impacts of extreme weather events, particularly those that result in power outages, which may expose food to ambient temHow Rising CO2 Affects Nutrition

Though rising CO2 stimulates plant growth and carbohydrate 

production, it reduces the nutritional value (protein and 

minerals) of most food crops (Figure 4).16, 127, 128, 129, 130, 131, 132, 133

This direct effect of rising CO2 on the nutritional value of crops 

represents a potential threat to human health.16, 133, 134, 135, 136

Protein

As CO2 increases, plants need less protein for photosynthesis, 

resulting in an overall decline in protein concentration in plant 

tissues.134, 135 This trend for declining protein levels is evident for 

wheat flour derived from multiple wheat varieties when grown 

under laboratory conditions simulating the observed increase 

in global atmospheric CO2 concentration since 1900.129 When 

grown at the CO2 levels projected for 2100 (540–958 ppm), major food crops, such as barley, wheat, rice, and potato, exhibit 

6% to 15% lower protein concentrations relative to ambient 

levels (315–400 ppm).16, 134, 135 In contrast, protein content is not 

anticipated to decline significantly for corn or sorghum.135

While protein is an essential aspect of human dietary needs, 

the projected human health impacts of a diet including plants 

with reduced protein concentration from increasing CO2 are not 

well understood and may not be of considerable threat in the 

United States, where dietary protein deficiencies are uncommon.

Micronutrients

The ongoing increase in atmospheric CO2 is also very likely to 

deplete other elements essential to human health (such as 

calcium, copper, iron, magnesium, and zinc) by 5% to 10% in 

most plants.16 The projected decline in mineral concentrations 

in crops has been attributed to at least two distinct effects of 

elevated CO2 on plant biology. First, rising CO2 increases carbohydrate accumulation in plant tissues, which can, in turn, dilute 

the content of other nutrients, including minerals. Second, high 

CO2 concentrations reduce plant demands for water, resulting 

in fewer nutrients being drawn into plant roots.133, 144, 145

The ongoing increase in CO2 concentrations reduces the 

amount of essential minerals per calorie in most crops, thus 

reducing nutrient density. Such a reduction in crop quality 

may aggravate existing nutritional deficiencies, particularly 

for populations with pre-existing health conditions (see Ch. 9: 

Populations of Concern).

Carbohydrate-to-Protein Ratio

Elevated CO2 tends to increase the concentrations of carbohydrates (starch and sugars) and reduce the concentrations 

of protein.134 The overall effect is a significant increase in the 

ratio of carbohydrates to protein in plants exposed to increasing CO2.

16 There is growing evidence that a dietary increase in 

this ratio can adversely affect human metabolism143 and body 

composition.141
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U.S. Global Change Research Program 200 Impacts of Climate Change on Human Health in the United States

peratures inadequate for safe storage.152 Storm-related power 

grid disruptions have steadily increased since 2000.157 Between 

2002 and 2012, extreme weather caused 58% of power outage 

events, 87% of which affected 50,000 or more customers.157

Power outages are often linked to an increase in illness. For 

example, in August of 2003, a sudden power outage affected 

over 60 million people in the northeastern United States and 

Canada. New York City’s Department of Health and Mental 

Hygiene detected a statistically significant citywide increase in 

diarrheal illness resulting from consumption of spoiled foods 

due to lost refrigeration capabilities.158

7.5 Populations of Concern 

Climate change, combined with other social, economic, and 

political conditions, may increase the vulnerability of many 

different populations to food insecurity or food-related illness.163 However, not all populations are equally vulnerable.7, 

62 Infants and young children, pregnant women, the elderly, 

low-income populations, agricultural workers, and those with 

weakened immune systems or who have underlying medical conditions are more susceptible to the effects of climate 

change on food safety, nutrition, and access. 

Children may be especially vulnerable because they eat more 

food by body weight than adults, and do so during important 

stages of physical and mental growth and development. Children are also more susceptible to severe infection or complications from E. coli infections, such as hemolytic uremic syndrome.164, 165, 166 Agricultural field workers, especially pesticide 

applicators, may experience increased exposure as pesticide 

applications increase with rising pest loads, which could also 

lead to higher pesticide levels in the children of these field 

workers.167, 168 People living in low-income urban areas, those 

with limited access to supermarkets,169, 170 and the elderly 

may have difficulty accessing safe and nutritious food after 

disruptions associated with extreme weather events. Climate 

change will also affect U.S. Indigenous peoples’ access to both 

wild and cultivated traditional foods associated with their 

nutrition, cultural practices, local economies, and community health171 (see also Ch. 6 Water-Related Illness and Ch. 9: 

Populations of Concern). All of the health impacts described 

in this chapter can have significant consequences on mental 

health and well-being (see Ch. 8 Mental Health).

The summer (June through August) of 2012 was the 

second hottest on record for the contiguous United 

States.159 High temperatures and a shortage of rain led 

to one of the most severe summer droughts the nation 

has seen and posed serious impacts to the Mississippi 

River watershed, a major transcontinental shipping route for Midwestern agriculture.160, 161 This drought resulted 

in significant food and economic losses due to reductions in barge traffic, the volume of goods carried, and the 

number of Americans employed by the tugboat industry.162 The 2012 drought was immediately followed by flooding 

throughout the Mississippi in the spring of 2013, which also resulted in disruptions of barge traffic and food 

transport. These swings in precipitation, from drought to flooding, are consistent with projected increases in the 

frequency or severity of some types of extreme weather under continued climate change.7, 62, 152

Case Study: Extreme Drought and the Mississippi River, 2012

Figure 5: Mississippi River gauge height at St. Louis, MO, 

from October 2007 through October 2014 showing low water 

conditions during the 2012 drought and water levels above 

flood stage in 2013. (Figure source: adapted from USGS 

2015)185

Low water conditions on Mississippi River near St. Louis, 

MO, on December 5, 2012. Photo source: St. Louis 

District, U.S. Army Corps of Engineers.

Mississippi River Level at St. Louis, Missouri
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U.S. Global Change Research Program 201 Impacts of Climate Change on Human Health in the United States

7.6 Emerging Issues 

Climate and food allergies. Food allergies in the United States 

currently affect between 1% and 9% of the population,172 but 

have increased significantly among children under age 18 since 

1997.173 Rising CO2 levels can reduce protein content and alter 

protein composition in certain plants, which has the potential 

to alter allergenic sensitivity. For example, rising CO2 has been 

shown to increase the concentration of the Amb a 1 protein—

the allergenic protein most associated with ragweed pollen.174

However, at present, the question of how rising levels of CO2

and climate change affect allergenic properties of food is uncertain and requires more research.175

Heavy metals. Arsenic and other heavy metals occur naturally 

in some groundwater sources.176 Climate change can exacerbate drought and competition for water, resulting in the use of 

poorer-quality water sources.177, 178 Because climate and rising 

CO2 levels can also influence the extent of water loss through 

the crop canopy, poorer water quality could lead to changes 

in the concentrations of arsenic and potentially other heavy 

metals (like cadmium and selenium) in plant tissues. Additional information is needed to determine how rising levels of CO2

and climate change affect heavy metal accumulation in food 

and the consequences for human exposure. 

Zoonosis and livestock. Zoonotic diseases, which are spread 

from animals to humans, can be transmitted through direct 

contact with an infected animal or through the consumption of 

contaminated food or water. Climate change could potentially 

increase the rate of zoonoses, through environmental change 

that alters the biology or evolutionary rate of disease vectors 

or the health of animal hosts. The impact of rising levels of CO2

and climate change on the transmission of disease through 

zoonosis remains a fundamental issue of potential global 

consequence. 

Foodborne pathogen contamination of fresh produce by insect 

vectors. Climate change will alter the range and distribution of 

insects and other microorganisms that can transmit bacterial pathogens such as Salmonella to fresh produce.179, 180, 181

Additional information is needed regarding the role of climate 

change on the transmission to and development of food 

pathogens through insect vectors. 

7.7 Research Needs

In addition to the emerging issues identified above, the 

authors highlight the following potential areas for additional 

scientific and research activity on food safety, nutrition and 

distribution, based on their review of the literature. Understanding climate change impacts in the context of the current 

food safety infrastructure will be improved by enhanced 

surveillance of foodborne diseases and contaminant levels, 

improved understanding of CO2 impacts on nutritional quality 

of food, and more accurate models of the impacts of extreme 

events on food access and delivery. 

Future assessments can benefit from research activities that:

• synthesize and assess efforts to identify and respond to current and projected food safety concerns and their impacts 

on human health within the existing and future food safety 

infrastructure;

• develop, test, and expand integrated assessment models 

to enhance understanding of climate and weather variability, particularly extreme events, and the role of human 

responses, including changes in farming technology and 

management, on health risks within the food chain; and

• examine the impacts of rising CO2 and climate change 

on human and livestock nutritional needs, as well as the 

impacts of changing nutritional sources on disease vulnerability.1
U.S. Global Change Research Program 202 Impacts of Climate Change on Human Health in the United States

Supporting Evidence

PROCESS FOR DEVELOPING CHAPTER

The chapter was developed through technical discussions 

of relevant evidence and expert deliberation by the report 

authors at several workshops, teleconferences, and email 

exchanges. The authors considered inputs and comments 

submitted by the public, the National Academies of Sciences, 

and Federal agencies. For additional information on the overall 

report process, see Appendices 2 and 3. The author team also 

engaged in targeted consultations during multiple exchanges 

with contributing authors, who provided additional expertise 

on subsets of the Traceable Accounts associated with each Key 

Finding.

Because the impacts of climate change on food production, 

prices, and trade for the United States and globally have 

been widely examined elsewhere, including in the most 

recent report from the Intergovernmental Panel on Climate 

Change,2, 3, 4, 5, 6, 7 this chapter focuses only on the impacts of 

climate change on food safety, nutrition, and distribution 

in the context of human health in the United States. Many 

nutritional deficiencies and food-related illnesses are of critical 

importance globally, particularly those causing diarrheal 

epidemics or mycotoxin poisoning, and affect U.S. interests 

abroad; but the primary focus of this chapter is to address 

climate impacts on the food safety concerns most important 

in the United States. Thus, the literature cited in this chapter 

is specific to the United States or of demonstrated relevance 

to developed countries. The placement of health threats from 

seafood was determined based on pre- and post-ingestion 

risks: while ingestion of contaminated seafood is discussed 

in this chapter, details on the exposure pathways of waterrelated pathogens (for example, through recreational or 

drinking water) are discussed in Chapter 6: Water-Related 

Illness.

KEY FINDING TRACEABLE ACCOUNTS

Increased Risk of Foodborne Illness

Key Finding 1: Climate change, including rising temperatures 

and changes in weather extremes, is expected to increase 

the exposure of food to certain pathogens and toxins [Likely, 

High Confidence]. This will increase the risk of negative health 

impacts [Likely, Medium Confidence], but actual incidence of 

foodborne illness will depend on the efficacy of practices that 

safeguard food in the United States [High Confidence]. 

Description of evidence base

Multiple lines of research have shown that changes in weather 

extremes, such as increased extreme precipitation (leading to 

flooding and runoff events), can result in increased microbial 

and chemical contamination of crops and water in agricultural 

environments, with increases in human exposure.55, 56, 72 During 

times of drought, plants become weaker and more susceptible 

to stress, which can result in mold growth and mycotoxin 

production if plants are held in warm, moist environments.32, 75

While studies that link climate change to specific outbreaks 

of foodborne illness are limited, numerous studies have 

documented that many microbial foodborne illnesses increase 

with increasing ambient temperature.18, 19 There is very strong 

evidence that certain bacteria grow more rapidly at higher 

temperatures and can increase the prevalence of pathogens 

and toxins in food.32, 34, 54 Case studies have demonstrated that 

lack of refrigerated storage, particularly during very warm 

weather, leads to increases in microbial growth and higher 

exposure to pathogens.5, 18, 19, 48, 60

Major uncertainties

Concentrations of pathogens and toxins in food are expected to 

increase, resulting in an increase in the risk of human exposure 

to infectious foodborne pathogens and toxins. However, the 

number or severity of foodborne illnesses due to climate change 

is uncertain. Much of this uncertainty is due to having controls 

in place to protect public health. For example, contaminated 

crops are likely to be destroyed before consumption, and 

certain pathogens in food, like mycotoxins, are highly regulated 

in the United States. Consequently, the extent of exposure 

and foodborne illness will depend on regulatory, surveillance, 

monitoring, and communication systems, and on how, and to 

what extent, climate change alters these adaptive capacities. 

Furthermore, for certain pathogens, it is not yet clear whether 

the impact of climate change on a pathogen will be positive 

or negative. For example, climate change could lead to fewer 

cases of norovirus infection in the winter, but worsening health 

outcomes are also possible due to elevated transmission of 

norovirus during floods. Similarly drought can reduce water 

quality, increase runoff, and increase pathogen concentration, 

but can also decrease the survivability of certain foodborne 

pathogens.

Assessment of confidence and likelihood based on evidence 

There is high confidence that rising temperature and increases 

in flooding, runoff events, and drought will likely lead to 

increases in the occurrence and transport of pathogens in 

agricultural environments, which will increase the risk of food 

contamination and human exposure to pathogens and toxins. 

However, the actual prevalence of disease will depend on the 

response of regulatory systems and, for certain pathogens, the 

relative importance of multiple climate drivers with opposing 

impacts on exposure. Thus there is medium confidence that 

these impacts of climate change on exposure to pathogens and 

toxins will likely lead to negative health outcomes. There is a 

high confidence that the actual incidence of foodborne illness 

will depend on the efficacy of practices that safeguard food in 

the United States.
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Chemical Contaminants in the Food Chain

Key Finding 2: Climate change will increase human 

exposure to chemical contaminants in food through several 

pathways [Likely, Medium Confidence]. Elevated sea surface 

temperatures will lead to greater accumulation of mercury 

in seafood [Likely, Medium Confidence], while increases in 

extreme weather events will introduce contaminants into the 

food chain [Likely, Medium Confidence]. Rising carbon dioxide 

concentrations and climate change will alter incidence and 

distribution of pests, parasites, and microbes [Very Likely, 

High Confidence], leading to increases in the use of pesticides 

and veterinary drugs [Likely, Medium Confidence].

Description of evidence base

There are a number of established pathways by which 

climate change will intensify chemical contaminants within 

the food chain. Multiple studies have shown that increases 

in ocean temperatures are likely to increase the potential 

for mercury exposure, likely due to the increased uptake 

and concentration of mercury in fish and mammals at 

higher metabolic rates associated with warmer ambient 

temperatures.28, 88, 89, 90 Another pathway includes extreme 

weather events, which can move chemical contaminants 

such as lead into agricultural fields and pastures (as well as 

into drinking or recreational water sources—see Chapter 6: 

Water-Related Illness).45, 46, 87 A final pathway is through rising 

minimum winter temperatures and longer growing seasons, 

which will very likely alter pest distribution and populations. 

A large body of literature shows that temperature, carbon 

dioxide (CO2) concentrations, and water availability are also 

likely to affect pest development, number of pest generations 

per year, changes in pest range, rate of infestation, and host 

plant and animal susceptibility.3, 50, 76, 96, 97 Empirical models 

and an analysis of long-term in situ data indicate that rising 

temperatures will result in increased pest pressures.100, 101, 

105 These changes are expected to result in increased use 

of pesticides,100, 102 which can lead to increased human 

exposure.86

Major uncertainties

Each of the pathways described in the evidence base has 

variable levels of uncertainty associated with each step of 

the exposure pathway.110 For all these pathways, projecting 

the specific consequences on human health in the Unites 

States is challenging, due to the variability in type of pathogen 

or contaminant, time and duration of exposures, individual 

sensitivity (for example, genetic predisposition) and individual 

or institutional adaptive capacity. While increasing exposure 

to chemicals will exacerbate potential health risks, the nature 

of those risks will depend on the specific epidemiological links 

between exposure and human health as well as availability 

and access to health services. Resulting incidence of illness will 

depend on the genetic predisposition of the person exposed, 

type of contaminant, and extent of exposure over time.86

Assessment of confidence and likelihood based on evidence 

Although it is likely that climate change will increase human 

exposure to chemical contaminants, the specific pathway(s) 

of exposure have varying levels of uncertainty associated 

with them and hence there is medium confidence regarding 

the overall extent of exposure. This chapter focuses on three 

such pathways. First, it is likely that elevated sea surface 

temperatures will result in increased bioaccumulation 

of mercury in seafood, but there is medium confidence

regarding human illness because rates of accumulation and 

exposure vary according to the type of seafood ingested, 

and because of the role of varying individual sensitivity and 

individual or institutional adaptive capacity (particularly 

behavioral choices). Similarly, it is likely that extreme events 

will increase contaminants into agricultural soil and the 

food chain. However, there is medium confidence regarding 

exposure because the specific nature of the contaminant 

and the food source will vary, and because the extent of 

exposure will depend on risk management, communication 

of public health threats, and the effectiveness of regulatory, 

surveillance, and monitoring systems within the current food 

safety network. There is high confidence that it is very likely

that rising CO2 and climate change will alter pest incidence 

and distribution. There is medium confidence that such 

changes in incidence and distribution are likely to increase 

chemical management and the use of veterinary drugs 

in livestock. However, in all these pathways, the specific 

consequences on human health in the Unites States are 

uncertain, due primarily to the variability in type of pathogen 

or contaminant, time and duration of exposures, individual 

sensitivity (for example, genetic predisposition), and 

individual or institutional adaptive capacity. 

Rising Carbon Dioxide Lowers Nutritional Value of Food

Key Finding 3: The nutritional value of agriculturally 

important food crops, such as wheat and rice, will decrease 

as rising levels of atmospheric carbon dioxide continue to 

reduce the concentrations of protein and essential minerals in 

most plant species [Very Likely, High Confidence].

Description of evidence base

The nutritional response of crops to rising carbon dioxide is 

well documented, particularly among C3 cereals such as rice 

and wheat, which make up the bulk of human caloric input. C3

species are about 95% of all plant species and represent those 

species most likely to respond to an increase in atmospheric 

CO2 concentrations.

There is strong evidence and consensus that protein 

concentrations in plants strongly correlate with nitrogen 

concentrations. CO2-induced declines in nitrogen 

concentrations have been observed in nearly a hundred 

individual studies and several meta-analyses.16, 133, 137, 139, 140 A 

meta-analysis of the effect of CO2 on protein by crop covers 
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U.S. Global Change Research Program 204 Impacts of Climate Change on Human Health in the United States

228 observations on wheat, rice, soybeans, barley and potato, 

134 and was recently repeated for the United States, Japan, 

and Australia,135 covering 138 mean observations on nitrogen/

protein in wheat, rice, peas, maize, and sorghum. There is very 

strong evidence that rising CO2 reduces protein content in nonleguminous C3 crops, including wheat, rice, potato, and barley. 

There is also good agreement across studies that the ongoing 

increase in CO2 elevates the overall carbohydrate content in C3

plants.16

Another meta-analysis quantifies the role of increasing CO2 in 

altering the ionome (the mineral nutrient and trace element 

composition of an organism) of plants, including major 

crops.16 This meta-analysis of 7,761 observations indicates 

that increasing CO2 also significantly reduces the mineral 

concentrations (calcium, magnesium, iron, zinc, copper, sulfur, 

potassium, and phosphorus) in C3 plants, including grains and 

edible parts of other crops, while also substantially increasing 

the ratio of total non-structural carbohydrates (starch and 

sugars) to minerals and to protein.

Furthermore, these studies show the quality of current crops 

to be lower relative to the crops raised in the past with respect 

to protein and minerals.16, 134 Direct experimental evidence 

shows that protein concentrations in wheat flour progressively 

declined with rising CO2 concentrations representing levels 

in 1900 (approximately 290 ppm), 2008 (approximately 385 

ppm), and the CO2 concentrations projected to occur later in 

this century (approximately 715 ppm).129

Major uncertainties

While the general response and the direction in the change 

of crop quality is evident; there is uncertainty in the extent of 

variation in both protein and ionome among different crop 

varieties. There is little evidence regarding the CO2 effects 

on complex micronutrients such as carotenoids (vitamin A, 

lutein, and zeaxanthin). Although protein, micronutrients, and 

ratio of carbohydrates to protein are all essential aspects of 

human dietary needs, the projected human health impacts 

of nutritional changes with increasing CO2 are still being 

evaluated. There remains a high level of uncertainty regarding 

how reductions in crop quality affect human nutrition 

by contributing to or aggravating existing chronic dietary 

deficiencies and obesity risks, particularly in the United States 

where dietary protein deficiencies are uncommon.

Assessment of confidence and likelihood based on evidence

Based on the evidence, there is high confidence that the 

rapid increase in atmospheric CO2 has resulted in a reduction 

in the level of protein and minerals relative to the amount of 

carbohydrates present for a number of important crop species 

(including a number of globally important cereals such as 

wheat, barley and rice), and will very likely continue to do so as 

atmospheric CO2 concentration continues to rise. 

Extreme Weather Limits Access to Safe Foods

Key Finding 4: Increases in the frequency or intensity of some 

extreme weather events associated with climate change 

will increase disruptions of food distribution by damaging 

existing infrastructure or slowing food shipments [Likely, 

High Confidence]. These impediments lead to increased risk 

for food damage, spoilage, or contamination, which will 

limit availability of and access to safe and nutritious food, 

depending on the extent of disruption and the resilience of 

food distribution infrastructure [Medium Confidence].

Description of evidence base

It is well documented in assessment literature that climate 

models project an increase in the frequency and intensity 

of some extreme weather events.14, 15 Because the food 

transportation system moves large volumes at a time, 

has limited alternative routes, and is dependent on the 

timing of the growing and harvest seasons, it is likely that 

the projected increase in the frequency and intensity of 

extreme weather events13, 14 will also increase the frequency 

of food supply chain disruptions (including risks to food 

availability and access)147, 148, 149, 150, 151, 152, 156 and the risk for 

food spoilage and contamination.152, 163 Recent extreme 

events have demonstrated a clear linkage to the disruption 

of food distribution and access.151, 161 Case studies show 

that such events, particularly those that result in power 

outages, may also expose food to temperatures inadequate 

for safe storage,152 with increased risk of illness. For example, 

New York City’s Department of Health and Mental Hygiene 

detected a statistically significant citywide increase in 

diarrheal illness resulting from consumption of spoiled foods 

due to lost refrigeration capabilities after a 2003 power 

outage.158

Major uncertainties

The extent to which climate-related disruptions to the food 

distribution system will affect food supply, safety, and human 

health, including incidences of illnesses, remains uncertain. 

This is because the impacts of any one extreme weather 

event are determined by the type, severity, and intensity 

of the event, the geographic location in which it occurs, 

infrastructure resiliency, and the social vulnerabilities or 

adaptive capacity of the populations at risk.

Assessment of confidence and likelihood based on evidence

Given the evidence base and current uncertainties, there is 

high confidence that projected increases in the frequency 

and severity of extreme events will likely lead to damage of 

existing food supplies and disruptions to food distribution 

infrastructure. There is medium confidence that these 

damages and disruptions will increase risk for food damage, 

spoilage, or contamination, which will limit availability and 

access to safe and nutritious foods because of uncertainties 

surrounding the extent of the disruptions and individual, 

community, or institutional sensitivity to impacts. There are 
7–FOOD SAFETY, NUTRITION, AND DISTRIBUTION

U.S. Global Change Research Program 205 Impacts of Climate Change on Human Health in the United States

further uncertainties surrounding how the specific dynamics of 

the extreme event, such as the geographic location in which it 

occurs, as well as the social vulnerabilities or adaptive capacity 

of the populations at risk, will impact human health. 

DOCUMENTING UNCERTAINTY

See Appendix 4: Documenting Uncertainty for more 

information on assessments of confidence and likelihood used 

in this report. 

PHOTO CREDITS

Pg. 189–Farmer holding wheat: © Dan Lamont/Corbis

Pg. 190–Family enjoying outdoor grilling party: © Hill Street

 Studios/Blend Images/Corbis

Pg. 196–Helicopter crop dusting: © Lucas Payne/AgStock 

 Images/Corbis

Pg. 197–Farmer holding wheat: © Dan Lamont/Corbis

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few sources, limited consistency, models 

incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts

Likelihood

Very Likely

≥ 9 in 10

Likely

≥ 2 in 3

As Likely As Not

≈ 1 in 2

Unlikely

≤ 1 in 3

Very Unlikely

≤ 1 in 10
U.S. Global Change Research Program 206 Impacts of Climate Change on Human Health in the United States

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170. Furey, S., C. Strugnell, and H. McIlveen, 2001: An 

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171. Bennett, T.M.B., N.G. Maynard, P. Cochran, R. Gough, 

K. Lynn, J. Maldonado, G. Voggesser, S. Wotkyns, and K. 

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172. Schneider Chafen, J.J., S.J. Newberry, M.A. Riedl, D.M. 

Bravata, M. Maglione, M.J. Suttorp, V. Sundaram, N.M. 

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177. Appleyard, S.J., J. Angeloni, and R. Watkins, 2006: Arsenic-rich groundwater in an urban area experiencing drought 

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178. Levin, R.B., P.R. Epstein, T.E. Ford, W. Harrington, E. 

Olson, and E.G. Reichard, 2002: U.S. drinking water challenges in the twenty-first century. Environmental Health Perspectives, 110, 43-52. PMC1241146 

179. Soto-Arias, J.P., R. Groves, and J.D. Barak, 2013: Interaction of phytophagous insects with Salmonella enterica on 

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Microbiology, 80, 5447-5456. http://dx.doi.org/10.1128/

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182. Rose, J.B. and F. Wu, 2015: Waterborne and foodborne 

diseases. Climate Change and Public Health. Levy, B. and J. 

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184. Meinshausen, M., S.J. Smith, K. Calvin, J.S. Daniel, M.L.T. 

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011-0156-z

185. USGS, 2015: USGS Water Resources: 07010000 Mississippi River at St. Louis, MO. National Water Information 

System, United States Geological Survey. http://waterdata.usgs.gov/nwis/inventory?agency_code=USGS&site_

no=07010000

End
MENTAL HEALTH AND 8 WELL-BEING

On the web: health2016.globalchange.gov

U.S. Global Change Research Program

*Chapter Coordinator

Lead Author

Daniel Dodgen

U.S. Department of Health and Human Services, Office of the 

Assistant Secretary for Preparedness and Response

Contributing Authors

Darrin Donato

U.S. Department of Health and Human Services, Office of the 

Assistant Secretary for Preparedness and Response

Nancy Kelly

U.S. Department of Health and Human Services, Substance Abuse 

and Mental Health Services Administration

Annette La Greca

University of Miami

Joshua Morganstein

Uniformed Services University of the Health Sciences

Joseph Reser

Griffith University

Josef Ruzek

U.S. Department of Veterans Affairs

Shulamit Schweitzer

U.S. Department of Health and Human Services, Office of the 

Assistant Secretary for Preparedness and Response

Mark M. Shimamoto*

U.S. Global Change Research Program, National Coordination Office

Kimberly Thigpen Tart

National Institutes of Health 

Robert Ursano

Uniformed Services University of the Health Sciences 

Recommended Citation: Dodgen, D., D. Donato, N. Kelly, A. La Greca, J. Morganstein, J. Reser, J. Ruzek, S. Schweitzer, M.M. Shimamoto, 

K. Thigpen Tart, and R. Ursano, 2016: Ch. 8: Mental Health and Well-Being. The Impacts of Climate Change on Human Health in 

the United States: A Scientific Assessment. U.S. Global Change Research Program, Washington, DC, 217–246. http://dx.doi.

org/10.7930/J0TX3C9H

THE IMPACTS OF CLIMATE CHANGE ON HUMAN HEALTH IN THE UNITED STATES

A Scientific Assessment

Acknowledgements: Anthony Barone, U.S. Department of Health and Human Services, Office of the Assistant Secretary for Preparedness 

and Response; Kathleen Danskin, U.S. Department of Health and Human Services, Office of the Assistant Secretary for Preparedness and 

Response; Trina Dutta, Formerly of the U.S. Department of Health and Human Services, Substance Abuse and Mental Health Services 

Administration; Ilya Fischhoff, U.S. Global Change Research Program, National Coordination Office; Lizna Makhani, Formerly of the U.S. 

Department of Health and Human Services, Substance Abuse and Mental Health Services Administration
218

8 MENTAL HEALTH AND WELL-BEING

Key Findings

Exposure to Disasters Results in Mental Health Consequences

Key Finding 1: Many people exposed to climate-related or weather-related disasters experience stress and 

serious mental health consequences. Depending on the type of the disaster, these consequences include 

post-traumatic stress disorder (PTSD), depression, and general anxiety, which often occur at the same time 

[Very High Confidence]. The majority of affected people recover over time, although a significant proportion 

of exposed individuals develop chronic psychological dysfunction [High Confidence].

Specific Groups of People Are at Higher Risk 

Key Finding 2: Specific groups of people are at higher risk for distress and other adverse mental health 

consequences from exposure to climate-related or weather-related disasters. These groups include children, 

the elderly, women (especially pregnant and post-partum women), people with preexisting mental illness, 

the economically disadvantaged, the homeless, and first responders [High Confidence]. Communities that 

rely on the natural environment for sustenance and livelihood, as well as populations living in areas most 

susceptible to specific climate change events, are at increased risk for adverse mental health outcomes 

[High Confidence].

Climate Change Threats Result in Mental Health Consequences and Social Impacts

Key Finding 3: Many people will experience adverse mental health outcomes and social impacts from the 

threat of climate change, the perceived direct experience of climate change, and changes to one’s local 

environment [High Confidence]. Media and popular culture representations of climate change influence 

stress responses and mental health and well-being [Medium Confidence].

Extreme Heat Increases Risks for People with Mental Illness

Key Finding 4: People with mental illness are at higher risk for poor physical and mental health due to 

extreme heat [High Confidence]. Increases in extreme heat will increase the risk of disease and death for 

people with mental illness, including elderly populations and those taking prescription medications that 

impair the body’s ability to regulate temperature [High Confidence].
8-MENTAL HEALTH AND WELL-BEING

U.S. Global Change Research Program 219 Impacts of Climate Change on Human Health in the United States

Introduction

The effects of global climate change on mental health and 

well-being are integral parts of the overall climate-related 

human health impacts. Mental health consequences of climate 

change range from minimal stress and distress symptoms to 

clinical disorders, such as anxiety, depression, post-traumatic 

stress, and suicidal thoughts.1, 2, 3, 4, 5 Other consequences include 

effects on the everyday life, perceptions, and experiences of 

individuals and communities attempting to understand and respond appropriately to climate change and its implications.3, 6, 7

The social and mental health consequences of extreme weather events have been the focus of research for more than three 

decades.3, 4, 5, 8, 9, 10 The mental health and well-being consequences of extreme events, particularly natural disasters, are 

common and form a significant part of the overall effects on 

health. These consequences of climate change related impacts 

rarely occur in isolation, but often interact with other social 

and environmental stressors. 

Figure 1: This conceptual diagram illustrates the key pathways by which humans are exposed to health threats from climate drivers, 

and potential resulting mental health and well-being outcomes (center boxes). These exposure pathways exist within the context of 

other factors that positively or negatively influence health outcomes (gray side boxes). Key factors that influence health outcomes and 

vulnerability for individuals are shown in the right box, and include social determinants of health and behavioral choices. Key factors 

that influence health outcomes and vulnerability at larger community or societal scales, such as natural and built environments, 

governance and management, and institutions, are shown in the left box. All of these influencing factors may also be affected by 

climate change. See Chapter 1: Introduction for more information.

Climate Change and Mental Health and Wellness
8-MENTAL HEALTH AND WELL-BEING

U.S. Global Change Research Program 220 Impacts of Climate Change on Human Health in the United States

Many people exposed to climate- or weather-related natural disasters experience stress reactions and serious mental 

health consequences, including symptoms of post-traumatic 

stress disorder (PTSD), depression, and general anxiety, which 

often occur simultaneously.29, 30, 31, 32, 33, 34 Mental health effects 

include grief/bereavement, increased substance use or misuse, 

and suicidal thoughts.19, 35, 36, 37, 38 All of these reactions have 

the potential to interfere with the individual’s functioning and 

well-being, and are especially problematic for certain groups 

(see "8.2 Populations of Concern" on page 223).

Exposure to life threatening 

events, like highly destructive 

hurricanes such as Hurricane 

Katrina in 2005, have been associated with acute stress, PTSD, 

and higher rates of depression 

and suicide in affected communities.18, 20, 23, 30, 39, 40, 41, 42, 43, 44, 45, 46, 

47 These mental health consequences are of particular concern for people facing recurring 

disasters, posing a cumulative psychological toll. Following 

exposure to Hurricane Katrina, veterans with preexisting mental 

illness had a 6.8 times greater risk for developing any additional 

mental illness, compared to those veterans without a preexisting mental illness.48 Following hurricanes, increased levels 

of PTSD have been experienced by individuals who perceive 

members of their community as being less supportive or helpful 

to one another.49

Depression and general anxiety are also common consequences 

of extreme events (such as hurricanes and floods) that involve 

a loss of life, resources, or social support and social networks 

or events that involve extensive relocation and life disruption.20, 

21, 23, 29, 30, 31, 33, 37, 41, 46, 50, 51, 52, 53, 54 For example, long-term anxiety 

The threat of climate change is a key psychological and emotional stressor. Individuals and communities are affected both 

by direct experience of local events attributed to climate change 

and by exposure to information regarding climate change and 

its effects.10, 11, 12, 13, 14, 15 For example, public communication 

and media messages about climate change and its projected 

consequences can affect perceptions of physical and societal 

risks and consequently affect mental health and well-being. 

The interactive and cumulative nature of climate change effects 

on health, mental health, and well-being are critical factors in 

understanding the overall consequences of climate change on 

human health.16

People have inherent capabilities to adjust to new information 

and experiences and adopt new behaviors to cope with change. 

There is also an array of interventions and treatments that mental health practitioners use to address mental health conditions 

and stress reactions. These interventions occur within the context of health systems that have finite resources to deliver these 

services. These considerations are not discussed in detail, as this 

chapter focuses on the state of the science regarding the effects 

of climate change on mental health and well-being, rather than 

potential actions that could be taken in response to the impacts 

and risks associated with climate change.

8.1 Effects of Climate Change on Mental Health and 

Well-being

The cumulative and interactive effects of climate change, as 

well as the threat and perception of climate change, adversely 

impact individual and societal 

health, mental health, and 

well-being. Figure 2 illustrates 

how climate change impacts 

create cascading and interrelated mental, physical, and 

community health effects. 

These impacts include exposures to higher temperatures 

and extreme weather events 

as well as vector-borne disease transmission, degraded air and 

water quality, and diminished food safety and security.

Extreme Weather Events

In the United States, the mental health impacts of extreme 

weather mainly have been studied in response to hurricanes 

and floods17, 18, 19, 20, 21, 22, 23, 24 and, to a lesser extent, wildfires.25, 26, 

27, 28 Though many studies discuss the mental health impacts of 

specific historical events, they are demonstrative of the types of 

mental health issues that could arise as climate change leads to 

further increases in the frequency, severity, or duration of some 

types of extreme weather (see Ch. 1: Introduction and Ch. 4: Extreme Events). The mental health impacts of these events, such 

as hurricanes, floods, and drought, can be expected to increase 

as more people experience the stress—and often trauma—of 

these disasters.

The mental health impacts of hurricanes, 

floods, and drought can be expected to increase 

as more people experience the stress–and often 

trauma–of these disasters.

Residents and volunteers in Queens, New York City, filter 

through clothes and food supplies from donors following 

Superstorm Sandy on November 3, 2012. A majority of 

individuals psychologically affected by a traumatic event 

recover over time, and some experience a set of positive 

changes that known as post-traumatic growth as a result 

of coping with or experiencing a traumatic event.
8-MENTAL HEALTH AND WELL-BEING

U.S. Global Change Research Program 221 Impacts of Climate Change on Human Health in the United States

and depression, PTSD, and increased aggression (in children) 

have been found to be associated with floods.55 First responders 

following a disaster also experience increased rates of anxiety 

and depression.37

Increases from pre-disaster rates have been observed in 

interpersonal and domestic violence, including intimate 

partner violence,5, 56 particularly toward women, in the wake 

of climate- or weather-related disasters.37, 57, 58 High-risk coping 

behaviors, such as alcohol abuse, can also increase following 

extreme weather events.37, 38, 59, 60, 61, 62 Individuals who use 

alcohol to cope with stress and those with preexisting alcohol 

use disorders are most vulnerable to increased alcohol use 

following extreme weather events.62

Persons directly affected by a climate- or weather-related 

disaster are at increased incidence of suicidal thoughts and 

behaviors. Increases in both suicidal thoughts (from 2.8% 

to 6.4%) and actual suicidal plans (from 1.0% to 2.5%) were 

observed in residents 18 months after Hurricane Katrina.19

Following Hurricanes Katrina and Rita, a study of internally 

displaced women living in temporary housing found reported rates of suicide attempt and completion to be 78.6 times 

and 14.7 times the regional average, respectively.63 In the 

six months following 1992’s Hurricane Andrew, the rate of 

homicide-suicides doubled to two per month in Miami-Dade 

County, where the hurricane hit, compared to an average of 

one per month during the prior five-year period that did not 

include hurricane activity of the same scale.64

Climate- or weather-related disasters can strain the resources available to provide adequate mental (or even immediate 

physical) health care, due to the increased number of individuals who experience severe stress and mental health reactions. 

Communities adversely affected by these events also have 

diminished interpersonal and social networks available to support mental health needs and recovery due to the destruction 

and disruption caused by the event.65

Drought

Many regions in the United States have experienced drought 

(see Ch 1: Introduction and Ch. 4: Extreme Events).66 Longterm drought, unlike sudden extreme weather events, has a 

slow onset and long duration.66, 67 Long-term drought interacts 

over time with multiple environmental and social stressors to 

disrupt lives and livelihoods and the functioning of individuals, 

households, and communities.68, 69, 70 Prolonged drought can 

have visible and long-term impacts on landscapes, on rural 

agricultural industries and communities, and on individual and 

community resilience.71, 72, 73

Cascading and interacting economic, social, and daily life 

circumstances have accompanied prolonged drought in rural 

regions. Drought-related worry and psychological distress 

Figure 2: At the center of the diagram are human figures representing adults, children, older adults, and people with disabilities. The 

left circle depicts climate impacts including air quality, wildfire, sea level rise and storm surge, heat, storms, and drought. The right 

circle shows the three interconnected health domains that will be affected by climate impacts—Medical and Physical Health, Mental 

Health, and Community Health. (Figure source: adapted from Clayton et al. 2014).5

Impact of Climate Change on Physical, Mental, and Community Health
8-MENTAL HEALTH AND WELL-BEING

U.S. Global Change Research Program 222 Impacts of Climate Change on Human Health in the United States

air quality limit people’s outdoor activities. For many, reductions in outdoor exercise and stress-reducing activities lead to 

diminished physical health, increased stress, and poor mental 

health.5

There may be a link between extreme heat (climate change 

related or otherwise) and increasing violence, aggressive 

motives, and/or aggressive behavior.80, 92, 93, 94 The frequency 

of interpersonal violence and intergroup conflict may increase 

with more extreme precipitation and hotter temperatures.83

These impacts can include heightened aggression, which may 

result in increased interpersonal violence and violent crime, 

negatively impacting individual and societal mental health and 

well-being.85 Given projections of increasing temperatures 

(see Ch. 2: Temperature-Related Death and Illness), there is 

potential for increases in human conflict, but the causal linkages between climate change and conflict are complex and the 

evidence is still emerging.83, 95, 96

Threat of Climate Change as a Stressor

Many people are routinely exposed to images, headlines, and 

risk messages about the threat of current and projected climate change. Forty percent of Americans report hearing about 

climate change in the media at least once a month.97

Noteworthy environmental changes associated with climate 

change constitute a powerful environmental stressor—an ongoing and stress-inducing condition or aspect of an individual’s 

everyday environment.69, 98, 99 Equally concerning are adverse 

impacts relating to people’s connections to place and identity, 

and consequent sense of loss and disconnection.11

About half of Americans reported being worried about climate 

change in a 2015 survey. However, these people tended to see 

climate change as a relatively distant threat: 36% said global 

warming would harm them personally, while more expected harm to come to people in other countries and to future 

generations.97 Public risk perceptions of the phenomenon and 

increased in drought-declared Australian regions, particularly 

for those experiencing loss of livelihood and industry.2, 72, 74, 75, 

76 Long-term drought has been linked to increased incidence of 

suicide among male farmers in Australia.2, 77

Extreme Heat

The majority (80.7%) of the U.S. population lives in cities and 

urban areas78 and urbanization is expected to increase in the 

future.79 People in cities may experience greater exposure to 

heat-related health effects during heat waves (see Ch. 2: Temperature-Related Death and Illness). The impact of extreme 

heat on mental health is associated with increased incidence 

of disease and death, aggressive behavior, violence, and suicide and increases in hospital and emergency room admissions for those with mental health or psychiatric conditions.80, 

81, 82, 83, 84, 85, 86, 87

Individuals with mental illness are especially vulnerable to extreme heat or heat waves. In six case-control studies involving 

1,065 heat wave-related deaths, preexisting mental illness was 

found to triple the risk of death due to heat wave exposure.88

The risk of death also increases during hot weather for patients with psychosis, dementia, and substance misuse.84 Hospital admissions have been shown to increase for those with 

mental illness as a result of extreme heat, increasing ambient 

temperatures, and humidity.81, 86, 87 An increased death rate has 

also been observed in those with mental illness among cases 

admitted to the emergency department with a diagnosis of 

heat-related pathology.82

People who are isolated and have difficulty caring for themselves—often characteristics of the elderly or those with a 

mental illness—are also at higher risk for heat-related incidence of disease and death.86, 88 Fewer opportunities for 

social interaction and increased isolation89, 90, 91 put people at 

elevated risk for not only heat-related illness and death but 

also decline in mental health and, in some cases, increases 

in aggression and violence.5 Hotter temperatures and poorer 

An elderly couple walk to the Superdome days after Hurricane Katrina made landfall. New Orleans, Louisiana, September 1, 2005.
8-MENTAL HEALTH AND WELL-BEING

U.S. Global Change Research Program 223 Impacts of Climate Change on Human Health in the United States

Disaster-related stress reactions and accompanying psychological impacts occur in many individuals directly exposed to 

the event and can continue over extended time periods (up to 

a year or more). For example, three months after Hurricane 

Andrew, 38% of children (age 8 to 12 years) living in affected 

areas of south Florida reported symptom levels consistent 

with a “probable diagnosis” of PTSD. At 10 months post-disaster, this proportion declined to about 18%,21, 44 representing a 

substantial decrease but still indicating a significant number 

of individuals with serious mental health issues resulting from 

the disaster event.

Emerging evidence shows that individuals who are actively 

involved in climate change adaptation or mitigation actions 

experience appreciable health and well-being benefit from 

such engagement.110, 136 These multiple psychological and 

environmental benefits do not necessarily minimize distress. 

However, when people do have distress related to relevant 

media exposure or to thinking about or discussing climate 

change, taking action to address the issue can buffer against 

distress.110, 136 Such engagement both addresses the threat and 

helps manage the emotional responses as people come to 

terms with—and adjust their understandings and lives in the 

context of—climate change. 

8.2 Populations of Concern

Populations of concern will be at higher risk for poor mental 

health outcomes as the negative effects of climate change 

progress.10, 137 In addition to the populations described below, 

farmers, those with limited mobility, immigrants, those 

living in coastal areas, those from Indigenous communities 

or tribes,138, 139 and veterans are also expected to experience 

higher risk of poor mental health outcomes (see also Ch. 9: 

Populations of Concern).1, 10, 140, 141, 142, 143, 144, 145

threat of climate change is associated with stigma, dread risk 

(such as a heightened fear of low-probability, high-consequence events), and uncertainty about the future.3, 7, 10, 70, 100, 

101, 102, 103, 104, 105, 106, 107

Many individuals experience a range of adverse psychological responses to the hybrid risk of climate change impacts. A 

hybrid risk is an ongoing threat or event, which is perceived 

or understood as reflecting both natural and human causes 

and processes. These responses include heightened risk perceptions, preoccupation, general anxiety, pessimism, helplessness, eroded sense of self and collective control, stress, 

distress, sadness, loss, and guilt.1, 4, 5, 16, 56, 108, 109, 110, 111, 112

Media representations of serious environmental risks, such 

as climate change, are thought to elicit strong emotional 

responses,7, 113 in part dependent on how climate change 

information is presented.114 People experience the threat of 

climate change through frequent media coverage describing 

events and future risks attributed to climate change. They 

also are directly exposed to increasingly visible changes in 

local environments and seasonal patterns, and in the frequency, magnitude, and intensity of extreme weather events.6, 115

Furthermore, between 2012 and 2013, roughly a third of U.S. 

survey respondents report that they have personally experienced the effects of global warming.12, 13 Exposure to climate 

change through the media could cause undue stress if the 

media coverage is scientifically inaccurate or discouraging. 

However, effective risk communication promotes adaptive 

and preventive individual or collective action.4, 5, 116, 117, 118, 119

Resilience and Recovery

A majority of individuals psychologically affected by a traumatic event (such as a climate-related disaster) will recover 

over time.120 A set of positive changes that can occur in a 

person as a result of coping with or experiencing a traumatic 

event is called post-traumatic growth.121, 122, 123, 124 An array of 

intervention approaches used by mental health practitioners 

also may reduce the adverse consequence of traumatic 

events. While most people who are exposed to a traumatic 

event can be expected to recover over time, a significant proportion (up to 20%) of individuals directly exposed develop 

chronic levels of psychological dysfunction, which may not get 

better or be resolved.21, 35, 47, 53, 125, 126, 127, 128 Multiple risk factors 

contribute to these adverse psychological effects, including 

disaster-related factors such as physical injury, death, or loss 

of a loved one;18, 23, 51, 129 loss of resources such as possessions 

or property;20, 30, 44, 46, 47 and displacement.32, 130, 131, 132, 133, 134 Life 

events and stressors secondary to extreme events also affect 

mental health, including loss of jobs and social connections, 

financial worries, loss of social support, and family distress or 

dysfunction.18, 20, 46, 47, 129, 135

People experience the threat of climate change through 

frequent media coverage.
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U.S. Global Change Research Program 224 Impacts of Climate Change on Human Health in the United States

Children

Children are at particular risk for distress, anxiety, and other 

adverse mental health effects in the aftermath of an extreme 

event. As children are constantly developing, their reactions 

will vary by age and developmental level. Children have been 

shown to possess an innate resilience to adverse events,146, 147, 

148, 149 but despite this resilience, children can and do exhibit 

various stress symptoms when exposed to a traumatic event. 

These symptoms will depend on the developmental stage 

of the child, the level and type of exposure, the amount of 

destruction seen, and that particular child’s risk factors and 

protective factors.150

Children are dependent on others for care and a significant 

predictor of mental health and well-being in a child is the 

mental health status of the primary caregiver.5, 151 If the primary caregiver’s mental health needs are being addressed, then 

a child will fare better after experiencing a disaster or other 

trauma.5, 150, 151, 152, 153

The potential exists for an array of difficult emotional and 

behavioral responses in children shortly after a disaster, such 

as depression, clinginess, aggressiveness, and social withdrawal, some of which are normal and expected and will resolve 

over time with proper support. However, children may be at 

a higher risk than adults of having symptoms persist in the 

long-term. Significantly more children than adults have shown 

continued PTSD symptoms more than two years post-disaster, 

and, in general, children are more likely to be impaired by a 

disaster.141 Chronic stress from the acute and ongoing impacts 

of climate change may alter biological stress response systems 

and make growing children more at risk for developing mental 

health conditions later in life, such as anxiety, depression, and 

other clinically diagnosable disorders.151

Women, Pregnant Women, and Post-partum Mothers

Post-disaster stress symptoms are often reported more 

frequently by women than men.154, 155 Women have higher 

prevalence of PTSD and other mental health disorders after 

disasters than do men,156 and are prone to greater worry 

and feelings of vulnerability,157 anxiety disorders, and other 

adverse mental health outcomes.141, 158 Increases in domestic 

violence towards women are also common after a disaster.5, 56

Pregnant and postpartum women can be quite resilient, but 

their resilience diminishes when social supports are reduced, 

when they have experienced injury, illness, or danger due to 

the disaster, and when they have lived through multiple disaster experiences.39, 57, 159 Estimates indicated that there were 

56,100 pregnant women and 74,900 infants directly affected 

by Hurricane Katrina160 and that pregnant women with high 

hurricane exposure and severe hurricane experiences were at 

a significantly increased risk for PTSD and depression.156 The 

increases in PTSD and depression found in pregnant women 

exposed to Hurricane Katrina were likely due to the severity of 

the event and the intensity of the disaster experience rather 

than a general exposure to the event.42, 156

The many consequences of natural disasters, such as destruction of homes, and of gradual climate change impacts, such as 

rising temperatures, incidence of vector-borne illness, waterborne illness, and even compromised food,160 can all contribute to the emotional stress that women have while pregnant, 

nursing, or responsible for young children. Nutrition is essential to women’s health and well-being, especially if pregnant 

or nursing. Access to clean water and food is critical, and the 

lack of either may affect women’s ability to cope with the 

impacts of climate change. Poor nutrition can lead to difficult 

pregnancies, delivery problems, low birth weight, and even 

death of a newborn, all of which can be immensely stressful to 

the mother.161

Children are at particular risk for distress, anxiety, and other adverse mental health effects in the aftermath of an extreme event.
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U.S. Global Change Research Program 225 Impacts of Climate Change on Human Health in the United States

Elderly

In the United States, the number of individuals 65 years of age 

and older is expected to climb from 47.8 million by the end 

of 2015 to 98 million in 2060, an increase from 14.9% of the 

population to 23.6%.162 The aging population may have difficulty responding to the challenges of climate change, as they 

tend to have higher rates of untreated depression and physical 

ailments that contribute to their overall vulnerability, such as 

increased susceptibility to heat and accompanying physical 

and mental health and well-being impacts.

Physical health problems are associated with the development 

of mental health problems,163, 164 particularly among older 

adults.137, 165 Long-term exposure to air pollution is linked with 

poorer cognitive function and an increased rate of cognitive decline among the elderly.166, 167, 168, 169, 170 Greater flood 

exposure, lack of social support, higher stoicism, and the use 

of maladaptive coping are all associated with greater deterioration in mental health after floods for seniors.17 The mental 

health consequences experienced by the elderly in response 

to a disaster may ultimately be due to challenges they face 

with physical health, mobility, and difficulty managing trauma 

in response to the disaster.142

Economically Disadvantaged

People living in poverty and with fewer socioeconomic resources have less capacity to adapt to the challenges brought 

by climate change. They are less able to evacuate should 

there be a natural disaster, and are more exposed to harmful 

conditions created by heat waves and poor air quality. Low-income people disproportionately experience the most negative 

impacts and weather-related mental distress due to more fragile overall health, reduced mobility, reduced access to health 

care, and economic limitations that reduce the ability to buy 

goods and services that could provide basic comfort and mitigate the effects of disasters.140, 143

Many low-income people in the United States are employed 

in climate-dependent sectors, such as agriculture and fishing, 

or live in weather- and temperature-vulnerable areas, such 

as cities, flood zones, and drought-prone areas (see Ch. 9: 

Populations of Concern). As observed internationally, such 

individuals also have higher levels of distress and are more 

vulnerable to experiencing poor mental health due to extreme 

weather events or other climate change impacts.137, 171 Farming 

or rural communities may be particularly vulnerable to the 

negative mental health outcomes associated with drought. For 

example, older farmers in Australia reported experiencing an 

overwhelming sense of loss as a result of chronic drought and 

its economic consequences.172

Emergency Workers and First Responders

Emergency workers and first responders, including healthcare 

workers and public safety workers, are exposed to deaths, injuries, diseases, and mental stress caused by climate and weather-related disasters. As some extreme weather events increase 

in frequency and severity (see Ch. 4: Extreme Events), there will 

be an increased need for emergency response workers involved 

in rescue and cleanup.173 Firefighters, emergency medical 

service providers, healthcare workers, those recovering human 

remains, and non-traditional first responders who may be 

involved with supporting the community after a natural disaster 

are all at increased risk for mental health consequences, including substance use, both in the short term and long term.174, 175

The very nature of the work, which involves being exposed 

to a traumatic event and helping others in crisis, frequently 

working long hours in difficult environments and away from 

loved ones, increases the susceptibility of first responders and 

emergency workers to experiencing negative mental health 

consequences. The level of stress and distress in responders increases when the injured are children or people they 

know.176 Vicarious trauma or identifying with the victim’s suffering, and being overwhelmed by the number and scope of 

injuries, can also adversely impact the general mental health 

and well-being of all responders.176, 177

Rates of PTSD among first responders have ranged from 

13% to 18% up to four years following large-scale response 

events.174 Among Australian firefighters with PTSD, a large proportion (77%) also presented with simultaneously occurring 

mental health conditions, such as depression, panic disorder, 

or phobic disorders.174 In a study of Coast Guard responders 

to Hurricanes Katrina and Rita, local responders were three 

times more likely to report depression than those who were 

not local.178

Extreme weather events and natural disasters can cause 

damage to infrastructure (such as power grids, roads, and 

transportation) and buildings and put response workers at increased risk of traumatic injury and death (see Ch. 4: Extreme 

A home owner reacts after firefighters arrive to take over the 

protection of his home and two of his neighbors' homes in Rim 

Forest, California, October 3, 2003.
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U.S. Global Change Research Program 226 Impacts of Climate Change on Human Health in the United States

Events).179 The impacts of more frequent and intense weather 

events result in increased stress for responders and threaten 

their overall mental health and well-being.37, 177, 180

People Who Are Homeless

About 30% of people who are chronically homeless suffer 

from some form of mental illness.181 The majority of homeless 

populations live in urban and suburban areas, where they are 

more vulnerable to health risks from exposure to heat waves 

due to the urban heat island effect.182 The combination of risk 

factors, including high rates of mental illness and the geographical location of the homeless, make the homeless very 

vulnerable to the effects of extreme heat.

Some extreme weather events are projected to become more 

frequent and severe, and those who become homeless due 

to these disasters are at increased risk for post-traumatic 

stress symptoms. People experiencing homelessness are also 

vulnerable to acquiring a vector-borne illness. Increases in human–mosquito exposure have been observed after hurricanes, 

such as after Hurricane Katrina.183 For the homeless population, Lyme disease and West Nile virus have the potential to 

compound already high rates of mental illness with additional 

cognitive, neurological, and mental health complications that 

can result from these vector-borne illnesses.184, 185

Individuals with Prior or Preexisting Mental Illness

As of 2013, there were an estimated 43.8 million adults aged 

18 or older in the United States who had any mental illness in 

the past year, representing 18.5% of all adults in the United 

States.186 An estimated 2.6 million youth age 12–17 had a major 

depressive episode during the past year.186 People with mental 

illness and those using medications to treat a variety of mental 

health disorders such as depression, anxiety, and other mood 

disorders are particularly vulnerable to extreme weather events 

and extreme heat.137 Between 2005 and 2010, approximately 

6% of the U.S. adolescents aged 12–19 reported using medications to treat a mental illness.187 As the U.S. population and 

average age increases, the total number of U.S. adults with 

depressive disorders is projected to increase from 33.9 million 

to 45.8 million from 2005 to 2050—a 35% increase, with those 

over 65 years old having the largest increase (117%) in depressive disorders.188 As the number of people with mental health 

disorders increases, so will the number taking medications for 

these disorders, giving rise to a larger population vulnerable to 

the effects of extreme heat and extreme weather events. 

Extreme weather events carry threats of psychological trauma 

and disruption to behavioral health services systems. Individuals 

with mental health and stress-related disorders, such as PTSD, 

depression, anxiety, sleep difficulties, and sometimes those 

who abuse drugs or alcohol, can experience an exacerbation 

of symptoms following a traumatic event. When infrastructure 

is damaged and communication lines are weakened, mental 

health services and personal support networks are also disrupted, leaving those with a mental illness vulnerable to experiencing additional negative mental health consequences (see Ch. 4: 

Extreme Events).

Many medications used to treat a variety of mental health 

disorders interfere with temperature regulation and heat 

elimination and may directly induce hyperthermia. Being dehydrated can also influence the way some medications such as 

lithium (used to stabilize mood)82, 189 or anti-epileptics work in 

the body.190 One of the major underlying risks for death due to 

extreme heat is the use of medications that affect the body’s 

ability to regulate heat or that have neurological effects, increasing susceptibility to the effect of heat.191

After the 2012 heat wave in Wisconsin, nearly 52% of the 

heat-related deaths studied occurred among people with 

at least one mental illness, and half of those were taking a 

medication that treats mental illness and sensitizes people to 

heat.192 Certain drugs prescribed for depression, sleep disorders, psychosis, and anxiety-related disorders were found to 

be independent risk factors for heat-related hospitalization 

cases at an emergency department studied after the 2003 

heat wave in France.82 Many studies have found increased 

susceptibility to heat for people taking certain classes of 

medication typically used to treat mental health disorders and 

other conditions, as well as for alcohol- and drug-dependent 

people.81, 89, 189, 193, 194, 195, 196, 197

Several other factors, besides the effects or side effects of 

medication use, might explain why people with mental illness 

are vulnerable to heat-related death.196, 198 Isolation and deficits 

in care, common to those with severe mental illness, are critical 

characteristics of those with the highest rates of heat-related 

illness and death, as these factors lower the likelihood of utilizing preventive strategies such as showers and cooling shelters 

during times of extreme heat.192 Those with mental illness 

often experience poorer overall health and have fewer social 

Many medications used to treat a variety of mental health 

disorders interfere with temperature regulation and heat 

elimination and may directly induce hyperthermia. 
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U.S. Global Change Research Program 227 Impacts of Climate Change on Human Health in the United States

supports. Persons with a combination of mental and physical 

disorders and who are taking more than one kind of medication 

are also at greater risk of heat-related death.

8.3 Emerging Issues

Multiple issues warrant further attention regarding the impact 

of climate change on individuals’ and communities’ mental 

health and well-being. Broadly, these include: 1) the impacts 

of mass evacuation and relocation before, during, and after 

extreme weather events; 2) the influence of individuals’ understandings and attitudes toward climate change and associated 

risk perceptions on their disaster-related psychological reactions; and 3) the cumulative effects of media presentations of 

extreme events on mental health and well-being. 

A more specific emerging issue is the effect of extreme 

temperatures on mental health, in particular suicide. Some 

studies report a connection between higher temperature and 

suicide;199 with some indicating increased risk of suicide.200, 201, 

202 The association between hotter temperatures and suicide 

appears to be stronger for violent suicide methods than for 

non-violent suicide methods,203 and there is emergent evidence 

that deaths by suicide may increase above certain temperatures, suggesting hot weather may trigger impulsive and 

aggressive behaviors.201, 204 More studies are needed to better 

understand the relationship, as negative correlations have been 

found,205, 206 as well as no correlation at all.207, 208, 209

Children who use methylphenidate (for example, to treat attention deficit disorder) and are engaging in physical activity in 

hot and humid environments may also be at heightened risk for 

heat-related illness.210 More studies are needed to assess what 

the impact will be on children who use behavior modification 

medications during extreme heat. In addition, more frequent 

and prolonged heat waves may increase the amount of time 

spent indoors, which could have an effect on mental health, 

particularly for children and those who use the outdoors for 

exercise and stress management. 

As more is learned about the relationship between climate 

change and vector-borne illnesses, it will be important to further understand the scope of mental health consequences for 

those who become infected. Chapter 5 (Vector-Borne Diseases) 

addresses the complex relationship between climate change 

and vector-borne illnesses, focusing primarily on West Nile 

Virus (WNV) and Lyme disease. Individuals infected with either 

WNV or Lyme disease may experience a range of mental health 

consequences following infection that can include reduced 

cognitive function as well depression associated with other 

symptoms, such as fatigue, pain, and muscle and joint aches.184, 

185 These mental health symptoms can last for months but 

usually resolve over time. 

Clinical depression has been observed in patients who are 

infected with WNV.211, 212 In a long-term observational study, 

35% of participants were found to have new-onset depression. Those with the more severe neuroinvasive forms of WNV 

are at greater risk for depression between 13 to 18 months 

post-infection.212 People who are left with limited mobility as 

a result of WNV infection can experience long-term mental 

health impacts.212 Patient experiences, such as undergoing an 

extended treatment process or experiencing stress in family or 

work life due to a lingering illness, can result in mental health 

consequences. 

Poor air quality may have an effect on depression and suicide.213, 214, 215 While the current literature is not robust enough 

to imply causation, studies have found significant associations 

between short-term exposure to air pollution (sulfur dioxide 

[SO2], particulate matter smaller than 2.5 microns [PM2.5], nitrogen dioxide [NO2], and carbon monoxide [CO]) and emergency 

department admissions for depressive episodes in Canada.213, 

214 Recent studies conducted outside of the United States also 

found associations between air pollution, including aeroallergens, and risk of suicide and emergency department admissions 

for suicide attempts.215, 216, 217 These emerging issues may prove 

to be a significant impact if air quality conditions worsen in the 

United States.

The severity of risks to mental health and well-being for Indigenous populations that have a close connection to the environment, and in some cases lower economic resources, is also a 

concern.144, 145, 218, 219 All of these areas will require further study.

With regard to the impact of climate change related food safety 

risks, increased CO2 levels could decrease the nutritional value 

of some foods (see Ch. 7: Food Safety). Malnutrition (specifically, iron deficiencies) can cause fatigue and depression in children and adolescents.220 More needs to be learned regarding 

the mental health and well-being impacts that will result from 

changes to food composition, quality, and safety due to climate 

change.

Climate change and rising CO2 levels may increase the incidence 

of food allergies.221 Such an increase in food allergies would 

have an impact on mental health status, where those with food 

allergies have higher rates of stress and anxiety.222 Food allergy 

in children and adolescents has been connected to psychological distress, including anxiety and depression. Parents of 

children with food allergies have been found to have higher 

rates of stress and anxiety than parents of children without food 

allergies.223 Those with food allergies have higher rates of major 

depression, bipolar disorder, panic disorder, and social phobia 

than those with no food allergy.224
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U.S. Global Change Research Program 228 Impacts of Climate Change on Human Health in the United States

8.4 Research Needs

In addition to the emerging issues identified above, the authors 

highlight the following potential areas for additional scientific 

and research activity on mental health and well-being, based 

on their review of the literature. Studies of the broad range 

of health effects of climate change should incorporate mental 

health effects and consequences, since many mental health 

impacts are secondary to other health problems. In addition, 

the U.S does not currently have sustained psychological and social impact assessments or monitoring programs and measures 

necessary to identify important changes in mental health and 

well-being associated with climate change. National psychosocial impact assessment and monitoring programs could enhance 

the development of standardized methodologies and measures 

of psychological and social pathways needed to better predict 

mental health and well-being outcomes. 

Future assessments can benefit from research activities that:

• better understand how other health risks from gradual 

climate change affect mental health, including exposures to 

extreme heat, poor air quality, diminished food safety and 

security, and increased vector-borne risks;

• explore the associations between extreme temperatures and 

violent behavior, including violent suicide;

• develop efficient questionnaires and other methods of 

collecting data on mental health, psychological, and social 

impacts for use in epidemiological studies of other health 

impacts of climate change;

• identify predictors or risk factors for adverse psychological 

outcomes following weather-related or climate-related disasters;

• further improve evidence-based practices to facilitate recovery and post-traumatic growth following extreme events;

• identify the best practices for adaptation and prevention 

strategies to reduce the impacts of extreme heat on people 

with mental illness, including patients taking medications that 

increase their vulnerability to heat stress;

• improve understanding of the effects of secondary exposure, including cumulative media representations of climate 

change, as well as how an individual’s understanding of the 

threat of climate change affects their psychological well-being 

and resilience; and

• enhance understanding of the mental health and psychosocial impacts of long-term displacement, relocation, or loss 

of culturally significant geographic features, particularly for 

Indigenous populations.
U.S. Global Change Research Program 229 Impacts of Climate Change on Human Health in the United States

Supporting Evidence

PROCESS FOR DEVELOPING CHAPTER

The chapter was developed through technical discussions 

of relevant evidence and expert deliberation by the report 

authors at several workshops, teleconferences, and email 

exchanges. The authors considered inputs and comments 

submitted by the public, the National Academies of Sciences, 

and Federal agencies. For additional information on the 

overall report process, see Appendices 2 and 3. 

Areas of focus for the Mental Health and Well-Being chapter 

were determined based on the most relevant available 

scientific literature relating to mental health, wellness, and 

climate change, as well as the mental health impacts of 

events associated with climate change. Much of the evidence 

on these impacts has been compiled in countries outside 

the United States; however, the scenarios are similar and 

the evidence directly relevant to the situation in the United 

States, and thus this literature has been considered in the 

chapter. The evidence-base on mental health and wellness 

following extreme weather disasters is both well-established 

and relevant to climate change. The existence of highly 

relevant scientific literature on specific concerns directly 

influenced by climate change—such as the effects of extreme 

heat, stress associated with the threat and perception of 

climate change, and special population risks—resulted in the 

inclusion of these more targeted topics. Although significant 

scientific literature for resilience exists, in-depth discussions 

of adaptation, coping, and treatment approaches are outside 

the scope of this chapter, but are discussed in brief in the 

Resilience and Recovery section.

KEY FINDING TRACEABLE ACCOUNTS

Exposure to Weather-Related Disasters Results in 

Mental Health Consequences

Key Finding 1: Many people exposed to climate-related or 

weather-related disasters experience stress and serious 

mental health consequences. Depending on the type of the 

disaster, these consequences include post-traumatic stress 

disorder (PTSD), depression, and general anxiety, which 

often occur at the same time [Very High Confidence]. The 

majority of affected people recover over time, although a 

significant proportion of exposed individuals develop chronic 

psychological dysfunction [High Confidence].

Description of evidence base

Very strong evidence from multiple studies shows a consensus 

that many people exposed to climate- or weather-related 

natural disasters experience stress reactions and serious 

psychological harm, which often occur simultaneously.30, 31, 32, 

33, 34 Though many of these studies describe the mental health 

impacts of specific historical events, they demonstrate the 

types of mental health issues that will continue to arise as 

climate change leads to increases in the frequency, severity, 

and duration of extreme climate- and weather-related events 

such as floods, hurricanes, droughts, and wildfires.17, 18, 19, 20, 

21, 22, 23, 24, 25, 29, 41, 50, 51, 68, 70 Strong support is found in a number 

of recent studies for the potential for climate change-related 

psychological effects, including grief/bereavement, increased 

substance use or misuse, and thoughts of suicide.19, 35, 36, 37, 38, 60

Research on individual resilience and recovery shows that a 

majority of individuals psychologically affected by a traumatic 

event will recover over time. However, a convincing body of 

recent research shows that a significant proportion (typically 

up to 20%) of individuals directly exposed to the event will 

develop chronic levels of psychological dysfunction, which 

may not get better or be resolved.21, 35, 47, 53, 125, 126, 127, 128

Major uncertainties

There remains some uncertainty about the degree to 

which future extreme weather and climate events will 

impact mental health and wellness. An increase in the 

scope, frequency, or severity of these events will increase 

the number of people impacted and the degree to which 

they are affected. However, efforts that effectively increase 

preparation for both the physical and psychological 

consequences of extreme weather- and climate-related 

events could decrease the impact on mental health and wellbeing.

Assessment of confidence and likelihood based on evidence

Numerous and recent studies have examined the mental 

health and wellness impacts of climate- and weather-related 

events among a variety of populations. Taken as a whole, 

the strength of this scientific evidence provides very high 

confidence regarding the adverse impacts of environmental 

changes and events associated with global climate change 

on individual and societal mental health and well-being, and 

high confidence that these impacts will be long-lasting for a 

significant portion of the impacted population.
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U.S. Global Change Research Program 230 Impacts of Climate Change on Human Health in the United States

Specific Groups of People Are at Higher Risk 

Key Finding 2: Specific groups of people are at higher risk for 

distress and other adverse mental health consequences from 

exposure to climate-related or weather-related disasters. 

These groups include children, the elderly, women (especially 

pregnant and post-partum women), people with preexisting 

mental illness, the economically disadvantaged, the homeless, 

and first-responders [High Confidence]. Communities that rely 

on the natural environment for sustenance and livelihood, as 

well as populations living in areas most susceptible to specific 

climate change events, are at increased risk for adverse 

mental health outcomes [High Confidence].

Description of evidence base

Multiple studies have identified specific populations within 

the United States that are particularly vulnerable to the 

mental health impacts of climate change events.1, 10, 137, 140, 142, 

143 Some evidence suggests that children are at particular risk 

for distress, anxiety, and PTSD.141, 150, 151 Highly cited studies 

of the elderly show that high rates of physical and mental 

health disorders leave them more vulnerable to the impacts 

of climate change.17, 142, 163, 164 A large body of post-disaster 

studies shows that women often have a higher prevalence 

of PTSD156 and other adverse psychological outcomes. 5, 39, 

56, 57, 141, 157, 158, 159, 160 Research strongly suggests that people 

who currently suffer from psychological disorders will face 

additional challenges from climate change impacts.81, 82, 84, 

86, 87 Strong evidence suggests that people living in poverty 

disproportionately experience the most negative impacts,140, 

143 in part because they have less capacity to evacuate to 

avoid natural disasters, and because they are more frequently 

exposed to harmful environmental conditions such as heat 

waves and poor air quality.162 Similarly, the majority (91%) 

of homeless populations live in urban and suburban areas, 

where they are more vulnerable to certain weather- and 

climate-related health risks.182

A number of studies of disaster responders point to an 

increased risk of mental and physical health problems 

following climate-related disasters.174, 175, 178, 179 More frequent 

and intense weather events will increase the likelihood of this 

threat.37, 177, 180

Several studies show that those living in drought-prone areas 

are vulnerable to high levels of distress.137, 171, 172 In addition, 

evidence suggests those living in Arctic or other coastal areas, 

such as Indigenous communities or tribes, tend to be more 

reliant on natural resources that could be diminished by 

climate change, which can lead to an increased risk of poor 

mental health outcomes.138, 139, 144, 145, 218, 219

Major uncertainties

While there is uncertainty around the magnitude of effect, 

there is general agreement that climate-related disasters 

cause emotional and behavioral responses that will increase 

the likelihood of a mental illness or effect. Understanding how 

exposure, sensitivity, and adaptive capacity change over time 

and location for specific populations of concern is challenging. 

Uncertainties remain with respect to the underlying social 

determinants of health, public health interventions or 

outreach, adaptation options, and climate impacts at fine 

local scales.

Assessment of confidence and likelihood based on evidence

The combined breadth and strength of the scientific literature 

supports high confidence that certain vulnerable populations 

will face psychological tolls in the aftermath of climate-related 

disasters. An increase in adverse climate-related events will 

result in increased exposure of such populations of concern 

and an increased likelihood of elevated risk for mental health 

consequences. There is also high confidence that naturalresource-dependent communities and populations living in 

areas most susceptible to specific climate change events are 

at increased risk for adverse mental health outcomes. 

Climate Change Threats Result in Mental Health 

Consequences and Social Impacts

Key Finding 3: Many people will experience adverse mental 

health outcomes and social impacts from the threat of 

climate change, the perceived direct experience of climate 

change, and changes to one’s local environment [High 

Confidence]. Media and popular culture representations of 

climate change influence stress responses and mental health 

and well-being [Medium Confidence].

Description of evidence base

A strong combination of mental health epidemiological 

research, social science-based national survey research, 

social and clinical psychology, environmental risk perception 

research, and disaster mental health research supports the 

finding that the threat of climate change and perceptions of 

its related physical environment changes and extreme events 

together constitute a significant environmental stressor.3, 7, 10, 

11, 69, 70, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107

A large number of recent studies that have evaluated 

responses to the hybrid risk (risk that is part natural and 

part human-caused) of climate change impacts specifically 

reveal that many individuals experience a range of adverse 

psychological responses.1, 4, 5, 16, 56, 108, 109, 110, 111, 112

Major uncertainties

Major uncertainties derive from the distinction between 

people’s objective and subjective exposure and experience 

of environmental threats. The multimedia information 

environment to which individuals are exposed and its 

coverage of climate change and related events can 

contribute to complicated public perceptions and strong 

emotional responses related to climate change as a social, 

environmental, and political issue.7, 113, 114 If media exposure 

is inaccurate or discouraging, that could cause undue stress. 
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U.S. Global Change Research Program 231 Impacts of Climate Change on Human Health in the United States

However, accurate risk information dissemination can result in 

adaptive and preventive individual and collective action.4, 5, 116, 

117, 118, 119 The relative dearth of long term impact assessment 

and monitoring programs relating to the psychosocial 

impacts of climate change necessitates reliance on smallerscale, typically cross-sectional studies and research surveys 

that are often limited by their use of single-item indicators 

rather than standardized, climate change-specific, multi-item 

psychometric measures. 

Assessment of confidence and likelihood based on evidence

The large body of well-documented scientific evidence 

provides high confidence that adverse mental health 

outcomes and social impacts can result from the threat 

of climate change, the perceived experience of climate 

change, and changes to one’s local environment. Emerging 

evidence suggests there is medium confidence that media 

representations of climate change influence stress responses 

and mental health and well-being. 

Extreme Heat Increases Risks for People with Mental 

Illness

Key Finding 4: People with mental illness are at higher risk for 

poor physical and mental health due to extreme heat [High 

Confidence]. Increases in extreme heat will increase the risk 

of disease and death for people with mental illness, including 

elderly populations and those taking prescription medications 

that impair the body’s ability to regulate temperature [High 

Confidence].

Description of evidence base

Mental, behavioral, and cognitive disorders can be triggered 

or exacerbated by heat waves. An increased susceptibility to 

heat due to medication use for psychiatric and other mental 

health disorders, as well as for alcohol- and drug-dependent 

people, is supported by numerous studies,81, 189, 193, 194, 195, 

196, 197 and the influence of dehydration on the effects of 

psychotropic medications is well-documented.82, 189, 190

A significant body of evidence shows that the combination 

of mental illness and extreme heat can result in increases 

in hospitalizations and even death.81, 82, 84, 86, 87 Furthermore, 

six case-control studies, involving 1,065 heat wave-related 

deaths, have found that preexisting mental illness tripled the 

risk of death.88 In a more recent heat wave study, close to 52% 

of the heat-related fatalities were of people with at least one 

mental illness and half of those were taking a psychotropic 

medication.192

Major uncertainties

Uncertainties include whether pharmaceutical companies 

will develop new medications to treat mental illness and 

other health conditions that make individuals less susceptible 

to heat, whether strategies for prevention of heat-related 

illness and death are implemented, and whether individuals 

begin to adapt over time to increases in heat. Prevention, 

detection, and treatment of mental illness without the use 

of medications that negatively impact the body’s ability to 

regulate heat could moderate the magnitude of extreme 

heat’s impact on those predicted to have psychiatric and 

stress related disorders.

Assessment of confidence and likelihood based on evidence

A large body of established scientific evidence shows there 

is high confidence that people with mental illness are at 

greater risk for poor physical and mental health outcomes 

from climate change. Similarly, there is high confidence that 

exposure to extreme heat will exacerbate such outcomes, 

particularly for the elderly and those who take certain 

prescription medications to treat their mental illnesses. Given 

predictions of growth in the subgroup of the population who 

have mental health conditions and who take pharmaceuticals 

that sensitize them to heat, increases in the number of people 

experiencing related negative health outcomes due to climate 

change is expected to occur.

DOCUMENTING UNCERTAINTY

This assessment relies on two metrics to communicate 

the degree of certainty in Key Findings. See Appendix 4: 

Documenting Uncertainty for more on assessments of 

likelihood and confidence.

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few sources, limited consistency, models 

incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts

Likelihood

Very Likely

≥ 9 in 10

Likely

≥ 2 in 3

As Likely As Not

≈ 1 in 2

Unlikely

≤ 1 in 3

Very Unlikely

≤ 1 in 10
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U.S. Global Change Research Program 232 Impacts of Climate Change on Human Health in the United States

PHOTO CREDITS

Pg. 217–Man in smoke: © Carlos Avila Gonzalez/San 

Francisco Chronicle/Corbis 

Pg. 218–Rescuer and resident hugging: © U.S. Air Force/Staff

Sgt. James L. Harper, Jr. 

Pg. 223–Remote control: ©flickr/flash.pro

Pg. 220–Clothing drive: © Alec McClure/Demotix/Corbis

Pg. 222–Elderly couple in flooded street: © Michael

Ainsworth Dallas Morning News/Corbis

Pg. 224–Children looking through window: © Aurora/Aurora

Photo/Corbis

Pg. 225–Man in smoke: © Carlos Avila Gonzalez/San 

Francisco Chronicle/Corbis

Pg. 226–Woman with medication: © JGI/Jamie Grill/Blend

Images/Corbis
U.S. Global Change Research Program 233 Impacts of Climate Change on Human Health in the United States

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0738-10NG-7157
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© U.S. Air Force photo/Staff Sgt. James L. Harper Jr.
247

9

On the web: health2016.globalchange.gov

U.S. Global Change Research Program

Recommended Citation: Gamble, J.L., J. Balbus, M. Berger, K. Bouye, V. Campbell, K. Chief, K. Conlon, A. Crimmins, B. Flanagan, 

C. Gonzalez-Maddux, E. Hallisey, S. Hutchins, L. Jantarasami, S. Khoury, M. Kiefer, J. Kolling, K. Lynn, A. Manangan, M. McDonald, 

R. Morello-Frosch, M.H. Redsteer, P. Sheffield, K. Thigpen Tart, J. Watson, K.P. Whyte, and A.F. Wolkin, 2016: Ch. 9: Populations of 

Concern. The Impacts of Climate Change on Human Health in the United States: A Scientific Assessment. U.S. Global Change 

Research Program, Washington, DC, 247–286. http://dx.doi.org/10.7930/J0Q81B0T 

THE IMPACTS OF CLIMATE CHANGE ON HUMAN HEALTH IN THE UNITED STATES

A Scientific Assessment

Acknowledgements: Larry Campbell, Swinomish Indian Tribal Community; Jean Paul Chretien, U.S. Department of Defense; Patricia A. 

Cochran, Alaska Native Science Commission; Jamie Donatuto, Swinomish Indian Tribal Community; James Persson, U.S. Department of 

Defense

*Chapter Coordinator

Jessica Kolling

Centers for Disease Control and Prevention

Kathy Lynn

University of Oregon

Arie Manangan

Centers for Disease Control and Prevention

Marian McDonald

Centers for Disease Control and Prevention

Rachel Morello-Frosch

University of California, Berkeley

Margaret Hiza Redsteer

U.S. Geological Survey

Perry Sheffield

Icahn School of Medicine at Mount Sinai, New York

Kimberly Thigpen Tart

National Institutes of Health

Joanna Watson

Centers for Disease Control and Prevention, National Institute for 

Occupational Safety and Health

Kyle Powys Whyte

Michigan State University

Amy Funk Wolkin

Centers for Disease Control and Prevention

Lead Authors

Janet L. Gamble

U.S. Environmental Protection Agency 

John Balbus

National Institutes of Health

Contributing Authors

Martha Berger

U.S. Environmental Protection Agency

Karen Bouye

Centers for Disease Control and Prevention

Vince Campbell

Centers for Disease Control and Prevention

Karletta Chief

The University of Arizona

Kathryn Conlon

Centers for Disease Control and Prevention

Allison Crimmins*

U.S. Environmental Protection Agency

Barry Flanagan

Centers for Disease Control and Prevention

Cristina Gonzalez-Maddux

formerly of the Institute for Tribal Environmental Professionals

Elaine Hallisey

Centers for Disease Control and Prevention

Sonja Hutchins

Centers for Disease Control and Prevention

Lesley Jantarasami*

U.S. Environmental Protection Agency

Samar Khoury

Association of Schools and Programs of Public Health

Max Kiefer

Centers for Disease Control and Prevention, National Institute for 

Occupational Safety and Health

POPULATIONS OF CONCERN
248

FPO

9

Key Findings

Vulnerability Varies Over Time and Is Place-Specific

Key Finding 1: Across the United States, people and communities differ in their exposures, their inherent 

sensitivity, and their adaptive capacity to respond to and cope with climate change related health threats 

[Very High Confidence]. Vulnerability to climate change varies across time and location, across communities, 

and among individuals within communities [Very High Confidence].

Health Impacts Vary with Age and Life Stage

Key Finding 2: People experience different inherent sensitivities to the impacts of climate change at different 

ages and life stages [High Confidence]. For example, the very young and the very old are particularly 

sensitive to climate-related health impacts. 

Social Determinants of Health Interact with Climate Factors to Affect Health Risks

Key Finding 3: Climate change threatens the health of people and communities by affecting exposure, 

sensitivity, and adaptive capacity [High Confidence]. Social determinants of health, such as those related 

to socioeconomic factors and health disparities, may amplify, moderate, or otherwise influence climaterelated health effects, particularly when these factors occur simultaneously or close in time or space [High 

Confidence]. 

Mapping Tools and Vulnerability Indices Identify Climate Health Risks 

Key Finding 4: The use of geographic data and tools allows for more sophisticated mapping of risk factors 

and social vulnerabilities to identify and protect specific locations and groups of people [High Confidence].

POPULATIONS OF CONCERN
9–POPULATIONS OF CONCERN

U.S. Global Change Research Program 249 Impacts of Climate Change on Human Health in the United States

9.1 Introduction 

Climate change is already causing, and is expected to continue 

to cause, a range of health impacts that vary across different 

population groups in the United States. The vulnerability 

of any given group is a function of its sensitivity to climate 

change related health risks, its exposure to those risks, and its 

capacity for responding to or coping with climate variability 

and change. Vulnerable groups of people, described here as 

populations of concern, include those with low income, some 

communities of color, immigrant groups (including those with 

limited English proficiency), Indigenous peoples, children 

and pregnant women, older adults, vulnerable occupational 

groups, persons with disabilities, and persons with preexisting 

or chronic medical conditions. Planners and public health officials, politicians and physicians, scientists and social service 

providers are tasked with understanding and responding 

to the health impacts of climate change. Collectively, their 

characterization of vulnerability should consider how populations of concern experience disproportionate, multiple, and 

complex risks to their health and well-being in response to 

climate change. 

Some groups face a number of stressors related to both 

climate and non-climate factors. For example, people living 

in impoverished urban or isolated rural areas, floodplains, 

coastlines, and other at-risk locations are more vulnerable not 

only to extreme weather and persistent climate change but 

also to social and economic stressors. Many of these stressors 

can occur simultaneously or consecutively. Over time, this 

“accumulation” of multiple, complex stressors is expected 

to become more evident1 as climate impacts interact with 

stressors associated with existing mental and physical health 

conditions and with other socioeconomic and demographic 

factors. 

9.2 A Framework for Understanding Vulnerability

Some populations of concern demonstrate relatively greater 

vulnerability to the health impacts of climate change. The 

definitions of the following key concepts are important to 

understand how some people or communities are disproportionately affected by climate-related health risks (Figure 1). 

Definitions are adapted from the Intergovernmental Panel 

on Climate Change (IPCC) and the National Research Council 

(NRC).2, 3

• Vulnerability is the tendency or predisposition to be 

adversely affected by climate-related health effects, and 

encompasses three elements: exposure, sensitivity or susceptibility to harm, and the capacity to adapt to or to cope 

with change. Exposure is contact between a person and one 

or more biological, chemical, or physical stressors, including 

stressors affected by climate change. Contact may occur in 

a single instance or repeatedly over time, and may occur in 

one location or over a wider geographic area. Sensitivity is 

the degree to which people or communities are affected, 

either adversely or beneficially, by climate variability and 

change. Adaptive capacity is the ability of communities, 

institutions, or people to adjust to potential hazards, to take 

advantage of opportunities, or to respond to consequences. 

A related term, resilience, is the ability to prepare and plan 

for, absorb, recover from, and more successfully adapt to 

adverse events. People and communities with strong adaptive capacity have greater resilience.

• Risk is the potential for consequences to develop where 

something of value (such as human health) is at stake and 

where the outcome is uncertain. Risk is often represented 

as the probability of the occurrence of a hazardous event 

multiplied by the expected severity of the impacts of that 

event. 

• Stressors are events or trends, whether related to climate 

change or other factors, that increase vulnerability to 

health effects.

People or communities can have greater or lesser vulnerability to health risks depending on social, political, and economic 

factors that are collectively known as social determinants of 

health.5 Some groups are disproportionately disadvantaged 

by social determinants of health that limit resources and 

opportunities for health-promoting behaviors and conditions of daily life, such as living/working circumstances and 

access to healthcare services.5 In disadvantaged groups, social 

determinants of health interact with the three elements of 

vulnerability by contributing to increased exposure, increased 

sensitivity, and reduced adaptive capacity (Figure 2). Health 

risks and vulnerability may increase in locations or instances 

where combinations of social determinants of health that amplify health threats occur simultaneously or close in time or 

space.6, 7 For example, people with limited economic resources living in areas with deteriorating infrastructure are more 

likely to experience disproportionate impacts and are less 

Food is distributed to people in need at Catholic Community 

Service in Wheaton, MD, November 23, 2010. Populations of 

concern experience disproportionate, multiple, and complex 

risks to their health and well-being in reponse to climate 

change.
9–POPULATIONS OF CONCERN

U.S. Global Change Research Program 250 Impacts of Climate Change on Human Health in the United States

able to recover following extreme events,8, 9 increasing their 

vulnerability to climate-related health effects. Understanding 

the role of social determinants of health can help characterize 

climate change impacts and identify public health interventions or actions to reduce or prevent exposures in populations 

of concern.6, 7, 10

Factors that Contribute to Exposure 

Exposures to climate-related variability and change are 

determined by a range of factors that individually and collectively shape the nature and extent of exposures. These factors 

include:

• Occupation: Certain occupations have a greater risk of 

exposure to climate impacts. People working outdoors or 

performing duties that expose them to extreme weather, 

such as emergency responders, utility repair crews, farm 

workers, construction workers, and other outdoor laborers, 

are at particular risk.11

• Time spent in risk-prone locations: Where a person lives, 

goes to school, works, or spends leisure time will contribute 

to exposure. Locations with greater health threats include 

urban areas (due to, for example, the “heat island” effect 

or air quality concerns), areas where airborne allergens and 

other air pollutants occur at levels that aggravate respiratory illnesses, communities experiencing depleted water supplies or vulnerable energy and transportation infrastructure, 

coastal and other flood-prone areas, and locations affected 

by drought and wildfire.12, 13, 14

• Responses to extreme events: A person’s ability or, in some 

cases, their choice whether to evacuate or shelter-in-place 

in response to an extreme event such as a hurricane, flood, 

or wildfire affects their exposure to health threats. Low-income populations are generally less likely to evacuate in 

response to a warning (see Ch. 4: Extreme Events).8

• Socioeconomic status: Persons living in poverty are more 

likely to be exposed to extreme heat and air pollution.15, 16

Poverty also determines, at least in part, how people perceive the risks to which they are exposed, how they respond 

to evacuation orders and other emergency warnings, and 

their ability to evacuate or relocate to a less risk-prone location (see Ch. 8: Mental Health).8

• Infrastructure condition and access: Older buildings may 

expose occupants to increased indoor air pollutants and 

mold, stagnant airflow, or high indoor temperatures (see Ch. 

3: Air Quality Impacts). Persons preparing for or responding 

to flooding, wildfires, or other weather-related emergencies may be hampered by disruption to transportation, 

utilities, medical, or communication infrastructure. Lack of 

access to these resources, in either urban or rural settings, 

can increase a person’s vulnerability (see Ch. 4: Extreme 

Events).17,18

Determinants of Vulnerability

Injury, acute and chronic illness (including 

mental health and stress-related illness), 

developmental issues, and death 

HEALTH IMPACTS

VULNERABILITY of Human Health to Climate Change

Exposure is contact between 

a person and one or more 

biological, psychosocial, 

chemical, or physical 

stressors, including stressors 

affected by climate change.

EXPOSURE

Adaptive capacity is the ability 

of communities, institutions, or 

people to adjust to potential 

hazards, to take advantage of 

opportunities, or to respond to 

consequences.

ADAPTIVE CAPACITY

Sensitivity is the degree to 

which people or communities 

are affected, either adversely 

or beneficially, by climate 

variability or change.

SENSITIVITY

Determinants of Vulnerability

Figure 1: Defining the determinants of vulnerability to health impacts associated with climate change, 

including exposure, sensitivity, and adaptive capacity. (Figure source: adapted from Turner et al. 2003)4
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U.S. Global Change Research Program 251 Impacts of Climate Change on Human Health in the United States

• Compromised mobility, cognitive function, and other 

mental or behavioral factors: These factors can lead to increased exposure to climate-related health impacts if people 

are not aware of health threats or are unable to take actions 

to avoid, limit, or respond to risks.19 People with access and 

functional needs may be particularly at risk if these factors 

interfere with their ability to access or receive medical care 

before, during, or after a disaster or emergency.

Characterizing Biological Sensitivity 

The sensitivity of human communities and individuals to 

climate change stressors is determined, at least in part, by 

biological traits. Among those traits are the overall health 

status, age, and life stage. From fetus, to infant, to toddler, to 

child, to adolescent, to adult, to the elderly, persons at every 

life stage have varying sensitivity to climate change impacts.12, 

20, 21 For instance, the relatively immature immune systems of 

very young children make them more sensitive to aeroallergen 

exposure (such as airborne pollens). In addition to life stage, 

people experiencing long-term chronic medical and/or psychological conditions are more sensitive to climate stressors. 

Persons with asthma or chronic obstructive pulmonary disease 

(COPD) are more sensitive to exposures to wildfire smoke and 

other respiratory irritants. Social and economic factors also affect disparities in the prevalence of chronic medical conditions 

that aggravate biological sensitivity.22, 23

Adaptive Capacity and Response to Climate Change

Many of the same factors that contribute to exposure or 

sensitivity also influence the ability of both individuals and 

communities to adapt to climate variability and change. Socioeconomic status, the condition and accessibility of infrastructure, the accessibility of health care, certain demographic 

characteristics, human and social capital (the skills, knowledge, 

experience, and social cohesion of a community), and other 

institutional resources all contribute to the timeliness and 

effectiveness of adaptive capacity (see Ch. 1: Introduction and 

Ch. 4: Extreme Events). 

Poverty; Education; 

Social Norms; 

Governance;

Social, Health, and 

Economic Policy

Underlying Health

Disparities

Poverty, 

Occupation, 

Racial Discrimination

CLIMATE DRIVERS

EXPOSURE PATHWAYS

HEALTH IMPACTS

HEALTH OUTCOMES

Exposure

Sensitivity

Adaptive Capacity

People in poorer neighborhoods are 

generally more likely to be exposed to 

climate change health threats

People with chronic medical conditions 

are more likely to have a serious 

health problem during a heat wave 

than healthy people

People with reduced access to care 

and preventative services are more 

likely to have a severe health outcome 

from their illness

Elements of Vulnerability Social Determinants of Health Examples

Intersection of Social Determinants of Health and Vulnerability Intersection of Social Determinants of Health and Vulnerability

Figure 2: Social determinants of health interact with the three elements of vulnerability. The left side boxes 

provide examples of social determinants of health associated with each of the elements of vulnerability. Increased 

exposure, increased sensitivity and reduced adaptive capacity all affect vulnerability at different points in the 

causal chain from climate drivers to health outcomes (middle boxes). Adaptive capacity can influence exposure 

and sensitivity and also can influence the resilience of individuals or populations experiencing health impacts 

by influencing access to care and preventive services. The right side boxes provide illustrative examples of the 

implications of social determinants on increased exposure, increased sensitivity, and reduced adaptive capacity.
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9.3 Populations of Concern

Communities of Color, Low Income, Immigrants, and 

Limited English Proficiency Groups

In the United States, some communities of color, low-income 

groups, people with limited English proficiency (LEP), and 

certain immigrant groups (especially those who are undocumented) live with many of the factors that contribute to their 

vulnerability to the health impacts of climate change (see Section 9.2). These populations are at increased risk of exposure 

given their higher likelihood of living in risk-prone areas (such 

as urban heat islands, isolated rural areas, or coastal and other 

flood-prone areas), areas with older or poorly maintained 

infrastructure, or areas with an increased burden of air pollution.24, 25, 26, 27 These groups of people also experience relatively 

greater incidence of chronic medical conditions, such as cardiovascular and kidney disease, diabetes, asthma, and COPD,28, 

29, 30 which can be exacerbated by climate-related health 

impacts.24, 31, 32, 33, 34 Socioeconomic and educational factors, 

limited transportation, limited access to health education, and 

social isolation related to language deficiencies collectively 

impede their ability to prepare for, respond to, and cope with 

climate-related health risks.24, 26, 34, 35, 36, 37, 38, 39, 40, 41 These populations also may have limited access to medical care and may 

not be able to afford medications or other treatments.30, 38 For 

LEP and undocumented persons, high poverty rates, language 

and cultural barriers, and citizenship status limit access to and 

use of health care and other social services and make these 

groups more hesitant to seek out help that might compromise 

their immigration status in the United States.39, 42, 43, 44, 45, 46

The number of people of color in the United States who may 

be affected by heightened vulnerability to climate-related 

health risks will continue to grow. Currently, Hispanics or Latinos, Blacks or African Americans, American Indians and Alaska 

Natives, Asian Americans, and Native Hawaiians and Pacific 

Islanders represent 37% of the total U.S. population.47, 48 By 

2042, they are projected to become the majority.49 People of 

color already constitute the majority in four states (California, 

Hawaii, New Mexico, and Texas) and in many cities.48 Numbers of LEP and undocumented immigrant populations have 

also increased. In 2011, LEP groups comprised approximately 

9% (25.3 million individuals) of the U.S. population aged five 

and older.50 In 2010, approximately 11.2 million people in the 

United States were undocumented.51

Vulnerability to Climate-Related Health Stressors

Key climate impacts for some communities of color and low-income, LEP, and immigrant populations include heat waves, 

other extreme weather events, poor air quality, food safety, 

infectious diseases, and psychological stressors. 

Race is an important factor in vulnerability to climate-related 

stress, but it can be difficult to isolate the role of race from other related socioeconomic and geographic factors. Some racial 

minorities are also members of low-income groups, immigrants, 

and people with limited English proficiency, and it is their 

socioeconomic status (SES) that contributes most directly to 

their vulnerability to climate change-related stressors. SES is a 

measure of a person’s economic and social status, often defined 

by income, education, and occupation. Additional factors such 

as age, gender, preexisting medical conditions, psychosocial 

factors, and physical and mental stress are also associated 

with vulnerability to climate change. Because many of these 

variables are highly related to one another, statistical models 

must account for these factors in order to accurately measure 

the relative importance of various risk factors.52, 53 For instance, 

minority race and low SES are jointly linked to increased prevalence of underlying health conditions that may affect sensitivity 

to climate change. When adjusted for age, gender, and level 

of education, the number of potential life-years lost from all 

causes of death was found to be 35% greater for Blacks than for 

Whites in the United States,54 indicating an independent effect 

of race. 

Extreme heat events. Some communities of color and some 

low-income, homeless, and immigrant populations are more exposed to heat waves,55, 56 as these groups often reside in urban 

areas affected by heat island effects.13, 15, 24, 57 In addition, these 

populations are likely to have limited adaptive capacity due to 

a lack of adequately insulated housing, inability to afford or to 

use air conditioning, inadequate access to public shelters such 

as cooling centers, and inadequate access to both routine and 

emergency health care.24, 26, 29, 34, 35, 38 These social, economic, and 

health risk factors give rise to the observed increase in deaths 

and disease from extreme heat in some immigrant and impoverished communities.24, 32, 33 Elevated risks for mortality associated with exposures to high ambient temperatures are also 

reported for Blacks as compared to Whites,32, 40, 58, 59 a finding 

that persists once air conditioning use is accounted for (see also 

Ch. 2: Temperature-Related Death and Illness).60 

Nursing students and faculty at Emory University School of 

Nursing in Atlanta, Georgia, volunteering to give checkups in 

migrant workers' camps, June 12, 2006.
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Other weather extremes. As observed during and after 

Hurricane Katrina and Hurricane/Post-Tropical Cyclone Sandy, 

some communities of color and low-income people experienced increased illness or injury, death, or displacement due 

to poor-quality housing, lack of access to emergency communications, lack of access to transportation, inadequate access 

to health care services and medications, limited post-disaster 

employment, and limited or no health and property insurance.61, 62, 63, 64, 65, 66 Following a 2006 flood in El Paso, Texas, 

Hispanic ethnicity was identified as a significant risk factor for 

adverse health effects after controlling for other important 

socioeconomic factors (for example, age and housing quality).67 Adaptation measures to address these risk factors—such 

as providing transportation during evacuations or targeted 

employment assistance during the recovery phase—may help 

reduce or eliminate these health impact disparities, but may 

not be readily available or affordable (see also Ch. 4: Extreme 

Events).61, 62, 63, 65, 66

Degraded air quality. Climate change impacts on outdoor air 

quality will increase exposure in urban areas where large proportions of minority, low-income, homeless, and immigrant 

populations reside. Fine particulate matter and ozone levels 

already exceed National Ambient Air Quality Standards in 

many urban areas.26, 27, 68, 69 Given the relatively higher rates of 

cardiovascular and respiratory diseases in low-income urban 

populations,26, 28, 30 these populations are more sensitive to 

degraded air quality, resulting in increases in illness, hospitalization, and premature death.70, 71, 72, 73, 74, 75, 76, 77, 78 In addition, 

climate change can contribute to increases in aeroallergens, 

which exacerbate asthma, an illness that is relatively more 

common among some communities of color and low-income 

groups. People of color are especially impacted by air pollution due to both disproportionate exposures for persons 

living in urban areas as well as higher prevalence of underlying diseases, such as asthma and COPD, which increase their 

inherent sensitivity. In 2000, the prevalence of asthma was 

122 per 1,000 Black persons and 104 per 1,000 White persons 

in the United States. At that time, asthma mortality was 

approximately three times higher among Blacks as compared 

to Whites (see also Ch. 1: Introduction; Ch. 3: Air Quality 

Impacts).59

Waterborne and vector-borne diseases. Climate change is 

expected to increase exposure to waterborne pathogens that 

cause a variety of illnesses—most commonly gastrointestinal 

illness and diarrhea (see also Ch. 6: Water-Related Illness). 

Health risks increase in crowded shelter conditions following 

floods or hurricanes,79 which suggests that some low-income 

groups living in crowded housing (particularly prevalent 

among foreign-born or Hispanic populations)80 may face 

increased exposure risk. Substandard or deteriorating water 

infrastructure (including sewerage, drainage, and storm water 

systems, and drinking water systems) in both urban and rural 

low-income areas also contribute to increased risk of exposure to waterborne pathogens.81, 82 Low-income populations 

in some regions may also be more vulnerable to the changes 

in the distribution of some vector-borne diseases that are 

expected to result from climate change. For example, higher 

incidence of West Nile virus disease has been linked to poverty and to urban location in the southeastern and northeastern United States, respectively (see also Ch. 5: Vector-Borne 

Diseases).83, 84

Food safety and security. Climate change affects food safety 

and is projected to reduce the nutrient and protein content 

of some crops, like wheat and rice. Some communities of color and low-income populations are more likely to be affected 

because they spend a relatively larger portion of their household income on food compared to more affluent households. 

These groups often suffer from poor-quality diets and limited 

access to full-service grocery stores that offer healthy and affordable dietary choices (see also Ch. 7: Food Safety).36, 37, 85, 86

Psychological stress. Some communities of color, low-income 

populations, immigrants, and LEP groups are more likely to 

experience stress-related mental health impacts, particularly 

during and after extreme events. Other contributing factors 

include barriers in accessing and affording mental health 

care, such as counseling in native languages, and the availability and affordability of appropriate medications (see also 

Ch. 8: Mental Health).87, 88

Indigenous Peoples in the United States

A number of health risks are higher among Indigenous 

populations, such as poor mental health related to historical or personal trauma, alcohol abuse, suicide, infant/child 

mortality, environmental exposures from pollutants or toxic 

substances, and diabetes caused by inadequate or improper 

diets.89, 90, 91, 92, 93, 94, 95, 96 Because of existing vulnerabilities, 

Indigenous people, especially those who are dependent on 

the environment for sustenance or who live in geographically 

isolated or impoverished communities, are likely to experience greater exposure and lower resilience to climate-related 

health effects. Indigenous Arctic communities have already 

experienced difficulty adapting to climate change effects such 

as reductions in sea ice thickness, thawing permafrost, increases in coastal erosion97, 98, 99, 100 and landslide frequency,101

alterations in the ranges of some fish,102 increased weather 

unpredictability,103 and northward advance of the tree line.104

These climate changes have disrupted traditional hunting and 

subsistence practices and may threaten infrastructure such 

as the condition of housing, transportation, and pipelines,103

which ultimately may force relocation of villages.105

Food safety and security. Examples of how climate changes 

can affect the health of Indigenous peoples include changes 

in the abundance and nutrient content of certain foodstuffs, 
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U.S. Global Change Research Program 254 Impacts of Climate Change on Human Health in the United States

such as berries for Alaska Native communities;106 declining 

moose populations in Minnesota, which are significant to 

many Ojibwe peoples and an important source of dietary 

protein;107, 108 rising temperatures and lack of available water 

for farming among Navajo people;109 and declines in traditional rice harvests among the Ojibwe in the Upper Great Lakes 

region.110 Traditional foods and livelihoods are embedded in 

Indigenous cultural beliefs and subsistence practices.111, 112, 113, 

114, 115, 116, 117 Climate impacts on traditional foods may result in 

poor nutrition and increased obesity and diabetes.118

Changes in aquatic habitats and species also affect subsistence 

fishing.119 Rising temperatures affect water quality and availability. Lower oxygen levels in freshwater and seawater degrade water quality and promote the growth of disease-causing bacteria, viruses, and parasites.120 Warming can exacerbate 

shellfish disease and make mercury more readily absorbed in 

fish tissue. Elevated sea surface temperatures, consistent with 

projected trends in climate warming, have been associated 

with increased accumulation of methylmercury in fish and 

increased human exposure.121 Mercury is a neurotoxin that 

adversely affects people at all life stages, particularly during 

the prenatal stage (see also Ch. 6: Water-Related Illness; Ch. 7: 

Food Safety).121, 122, 123 In addition, oceans are becoming more 

acidic as they absorb some of the carbon dioxide (CO2) added 

to the atmosphere by fossil fuel burning and other sources, 

and this change in acidity can lower shellfish survival.120 This 

affects Indigenous peoples on the West and Gulf Coasts and 

Alaska Natives whose livelihoods depend on shellfish harvests.124 Rising sea levels will 

also destroy fresh and saltwater 

habitats that some Indigenous 

peoples located along the Gulf 

Coast rely upon for subsistence 

food.125

Water security. Indigenous peoples may lack access to water 

resources and to adequate infrastructure for water treatment 

and supply. A significant number 

of Indigenous persons living on 

remote reservations lack indoor plumbing and rely on unregulated water supplies that are vulnerable to drought, changes in 

water quality, and contamination of water in local systems.109, 

126 Existing infrastructure may be poorly maintained or in need 

of significant and costly upgrades.127 Heavy rainfall events and 

warm temperatures have been linked to diarrheal outbreaks 

and bacterial contamination of drinking water sources (see 

Ch. 6: Water-Related Illness). Acute diarrheal disease has 

been shown to disproportionately affect children on the Fort 

Apache reservation in Arizona,128 and result in higher overall hospitalization rates for American Indian/Alaska Native 

infants.129 Increased extreme precipitation and potential 

increases in cyanobacterial blooms (see Ch. 6: Water-Related 

Illness) are also expected to stress existing water infrastructure on tribal lands and increase exposure to waterborne 

pathogens.122, 130

Loss of cultural identity. Climate change threatens sacred 

ceremonial and cultural practices through changing the 

availability of culturally relevant plant and animal species.95, 

130 Climate-related threats may compound historical impacts 

associated with colonialism, as well as current effects on tribal 

culture as more young people leave reservations for education and employment opportunities. Loss of tribal territory 

and disruption of cultural resources and traditional ways of 

life131, 132 lead to loss of cultural 

identity.133, 134, 135 The loss of 

medicinal plants due to climate 

change may leave ceremonial 

and traditional practitioners 

without the resources they need 

to practice traditional healing.114, 

136 The relocation of young 

people may reduce interactions 

across generations and undermine the sharing of traditional 

knowledge, tribal lore, and oral 

history.137, 138

Degraded infrastructure and other impacts. Rising temperatures may damage transportation infrastructure on tribal 

lands. Changing ice or thawing permafrost, flooding, and 

drought-related dust storms may block roads and cut off 

communities from access to evacuation routes and emergency 

medical care or social services.139 Poor air quality from blowing 

dust affects southwestern Indigenous communities, particularly in Arizona and New Mexico, and is likely to worsen with 

drought conditions.140 Exposure to impaired air quality also 

affects Indigenous communities, especially those downwind 

from urban areas or industrial complexes.

Indigenous deckhand pulls in net of geoducks near Suquamish, 

Washington, January 17, 2007. Traditional foods and 

livelihoods are embedded in Indigenous cultural beliefs and 

subsistence practices.

Because of existing vulnerabilities, Indigenous 

people, especially those who are dependent on 

the environment for sustenance or who live 

in geographically isolated or impoverished 

communities, are likely to experience greater 

exposure and lower resilience to climate-related 

health effects.
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Children and Pregnant Women 

Children are vulnerable to adverse health effects associated 

with environmental exposures due to factors related to their 

immature physiology and metabolism, their unique exposure 

pathways, their biological sensitivities, and limits to their 

adaptive capacity. Children pass through a series of windows 

of vulnerability that begin in the womb and continue through 

their second decade of life. Children have a proportionately 

higher intake of air, food, and water relative to their body 

weight compared to adults.20 They also share unique behaviors 

and interactions with their environment that may increase 

their exposure to environmental contaminants. For example, 

small children often play indoors on the floor or outdoors on 

the ground and place hands and other objects in their mouths, 

increasing their exposure to dust and other contaminants, such 

as pesticides, mold spores, and allergens.141 There is, however, 

large variation in vulnerability among children at different life 

stages due to differing physiology and behaviors (Figure 3). Climate change—interacting with factors such as economic status, 

diet, living situation, and stage of development—will increase 

children’s exposure to health threats.12, 21, 142, 143, 144 The impact 

of poverty on children’s health is a critical factor to consider in 

ascertaining how climate change will be manifest in children. 

Poor and low-income households have difficulty accessing 

health care and meeting the basic needs that are crucial for 

healthy child development. In addition, children in poverty 

are less likely to have access to air conditioning to mitigate the 

effects of extreme heat. Children living in poverty are also less 

likely to be able to respond to or escape from extreme weather 

events.12, 21, 142, 143, 144

Vulnerability to Climate-Related Health Stressors 

Extreme heat events. An increase in the frequency and 

intensity of extreme heat events (see Ch. 2: Temperature-Related Death and Illness) will affect children who spend 

time outdoors or in non-climate-controlled indoor settings. 

Student athletes and other children who are susceptible to 

heat-related illnesses when they exercise or play outdoors in 

hot and humid weather may be poorly acclimated to physical 

exertion in the heat. Some 9,000 high school athletes in the 

United States are treated for exertional heat illness (such as 

heat stroke and muscle cramps) each year, with the greatest 

risk among high school football players.145, 146 This appears to 

be a worsening trend. Between 1997 and 2006, emergency 

department visits for all heat-related illness increased 133% 

and youth made up almost 50% of those cases.147 From 2000 

through 2013, the number of deaths due to heat stroke doubled among U.S. high school and college football players.148

Other data show effects of extreme heat on children of all 

ages, including increases in heat illness, fluid and electrolyte imbalances, and asthma. Children in homes or schools 

without air conditioning are also more vulnerable during heat 

events. 

Other weather extremes. Climate change is likely to affect 

the mental health and well-being of children, primarily by 

increasing exposure to traumatic weather events that result in 

injury, death, or displacement. In 2003, more than 10% of U.S. 

children from infancy to 18 years of age reported experiencing a disaster (fire, tornado, flood, hurricane, earthquake, 

etc.) during their lifetimes.149 Exposures to traumatic events 

can impact children’s capacity to regulate emotions, undermine cognitive development and academic performance, and 

contribute to post-traumatic stress disorder (PTSD) and other 

psychiatric disorders (such as depression, anxiety, phobia, and 

panic).150 Children’s ability to cope with disasters is affected 

by factors such as socioeconomic status, available support 

systems, and timeliness of treatment. Negative mental health 

effects in children, if untreated, can extend into adulthood.150

(See Ch. 4: Extreme Events; Ch. 8: Mental Health).

Vulnerability to the Health Impacts of Climate Change at Different Life Stages.

Figure 3: Children’s vulnerability to climate change results from distinct exposures, biological sensitivities (developing bodies and 

immune systems), and limitations to adaptive capacity (dependency on caregivers) at different life stages.
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U.S. Global Change Research Program 256 Impacts of Climate Change on Human Health in the United States

Degraded air quality. Several factors make children more sensitive to the effects of respiratory hazards, including lung development that continues through adolescence, the size of the 

child’s airways, their level of physical activity, and body weight. 

Climate change has the potential to affect future ground-level 

ozone concentrations, particulate matter concentrations, and 

levels of some aeroallergens. Ground-level ozone and particulate matter are associated with increases in asthma episodes 

and other adverse respiratory effects in children.151, 152, 153 Nearly seven million, or about 9%, of children in the United States, 

suffer from asthma.154 Asthma accounts for 10 million missed 

school days each year.155 Particulate matter such as dust and 

emissions from coal-fired electricity generation plants is also 

associated with decreases in lung maturation in children.156

Changes in climate also contribute to longer, more severe 

pollen seasons that may be associated with increases in childhood asthma episodes and other allergic illnesses. Children 

may also be exposed to indoor air pollutants, including both 

particulate matter originating outdoors and indoor sources 

such as tobacco smoke and mold. In addition, high outdoor 

temperatures may increase the amount of time children spend 

indoors. Homes, childcare centers, and schools—places where 

children spend large amounts of their time—are all settings 

where indoor air quality issues may affect children’s health. In 

communities where these buildings are insufficiently supplied 

with screens, air conditioning, humidity controls, or pest control, children’s health may be at risk.157 (See Ch. 3: Air Quality 

Impacts).

Waterborne illnesses. Climate change induced increases in 

heavy rainfall, flooding, and coastal storm events are expected to increase children’s risk of gastrointestinal illness from 

ingestion of or contact with contaminated water.61, 142, 143, 158 An 

increased association between heavy rainfall and increased 

acute gastrointestinal illness has already been observed in 

children in the United States.159 Children may be especially vulnerable to recreational exposures to waterborne pathogens, 

in part because they swallow roughly twice as much water 

as adults while swimming.160 In addition, children comprised 

40% of swimming-related eye and ear infections from the 

waterborne bacteria Vibrio alginolyticus during the period 

1997−2006161 and 66% (ages 1–19) of those seeking treatment for illness associated with harmful algal bloom toxins in 

2009–2010.162 (See Ch. 6. Water-Related Illness).

Vector-Borne and other infectious diseases. The changes in 

the distribution of infectious diseases that are expected to 

result from climate change may introduce new exposures to 

children (see Ch. 5: Vector-Borne Disease). Due to physiological vulnerability or changes in their body’s immune system, 

fetuses, pregnant women, and children are at increased risk 

of acquiring or having complications from certain infectious 

diseases such as listeriosis,163 dengue fever,164 and influenza.165

Children spend more time outdoors than adults, increasing 

their exposure to mosquito and tick bites that can cause 

vector-borne diseases that disproportionately affect children 

such as La Crosse encephalitis or Lyme disease.21, 143, 166 Lyme 

disease is most frequently reported among male children 

aged 5 to 9 years, and a disproportionate increasing trend was 

observed in all children from 1992 to 2006.167, 168

Food safety and security. Climate change, including rising 

levels of atmospheric CO2, significantly reduces food quality 

and threatens availability and access for children. Because of 

the importance of nutrition during certain stages of physical 

and mental growth and development, the direct effect of 

the continued rise of CO2 on reducing food quality will be an 

increasingly significant issue for children globally.169, 170, 171 For 

the United States, disruptions in food production or distribution due to extreme events such as drought can increase costs 

and limit availability or access,172, 173 particularly for food-insecure households, which include nearly 16% of households 

with children in the United States.174 Children are also more 

susceptible to severe infection or complications from Escherichia coli infections, such as hemolytic uremic syndrome.175

(See Ch. 7: Food Safety).

Vulnerability Related to Life Stage

Prenatal and pregnancy outcomes for mothers and babies.

Climate-related exposures may lead to adverse pregnancy and 

newborn health outcomes, including spontaneous abortion, low birth weight (less than 5.5 pounds), preterm birth 

(birth before 37 weeks of pregnancy), increased neonatal 

death, dehydration and associated renal failure, malnutrition, 

diarrhea, and respiratory disease.21, 176 Other risk factors that 

may influence maternal and newborn health include water 

scarcity, poverty, and population displacement.21, 176 The rate 

of preterm births is relatively high in the United States (1 of 

every 9 infants born),177 where they contribute substantially to 

neonatal death and illness. Of the 1.2 million preterm births 

estimated to occur annually in high-income countries, more 

than 500 thousand (42% of the total) occur in the United 

States.178 Extreme heat events have been associated with adverse birth outcomes such as low birth weight, preterm birth, 

Climate-related exposures may lead to adverse pregnancy and 

newborn health outcomes.
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U.S. Global Change Research Program 257 Impacts of Climate Change on Human Health in the United States

and infant mortality,179, 180, 181 as well as congenital cataracts.182

Newborns are especially sensitive to ambient temperatures 

that are too high or too low because their capacity for regulating body temperature is limited.183

In addition, exposure of pregnant women to inhaled particulate matter is associated with negative birth outcomes.184, 185, 

186, 187, 188, 189 Incidences of diarrheal diseases and dehydration 

may increase in extent and severity, which can be associated 

with adverse effects on pregnancy outcomes and the health 

of newborns.176 Floods are associated with an increased risk of 

maternal exposure to environmental toxins and mold, reduced 

access to safe food and water, psychological stress, and disrupted health care. Other flood-related health outcomes for mothers and babies include maternal risk of anemia (a condition 

associated with low red blood cell counts sometimes caused by 

low iron intake), eclampsia (a condition that can cause seizures 

in pregnant women), and spontaneous abortion.190, 191, 192, 193

Infants and toddlers. Infants and toddlers are particularly 

sensitive to air pollutants, extreme heat, and microbial water 

contamination, which are all affected by climate change. Ozone 

exposure in young children and exposure to air pollutants and 

toxins in wildfire smoke are associated with increased asthma 

risk and other respiratory illnesses.78, 142 Young children and 

infants are particularly vulnerable to heat-related illness and 

death, as their bodies are less able to adapt to heat than are 

adults.32, 40, 58, 143, 194 Children under four years of age experience 

higher hospital admissions for respiratory illnesses during heat 

waves.195 Rates of diarrheal illness have been shown to be higher in children under age five in the United States,196 and climate 

change is expected to increase children’s risk of gastrointestinal 

illness from ingestion or contact with contaminated water (see 

also Ch. 6: Water-Related Illness).61, 142, 143, 158

Older Adults

Older adults (generally defined as persons aged 65 and older) 

are vulnerable to the health impacts associated with climate 

change and weather extremes.12, 197, 198, 199 The number of older 

adults in the United States is projected to grow substantially in 

the coming decades. The nation’s older adult population (ages 

65 and older) will nearly double in number from 2015 through 

2050, from approximately 48 million to 88 million.200 Of those 

88 million older adults, a little under 19 million will be 85 years 

of age and older.201 This projected population growth is largely 

due to the aging of the Baby Boomer generation (an estimated 

76 million people born in the United States between 1946 and 

1964), along with increases in lifespan and survivorship.19 Older 

adults in the United States are not uniform with regard to their 

climate-related vulnerabilities, but are a diverse group with 

distinct subpopulations that can be identified not only by age 

but also by race, educational attainment, socioeconomic status, 

social support networks, overall physical and mental health, 

and disability status.198, 202

Vulnerability to Climate-Related Health Stressors

The potential climate change related health impacts for older 

adults include rising temperatures and heat waves; increased 

risk of more intense hurricanes (Categories IV and V), floods, 

droughts, and wildfires; degraded air quality; exposure to infectious diseases; and other climate-related hazards.120

Extreme heat events. Older adults exposed to extreme heat 

can experience multiple adverse effects.203 In the coming 

decades, extreme heat events are projected to become more 

frequent, more intense, and of longer duration, especially in 

higher latitudes and large metropolitan areas.24, 204 Between 

1979 and 2004, 5,279 deaths were reported in the United 

States related to heat exposure, with those deaths reported 

most commonly among adults aged 65 and older.205 Disease 

incidence among older adults is expected to increase even 

in regions with relatively modest temperature changes (as 

demonstrated by case studies of a 2006 California heat wave).40

In New York City, extreme high temperatures were associated 

with an increase in hospital admissions for cardiovascular and 

respiratory disorders, with the elderly among the most affected. Hospital admissions for respiratory illness were greatest 

for the elderly, with a 4.7% increase per degree Centigrade 

increase.33 Future climate-related increases in summertime 

temperatures may increase the risk of death in older people 

with chronic conditions, particularly those suffering from congestive heart failure and diabetes.206 The percentage of older 

adults with diabetes, which puts individuals at higher risk for 

heat-related illness and death, has increased from 9.1% in 1980 

to 19.9% in 2009.207

Other weather extremes. Hurricanes and other severe weather 

events lead to physical, mental, or emotional trauma before, 

during, and after the event.208 The need to evacuate an area 

can pose increased health and safety risks for older adults, 

especially those who are poor or reside in nursing or assisted-living facilities.209, 210 Moving patients to a sheltering facility 

is complicated, costly, and time-consuming and requires concurrent transfer of medical records, medications, and medical 

equipment (see also Ch. 4: Extreme Events).210, 211

Degraded air quality. Climate change can affect air quality 

by increasing ground-level ozone, fine particulate matter, 

aeroallergens, wildfire smoke, and dust (see Ch. 3: Air Quality 

Impacts).212, 213 Exposure to ground-level ozone varies with age 

and can affect lung function and increase emergency department visits and hospital admissions, even for healthy adults. 

Air pollution can also exacerbate asthma and COPD and can 

increase the risk of heart attack in older adults, especially those 

who are also diabetic or obese.214

Vector-Borne and waterborne diseases. The changes in the 

distribution of disease vectors like ticks and mosquitoes that 

are expected to result from climate change may increase 

exposures to pathogens in older adult populations (see Ch. 5: 

FPO
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U.S. Global Change Research Program 258 Impacts of Climate Change on Human Health in the United States

Vector-Borne Diseases). Some vector-borne diseases, notably 

mosquito-borne West Nile and St. Louis encephalitis viruses,215, 

216 pose a greater health risk among sensitive older adults with 

already compromised immune systems. Climate change is also 

expected to increase exposure risk to waterborne pathogens in 

sources of drinking water and recreational water. Older adults 

have a higher risk of contracting gastrointestinal illnesses from 

contaminated drinking and recreational water and suffering 

severe health outcomes and death (see Ch. 6: Water-Related 

Illness).217, 218, 219, 220

Interactions with Non-Climate Stressors

Vulnerability related to locations and condition of the built 

environment. Older adults are particularly vulnerable to climate 

change related health effects depending on their geographic location and characteristics of their homes, such as the quality of 

construction and amenities. More than half of the elderly U.S. 

adult population is concentrated in 170 counties (5% of all U.S. 

counties), and approximately 20% of older Americans live in a 

county in which a hurricane or tropical storm made landfall over 

the last decade.221 For example, Florida is a traditional retirement destination with an older adult population accounting for 

16.8% of the total in 2010, nearly four percentage points higher 

than the national average.222 The increasing severity of tropical storms may pose particular risks for older adults in coastal 

zones.223 Other geographic risk factors common to older adults 

are the urban heat island effect, urban sprawl (which affects 

mobility), characteristics of the built environment, and perceptions of neighborhood safety.224, 225

In neighborhoods where safety and crime are a concern, older 

residents may fear venturing out of their homes, thus increasing 

their social isolation and risk of health impacts during events 

such as heat waves.224 Degraded infrastructure, including the 

condition of housing and public transportation, is associated 

with higher numbers of heat-related deaths in older adults. 

In multi-story residential buildings in which residents rely on 

elevators, electricity loss makes it difficult, if not impossible, for 

elderly residents and those with disabilities to leave the building 

to obtain food, medicine, and other needed services.226 Also, 

older adults who own air-conditioning units may not utilize them 

during heat waves due to high operating costs.12, 227, 228, 229

Vulnerability related to physiological factors. Older adults are 

more sensitive to weather-related events due to age-related 

physiological factors. Elevated risks for cardiovascular deaths 

related to exposure to extreme heat have been observed in 

older adults.32, 230 Generally poorer physical health conditions, 

such as long-term chronic illnesses, are exacerbated by climate 

change.227, 228, 231, 232 In addition, aging can impair the mechanisms that regulate body temperature, particularly for those 

taking medications that interfere with regulation of body 

temperature, including psychotropic medications used to treat a 

variety of mental illnesses such as depression, anxiety, and psychosis.233 Respiratory impairments already experienced by older 

adults will be exacerbated by increased exposure to outdoor air 

pollutants (especially ozone and fine particulate matter), aeroallergens, and wildfire smoke—all of which may be exacerbated 

by climate change.199, 213

Vulnerability related to disabilities. Some functional limitations 

and mobility impairments increase older adults’ sensitivity to 

climate change, particularly extreme events. In 2010, 49.8% of 

older adults (over 65) were reported to have a disability, compared to 16.6% of people aged 21–64.234 Dementia occurs at a 

rate of 5% of the U.S. population aged 71 to 79 years, with an 

increase to more than 37% at age 90 and older.235 Older adults 

with mobility or cognitive impairments are likely to experience 

greater vulnerability to health risks due to difficulty responding 

to, evacuating, and recovering from extreme events.12, 231

Occupational Groups 

Climate change may increase the prevalence and severity of 

known occupational hazards and exposures, as well as the 

emergence of new ones. Outdoor workers are often among 

the first to be exposed to the effects of climate change. Climate 

change is expected to affect the health of outdoor workers 

through increases in ambient temperature, degraded air quality, 

extreme weather, vector-borne diseases, industrial exposures, 

and changes in the built environment.11 Workers affected by 

climate change include farmers, ranchers, and other agricultural workers; commercial fishermen; construction workers; 

paramedics, firefighters and other first responders; and transportation workers. Also, laborers exposed to hot indoor work 

environments (such as steel mills, dry cleaners, manufacturing 

facilities, warehouses, and other areas that lack air conditioning) are at risk for extreme heat exposure.236, 237, 238

For some groups, such as migrant workers and day laborers, the 

health effects of climate change can be cumulative, with occupational exposures exacerbated by exposures associated with 

poorly insulated housing and lack of air conditioning. Workers 

may also be exposed to adverse occupational and climate-related conditions that the general public may altogether avoid, such 

as direct exposure to wildfires. 

Extreme heat events. Higher temperatures or longer, more 

frequent periods of heat may result in more cases of heat-related illnesses (for example, heat stroke and heat exhaustion) 

and fatigue among workers,237, 238, 239, 240, 241 especially among 

more physically demanding occupations. Heat stress and 

fatigue can also result in reduced vigilance, safety lapses, 

reduced work capacity, and increased risk of injury. Elevated 

temperatures can increase levels of air pollution, including 

ground-level ozone, resulting in increased worker exposure 

and subsequent risk of respiratory illness (see also Ch. 2: Temperature-Related Death and Illness).11, 236, 237, 242
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U.S. Global Change Research Program 259 Impacts of Climate Change on Human Health in the United States

Other weather extremes. Some extreme weather events and 

natural disasters, such as floods, storms, droughts, and wildfires, are becoming more frequent and intense (see also Ch. 4: 

Extreme Events).120 An increased need for complex emergency 

responses will expose rescue and recovery workers to physical 

and psychological hazards.205, 243 The safety of workers and 

their ability to recognize and avoid workplace hazards may be 

impaired by damage to infrastructure and disrupted communication. 

From 2000 to 2013, almost 300 U.S. wildfire firefighters 

were killed while on duty.244 With the frequency and severity of wildfires projected to increase, more firefighters will 

be exposed. Common workplace hazards faced on the fire 

line include being overrun by fire (as happened during the 

Yarnell Hill Fire in Arizona in 2013 that killed 19 firefighters);245

heat-related illnesses and injuries; smoke inhalation; vehicle-related injuries (including aircraft); slips, trips, and falls; 

and exposure to particulate matter and other air pollutants in 

wildfire smoke. In addition, wildland fire fighters are at risk of 

rhabdomyolysis (a breakdown of muscle tissue) that is associated with prolonged and intense physical exertion.246

Other workplace exposures to outdoor health hazards. Other 

climate-related health threats for outdoor workers include 

increased waterborne and foodborne pathogens, increased 

duration of aeroallergen exposure with longer pollen seasons,247, 248 and expanded habitat ranges of disease-carrying 

vectors that may influence the risk of human exposure to 

diseases such as West Nile virus or Lyme disease (see also Ch. 

5: Vector-Borne Diseases).249

FPO

Another emerging area of interest, but one where research 

is limited and key research questions remain, is the 

relationship between climate change and occupational 

safety and health hazards posed to members of the U.S. 

Armed Forces. The U.S. Department of Defense (DoD) 

recognizes that climate change will affect its operating 

environment, roles, and missions both within the United 

States and abroad.250, 251, 252 The DoD faces unique 

challenges in protecting the health of its personnel from 

climate change impacts.

Military personnel who train and conduct operations in 

hot environments are at risk for heat-related illness. The 

incidence of heat illness among active duty U.S. military 

personnel is several-fold higher than the summertime 

incidence in the general U.S. population (147 per 

100,000 among the military versus 21.5 per 100,000 in the general population per year).253, 254 A large proportion 

of military heat illness cases occur in training settings in the southern United States,253 where climate change may 

increase future risk. 

Exposure to some climate-sensitive infectious diseases also may be increased among military personnel who work 

extensively in field settings. For example, Lyme disease is the most commonly reported vector-borne disease in the 

list of Armed Forces Reportable Medical events, which covers diseases that may represent significant threats to public 

health and military operations. Lyme disease incidence is highest in military units in the Northeast United States, 

and in some cases is substantially higher than the Centers for Disease Control and Prevention (CDC) estimates for 

the population of the state in which the unit is located.255 Coccidioidomycosis, or “valley fever,” caused by inhalation 

of fungal spores, is an occupational hazard for military personnel training in the southwestern United States or other 

endemic areas (see also Ch 4: Extreme Events).256, 257 Military personnel are stationed and deployed globally, and 

may face higher risk of climate-sensitive infections that are rare in the United States. Recent examples include 

chikungunya,258, 259 dengue fever,260 leishmaniasis,261 and malaria.262, 263, 264, 265

The DoD’s climate change adaptation plan includes several health-related initiatives to understand and mitigate 

such threats, including assessment of projected climate change on health risks to DoD personnel, health surveillance 

demands, and distribution of disease vectors, among others.250

Vulnerability of the U.S. Armed Forces

Soldiers race for first place during an annual physical training 

competition in Fort Riley, Kansas.
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Persons with Disabilities 

Disability refers to any condition or impairment of the body 

or mind that limits a person’s ability to do certain activities or 

restricts a person’s participation in normal life activities, such 

as school, work, or recreation.266 The term “disability” covers a 

wide variety and range of functional limitations related to expressive and receptive communication (hearing and speech), 

vision, cognition, and mobility. 

These factors, if not anticipated 

and accommodated before, 

during, and after extreme 

events, can result in illness and 

death.267 The extent of disability, or its severity, is reflected 

in the affected person’s need 

for environmental accessibility 

and accommodations for their 

impairment(s).268

Disability can occur at any age and is not uniformly distributed across populations. Disability varies by gender, race, 

ethnicity, and geographic location.269 Approximately 18.7% of 

the U.S. population has a disability.234 In 2010, the percent of 

American adults with a disability was approximately 16.6% for 

those aged 18–64 and 49.8% for persons 65 and older.234 In 

2014, working-age adults with disabilities were substantially 

less likely to participate in the labor force (30.2%) than people 

without disabilities (76.2%), and experience more than twice 

the rate of unemployment (13.9% and 6.0%, respectively).270

People with disabilities experience disproportionately higher 

rates of social risk factors, such as poverty and lower educational attainment, that contribute to poorer health outcomes 

during extreme events or climate-related emergencies. These 

factors compound the risks posed by functional impairments 

and disrupt planning and emergency response. Of the climate-related health risks experienced by people with disabilities, perhaps the most fundamental is their “invisibility” to 

decision-makers and planners.271 There has been relatively 

limited empirical research documenting how people with 

disabilities fare during or after an extreme event.272

An increase in extreme weather can be expected to disproportionately affect populations with disabilities unless emergency 

planners make provisions to address their functional needs 

in preparing emergency response plans. In 2005, Hurricane 

Katrina had a significant and disproportionate impact on 

people with disabilities. Of the 986 deaths in Louisiana directly 

attributable to the storm, 103 occurred among individuals in 

nursing homes, presumably with a disability.273 Strong social 

capital and societal connectedness to other people, especially 

through faith-based organizations, family networks, and work 

connections, were considered to be key enabling factors that 

helped people with disabilities to cope before, during, and 

after the storm.274 In the aftermath of Hurricane Sandy, the 

City of New York lost a lawsuit filed by the Brooklyn Center for 

Independence of the Disabled (Brooklyn Center for Independence of the Disabled et al. v. Bloomberg et al., Case 1.11-cv06690-JMF 2013), with the finding that the city had not 

adequately prepared to accommodate the social and medical 

support needs of New York residents with disabilities.

Risk communication is not 

always designed or delivered in 

an accessible format or media 

for individuals who are deaf or 

have hearing loss, who are blind 

or have low vision, or those with 

diminished cognitive skills.275, 276

Emergency communication and 

other important notifications 

(such as a warning to boil contaminated water) simply may 

not reach persons with disabilities. In addition, persons with 

disabilities often rely on medical equipment (such as portable 

oxygen) that requires an uninterrupted source of electricity. 

Portable oxygen supplies must be evacuated with the patient.277

Persons with Chronic Medical Conditions 

Preexisting medical conditions present risk factors for increased illness and death associated with climate-related 

stressors, especially exposure to extreme heat. In some cases, 

risks are mediated by the physiology of specific medical conditions that may impair responses to heat exposure. In other 

cases, the risks are related to unintended side effects of medPeople with disabilities experience 

disproportionately higher rates of social risk 

factors, such as poverty and lower educational 

attainment, that contribute to poorer health 

outcomes during extreme events or 

climate-related emergencies.

Persons with disabilities often rely on medical equipment (such 

as portable oxygen) that requires an uninterrupted source of 

electricity.
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U.S. Global Change Research Program 261 Impacts of Climate Change on Human Health in the United States

ical treatment that may impair body temperature, fluid, or 

electrolyte balance and thereby increase risks. Trends in the 

prevalence of chronic medical conditions are summarized in 

Table 1 in Chapter 1: Introduction. In general, the prevalence 

of common chronic medical conditions, including cardiovascular disease, respiratory disease, diabetes, asthma, and obesity, is anticipated to increase over the coming decades (see 

Table 1 in Ch. 1: Introduction), resulting in larger populations 

at risk of medical complications from climate change related 

exposures.

Excess heat exposure has been shown to increase the risk 

of disease exacerbation or death for people with various 

medical conditions. Hospital admissions and emergency room 

visits increase during heat waves for people with diabetes, 

cardiovascular diseases, respiratory diseases, and psychiatric 

illnesses.40, 58, 195, 278, 279, 280, 281, 282 Medical conditions like Alzheimer’s disease or mental illnesses can impair judgment and behavioral responses in crisis situations, which can place people 

with those conditions at greater risk.228

Medications used to treat chronic medical conditions are 

associated with increased risk of hospitalization, emergency 

room admission, and in some cases, death from extreme 

heat. These medicines include drugs used to treat neurologic 

or psychiatric conditions, such as anti-psychotic drugs, anti-cholinergic agents, anxiolytics (anti-anxiety medicines), and 

some antidepressants (such as selective serotonin reuptake 

inhibitors or SSRIs; see also Ch. 8: Mental Health).233, 283, 284 In 

addition, drugs used to treat cardiovascular diseases, such 

as diuretics and beta-blockers, may impair resilience to heat 

stress.283, 285

People with chronic medical conditions also can be more vulnerable to interruption in treatment. For example, interrupting treatment for patients with addiction to drugs or alcohol 

may lead to withdrawal syndromes.286, 287, 288 Treatment for 

chronic medical conditions represents a significant proportion 

of post-disaster medical demands.289 Communities that are 

both medically underserved and have a high prevalence of 

chronic medical conditions can be especially at risk.290 While 

most studies have assessed adults, and especially the elderly, with chronic medical conditions, children with medical 

conditions such as allergic and respiratory diseases are also at 

greater risk of symptom exacerbation and hospital admission 

during heat waves.144

9.4 Measures of Vulnerability and Mapping

Vulnerability associated with exposures to climate-related 

hazards is closely tied to place. While an understanding of 

the individual-level factors associated with vulnerability is essential to assessing population risks and considering possible 

protective measures, understanding how potential exposures overlap with the geographic location of populations of 

concern is critical for designing and implementing appropriate adaptations. Analytic capabilities provided by mapping 

tools allow public health and emergency response workers to 

consider multiple types of vulnerability and how they interact 

with place. The development of indices that combine different elements of vulnerability and allow visualization of areas 

and populations experiencing the highest risks is related to 

improved geographic information systems (GIS) capabilities.291

There are multiple approaches for developing vulnerability 

indices to identify populations of concern across large areas, 

such as state or multistate regions, or small areas, such as 

households within a county or several counties within a 

state.293 The Social Vulnerability Index (SVI) developed by 

the CDC aggregates U.S. census data to estimate the social 

vulnerability of census tracts (which are generally subsets 

of counties; Figure 4). The SVI provides a measure of overall 

social vulnerability in addition to measures of elements that 

comprise social vulnerability (including socioeconomic status, 

household composition, race or ethnicity, native language, 

and infrastructure conditions). Each census tract receives a 

separate ranking for overall vulnerability and for each of the 

four elements, which are available at the census-tract level 

for the entire United States. A similar methodology has been 

Mapping Social Vulnerability

Figure 4: CDC Social Vulnerability Index (SVI): This interactive 

web map shows the overall social vulnerability of the U.S. 

Southwest in 2010. The SVI provides a measure of four social 

vulnerability elements: socioeconomic status; household 

composition; race, ethnicity, and language; and housing/

transportation. Each census tract receives a separate ranking for 

overall vulnerability at the census-tract level. Dark blue indicates 

the highest overall vulnerability (the top quartile) with the lowest 

quartile in pale yellow. (Figure source: ATSDR 2015)292
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U.S. Global Change Research Program 262 Impacts of Climate Change on Human Health in the United States

FPO

Mapping Heat Vulnerability in Georgia

Figure 5: Vulnerability to heat-related illness in Georgia extends beyond urban zones. The map on top shows a composite 

measure of social vulnerability for the Atlanta, Georgia Metropolitan Area (darkest colors indicate the most vulnerable 

areas). The six state-wide maps below show the following six vulnerability factors: 1) percent population below the poverty 

level, 2) percent aged 65 and older living alone, 3) heat event exposure with Heat Index over 100oF for two consecutive 

days, 4) percent dialysis patients on Medicare, 5) hospital insufficiency based upon accessibility of hospital infrastructure, 

and 6) percent impervious surface. Areas located in rural southern Georgia experienced more hazardous heat events, had 

less access to health care, and had a higher percentage of people living alone. (Figure source: adapted from Manangan 

et al. 2014)294
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U.S. Global Change Research Program 263 Impacts of Climate Change on Human Health in the United States

used to develop a vulnerability index for climate-sensitive 

health outcomes which, in addition to socioeconomic data, 

incorporates data on climate-related exposures and adaptive 

capacity.294

Application of Vulnerability Indices

GIS—data management systems used to capture, store, manage, retrieve, analyze, and display geographic information—can 

be used to quantify and visualize factors that contribute to 

climate-related health risks. By linking together census data, 

data on the determinants of health (social, environmental, preexisting health conditions), measures of adaptive capacity (such 

as health care access), and climate data, GIS mapping helps 

identify and position resources for at-risk populations.4, 10, 294, 295, 

296, 297 For instance, heat-related illnesses have been associated 

with social isolation in older adults, which can be mapped by 

combining data for persons living alone (determinants of health 

data), distribution of people aged 65 and older (census data), 

and frequency and severity of heat waves (climate data). 

Vulnerability mapping can also enhance emergency and disaster risk management.298, 299 Vulnerability mapping conducted 

at finer spatial resolution (for example, census tracts or census 

blocks) allows public health departments to target vulnerable 

communities for emergency preparedness, response, recovery, 

and mitigation.300 Geographic characteristics of vulnerability 

can be used to determine where to position emergency medical and social response resources that are most needed before, 

during, and after climate change related events.298, 299, 300

Emergency response agencies can apply lessons learned by 

mapping prior events. For example, vulnerability mapping 

has been used to assess how social disparities affected the 

geography of recovery in New Orleans following Hurricane Katrina.9

 Maps displaying the intersection of social vulnerability 

(low, medium, high scores) and flood inundation (none, low, 

medium, high levels) showed that while the physical manifestation of the disaster had few race or class distinctions, the 

social vulnerability of communities influenced both pre-impact responses, such as evacuation, and post-event recovery.9

As climate change increases the probability of more frequent 

or more severe extreme weather events, vulnerability mapping is an important tool for preparing for and responding to 

health threats.

9.5 Research Needs 

A number of research needs related to populations of concern 

have been identified. There are some limitations with current 

public health surveillance and monitoring of risk factors that 

impede the development of projections of vulnerability to 

climate change impacts. Obtaining detailed data on social, 

economic, and health factors that contribute to vulnerability 

is challenging, especially at the small spatial scales required 

for analyzing climate change impacts. Privacy concerns often 

limit the collection and use of personal health and socioeconomic data. Ultimately, data limitations determine the feasibility of developing alternative vulnerability indicators using 

existing data sources. The science requires comprehensive 

and standardized measures of vulnerability that combine data 

identification and collection with the development of appropriate vulnerability indices.

More comprehensive and robust projections of factors that 

contribute to population vulnerability would also enhance the 

value of predictive models. At present, there are only limited 

projections of health status of the U.S. population, and the 

U.S. Census no longer provides population projections at the 

state level. Projecting population vulnerability into the future, 

as well as the development of consensus storylines that characterize alternative socioeconomic scenarios, will facilitate 

more robust and useful assessments of future health impacts 

of climate change.

Future assessments can benefit from research activities that:

• improve understanding of the relative contributions and 

causal mechanisms of vulnerability factors (for example, 

genetic, physiological, social, behavioral) to risks of specific 

health impacts of climate change;

Mapping Heat Vulnerability in Georgia, continued

The CDC conducted a case study of heat-related vulnerability in Georgia using data from 2002 to 2008. This 

climate and health vulnerability assessment, which identifies people and places that are most susceptible to 

hazardous exposures from climate change, uses GIS to overlay six maps depicting population-level sensitivity 

(poverty levels, elderly people living alone, preexisting health conditions, and people living in urban areas), 

adaptive capacity (a measure of access to healthcare), and exposure (a measure of heat events). The study found 

that vulnerability to heat-related illness in Georgia extends beyond urban zones. In fact, areas located in the 

southern portion of Georgia, which is more rural, experienced more hazardous heat events, had less access to 

health care, and had a higher percentage of people living alone. These types of studies allow researchers to use 

GIS to identify vulnerable communities, which can aid in the development of public health interventions and 

other adaptation strategies294 (Figure 5).
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U.S. Global Change Research Program 264 Impacts of Climate Change on Human Health in the United States

• investigate how available sources of data on population 

characteristics can be used to create valid indicators and 

help map vulnerability to the health impacts of climate 

change; 

• understand how vulnerability to both medical and psychological health impacts of climate change affect cumulative 

stress and health status; and

• evaluate the efficacy of measures designed to enhance resilience and reduce the health impacts from climate change at 

the individual, institutional, and community levels.
U.S. Global Change Research Program 265 Impacts of Climate Change on Human Health in the United States

PROCESS FOR DEVELOPING CHAPTER

The chapter was developed through technical discussions 

of relevant evidence and expert deliberation by the report 

authors at several workshops, teleconferences, and email 

exchanges. The authors considered inputs and comments 

submitted by the public, the National Academies of Sciences, 

and Federal agencies. For additional information on the 

overall report process, see Appendices 2 and 3. 

The author team identified a number of populations affected 

by climate change health impacts, including communities 

of color and low-income, immigrant, and limited English 

proficiency groups; Indigenous populations; children 

and pregnant women; older adults; certain occupational 

groups; persons with disabilities; and persons with chronic 

medical conditions. This list of populations was identified to 

reflect current understandings related to how the health of 

particular groups of people or particular places are affected 

by climate change in the United States. While not exhaustive, 

these populations of concern are those most commonly 

identified and discussed in reviews of climate change health 

impacts on vulnerable populations. In this chapter, the order 

of these populations is not prioritized. While there are other 

populations that may be threatened disproportionately 

by climate change, the authors focused the sections of 

this chapter on populations for which there is substantive 

literature. In addition to this chapter’s summary of vulnerable 

populations, each of the health outcome chapters in the 

report includes discussion of populations of concern. Some 

populations may be covered more extensively in these other 

chapters; for instance, homeless populations are discussed in 

Chapter 8: Mental Health, as the literature on this population 

focuses primarily on mental health.

KEY FINDING TRACEABLE ACCOUNTS

Vulnerability Varies Over Time and Is Place-Specific

Key Finding 1: Across the United States, people and 

communities differ in their exposures, their inherent 

sensitivity, and their adaptive capacity to respond to and 

cope with climate change related health threats [Very High 

Confidence]. Vulnerability to climate change varies across 

time and location, across communities, and among 

individuals within communities [Very High Confidence].

Description of evidence base

There is strong evidence from multiple current 

epidemiological studies on climate-sensitive health outcomes 

in the United States that health impacts will differ by 

location, pathways of exposure, underlying susceptibility, 

and adaptive capacity. The literature consistently finds 

that these disparities in health impacts will largely result 

from differences in the distribution of individual attributes 

in a population that confers vulnerability (such as age, 

socioeconomic status, and race), attributes of place that 

reduce or amplify exposure (such as floodplain, coastal zone, 

and urban heat island), and the resilience of public health 

infrastructure.

Across multiple studies, the following factors are consistently 

identified that contribute to exposure: occupation,11 time 

spent in risk-prone locations,12, 13, 14 displacement by weather 

extremes,64 economic status,15, 16 condition of infrastructure,17, 

18 and compromised mobility, cognitive function, and other 

mental or behavioral factors.19

There is consensus within the scientific literature that biologic 

sensitivity and adaptive capacity are tied to many of the 

same factors that contribute to exposures, and that all of 

these factors can change across time and life stage.12, 20, 21

There is also strong evidence from multiple studies that social 

and economic factors affect disparities in the prevalence 

of chronic medical conditions that aggravate biological 

sensitivity.22, 23

Major uncertainties 

Understanding how exposure, sensitivity, and adaptive 

capacity change over time and location for specific 

populations of concern is challenging, particularly when 

attempting to project impacts of climate change on health 

across long time frames (such as in the year 2100, a year 

for which climate projections often estimate impacts) or 

vast geographic areas. Uncertainties remain with respect to 

the underlying social determinants of health, public health 

interventions or outreach, adaptation options, and climate 

impacts at fine local scales.

Assessment of confidence and likelihood based on evidence 

Based on the evidence presented in the peer-reviewed 

literature, there is very high confidence that climate change 

impacts on health will vary across place and time, as 

demonstrated by the complex factors driving vulnerability. 

Many qualitative and quantitative studies have been 

published with consistent findings and strong consensus that 

the impacts of climate change on human health will vary 

according to differential exposure, sensitivity, and adaptive 

capacity, which change over time and across places. These 

conclusions are well-documented and supported by highquality evidence from multiple sources. 

Supporting Evidence
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U.S. Global Change Research Program 266 Impacts of Climate Change on Human Health in the United States

Health Impacts Vary with Age and Life Stage 

Key Finding 2: People experience different inherent 

sensitivities to the impacts of climate change at different ages 

and life stages [High Confidence]. For example, the very young 

and the very old are particularly sensitive to climate-related 

health impacts.

Description of evidence base 

There is strong, consistent evidence from multiple studies 

that children have inherent sensitivities to climate-related 

health impacts. There are multiple, high-quality studies concerning the impact of changes in ground-level ozone, particulate matter, and aeroallergens on increases in childhood 

asthma episodes and other adverse respiratory effects in 

children.151, 152, 153, 156 In addition, the literature supports a finding that children are vulnerable to waterborne pathogens in 

drinking water and through exposures while swimming. There 

is a positive and statistically significant association between 

heavy rain and emergency department visits for children with 

gastrointestinal illness, though evidence comes from regional 

studies and is not at the national scale.159, 160 The science also 

supports a finding that children’s mental health is affected by 

exposures to traumatic weather events, which can undermine 

cognitive development and contribute to psychiatric disorders.149, 150

There is also strong, consistent evidence from multiple studies 

that older adults have inherent sensitivities to climate-related 

health impacts. In particular, exposure to extreme ambient 

temperature is an important determinant of health in older 

adults24, 203 and has been associated with increased hospital 

admissions for cardiovascular, respiratory, and metabolic 

disorders.26, 33 In addition, older adults are particularly 

affected by extreme weather events that compromise the 

availability and safety of food and water supplies; interrupt 

communications, utilities, and emergency services; and 

destroy or damage homes and the built environment.209, 210, 

211, 226 Some functional and mobility impairments make older 

adults less able to evacuate when necessary.231, 301

Major uncertainties

There is less information with which to quantify climaterelated impacts on children and older adults at a national 

level given limited data availability. Some studies of agerelated vulnerability have limited geographic scope or focus 

on single events in particular locations. Nevertheless, multiple 

factors, all with some degree of uncertainty, converge to 

determine climate-related vulnerability across age groups.

Assessment of confidence and likelihood based on evidence

Based on the evidence presented in the peer-reviewed 

literature, there is high confidence that a wide range 

of health effects exacerbated by climate change will be 

experienced by vulnerable age groups, especially young 

children and older adults. Both qualitative and quantitative 

studies have been published about the effects of age or life 

stage on vulnerability to health impacts, and that evidence is 

consistent and of good quality.

Social Determinants of Health Interact with Climate 

Factors to Affect Health Risks

Key Finding 3: Climate change threatens the health of people 

and communities by affecting exposure, sensitivity, and 

adaptive capacity [High Confidence]. Social determinants 

of health, such as those related to socioeconomic factors 

and health disparities, may amplify, moderate, or otherwise 

influence climate-related health effects, particularly when 

these factors occur simultaneously or close in time or space 

[High Confidence].

Description of evidence base

The literature is consistent and the results are compelling 

that social determinants of health, such as those related to 

socioeconomic factors and health disparities, will contribute 

to the nature and extent of vulnerability and health effects 

due to climate change. The following factors illustrate the 

depth of the literature supporting the conclusions above 

regarding the relationship between climate change health 

threats, vulnerability (comprised of exposure, sensitivity, and 

adaptive capacity), and social determinants of health:

• Occupation: where workers are at risk due to their place of 

employment or the nature of their duties.11

• Time spent in risk-prone locations: There is an extensive 

literature base and broad consensus to support a finding 

that locations that experience greater risks include urban 

heat islands where exposed populations are likely to have 

limited adaptive capacity due to poor housing conditions, 

and inability to use or to afford air conditioning.15, 24, 26, 34, 35, 

38, 55, 56, 57 . 

• Economic status: In the literature, a significant relationship 

has been observed that links people living in poverty with 

being less likely to have adequate resources to prepare 

for or respond to extreme events or to access and afford 

necessary health or supportive services to cope with 

climate-related health impacts.39, 42, 43, 44, 45, 46

• Condition of infrastructure: Deteriorating infrastructure 

exposes people to increased health risks. The literature is 

consistent and of good quality to support a finding that 

persons who evacuate may be hampered by damage to 

transportation, utilities, and medical or communication 

facilities and by a lack of safe food or drinking water 

supplies.12, 139, 226, 229

• Disparities in health conditions: Health disparities 

contribute to the sensitivity of people to climate change. 

Numerous studies indicate increased sensitivity and health 

risk for people with chronic or preexisting medical or 

psychological illnesses, people of certain age or stage of 

life; and people with compromised mobility or cognitive 

functioning.143, 289, 290 Social determinants of health 

contributing to disparities in rates of these conditions 

increase sensitivity of affected populations.32, 206, 289, 290

Health risks and vulnerability may increase in locations or 

instances where combinations of social determinants of 

health that amplify health threats occur simultaneously or 

close in time or space.6, 7 For example, people with limited 
9–POPULATIONS OF CONCERN

U.S. Global Change Research Program 267 Impacts of Climate Change on Human Health in the United States

economic resources living in areas with deteriorating 

infrastructure are more likely to experience disproportionate 

impacts and are less able to recover following extreme 

events,8, 9 increasing their vulnerability to climate-related 

health effects. 

Major uncertainties

A wide range of non-climate factors are expected to interact 

with climate change health impacts to determine population 

vulnerability, all with some degree of uncertainty. The extent 

to which social determinants of health individually and 

collectively affect the different components of vulnerability is, 

in many cases, not well understood and not readily amenable 

to measurement or quantification. Assessing the extent and 

nature of non-climate impacts as compared to impacts related 

to climate change is limited by data availability. Many studies 

of climate change vulnerability have limited geographic scope 

or focus on single events in particular locations, which makes 

drawing national-level conclusions more challenging.

Assessment of confidence and likelihood based on evidence

Based on the evidence presented in the peer-reviewed 

literature, there is high confidence that climate change 

threatens the health of people and communities by affecting 

exposure, sensitivity, and adaptive capacity. This conclusion 

takes into account the consistent evidence presented 

in multiple studies regarding the causes of vulnerability 

to climate-related health effects and the role of social 

determinants of health. There is high confidence based on 

many peer-reviewed studies that social determinants of 

health, such as those related to socioeconomic factors and 

health disparities, may amplify, moderate, or otherwise 

influence climate-related health effects across populations 

of concern, and the evidence presented is of good quality, 

consistent, and compelling.

Mapping Tools and Vulnerability Indices Identify 

Climate Health Risks

Key Finding 4: The use of geographic data and tools allows 

for more sophisticated mapping of risk factors and social 

vulnerabilities to identify and protect specific locations and 

groups of people [High Confidence].

Description of evidence base

Over the past decade, the literature on the use of GIS in a 

public health and vulnerability context has been steadily 

growing. Multiple studies provide strong, consistent evidence 

that spatial-analytic tools help facilitate analyses that link 

together spatially resolved representations of census data, 

data on the determinants of health (social, environmental, 

preexisting health conditions), measures of adaptive capacity 

(such as health care accessibility), and environmental data 

for the identification of at-risk populations.4, 10, 294, 295, 296, 297

Similarly, the more recent additions to the literature indicate 

that demographic and environmental data can be integrated 

to create an index that allows for analysis of the factors 

contributing to social vulnerability in a given geographic 

area.292, 294 Multiple studies conclude that spatial mapping 

that identifies factors associated with relative vulnerability 

is an important step in developing prevention strategies or 

determining where to focus or position health or emergency 

response resources.298, 299, 300 Fewer studies explicitly focus 

on vulnerability mapping in a climate change context, with 

the notable exception of the case study of heat-related 

vulnerability in Georgia.294

Major uncertainties

Multiple factors, all with some degree of uncertainty, 

determine geographic vulnerability to the health impacts 

of climate change. Although the literature indicates that 

mapping tools and vulnerability indices are useful in 

characterizing geographically based exposures, geocoded 

health data (particularly those data relevant to an analysis of 

climate change vulnerability) are not always available in some 

locations of interest. In addition, the extent of uncertainty 

increases at smaller spatial scales, which is typically the 

scale most relevant for targeting vulnerable communities. 

For instance, mental health outcome data are particularly 

challenging to obtain and geocode, partly because the 

majority of cases are underdiagnosed or underreported (see 

Ch. 8: Mental Health). 

Assessment of confidence and likelihood based on evidence

Based on the evidence presented in the peer-reviewed 

literature, there is high confidence that geographic data 

used in mapping tools and vulnerability indices can help 

to identify where and for whom climate health risks are 

greatest. A number of published studies provide consistent 

and good quality evidence to support a finding regarding the 

utility of mapping tools and vulnerability indices in a public 

health context, but methods are still emerging to support 

the application of these tools in the context of climate 

change. Overall, evidence is strong that mapping tools and 

vulnerability indices can help to identify at-risk locations 

and populations for whom climate health risks are greatest. 

As the state of the science continues to evolve, substantial 

improvements in mapping and spatial analytic tools and 

methodologies are expected that will allow researchers to 

predict, for a certain geographic area, the probability that 

human health impacts will occur across time.
9–POPULATIONS OF CONCERN

U.S. Global Change Research Program 268 Impacts of Climate Change on Human Health in the United States

DOCUMENTING UNCERTAINTY

This assessment relies on two metrics to communicate 

the degree of certainty in Key Findings. See Appendix 4: 

Documenting Uncertainty for more on assessments of 

likelihood and confidence.

PHOTO CREDITS

Pg. 247–Nurse and migrant worker: © Karen Kasmauski/

Corbis 

Pg. 248–Elderly Navajo woman and her niece: © Alison 

Wright/Corbis

Pg. 249–People waiting in food line: © Brooks Kraft/Corbis

Pg. 252–Nurse and migrant worker: © Karen Kasmauski/

Corbis

Pg. 254–Indigenous deckhand: © Mike Kane/Aurora Photos/

Corbis

Pg. 256–Pregnant woman: © JGI/Tom Grill/Blend Images/

Corbis

Pg. 259–Soldiers training: The U.S. Army/Staff Sgt. Robert 

DeDeaux/Flickr

Pg. 260–Woman in wheelchair: © iStockPhotos.com/

ozgurkeser

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few sources, limited consistency, models 

incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts

Likelihood

Very Likely

≥ 9 in 10

Likely

≥ 2 in 3

As Likely As Not

≈ 1 in 2

Unlikely

≤ 1 in 3

Very Unlikely

≤ 1 in 10
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eca.0.0067
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© Alison Wright/Corbis
TECHNICAL SUPPORT DOCUMENT: 

MODELING FUTURE CLIMATE IMPACTS 1 ON HUMAN HEALTH

U.S. Global Change Research Program

Lead Authors

Allison Crimmins*

U.S. Environmental Protection Agency

John Balbus

National Institutes of Health

Janet L. Gamble

U.S. Environmental Protection Agency

Contributing Authors

David R. Easterling

National Oceanic and Atmospheric Administration

Kristie L. Ebi

University of Washington

Jeremy Hess

University of Washington 

Kenneth E. Kunkel

Cooperative Institute for Climate and Satellites–North Carolina 

David M. Mills

Abt Associates

Marcus C. Sarofim

U.S. Environmental Protection Agency

THE IMPACTS OF CLIMATE CHANGE ON HUMAN HEALTH IN THE UNITED STATES

A Scientific Assessment

APPENDIX

On the web: health2016.globalchange.gov *Chapter Coordinator

Recommended Citation: Crimmins, A., J. Balbus, J.L. Gamble, D.R. Easterling, K.L. Ebi, J. Hess, K.E. Kunkel, D.M. Mills, and M.C. 

Sarofim, 2016: Appendix 1: Technical Support Document: Modeling Future Climate Impacts on Human Health. The Impacts 

of Climate Change on Human Health in the United States: A Scientific Assessment. U.S. Global Change Research Program, 

Washington, DC, 287–300. http://dx.doi.org/10.7930/J0KH0K83

Acknowledgements: Jennifer Parker, Centers for Disease Control and Prevention
U.S. Global Change Research Program 288 Impacts of Climate Change on Human Health in the United States

Models are an important component of climate change impact 

projections. In general, quantitative evaluations of health impacts 

require projections of 1) physical climate changes, 2) future 

socioeconomic characteristics, and 3) the relationships between 

these factors and the health outcome of interest. Uncertainties 

exist in each of these areas, and aligning the spatial and temporal 

parameters used in climate models with epidemiological data to 

assess health outcomes can be challenging. Despite these challenges, health impact modeling continues to improve, increasing 

our understanding of the quantitative impacts associated with 

climate change (for example, Melillo et al. 2014; Tamerius et al. 

2007; Post et al. 2012).1, 2, 3

A1.1 Quantitative Evaluations of Health Impacts 

Projecting Climate Change Impacts 

Climate models are used to analyze past changes in the long-term 

averages and variations in temperature, precipitation, and other 

climate indicators and to make projections of how these trends 

may change in the future. Since there is no universally accepted 

set of metrics to identify the “best” climate models, it is standard 

practice to use an ensemble (a collection of simulations from different models) in order to present a range of results and provide 

a measure of the certainty in the results. In addition, because 

climate model results can depend on initial conditions (the state 

of the atmosphere and ocean at the moment the modeling run 

begins), even for a single model, multiple model simulations 

can be used to similarly present a range of results and improve 

understanding of variability. Climate model outputs may require 

additional processing, such as the use of downscaling methods 

when higher resolutions are needed, or coupling to an atmospheric chemistry model in order to examine and incorporate 

changes in local air quality.

Projections of climate changes are usually based on scenarios (or 

sets of assumptions) regarding how future emissions may change 

as a result of population, energy, technology, and economics. 

Over the past decade, climate change simulations were based primarily on emissions scenarios developed in the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions 

Scenarios (SRES).4 These scenarios were used as inputs to climate 

models in order to develop projections used in the Coupled Model Intercomparison Project Phase 3 (CMIP3). The global climate 

model (GCM) simulations included in CMIP use a standard experimental protocol so that their outputs can be compared. The IPCC 

Fifth Assessment Report5 drew upon model simulations from 

the Coupled Model Intercomparison Project Phase 5 (CMIP5), 

which collected simulation data from more recent models, used 

Representative Concentration Pathways (RCPs) in place of SRES 

scenarios, and incorporated updated historical forcing trends and 

other exogenous model inputs. 

CMIP5 contains approximately 60 climate representations from 

28 different modeling centers.6 The spatial resolution of most 

model grid cells is about 1° to 2° of latitude and longitude, or 

about 60 to 130 square miles. CMIP5 experiments simulate both 

a. the 20th century climate using the best available estimates 

of the temporal variations in external forcing factors (such 

as greenhouse gas concentrations, solar output, and volcanic aerosol concentrations); and

b. the 21st century climate based on future greenhouse gas 

concentration pathways resulting from various emissions 

scenarios.

Four RCP emissions pathways were used for the CMIP5 simulations: RCP2.6, RCP4.5, RCP6.0, and RCP8.5. These pathways are 

named according to the increase in radiative forcing (a measure 

of the total change in Earth’s energy balance) projected for that 

pathway in the year 2100 relative to preindustrial levels, measured in Watts per square meter (Wm−2). For example, RCP6.0 

projects that the end-of-century radiative forcing increase will 

be 6.0 Wm−2 above preindustrial levels. The range of simulated 

global average surface temperature changes under both SRES and 

RCPs is shown in Figure 1. 

Projecting Socioeconomic Development 

Along with the RCPs, used to provide a range of possible 

future greenhouse gas concentrations for climate models, the 

modeling of climate change impacts can be improved by acknowledging scenarios that describe future societal characteristics. For the IPCC’s Fifth Assessment Report,5

 impact modelers discussed the use of new scenarios constructed from three 

building blocks:

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• Representative Concentration Pathways (RCPs)

• Shared Socioeconomic Pathways (SSPs) 

• Shared Climate Policy Assumptions (SPAs) 

Shared Socioeconomic Pathways define plausible alternative states of global human and natural societies at a macro 

scale, including qualitative and quantitative factors such as 

demographic, political, social, cultural, institutional, lifestyle, 

economic, and technological variables and trends. Also included are the human impacts on ecosystems and ecosystem 

services, such as air and water quality.7, 8, 9

As with the IPCC Fifth Assessment Report, SSPs are not 

explicitly used in the analyses highlighted in this assessment. 

However, because these scenarios are likely to be used by the 

impacts modeling community over the next few years, placing 

the approach taken in this assessment in context is a valuable 

exercise. 

Five reference SSPs, referred to as SSP1 through SSP5,9 describe challenges to adaptation (efforts to adapt to climate 

change) and mitigation (efforts to reduce the amount of 

climate change) that change over time irrespective of climate 

change.7, 8, 9 Although the SSPs describe broad-scale global 

trends across multiple sectors, these trends are relevant to 

projections of health impacts in the United States; trends 

within each SSP represent different challenges for maintaining 

and improving the health of Americans. For example, future 

vulnerability to changing concentrations of air pollutants, 

particularly ozone, will in part depend on demographics, urbanization, policies to control air pollutants, and hemispheric 

transport of emissions from areas outside the region. 

The combination of RCP6.0 (used by most of the analyses 

highlighted in the Temperature-Related Death and Illness, Air 

Quality Impacts, Vector-Borne Diseases, and Water-Related 

Illness chapters—see Section A1.2) and the population parameters for the SRES B2 emissions pathway (used in the extreme 

heat and ozone analyses highlighted in Ch. 2: Temperature-Related Death and Illness and Ch. 3: Air Quality Impacts) can 

be partially mapped to the SSP2 storyline.9, 10 SSP2 depicts a 

Scenarios of Future Temperature Rise

Figure 1: Projected global average temperature rise for specific emissions pathways (left) and concentration pathways (right) relative 

to the 1901−1960 average. Shading indicates the range (5th to 95th percentile) of results from a suite of climate models. Projections 

in 2099 are indicated by the bars to the right of each panel. In all cases, temperatures are expected to rise, although the difference 

between lower and higher pathways is substantial.

The left panel shows the two main CMIP3 scenarios (SRES) used in this assessment: A2 assumes continued increases in emissions 

throughout this century, and B1 assumes significant emissions reductions beginning around 2050. The right panel shows the newer 

CMIP5 scenarios using Representative Concentration Pathways (RCPs). CMIP5 includes both lower and higher pathways than 

CMIP3. The lowest concentration pathway shown here, RCP2.6, assumes immediate and rapid reductions in emissions and would 

result in about 2.5°F of warming in this century. The highest pathway, RCP8.5, roughly similar to a continuation of the current path 

of global emissions increases, is projected to lead to more than 8°F warming by 2100, with a high-end possibility of more than 11°F. 

(Data from CMIP3, CMIP5, and NOAA NCEI). (Figure source: adapted from Melillo et al. 2014)1
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world where global health improves at an intermediate pace. 

Under SSP2, multiple factors contribute to some countries 

making slower progress in reducing health burdens, including, 

in some low-income countries, high burdens of climate-related 

diseases combined with moderate to high population growth. 

In the United States, challenges to public health infrastructure 

and health care under this socioeconomic pathway could include inadequate resources and international commitment for 

1) integrated monitoring and surveillance systems, 2) research 

on and modeling of the health risks of climate change, 3) iterative management approaches, 4) training and education of 

health care and public health professionals and practitioners, 

and 5) technology development and deployment.7

The SSPs do not include any explicit climate policy assumptions. This role is reserved for the Shared Climate Policy 

Assumptions (SPAs) which capture key policy attributes such 

as the goals, instruments, and obstacles of mitigation and 

adaptation measures up to the global and century scale.11 In 

this way, the SPAs provide the link between RCPs and SSPs 

by allowing for a variety of alternative socioeconomic evolutionary paths to be coupled with a library of climate model 

simulations created using the RCPs. SPAs are also not used in 

the analyses highlighted in this assessment. 

Projecting Health Outcomes 

Public health officials often require information on health risks at 

relatively local geographic scales. Climate models, on the other 

hand, are better at projecting changes on national to global scales 

and over timescales of decades to centuries. Figure 2 shows two 

illustrative resolutions for eastern North American topography. 

The top figure has a grid cell resolution of 68 miles by 68 miles, 

which is comparable to high resolution global models with 

projections at a 1° latitude by 1° longitude resolution. The lower 

figure shows how the same topography would look using smaller 

grid cells with a resolution of 19 miles by 19 miles. The finer detail 

at the higher resolution (note, for example, the better representation of the elevation changes of the Appalachian Mountains) 

would potentially improve a model’s ability to provide local 

information, as temperature, winds, and other features of the 

model simulation are all influenced by topography. On the other 

hand, models with higher resolution are not necessarily better at 

capturing large-scale climate changes and weather patterns.

In addition to higher spatial resolutions, public health officials are 

also generally most interested in short-term projections of future 

conditions (for example, one to five years). This is in part due to 

the fact that these officials work in resource-constrained environments where relative priorities and associated funding decisions 

can shift, often quickly. In addition, they provide services to populations with characteristics that are likely to change in response to 

changing economic conditions, immigration patterns, or impacts 

of extreme weather events. In this short timeframe, public health 

officials typically focus on information regarding the timing and 

magnitude of specific events or combinations of events that 

would stress existing programs and systems (for example, heat 

waves, tropical storms, wildfires, and air quality events). The oneto five-year information requirements of public health providers 

can contrast with the information climate modelers can develop, 

which project future conditions for timescales of decades to centuries and often derive impacts in 2050 or 2100. Climate models 

provide less guidance in terms of changes in near-term impacts 

because short-term variability from natural sources such as ocean 

circulation can obscure the long-term climate trends produced by 

increasing greenhouse gas concentrations. As such, climate projections over longer time periods typically serve more as a guide 

to emerging issues and as an input to longer-range planning. 

Example of Increasing Spatial Resolution of 

Climate Models

Figure 2: Top: Illustration of eastern North American topography 

in a resolution of 68 miles x 68 miles (110 x 110 km). Bottom: 

Illustration of eastern North America at a resolution of 19 miles 

x 19 miles (30 x 30 km). Global climate models are constantly 

being enhanced as scientific understanding of climate improves 

and as computational power increases. For example, in 1990, the 

average model divided up the world into grid cells measuring more 

than 300 miles per side. Today, most models divide the world up 

into grid cells of about 60 to 100 miles per side, and some of the 

most recent models are able to run short simulations with grid 

cells of only 15 miles per side. Supercomputer capabilities are the 

primary limitation on grid cell size. Newer models also incorporate 

more of the physical processes and components that make up the 

Earth’s climate system. (Figure source: Melillo et al. 2014)1
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A1.2 Modeling Highlighted in the Assessment 

The four chapters that highlight modeling studies conducted 

for this assessment (Temperature-Related Death and Illness, 

Air Quality Impacts, Vector-Borne Diseases, and Water-Related 

Illness) analyzed a subset of the full CMIP5 dataset (see Table 1). 

The air quality analyses required the most intensive processing 

of the CMIP5 model output; calculating air quality changes at the 

appropriate geographic scale requires modelers to use a technique known as dynamical downscaling to generate climate data 

at the desired small-scale resolution, and then run an atmospheric chemistry model, both of which are computationally intensive 

processes. Thus the ozone analysis was limited to two model–scenario examples (see Table 1). By contrast, the water-related illness 

analyses examined results from 21 of the CMIP5 models, though 

only for one particular scenario. 

In general, the authors of the studies highlighted in this assessment used historical data in order to calibrate their historical 

results and to improve geographic resolution. These downscaling 

approaches determine the climate signal by taking the difference 

between the modeled future and the modeled historical period 

at the grid cell resolution (often averaged over 30 years). This 

climate signal can then be added to observed historical data at a 

resolution potentially much finer than the model grid cell scale. 

For example, any given weather station might be, on average, 

cooler in the summer than the grid cell average because it is 

located next to a lake. By adding the modeled climate signal to 

the historical data from the weather station, the projected future 

temperatures can more effectively account for microclimate effects, from lakes or hills for example, that are consistent with historical variation at a spatial resolution smaller than the modeled 

grid scale. More sophisticated calibrations can also adjust model 

variability to match historical variability by using a technique 

known as quantile mapping.12

The modeling studies highlighted in this assessment use several approaches. The three different historical reference periods 

used in the highlighted studies (1985–2000, 1992–2007, and 

1976–2006) are slightly warmer than the 1971–2000 period used 

in the 2014 National Climate Assessment (NCA), by 0.3°F to 0.8°F. 

In addition, different sets of climate models were used. 

A sensitivity analysis was conducted to test for two potential impacts of using different modeling approaches: the use of different 

historical reference periods and the use of different sets of CMIP5 

models. Figure 3 shows the change in temperature from the 2014 

NCA reference period (1971–2000) for three historical reference 

periods used in the studies highlighted (first column). The differences among these three historical reference periods are small 

compared with the warming projected for the middle of this 

century by the different sets of models used (second column). 

For the sets of 21, 11, and 5 models used in the studies of Vibrio/

Alexandrium species, Gambierdiscus species, and Lyme disease, 

respectively, the multi-model mean warming for the middle of 

the 21st century are within 0.5°F of each other, although the set 

of 11 models does not include a few of the cooler models and the 

set of 5 models spans a narrower range. The two models used 

in the extreme temperature study are slightly warmer than the 

mean of the entire set of models, while the single model used 

in the air quality (ozone) study is slightly cooler. However, these 

differences in mean warming among the five approaches shown 

in the second column are small compared to projected warming. 

Figure 3: A sensitivity analysis was 

conducted to test for potential impacts of 

differences in the modeling approaches 

(use of different historical reference 

periods and use of different sets of 

CMIP5 models) in the research studies 

highlighted in this assessment (see 

Research Highlights in Chapters 2, 3, 

5, and 6). The values in the first column 

are temperature changes for three 

different reference periods used in this 

assessment, relative to the 1971–2000 

reference period used in the 2014 NCA. 

The sets of values in the second column 

show future temperature changes for 

individual climate models for 2050–2059, 

relative to 1971–2000, for those studies 

that used the RCP6.0 scenario. 

From left to right, the vertical sets of 

values represent (a) 21 models used in the Vibrio/Alexandrium bacteria study (red), (b) 11 models used in the Gambierdiscus

study (green), (c) the 5 models used in the Lyme disease study (purple), (d) the 2 models used in the extreme temperature study 

(blue), and (e) the single model used in the air quality study (orange). Each “x” represents a single model. The filled-in circle is the 

mean temperature change for all models in the column. (Figure source: NOAA NCEI / CICS-NC)

Sensitivity Analysis of Differences in Modeling Approaches
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Each modeling approach requires different input from the climate 

models. For example, the temperature mortality analysis required 

only temperature data, while the analysis in the Water-Related 

Illness chapter used sea surface temperature data. However, the 

ambient air quality modeling required temperature, precipitation, 

ventilation, and other data in order to provide boundary conditions for the dynamical downscaling approach. Besides climate 

data, modeling teams also used other inputs. The main sources 

of additional data were the Integrated Climate and Land Use Scenarios (ICLUS) model for population projections and the Environmental Benefits Mapping and Analysis Program (BenMAP) model 

for baseline mortality data, which were used for the extreme 

temperature and air quality modeling efforts.13, 14 The analysis 

in the Water-Related Illness chapter required salinity, light, and 

other oceanographic data not provided by the CMIP5 models. 

Table 1: See Research Highlights in Ch. 2: Temperature-Related Death and Illness; Ch. 3: Air Quality Impacts; 

Ch. 6: Water-Related Illness; Ch. 5: Vector-Borne Diseases.

Chapter Modeled 

Endpoint

TimeframeTemporal 

Resolution

Scenarios/

Pathways

Models Bias Correction and/or 

Downscaling

Geographic 

Scope

Climate 

Variables

Additional 

Data Inputs

TemperatureRelated Death 

and Illness

Mortality15 2030, 2050, 

2100

30 years RCP6.0 GFDL–

CM3, 

MIROC5

Statistical 

downscaling, 

then delta 

approach

209 U.S. 

cities

Temperature 

(0–5 day lags)

BenMAP 

baseline 

mortality 

data

Air Quality Mortality/ 

Morbidity from 

changes in 

ozone16

2030 3 years within 

11 year span

RCP6.0 GISS-E2 Dynamic 

downscaling

National Temperature, 

precipitation, 

ventilation, 

others

ICLUS population data, 

BenMAP 

health model, SES, air 

condition 

prevalence, 

baseline 

health 

status data

2030 11 year 

average

RCP8.5 CESM Dynamic 

downscaling

National Temperature, 

precipitation, 

ventilation, 

others

Changes 

in air exchange that 

drive indoor 

air quality17

2040–70 30 years SRES A2 CCSM, 

CGM3, 

GFDL, 

HadCM3

Dynamic 

downscaling 

9 U.S. cities Temperature, 

wind speed 

at 3-hour 

resolution

NA

Water-Related 

Illness

Seasonality and 

geographic 

range 

of Vibrio

bacteria18

2030, 2050, 

2095

Decadal 

average of 

monthly data

RCP6.0 21 CMIP5 

models (4 

used for 

Alaska)

Statistical 

downscaling; 

bias correction 

& quantile 

mapping

Chesapeake 

bay, Alaskan 

Coast

SST (driven 

by surface air 

temperature)

NA

Seasonality of 

Alexandrium

bacteria18

2030, 2050, 

2095

Decadal 

average of 

monthly data

RCP6.0 21 CMIP5 

models

Statistical downscaling; mean 

and variance 

bias correction

Puget Sound SST (driven 

by surface air 

temperature)

NA

Growth 

rates of 3 

Gambierdiscus algae 

species19

2000–2099 Annual RCP6.0 11 CMIP5 

models

Mean and 

variance bias 

correction, then 

temporal 

disaggregation

Gulf of 

Mexico and 

Caribbean

SST Salinity, 

light, and 

other 

biological 

and oceanographic 

variables 

Vector-Borne 

Disease

Lyme 

disease 

onset 

week20

2025–2040 

and 

2065–2080

16 year 

periods

RCP2.6, 

RCP4.5, 

RCP6.0, 

RCP8.5

CESM1

(CAM5), 

GFDL–

CM3, 

GISS–

E2–R, HadGEM2-ES, 

MIROC5

Statistical downscaling, then 

delta approach

12 U.S. 

states where 

Lyme is 

prevalent

Temp (growing 

degree days) 

precipitation, 

and saturation 

deficit (assume 

constant 

relative

humidity)

Distance 

to coast 

in decimal 

degrees
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a 1.5°C and 4.5°C (2.7°F to 8.1°F) increase in global average 

temperature (see Figure 1).5

Climate scientists have greater confidence in predicting the 

average temperature of the whole planet than what the temperature will be in any given region or locale. Global average 

temperatures may not, however, be particularly informative 

for determining health impacts at a local scale. An increase 

in global temperatures will, at local scales, result in different 

warming rates in different locations, different seasonal warming rates, different warming rates during the day compared to 

the night, and different changes in day-to-day or year-to-year 

variability. Despite these possible differences, it is highly likely 

that warming will occur almost everywhere.21 In addition to 

temperature, changes in precipitation, humidity, and weather 

systems are all important drivers of local impacts. However, 

future changes in these variables are less certain than changes 

in temperature.

Uncertainty in Public Health Surveillance and 

Monitoring 

Improvements in understanding future health impacts can result 

from better understanding current health impacts. Obtaining 

this understanding is complicated by the fact that, in the United 

States, there is no single source of health data and surveillance 

often involves acquiring, analyzing, and interpreting data from 

several sources collected using potentially different techniques 

and systems.22, 23 This is further complicated by a number of additional limitations, including the fact that data are often incomplete, may not include a representative sample of all members 

of society, and rely on reporting of disease status. Estimates of 

disease patterns or trends may also vary across geographic locations.23 Understanding the surveillance and monitoring limitations regarding population health data and spatial variability can 

enable more accurate estimations of the confidence in the links 

between health impacts and climate drivers, and this can be used 

to estimate uncertainty in future projections of health impacts.

Having complete socioeconomic, geographic, demographic, and 

health data at an individual level for everyone would improve 

our understanding of connections between these attributes 

and deaths and illnesses. However, such complete data are not 

available for both practical and confidentiality reasons. Mandatory reporting, disease records, and administrative sources, 

including data from medical records or vital records, can be used 

to estimate counts of given health impacts and these counts can 

be divided by population estimates to produce health impact 

rates. Uncertainty in the data can differ depending on the type 

of population health estimate and the existing surveillance data 

source used (such as using registries versus surveys). 

In addition to uncertainty regarding the quality and usefulness of 

data, confidence in estimates of health impacts depends on the 

extent of useable data. In general, the larger the data set (larger 

The modeling approaches also included different geographic 

scales. The Water-Related Illness analyses examined individual 

bodies of water such as the Chesapeake Bay, Puget Sound, and 

the Gulf of Mexico. The vector-borne disease projections of Lyme 

disease concentrated on the 12 U.S. states where Lyme disease 

is most prevalent. The temperature mortality analysis examined 

209 U.S. cities that had sufficient data for a historical epidemiology analysis. The ozone analysis was able to address the entire 

contiguous United States.

A1.3 Sources of Uncertainty 

The use of the term “uncertainty” in climate assessments refers 

to a range of possible futures. Uncertainty about the future 

climate arises from the complexity of the climate system and the 

ability of models to represent timing, magnitude, and location of 

changes, as well as the difficulties in predicting the decisions that 

society will make. There is also uncertainty about how climate 

change, in combination with other stressors, will affect people 

and natural systems.1

Though quantitative evaluations of climate change impacts on 

human health are continually improving, there is always some 

degree of uncertainty when using models to gain insight into future conditions (see Figure 4). The presence of uncertainty, or the 

fact that there is a range in potential outcomes, does not negate 

the knowledge we have, nor does it mean that actions cannot be 

taken. Everyone makes decisions, in all aspects of their life, based 

on limited knowledge or certainty about the future. Decisions 

like where to go to college or what job to take, what neighborhood to live in or which restaurant to eat in, whom to befriend or 

marry, and so on are all made in light of uncertainty, which can 

sometimes be considerable. Recent years have seen considerable 

progress in the development of improved methods to describe 

and deal with uncertainty in modeling climate change impacts 

on human health (for example, Melillo et al. 2014; Tamerius et al. 

2007; Post et al. 2012).1, 2, 3 

Uncertainty in Projecting Climate Change 

Two of the key uncertainties in projecting future global temperatures are 1) uncertainty about future concentrations of 

greenhouse gases, and 2) uncertainty about how much warming will occur for a given increase in greenhouse gas concentrations. Future concentrations depend on both future emissions and how long these emissions remain in the atmosphere 

(which can vary depending on how natural systems process 

those emissions). Because of uncertainty in future greenhouse 

gas concentrations, climate modelers analyze multiple future 

emissions pathways in order to determine the range of varying 

impacts of lower emissions compared to higher emissions. In 

terms of how much warming will occur for a given increase in 

greenhouse gas concentrations, the most recent assessment 

by the IPCC found the most likely response of the climate 

system to a doubling of carbon dioxide (CO2) concentrations, 

referred to as the sensitivity in climate models, lies between 
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populations or longer time periods), and the more common the 

health condition, the more confidence there is in estimated rates, 

and changes in those rates, across time periods, demographic 

groups, or other attributes.22 

Uncertainty in Estimating Stressor-Response 

Relationships

Exposure–response or stressor–response relationships describe 

the change in the health status associated with different levels 

of exposure to a stressor or concentration of a stressor (also see 

Ch. 1: Introduction, Section 1.4). Some environmental exposures, 

such as air quality and ambient temperature, have a relatively 

direct effect on deaths and illness, which is captured in stressor-response relationships in epidemiological studies. For example, 

increases in temperature can affect a range of chronic illnesses 

and infectious diseases. In other situations, climate change will 

have health effects through intermediaries such as changes in 

ecological conditions like pollen distribution (causing allergies) 

and the distribution of infectious disease pathogens and vectors 

(causing vector-borne, foodborne, and waterborne infectious 

diseases). Modeling exposure–response relationships can be 

particularly challenging for outcomes involving multiple intermediary stressors along an exposure pathway, each of which may be 

influenced by climate change. Even for relatively direct impacts, 

the same exposure can produce different responses for different 

health outcomes. Moreover, responses for a given exposure can 

vary by location (for example, different impacts of extreme heat 

in dry areas versus humid areas) and across sub-populations (different socioeconomic and demographic groups). For each pairing 

of exposure and health response, the exposure–response relationship may be represented as a quantitative estimate (such as 

the increase in number of deaths for a 1°F increase in maximum 

temperature) or in a qualitative manner (such as a determination 

that increases in extreme precipitation events may increase exposure to indoor molds). 

Sources of Uncertainty

Figure 4: Examples of sources of uncertainty in projecting impacts of climate change on human health. The left column 

illustrates the exposure pathway through which climate change can affect human health. The right column lists examples 

of key sources of uncertainty surrounding effects of climate change at each stage along the exposure pathway.
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In recent decades, progress has been made in modeling exposure–response relationships for a wide range of climate-sensitive 

environmental exposures and health responses. For example, 

we have gained a better understanding in recent years of the 

relationships between exposure to varying temperatures, concentrations of ozone and fine particulate matter, and the health 

response in terms of a range of illnesses and premature death 

(for example, Samoili et al. 2005. Bobb et al. 2014; see also Ch. 

2: Temperature-Related Death and Illness and Ch. 3: Air Quality 

Impacts).24, 25 Quantitative exposure–response functions are often 

used in understanding how health risks from these exposures 

vary across locations; these are also used in modeling efforts to 

project the health impacts of climate change in specific locations. 

However, it is important to carefully consider uncertainty when 

developing and using exposure–response functions, as the environmental processes affecting human health are complex. 

Exposure–response functions may not remain constant over time 

or space. One source of uncertainty arises from the potential that 

high levels of exposure could be associated with proportionately 

larger effects compared to low levels of exposure (non-linearity, see for example Gasparrini 2014 and Burnett et al 2014).26, 

27 Further, as the nature of the exposure and the potential for 

changes in human behavior and adaptive capacity change over 

time, so can the response function change. Representing health 

response for a singular point estimate of exposure instead of a 

range of exposure values could lead to imprecise assessment of 

the health risk. The large amounts of data required for reliable 

and accurate estimation of exposure–response functions may not 

be available at suitable resolutions for all locations (for example, 

Hubbell et al 2009).28 In some cases, estimating health outcomes 

by using exposure–response functions from other locations in 

the absence of reliable locally specific exposure–response relationships introduces uncertainty (for example, Wardekker et al. 

2012).29 The exposure–response estimates may also vary within 

sub-populations in a location, being relatively high for particularly 

vulnerable communities (for example, the elderly population will 

have a higher exposure–response relationship from extreme heat 

compared to the rest of the population). 

Another challenge in characterizing the relationship between 

exposure and health impacts is determining when a relationship 

is correlative, as opposed to causative. For example, statistical 

analyses would adjust for other factors that could be influencing health outcomes, such as age, race, year, day of the week, 

insurance status, and the concentrations of other air pollutants. 

Evaluating and integrating evidence across epidemiological, 

toxicological, and controlled human exposure studies allows 

researchers to conclude whether there is a causal relationship 

between human exposure to air pollution and a given health 

outcome. As evidence mounts, as is the case for associations 

between ozone concentration and adverse health impacts,30, 31, 

32, 33, 34, 35 the hypothesis of a causal relationship is strengthened, 

and observed exposure–response associations can be used with 

greater confidence. 

Users of exposure–response relationships in risk assessments or 

disease burden projection need to carefully consider the context 

in which the estimates were derived prior to their use. Carefully 

designed meta-analyses, leveraging the information obtained 

from multiple studies, can provide summary estimates of relationships and ensure consistency in application (for example, 

Normand 1999).36

Approach to Reporting Uncertainty in Key Findings

Despite the sources of uncertainty described above, the current 

state of the science allows an examination of the likely direction 

of and trends in the health impacts of climate change. Over the 

past ten years, the models used for climate and health assessments have become more useful and more accurate (for example, Melillo et al. 2014; Tamerius et al. 2007; Post et al. 2012).1, 

2, 3 This assessment builds on that improved capability. A more 

detailed discussion of the approaches to addressing uncertainty 

from the various sources can be found in the Guide to the Report 

(Front Matter) and Appendix 4: Documenting Uncertainty: Confidence and Likelihood.

Two kinds of language are used when describing the uncertainty 

associated with specific statements in this report: confidence 

language and likelihood language. Confidence in the validity of 

a finding is based on the type, amount, quality, strength, and 

consistency of evidence and the degree of expert agreement on 

the finding. Confidence is expressed qualitatively and ranges from 

low confidence (inconclusive evidence or disagreement among 

experts) to very high confidence (strong evidence and high consensus). 

Likelihood language describes the likelihood of occurrence based 

on measures of uncertainty expressed probabilistically (in other 

words, based on statistical analysis of observations or model 

results or on expert judgment). Likelihood, or the probability 

of an impact, is a term that allows a quantitative estimate of 

uncertainty to be associated with projections. Thus likelihood 

statements have a specific probability associated with them, 

ranging from very unlikely (less than or equal to a 1 in 10 chance 

of the outcome occurring) to very likely (greater than or equal to 

a 9 in 10 chance). The likelihood rating does not consider severity 

of the health risk or outcome, particularly as it relates to health 

risk factors not associated with climate change, unless otherwise 

stated in the Key Finding. 

Each Key Finding includes confidence levels; where possible, separate confidence levels are reported for 1) the impact of climate 

change, 2) the resulting change in exposure or risk, and 3) the 

resulting change in health outcomes. Where projections can be 

quantified, both a confidence and likelihood level are reported. 

Determination of confidence and likelihood language involves the 

expert assessment and consensus of the chapter author teams. 

The author teams determine the appropriate level of confidence 

or likelihood by assessing the available literature, determining 

the quality and quantity of available evidence, and evaluating the 
APPENDIX 1: TECHNICAL SUPPORT DOCUMENT: MODELING FUTURE CLIMATE IMPACTS ON HUMAN HEALTH

U.S. Global Change Research Program 296 Impacts of Climate Change on Human Health in the United States

level of agreement across different studies. Often, the underlying studies will provide their own estimates of uncertainty and 

confidence intervals. When available, these confidence intervals 

are used by the chapter authors in making their own expert 

judgments.

DOCUMENTING UNCERTAINTY

This assessment relies on two metrics to communicate the degree of certainty in Key Findings. See Appendix 4: Documenting 

Uncertainty for more on assessments of likelihood and confidence.

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few sources, limited consistency, models 

incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts

Likelihood

Very Likely

≥ 9 in 10

Likely

≥ 2 in 3

As Likely As Not

≈ 1 in 2

Unlikely

≤ 1 in 3

Very Unlikely

≤ 1 in 10
U.S. Global Change Research Program 297 Impacts of Climate Change on Human Health in the United States

References 

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2. Post, E.S., A. Grambsch, C. Weaver, P. Morefield, J. Huang, 

L.-Y. Leung, C.G. Nolte, P. Adams, X.-Z. Liang, J.-H. Zhu, 

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E. Hawkins, G. Hegerl, D. Karoly, N. Keenlyside, M. Kimoto, B. Kirtman, A. Navarra, R. Pulwarty, D. Smith, D. Stammer, and T. Stockdale, 2009: Decadal Prediction. Bulletin of 

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17. Ilacqua, V., J. Dawson, M. Breen, S. Singer, and A. Berg, 

2015: Effects of climate change on residential infiltration 

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framework for examining climate-driven changes to the seasonality and geographical range of coastal pathogens and 

harmful algae. Climate Risk Management, 8, 16-27. http://

dx.doi.org/10.1016/j.crm.2015.03.002

19. Kibler, S.R., P.A. Tester, K.E. Kunkel, S.K. Moore, and R.W. 

Litaker, 2015: Effects of ocean warming on growth and distribution of dinoflagellates associated with ciguatera fish 

poisoning in the Caribbean. Ecological Modelling, 316, 194-

210. http://dx.doi.org/10.1016/j.ecolmodel.2015.08.020

20. Monaghan, A.J., S.M. Moore, K.M. Sampson, C.B. Beard, 

and R.J. Eisen, 2015: Climate change influences on the annual onset of Lyme disease in the United States. Ticks and TickBorne Diseases, 6, 615-622. http://dx.doi.org/10.1016/j.

ttbdis.2015.05.005

21. Walsh, J., D. Wuebbles, K. Hayhoe, J. Kossin, K. Kunkel, 

G. Stephens, P. Thorne, R. Vose, M. Wehner, J. Willis, D. 

Anderson, S. Doney, R. Feely, P. Hennon, V. Kharin, T. 

Knutson, F. Landerer, T. Lenton, J. Kennedy, and R. Somerville, 2014: Ch. 2: Our changing climate. Climate Change 

Impacts in the United States: The Third National Climate 

Assessment. Melillo, J.M., T.C. Richmond, and G.W. Yohe, 

Eds. U.S. Global Change Research Program, Washington, 

D.C., 19-67. http://dx.doi.org/10.7930/J0KW5CXT

22. Klein, R.J., S.E. Proctor, M.A. Boudreault, and K.M. 

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Health Statistics, Hyattsville, MD. http://198.246.124.22/

nchs/data/statnt/statnt24.pdf

23. NCHS, 2015: Health, United States, 2014: With Special 

Feature on Adults Aged 55-64. 473 pp. National Center for 

Health Statistics, Centers for Disease Control and Prevention, Hyattsville, MD. http://www.cdc.gov/nchs/data/hus/

hus14.pdf

24. Bobb, J.F., R.D. Peng, M.L. Bell, and F. Dominici, 2014: 

Heat-related mortality and adaptation to heat in the United States. Environmental Health Perspectives, 122, 811-816. 

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25. Samoli, E., A. Analitis, G. Touloumi, J. Schwartz, H.R. 

Anderson, J. Sunyer, L. Bisanti, D. Zmirous, J.M. Vonk, 

J. Pekkanen, P. Goodman, A. Paldy, C. Schindler, and K. 

Katsouyanni, 2005: Estimating the exposure-response relationships betwen particulate matter and mortality within the 

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sim.5963

27. Burnett, R.T., C.A. Pope, III, M. Ezzati, C. Olives, S.S. Lim, 

S. Mehta, H.H. Shin, G. Singh, B. Hubbell, M. Brauer, H.R. 

Anderson, K.R. Smith, J.R. Balmes, N.G. Bruce, H. Kan, F. 

Laden, A. Pruss-Ustun, M.C. Turner, S.M. Gapstur, W.R. 

Diver, and A. Cohen, 2014: An integrated risk function for 

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33. Fann, N., A.D. Lamson, S.C. Anenberg, K. Wesson, D. 

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0258(19990215)18:3%3C321::AID-SIM28%3E3.0
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© Ringo Chiu/ZUMA Press/Corbis
The systematic literature review included a comprehensive 

search of the literature, collection and incorporation of information submitted by the public, screening and assessment 

of the eligibility of the collected literature, and synthesis of 

the collected literature. Authors were provided with detailed 

guidance, including Information Quality Act (IQA) procedures 

and the following process for the literature review.

A2.1 Identification of Literature Sources 

The sources of literature and information assessed for this 

report were derived from a comprehensive literature search 

conducted by the National Institute of Environmental Health 

Sciences (NIEHS), literature submitted for consideration during 

public engagement opportunities, references included in the 

Third National Climate Assessment (2014 NCA),1 and additional 

sources of information or data identified by the chapter authors. 

NIEHS, coordinating closely with the Interagency Crosscutting 

Group on Climate Change and Human Health (CCHHG), developed an updated (2012–2014) Health Sector Literature Review 

and Bibliography as part of the larger literature review for the 

2014 NCA. The NIEHS search covered multiple electronic databases (such as PubMed, Scopus, and Web of Science) as well as 

web search engines such as Google Scholar. Overall, searches 

were limited to publication dates of 2007 or later and to English-language citations. NIEHS conducted an eligibility screening of the information retrieved from the citation databases.2

A Federal Register Notice (FRN) published by the U.S. Environmental Protection Agency (EPA) on behalf of the U.S. Global 

Change Research Program (USGCRP) on February 7, 2014, called 

for submissions of relevant, peer-reviewed, scientific and/

or technical research studies on observed and/or projected 

climate change impacts on human health in the United States.3

A second FRN was published on April 7, 2015, announcing a 

public comment period, in which many commenters suggested 

additional sources of literature for consideration.4 Chapter authors were responsible for screening and assessing the eligibility 

of literature submitted by the public using the same process 

developed by NIEHS.

In the process of performing the review and evaluating the 

literature, authors identified additional relevant literature, not 

captured in the NIEHS literature search or public call for information. Chapter authors screened and assessed the eligibility 

of these sources using the same process developed by NIEHS.

A2.2 Screening for Eligibility

Throughout the process of drafting this assessment, guidance 

was provided to authors regarding the requirements of the 

IQA. In accordance with these requirements, chapter authors 

considered information quality when deciding whether or not 

to use source material in their chapter. The literature review 

guidance provided to authors required consideration of the 

following criteria for each source of information used in the 

assessment:

• Utility: Is the particular source important to the topic of 

your chapter? 

• Transparency and traceability: Is the source material identifiable and publicly available? 

• Objectivity: Why and how was the source material created? 

Is it accurate and unbiased? 

• Information integrity and security: Will the source material 

remain reasonably protected and intact over time?

The Supporting Evidence sections of each chapter include 

“Traceable Accounts” for the Key Findings. The Traceable Accounts identify the key studies for explaining a particular issue 

or answering a particular question, and which form the basis 

of support for Key Findings. Key studies exhibit the general 

attributes defined below:

• Focus: the work not only addresses the area of inquiry under consideration but also contributes to its understanding;

• Verify: the work is credible within the context of the wider 

body of knowledge/literature or, if not, the new or varying 

information is documented within the work;

U.S. Global Change Research Program 301 Impacts of Climate Change on Human Health in the United States

2 PROCESS FOR

LITERATURE REVIEW

APPENDIX
• Integrity: the work is structurally sound; in a piece of 

research, the design or research rationale is logical and 

appropriate;

• Rigor: the work is important, meaningful, and non-trivial 

relative to the field and exhibits sufficient depth of intellect 

rather than superficial or simplistic reasoning;

• Utility: the work is useful and professionally relevant; it 

makes a contribution to the field in terms of the practitioners’ understanding or decision-making on the topic; and

• Clarity: it is written clearly and appropriately for the nature 

of the study.

Authors were responsible for certifying adherence to IQA 

requirements by applying the process outlined in the Author’s 

Guidance documents (see Appendix 3: Report Requirements, 

Development Process, Review, and Approval). 

Recommended Citation: USGCRP, 2016: Appendix 2: Process for 

Literature Review. The Impacts of Climate Change on Human 

Health in the United States: A Scientific Assessment. U.S. 

Global Change Research Program, Washington, DC, 301–302. 

http://dx.doi.org/10.7930/J0FQ9TJT

References

1. Melillo, J.M., T.C. Richmond, and G.W. Yohe, Eds., 2014: 

Climate Change Impacts in the United States: The Third 

National Climate Assessment. U.S. Global Change Research 

Program, Washington, D.C., 842 pp. http://dx.doi.

org/10.7930/J0Z31WJ2

2. USGCRP, 2012: National Climate Assessment Health Sector Literature Review and Bibliography. Technical Input 

for the Interagency Climate Change and Human Health 

Group. National Institute of Environmental Health Sciences. http://www.globalchange.gov/what-we-do/assessment/

nca-activities/available-technical-inputs

3. 40 CFR Part 82, 2014: Request for Public Engagement in 

the Interagency Special Report on the Impacts of Climate 

Change on Human Health in the United States. U.S. Environmental Protection Agency on behalf of the United States 

Global Change Research Program. http://www.gpo.gov/

fdsys/pkg/FR-2014-02-07/pdf/2014-02304.pdf

4. 80 FR 18619, 2015: Notice of Availability of Draft Scientific Assessment for Public Comment. U.S. Environmental Protection Agency on behalf of the U.S. Global 

Change Research Program. https://www.federalregister.

gov/articles/2015/04/07/2015-07629/notice-of-availability-of-draft-scientific-assessment-for-public-comment

APPENDIX 2: PROCESS FOR LITERATURE REVIEW

U.S. Global Change Research Program 302 Impacts of Climate Change on Human Health in the United States
A3.1 Scoping the Report

In early 2013, the Interagency Crosscutting Group on Climate 

Change and Human Health (CCHHG), a working group of the 

U.S. Global Change Research Program (USGCRP), established 

that developing a climate and health assessment was a priority action and convened a Climate and Health Assessment 

Steering Committee in June 2013. The Steering Committee 

determined the scope of the Climate and Health Assessment 

with input from a scoping workshop, held November 21, 2013. 

The CCHHG participants in this workshop discussed the focus 

and breadth of the report outline, roles and responsibilities 

of authors and contributors, the process and timing for report 

development, and the goals of leveraging federal expertise 

and ongoing research/analyses across CCHHG agencies and 

synthesizing multiple efforts into a single robust product. A 

draft prospectus outlining the proposed focus areas and scope 

of the report was developed by the Steering Committee and 

published in a Federal Register Notice (FRN) on February 7, 

2014.1 The prospectus proposed plans for scoping, drafting, 

reviewing, producing, and disseminating the report.

A3.2 Author Selection

A team of more than 100 experts was involved in writing this 

report. The selection of authors was limited to Federal employees and their contractors or affiliates. Each chapter had an 

author team consisting of Lead and Contributing Authors, who 

were responsible for a chapter or subsection of a chapter based 

on their expertise. Lead and Contributing Authors came from 

multiple agencies across the government, including the U.S. 

Department of Health and Human Services (HHS; National Institutes of Health [NIH], Centers for Disease Control and Prevention [CDC], National Institute for Occupational Safety and Health 

[NIOSH], the Assistant Secretary for Preparedness and Response 

[ASPR], U.S. Food and Drug Administration [FDA], and Substance 

Abuse and Mental Health Services Administration [SAMHSA]), 

National Oceanic and Atmospheric Administration (NOAA), U.S. 

Environmental Protection Agency (EPA), U.S. Department of 

Agriculture (USDA), National Aeronautics and Space Administration (NASA), U.S. Geological Survey (USGS), U.S. Department of 

Defense (DOD; the Uniformed Services University of the Health 

Sciences [USUHS]), and the U.S. Department of Veterans Affairs 

(VA) (see author lists in the front matter and in each chapter for 

full affiliations). Lead Authors were nominated and selected by 

the CCHHG and include CCHHG members, attendees of the first 

scoping workshop, and other Federal experts and contractors/

grantees with relevant expertise. Contributing Authors were 

nominated by the Lead Authors, CCHHG or other interagency 

members, and the general public. Public nominations were accepted through the FRN dated February 7, 2014, which provided 

an opportunity for external (non-Federal) subject matter experts 

to be hired under a Federal contract as Contributing Authors. 

These nominees were screened according to criteria established 

by the Steering Committee and selected through an independent process. 

A3.3 Drafting the Report

The report was drafted between spring 2014 and spring 2015. 

Guidance and resources provided to authors included:

• Literature Review Guidance. Guidance was provided to 

authors on reviewing and assessing the literature, screening 

for eligibility and information quality, and documenting their 

process for inclusion in the assessment. Please see Appendix 

2: Process for Literature Review for more information on the 

literature review and selection process.

• Author Guidance. Guidance was provided to authors on 

chapter development, including basic and technical guidance on scope, chapter preparation and outlines, and 

meeting information quality guidance. Guidelines were also 

provided for transparent reporting of likelihood, confidence, 

and uncertainty.

• Modeling Guidance. Guidance was provided to the authors 

for the four chapters within the assessment that highlight 

recent peer-reviewed modeling and/or quantitative analyses. These analyses were conducted by the chapter authors 

for the purpose of this assessment, in addition to their 

assessment of the broader body of literature. Please see 

Appendix 1: Technical Support Document, for more information on modeling approaches.

U.S. Global Change Research Program 303 Impacts of Climate Change on Human Health in the United States

3 REPORT REQUIREMENTS, DEVELOPMENT 

PROCESS, REVIEW, AND APPROVAL

APPENDIX
• Style and Language Guidance. The Steering Committee, 

in conjunction with USGCRP staff and the NOAA Technical 

Support Unit (TSU), developed a style guide to ensure consistent style, tone, formatting, use of graphics, and documentation of metadata across the report. 

• Author Resource Portal. An online platform was developed 

by the NOAA TSU to provide author teams with a shared 

online workspace, help structure the drafting and revising 

process, and document metadata on report figures.

• Drafting Workshop. An all-authors workshop was held on 

September 10–11, 2014, to review guidelines and timelines 

and to discuss cross-cutting issues among and between 

author teams. 

A3.4 Public Engagement

The Steering Committee provided a number of opportunities 

for public engagement in scoping, informing, and reviewing 

the report. On February 7, 2014, EPA released a FRN on behalf 

of USGCRP announcing a request for public engagement in a 

Public Forum (held March 13, 2014) and establishing a 30-day 

period to submit public comments on the draft prospectus, 

suggestions for scientific information to inform the assessment, and nominations for Contributing Authors. A second 

FRN, released by EPA on behalf of USGCRP on April 7, 2015,2

announced a 60-day period to submit public comments on 

the draft assessment. Responses to each comment are posted 

on the USGCRP Climate and Health Assessment website 

(http://www.globalchange.gov/health-assessment). Finally, 

Steering Committee members and authors further engaged 

the community of experts and the general public about the 

report and public comment periods at scientific meetings, 

conferences, and symposia. 

A3.5 Peer-Review and Clearance

The draft assessment was peer-reviewed by a committee 

convened by the National Academies of Sciences, Engineering, and Medicine. Based on comments from the public and 

the National Academies’ report,3 the authors extensively 

reviewed and revised the assessment. The assessment was 

reviewed and approved by the USGCRP agencies and the 

Federal Committee on Environment, Natural Resources, and 

Sustainability (CENRS). This report meets all Federal requirements associated with the Information Quality Act (see 

Appendix 2: Process for Literature Review), including those 

pertaining to public comment and transparency.

Recommended Citation: USGCRP, 2016: Appendix 3: Report 

Requirements, Development Process, Review, and Approval. The 

Impacts of Climate Change on Human Health in the United 

States: A Scientific Assessment. U.S. Global Change Research 

Program, Washington, DC, 303–304. http://dx.doi.org/10.7930/

J09Z92TG

References:

1. 40 CFR Part 82, 2014: Request for Public Engagement in 

the Interagency Special Report on the Impacts of Climate 

Change on Human Health in the United States. U.S. Environmental Protection Agency on behalf of the United States 

Global Change Research Program. http://www.gpo.gov/

fdsys/pkg/FR-2014-02-07/pdf/2014-02304.pdf

2. 80 FR 18619, 2015: Notice of Availability of Draft Scientific Assessment for Public Comment. U.S. Environmental Protection Agency on behalf of the U.S. Global 

Change Research Program. https://www.federalregister.

gov/articles/2015/04/07/2015-07629/notice-of-availability-of-draft-scientific-assessment-for-public-comment

3. National Academies of Sciences Engineering and Medicine, 

2015: Review of the Draft Interagency Report on the Impacts 

of Climate Change on Human Health in the United States. 

National Academies Press, Washington, D.C. http://www.

nap.edu/catalog/21787/review-of-the-draft-interagency-report-on-the-impacts-of-climate-change-on-human-healthin-the-united-states

APPENDIX 3: REPORT REQUIREMENTS, DEVELOPMENT PROCESS, REVIEW, AND APPROVAL

U.S. Global Change Research Program 304 Impacts of Climate Change on Human Health in the United States
The authors have assessed a wide range of information in the 

scientific literature and various technical reports to arrive at 

their Key Findings. Similar to the 2014 NCA1 and the Intergovernmental Panel on Climate Change’s Fifth Assessment 

Report,2 this assessment relies on two metrics to communicate the degree of certainty in Key Findings: 

1. Confidence in the validity of a finding based on the type, 

amount, quality, strength, and consistency of evidence (such 

as mechanistic understanding, theory, data, models, and 

expert judgment); the skill, range, and consistency of model 

projections; and the degree of agreement within the body 

of literature.

2. Likelihood, or probability of an effect or impact occurring, is 

based on measures of uncertainty expressed probabilistically (in other words, based on statistical analysis of observations or model results or on the authors’ expert judgment).

Key sources of information used to develop these characterizations are referenced in the Supporting Evidence section 

found at the end of each chapter. The Supporting Evidence 

sections include “Traceable Accounts” for each Key Finding 

that 1) document the process and rationale the authors used 

in reaching the conclusions in their Key Finding, 2) provide additional information to readers about the quality of the information used, 3) allow traceability to resources and data, and 

4) describe the level of likelihood and confidence in the Key 

Finding. Thus, the Traceable Accounts represent a synthesis of 

the chapter author team’s judgment of the validity of findings, 

as determined through evaluation of evidence and agreement 

in the scientific literature. The Traceable Accounts also identify 

areas where data are limited or emerging or where scientific uncertainty limits the authors’ ability to estimate future 

climate change impacts. Each Traceable Account includes 1) a 

description of the evidence base, 2) major uncertainties, and 

3) an assessment of confidence based on evidence. 

A4.1 Evaluation of Confidence in the Validity of a 

Finding 

Assessments of confidence in the Key Findings are based on 

the expert judgment of the chapter authors. Authors provide 

supporting evidence for each of the chapter’s Key Findings in 

the Traceable Accounts. Confidence is expressed qualitatively 

and ranges from low confidence (inconclusive evidence or 

disagreement among experts) to very high confidence (strong 

evidence and high consensus) (see Figure 1). Confidence levels 

are reported even where confidence is low. Confidence should 

not be interpreted probabilistically, as it is distinct from statistical likelihood. 

Figure 1: Likelihood and Confidence Evaluation

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few sources, limited consistency, models 

incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts

Likelihood

Very Likely

≥ 9 in 10

Likely

≥ 2 in 3

As Likely As Not

≈ 1 in 2

Unlikely

≤ 1 in 3

Very Unlikely

≤ 1 in 10

APPENDIX

U.S. Global Change Research Program 305 Impacts of Climate Change on Human Health in the United States

4 DOCUMENTING UNCERTAINTY: 

CONFIDENCE AND LIKELIHOOD
A4.2 Evaluation of Likelihood of Risk 

For the purposes of this assessment, likelihood is the chance 

of occurrence of an effect or impact based on measures of 

uncertainty expressed probabilistically (in other words, based 

on statistical analysis of observations or model results or on 

expert judgment). Authors came to a consensus using expert 

judgment, based on the synthesis of the literature assessed, 

to arrive at an estimation of the likelihood that a particular 

impact will occur within the range of possible outcomes. 

Where it is considered justified to report the likelihood of 

particular impacts within the range of possible outcomes, 

this report takes a plain-language approach to expressing the 

expert judgment of the chapter team, based on the best available evidence. For example, an outcome termed “likely” has 

at least a 66% chance of occurring; an outcome termed “very 

likely,” at least a 90% chance (see Figure 1). 

A4.3 Uncertainty Language in Key Findings

All Key Findings include a description of confidence. Where it 

is considered scientifically justified to report the likelihood of 

particular impacts within the range of possible outcomes, Key 

Findings also include a likelihood designation. Where possible, 

levels of confidence and likelihood are provided for different 

steps along the exposure pathway to enable separate reporting of levels of uncertainty in understanding climate impacts, 

changes in exposure, the role of moderating or exacerbating 

factors, and observed or projected health outcomes. 

Confidence and likelihood levels are based on the expert 

assessment and consensus of the chapter author teams. 

These teams determined the appropriate level of confidence 

or likelihood by assessing the available literature, determining 

the quality and quantity of available evidence, and evaluating 

the level of agreement across different studies. Often, the 

underlying studies provided their own estimates of uncertainty and confidence intervals. When available, these confidence 

intervals were assessed by the chapter authors in making 

their own expert judgments. For specific descriptions of the 

process by which each chapter author team came to consensus on the Key Findings and the assessment of confidence 

and likelihood, see the Traceable Accounts in the Supporting 

Evidence section of each chapter.

Recommended Citation: USGCRP, 2016: Appendix 4: 

Documenting Uncertainty: Confidence and Likelihood. The 

Impacts of Climate Change on Human Health in the United 

States: A Scientific Assessment. U.S. Global Change Research 

Program, Washington, DC, 305–306. http://dx.doi.org/10.7930/

J0668B3G

References

1. Melillo, J.M., T.C. Richmond, and G.W. Yohe, Eds., 2014: 

Climate Change Impacts in the United States: The Third 

National Climate Assessment. U.S. Global Change Research 

Program, Washington, D.C., 842 pp. http://dx.doi.

org/10.7930/J0Z31WJ2

2. IPCC, 2014: Climate Change 2014: Impacts, Adaptation, 

and Vulnerability. Part A: Global and Sectoral Aspects. 

Contribution of Working Group II to the Fifth Assessment 

Report of the Intergovernmental Panel on Climate Change. 

Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. 

Mastrandrea, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L. White (Eds.), 1132 

pp. Cambridge University Press, Cambridge, UK and New 

York, NY. http://www.ipcc.ch/report/ar5/wg2/

APPENDIX 4: DOCUMENTING UNCERTAINTY: CONFIDENCE AND LIKELIHOOD

U.S. Global Change Research Program 306 Impacts of Climate Change on Human Health in the United States
Glossary 

Acclimatization. Physiological and behavioral adjustments to a 

change of climatic environment.

Acute. Occurring over a short period of time (as opposed to 

chronic).

Adaptive capacity. The ability of communities, institutions, 

or people to adjust to potential hazards, to take advantage of 

opportunities, or to respond to consequences.

Aeroallergens. Various airborne substances, such as pollen or 

spores, which can cause an allergic response.

Aerosol (atmospheric). Aerosols are fine solid or liquid 

particles, caused by people or occurring naturally, that are 

suspended in the atmosphere. Aerosols can cause cooling by 

scattering incoming radiation or by affecting cloud cover. Aerosols can also cause warming by absorbing radiation. Related 

terms: Aerosolize, aerosols.

Algae. Photosynthetic organisms forming the base of the food 

chain in freshwater and marine ecosystems. Algae range in in 

size from single-celled microalgae to large macroalgae, like 

kelp. Related term: Harmful algal blooms (HABs).

Algal bloom. A sudden, rapid growth of algae in lakes and 

coastal oceans caused by a variety of factors including, for 

example, warmer surface waters or increased nutrient levelsSome algal blooms may be toxic or harmful to humans and 

ecosystems.

Allergy/allergic. Reactions of the immune system to substances that, in most people, do not cause symptoms. Allergenicity 

refers to a substance being able to cause an allergic response.

Anxiety. Feelings of worry, nervousness, distress or a sense of 

apprehension.

Asthma. A chronic respiratory disease or condition characterized by recurrent breathing problems.

Bacteria. Small single-celled organisms. Though common and 

vital to ecosystems, particular species or groups may cause 

illness in humans and other organisms. See Cyanobacteria.

Baseline. A starting point or reference used as the basis for 

comparison.

Carbon dioxide (CO2). A colorless, odorless, greenhouse gas 

produced by combustion, respiration, and organic decomposition.

Carbon monoxide (CO). A colorless, odorless, poisonous gas 

produced by incomplete combustion. Related term: Carbon 

monoxide poisoning.

Cardiovascular. Referring to the heart and blood vessels. Cardiovascular disease (CVD) includes all diseases and conditions 

of the cardiovascular system.

Chronic. Occurring over a long period of time (as opposed to 

acute).

Chronic obstructive pulmonary disease (COPD). A group of 

diseases that cause airflow blockage and breathing-related 

problems.

Climate. The long-term statistical average of weather. Climate 

typically refers to the mean and variability of relevant weather 

variables, such as temperature, precipitation, and wind, over 

long time scales (30 years or more).

Climate change. Changes in average weather conditions 

that persist over multiple decades or longer. Climate change 

encompasses both increases and decreases in temperature, as 

U.S. Global Change Research Program 307 Impacts of Climate Change on Human Health in the United States

5 GLOSSARY AND ACRONYMS

APPENDIX
well as shifts in precipitation, changing risk of certain types of 

severe weather events, and changes to other features of the 

climate system.

Climate variability. Natural changes in climate that fall within 

the observed range of extremes for a particular region, as 

measured by temperature, precipitation, and frequency of 

events. Drivers of climate variability include the El Niño Southern Oscillation and other phenomena. Related terms: Natural 

variability.

Cognitive. Referring to intellectual activity like thinking, reasoning, remembering, imagining, or learning.

Cold wave. A period of abnormally cold weather lasting days 

to weeks.

Contaminant. A contaminant is any physical, chemical, biological, or radiological substance or matter found in any media 

where it does not belong, particularly at concentrations that 

may pose a threat to human health or the environment.

Cryptosporidium. A one-celled (protozoan) parasite that 

infects the intestines of people and animals. Cryptosporidiosis 

is an infection caused by Cryptosporidium.

Cumulative (health effects). The combination of successive or 

concurrent impacts on health.

Cyanobacteria. A photosynthetic group of bacteria that are 

functionally similar to algae.

Demographic. Related to the characteristics of a population 

such as age, gender, ethnicity, and race.

Dengue fever. A viral disease spread by mosquitoes.

Depression. A common, but serious, illness that interferes 

with daily life and is characterized by a sustained sad mood or 

inability to experience pleasure.

Diabetes. A group of diseases that affect the ability of the 

pancreas to produce insulin and thus affect how the body uses 

blood sugar (glucose).

Disability. A physical or mental condition that limits a person 

from doing one or more major life activities, including walking, talking, hearing, seeing, breathing, learning, performing 

manual tasks, and caring for oneself. A functional disability is 

any long-term limitation in activity resulting from a condition 

or health problem. Related term: Functional limitations.

Downscaling. Methods that use models to estimate future 

climate at local scales (for example, county, state, region).

Drought. A period of abnormally dry weather marked by little 

or no rain that lasts long enough to cause water shortage for 

people and natural systems.

Ecosystem. All the living things in a particular area as well 

as components of the physical environment with which they 

interact, such as air, soil, water, and sunlight.

Ecosystem services. The benefits produced by ecosystems on 

which people depend, including, for example, fisheries, drinking water, fertile soils for growing crops, climate regulation, 

and aesthetic and cultural value.

Electrolyte imbalance. Minerals (such as sodium, calcium, and 

potassium) in the body that have an electric charge. Electrolyte imbalance is when levels of these minerals are too high or 

too low.

Emissions. The release of climate-altering gases and particles 

into the atmosphere from human and natural sources.

Emissions scenarios. Quantitative illustrations of how the 

release of different amounts of climate-altering gases and particles into the atmosphere from human and natural sources 

will produce different future climate conditions. Scenarios are 

developed using a wide range of assumptions about population growth, economic and technological development, and 

other factors. Related term: emissions scenario, emission 

scenario. See Scenario.

Endemic. The constant or usual presence of a disease or infectious agent within a geographic area or population.

Enteric. Relating to the intestines of humans and animals. See 

Gastrointestinal.

Environmental justice. The fair treatment and meaningful 

involvement of all people regardless of race, color, national 

origin, or income with respect to the development, implementation, and enforcement of environmental laws, regulations, 

and policies.

Epidemiology. The study of the distribution and determinants 

of health conditions, states, or events in specified populations. 

Related term: Epidemiological

Exposure. Contact between a person and one or more biological, psychosocial, chemical, or physical stressors, including 

stressors affected by climate change.

Extreme events. A weather event that is rare at a particular 

place and time of year, including, for example, heat waves, 

cold waves, heavy rains, periods of drought and flooding, and 

APPENDIX 5: GLOSSARY AND ACRONYMS

U.S. Global Change Research Program 308 Impacts of Climate Change on Human Health in the United States
severe storms. Related terms: Extreme weather, Extreme 

weather event.

Food security. When all people at all times have both physical 

and economic access to sufficient food to meet their dietary 

needs for a productive and healthy life.

Foodborne illness. Illness or disease caused by foods or drinks 

contaminated with biological or chemical toxins or pathogens, 

including disease-causing microbes or toxic chemicals. Related 

terms: Foodborne disease, Foodborne infection.

Forcing. Factors that affect the Earth’s climate. For example, 

natural factors such as volcanoes and human factors such as 

the emission of heat-trapping gases and particles through 

fossil fuel combustion.

Gastrointestinal. Gastrointestinal refers to the stomach and 

intestinal tract. Gastroenteritis is inflammation of the stomach and intestines. Related term: Enteric.

Global Climate Models (GCM). Mathematical models that 

simulate the physics, chemistry, and biology that influence 

the climate system. Related term: General Circulation Model.

Greenhouse gases. Gases that absorb heat in the atmosphere 

near the Earth’s surface, preventing it from escaping into 

space. If the atmospheric concentrations of these gases rise, 

the average temperature of the lower atmosphere will gradually increase, a phenomenon known as the greenhouse effect. 

Greenhouse gases include, for example, carbon dioxide, water 

vapor, and methane.

Health. A state of physical, mental and social well-being, and 

not just the absence of disease.

Heat wave. A period of abnormally hot weather lasting days 

to weeks.

Heatstroke. A serious health condition that occurs when the 

body’s heat regulating mechanisms—such as sweating and 

respiration—fail.

Hypertension. Abnormally high arterial blood pressure.

Hyperthermia. Unusually high body temperature.

Hypothermia. Unusually low body temperature that causes a 

rapid, progressive mental and physical collapse.

Incidence. A measure of the frequency with which an event, 

such as a new case of illness, occurs in a population over a 

period of time.

Indicator. An observation or calculation that allows scientists, 

analysts, decision makers, and others to track environmental 

trends, understand key factors that influence the environment, 

and identify effects on ecosystems and society.

Infectious. A characterization of a disease indicating it can be 

transmitted between organisms.

Infrastructure. The physical structures, services, and institutions (for example, roads, electric utilities, legal systems) needed by a community, organization or country.

Land cover. The physical characteristics of the land surface, 

such as crops, trees, or concrete. See Land use.

Land use. Activities taking place on land, such as growing 

food, cutting trees, or building cities. Related term: Land-use 

patterns. See Land cover.

Lyme disease. A bacterial disease caused by microorganism 

Borrelia burgdorferi and transmitted by Ixodes ticks, commonly known as deer ticks.

Mental illnesses. Conditions that affect a person’s thinking, 

feeling, mood, or behavior.

Metabolic rate. The rate at which a person or animal uses 

calories over time, especially as estimated by food consumption, energy released as heat, or oxygen used in processes of 

the body.

Meteorological. Referring to the atmosphere and its phenomena, particularly weather and weather forecasting.

Microbial. Referring to microbes, also known as microorganisms, including disease-causing bacteria, viruses or parasites.

Mitigation. Measures to reduce the amount and speed of 

future climate change by reducing emissions of heat-trapping 

gases or removing carbon dioxide from the atmosphere. Related terms: Mitigate.

Morbidity. A disease or condition that reduces health and 

quality of life.

Mortality. Death as a health outcome. The mortality rate is 

the number of deaths in a defined population during a specified time period.

APPENDIX 5: GLOSSARY AND ACRONYMS

U.S. Global Change Research Program 309 Impacts of Climate Change on Human Health in the United States
Neurologic/neurological. Referring to the nervous system 

(including the brain, spinal cord, and nerves), particularly its 

structure, functions, and diseases.

Nutrients. Chemicals (such as nitrogen and phosphorus) that 

plants and animals need to live and grow. At high concentrations, particularly in water, nutrients can become pollutants.

Obesity. Having greater body fat relative to lean body mass 

than what is considered healthy. Related Term: Obese.

Ozone (O3). A colorless gas consisting of three atoms of oxygen, readily reacting with many other substances. Ozone in 

the upper atmosphere protects the Earth from harmful levels 

of ultraviolet radiation from the Sun. In the lower atmosphere 

ozone is an air pollutant with harmful effects on human 

health.

Parasite. An organism that lives inside or on a host organism, 

while causing harm to the host organism.

Particulate matter. Tiny airborne pieces of solid or liquid 

matter such as soot, dust, fumes, mists, aerosols, haze, and 

smoke.

Pathogen. Microorganisms (such as bacteria or viruses) that 

cause disease.

Permafrost. Ground that remains at or below freezing for at 

least two consecutive years.

Populations of concern. Vulnerable groups of people. Related 

Terms: Vulnerable populations, Populations at risk.

Postpartum. The time period after a woman gives birth.

Post-traumatic stress disorder (PTSD). A mental health 

problem that can occur after war, assault, accident, natural 

disaster, or other trauma.

Premature (early) death. Death that occurs earlier than a 

specified age, often the average life expectancy at birth.

Preparedness. Actions taken to build, apply, and sustain the 

capabilities necessary to prevent, protect against, and ameliorate negative effects.

Prevalence (in health context). A measure of the number or 

proportion of people with a specific disease or condition at a 

specific point in time.

Protozoa. A kind of single-celled microorganism that can be 

free-living or parasitic. See Parasite.

Psychiatric. Referring to mental illnesses and treatment. 

Psychiatric illnesses are mental health conditions affecting 

a person’s thinking, feeling, mood, or behavior. See Mental 

illness, Psychological.

Psychological. Of, affecting, or arising in the mind; refers to 

the mental and emotional state of a person.

Renal. Renal refers to the kidneys and surrounding region. 

Related terms: Kidney disease/disorder, Kidney/renal failure.

Representative Concentration Pathways (RCPs). Greenhouse 

gas concentration trajectories from the Intergovernmental 

Panel on Climate Change’s (IPCC’s) Fifth Assessment Report 

(2014) that reflect possible increases in radiative forcing associated with emissions over time. See Forcing.

Resilience. A capability to anticipate, prepare for, respond 

to, and recover from significant multi-hazard threats with 

minimum damage to social well-being, the economy, and the 

environment.

Respiratory. Related to the system of organs and tissue the 

body uses for breathing, including the airways, the lungs and 

linked blood vessels, and the muscles that enable breathing.

Risk. Risks are threats to life, health and safety, the environment, economic well-being, and other things of value. Risks 

are often evaluated in terms of how likely they are to occur 

(probability) and the damages that would result if they did 

happen (consequences).

Risk assessment. Studies that estimate the likelihood of 

specific sets of events occurring and their potential positive or 

negative consequences.

Risk perception. The psychological and emotional factors that 

affect people’s behavior and beliefs about potential negative 

hazards or consequences.

Salmonellosis. An infection with the Salmonella enterica bacteria that causes diarrhea, fever, and abdominal cramps.

Scenario. Sets of assumptions used to help understand potential future conditions such as population growth, land use, and 

sea level rise. Scenarios are neither predictions nor forecasts. 

Scenarios are commonly used for planning purposes. See 

Emissions scenarios.

Sensitivity. The degree to which people or communities are 

affected, either adversely or beneficially, by climate variability 

and change.

APPENDIX 5: GLOSSARY AND ACRONYMS

U.S. Global Change Research Program 310 Impacts of Climate Change on Human Health in the United States
Social determinants of health. The conditions in which 

people are born, grow, live, work, and age as shaped by the 

distribution of money, power, and resources.

Socioeconomic. Referring to a combination of social and economic factors, such as the education, income, and work status 

of individuals or communities.

Special Report on Emissions Scenarios (SRES). A set of emission scenarios from the IPCC Special Report on Emission Scenarios released in 2000 that describe a wide range of potential future socioeconomic conditions and resulting emissions. 

See Emissions scenario, Scenario.

Storm surge. The sea height during storms such as hurricanes 

that is above the normal level expected at that time and place 

based on the tides alone.

Stratification. The layering of water by temperature and 

density that can occur in lakes or other bodies of water, often 

seasonally.

Stressor. Something that has an effect on people and on natural, managed, and socioeconomic systems. Multiple stressors 

can have compounded effects, such as when economic or 

market stress combines with drought to negatively impact 

farmers.

Surveillance. The collection, analysis, interpretation, and 

dissemination of health data.

Thermoregulation. The process of maintaining the core 

internal temperature of the body. Normally, a person’s core 

temperature remains relatively constant at 98.6°F (37°C).

Toxin. Biological, chemical, or physical agents (such as radiation) that can cause harmful effects on people. Related term: 

Toxic.

Trauma. An adverse physical or psychological state caused 

by physical injury or mental stress. Related terms: Traumatic 

injury, Psychological trauma.

Uncertainty (climate change). An expression of the degree 

to which future climate is unknown. Uncertainty about the 

future climate arises from the complexity of the climate 

system and the ability of models to represent it, as well as the 

inability to predict the decisions that society will make. There 

is also uncertainty about how climate change, in combination 

with other stressors, will affect people and natural systems.

Urban heat island effect. The tendency for higher air temperatures to persist in urban areas as a result of heat absorbed 

and emitted by buildings and asphalt, tending to make cities 

warmer than the surrounding countryside.

Vector (disease). An organism, such as an insect or a tick, 

which transmits disease-causing microorganisms such as 

viruses, bacteria, or protozoa. Vector-Borne diseases include, 

for example, malaria, dengue fever, West Nile virus, and Lyme 

disease. Related terms: Vector-Borne disease.

Virus. A microorganism that can cause disease by infecting 

and then growing and multiplying in cells. Related terms: 

Enterovirus, Rotavirus, Norovirus, Hantavirus.

Vulnerability. The tendency or predisposition to be adversely 

affected by stressors or impacts, including climate-related 

health effects.

Waterborne illness. Diseases contracted through contact with 

water that is infected with pathogens such as Vibrio cholerae, 

Campylobacter, Salmonella, Shigella, and the diarrhea-causing 

Escherichia coli.

Watershed. An area of land that drains water to a particular 

stream, river, lake, bay, or ocean.

Weather. The day-to-day variations in temperature, precipitation, and other aspects of the atmosphere around us.

West Nile virus. A virus carried by birds and most often transmitted to people by infected mosquitos.

Wildfire. An unplanned fire that occurs in forest, shrubland, or 

grassland.

Zoonotic disease. A disease that can spread to people from 

other vertebrate animals. Examples of zoonotic diseases 

include dengue fever, avian flu, West Nile virus, and bubonic 

plague. Related term: Zoonoses.

APPENDIX 5: GLOSSARY AND ACRONYMS

U.S. Global Change Research Program 311 Impacts of Climate Change on Human Health in the United States
Abbreviations and Acronyms

BenMAP – Benefits Mapping and Analysis Program

CCHHG – Interagency Crosscutting Group on Climate Change 

and Human Health

CDC – Centers for Disease Control and Prevention

CENRS – Committee on Environment, Natural Resources, and 

Sustainability

CICS-NC – Cooperative Institute for Climate and Satellites–

North Carolina

CMIP – Coupled Model Intercomparison Project 

CO – carbon monoxide

CO2 – carbon dioxide

COPD – chronic obstructive pulmonary disease 

CVD – cardiovascular disease

DoD – U.S. Department of Defense

DOE – U.S. Department of Energy

EPA – U.S. Environmental Protection Agency

FDA – U.S. Food and Drug Administration

GCM – global climate model, also referred to as general circulation model

GHG – greenhouse gas

GIS – geographic information systems

HAB – harmful algal bloom

HHS – U.S. Department of Health and Human Services

ICLUS – Integrated Climate and Land Use Scenarios

ICS – inhaled corticosteroids

IPCC – Intergovernmental Panel on Climate Change

IQA – Information Quality Act

LEP – limited English proficiency

NASA – National Aeronautics and Space Administration

NCA – National Climate Assessment

NCEI – National Centers for Environmental Information, formerly the National Climatic Data Center 

NIEHS – National Institute of Environmental Health Sciences

NIH – National Institutes of Health

NIOSH – National Institute for Occupational Safety and Health

NOAA – National Oceanic and Atmospheric Administration

NOx – nitrogen oxides

NRC – National Research Council

O3 – ozone

PM – particulate matter

PTSD – post-traumatic stress disorder

RCP – Representative Concentration Pathway

SES – socioeconomic status

SO2 – sulfur dioxide

SRES – Special Report on Emissions Scenarios

USDA – U.S. Department of Agriculture

USGCRP – U.S. Global Change Research Program

USGS – U.S. Geological Survey

VA – U.S. Department of Veterans Affairs

WNV – West Nile virus

Recommended Citation: USGCRP, 2016: Appendix 5: Glossary and 

Acronyms. The Impacts of Climate Change on Human Health in 

the United States: A Scientific Assessment. U.S. Global Change 

Research Program, Washington, DC, 307–312. http://dx.doi.

org/10.7930/J02F7KCR

APPENDIX 5: GLOSSARY AND ACRONYMS A.1.1 End

U.S. Global Change Research Program 312 Impacts of Climate Change on Human Health in the United States

U.S. Global Change

Research Program

Climate change is affecting the health of Americans. As 

the climate continues to change, the risks to human health 

will grow, exacerbating existing health threats and creating 

new public health challenges. This assessment significantly 

advances what we know about the impacts of climate change 

on public health, and the confidence with which we know it.

health2016.globalchange.gov