U.S. National Climate Assessment

U.S. Global Change Research Program

Climate Change Impacts 

in the United States

OVERVIEW
2 U.S. GLOBAL CHANGE RESEARCH PROGRAM OVERVIEW of Climate Change Impacts in the United States 3

The National Climate Assessment assesses the science of climate change and its impacts 

across the United States, now and throughout this century. It 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. 

A team of more than 300 experts (see full report), guided by a 60-member National 

Climate Assessment and Development Advisory Committee (listed on the inside back 

cover) produced the full report – the largest and most diverse team to produce a U.S. 

climate assessment. Stakeholders involved in the development of the assessment 

included decision-makers from the public and private sectors, resource and environmental 

managers, researchers, representatives from businesses and non-governmental 

organizations, and the general public. More than 70 workshops and listening sessions were 

held, and thousands of public and expert comments on the draft report provided additional 

input to the process. 

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

input reports, and other publicly available sources; all sources meet the standards of the 

Information Quality Act. The report was extensively reviewed by the public and experts, 

including a panel of the National Academy of Sciences, the 13 Federal agencies of the 

U.S. Global Change Research Program, and the Federal Committee on Environment, 

Natural Resources, and Sustainability.

About the HIGHLIGHTS

The 148-page Highlights book presents the major findings and selected highlights from 

Climate Change Impacts in the United States, the third National Climate Assessment.

The Highlights report is organized around the National Climate Assessment’s 12 Report 

Findings, which take an overarching view of the entire report and its 30 chapters. All 

material in the Highlights report is drawn from the full report. The Key Messages from 

each of the 30 report chapters appear throughout the Highlights. 

Online at:

nca2014.globalchange.gov

Online at:

nca2014.globalchange.gov/highlights

About the NATIONAL CLIMATE ASSESSMENT

About the OVERVIEW

This booklet provides a high level compendium of Climate Change Impacts in the United 

States, the Third National Climate Assessment. The Overview covers the most important 

impacts at the national level but does not attempt to provide a comprehensive summary 

of the entire assessment. Numbered references can be found in the Highlights. 

To supplement this Overview, regional fact sheets are available that offer highlights 

from each of the eight regions. These and other resources can found at the website 

nca2014.globalchange.gov.

Our Changing Climate

Water Resources

Energy Supply and Use

Transportation

Agriculture

Forests

Ecosystems and Biodiversity

Human Health

Energy, Water, and Land Use

Urban Systems and Infrastructure

Indigenous Peoples, Lands, and Resources

Land Use and Land Cover Change

Rural Communities

Biogeochemical Cycles

Oceans and Marine Resources

Coastal Zones

Decision Support

Mitigation

Adaptation

Climate Change Impacts in the United States describes current and future impacts on various U.S. 

regions and sectors. It also describes some of the responses underway and planned. The eight regions 

are shown on the map, and icons and titles below identify the various subjects, sectors, cross-sector 

topics, and responses to climate change covered in the assessment.

Photo Credits:

Cover–Sandbagging: DoD photo by Staff Sgt. Michael Crane, U.S. Air Force; Oil rig and wind turbine: ©Jim West/imagebroker/Corbis; Fireman: ©AP Photo/The Press-Enterprise/Terry 

Pierson; Solar panel: ©Dennis Schroeder, NREL; Blue-green textured background on front and back covers and on page vii: ©iStockPhoto.com/javaman3. Pg. 14–Chicago sunset: ©Bill 

Ross/Corbis; Farm during drought: ©Scott Olson/Getty Images; North Atlantic hurricane: Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC caption; Supercell thunderstorm over a plain: ©Roger Hill/Science Photo Library/Corbis; Blue marble globe: courtesy NASA. Pg. 15– Blue marble globe: courtesy NASA; Clouds with precipitation: ©Eric Raptosh 

Photography/Blend Images/Corbis; Car in flooded road: ©James Borchuck/ZUMA Press/Corbis; field: ©AgStock Images/Corbis; Ice melt: ©Steve Morgan/epa/Corbis; Beach waves near 

city: ©Joe Raedle/Getty Images; Dissolved shell in acidified ocean water: David Liittschwager, National Geographic Images. Pg. 16–Person pumping gas: Charles Minshew/KOMU; People 

cooling off during heatwave: ©Julie Jacobson/AP/Corbis; Smog over city: ©iStockPhoto.com/Daniel Stein; Child blowing nose: ©Stockbyte/Getty Images. Pg. 17–Mosquito biting finger: 

©James Gathany, CDC; Road washed out due to flooding: ©John Wark/AP/Corbis; Mountain stream: ©Dan Sherwood/Design Pics/Corbis; Farmer with corn: ©iStockPhoto.com/ValentinRussanov. Pg. 18–Person building house: ©Aaron Huey/National Geographic Society/Corbis; Bear: ©Chase Swift/Corbis; Manatee: US Fish and Wildlife Service; Person with solar panels: 

©Dennis Schroeder, NREL. Back cover: Blue-green textured background on front and back covers and on page vii: ©iStockPhoto.com/javaman3, Field: ©Timothy Hearsum/AgStock 

Images/Corbis; Woman and solar panel: ©Bill Miles/Mint Images/Corbis; Sea ice melt: ©Steve Morgan/epa/Corbis; Flood rescue workers and victim: ©Adam Hunger/Reuters/Corbis

Climate Science 

Supplement

Frequently 

Asked 

Questions
4 U.S. GLOBAL CHANGE RESEARCH PROGRAM OVERVIEW of Climate Change Impacts in the United States 5

CLIMATE CHANGE AND 

Climate change, once considered an issue for a distant future, has moved firmly into the present. Corn producers in Iowa, oyster growers in Washington State, and maple syrup producers in 

Vermont are all observing climate-related changes that are outside of recent experience. So, 

too, are coastal planners in Florida, water managers in the arid Southwest, city dwellers from 

Phoenix to New York, and Native Peoples on tribal lands from Louisiana to Alaska. This National 

Climate Assessment concludes that the evidence of human-induced climate change continues to 

strengthen and that impacts are increasing across the country.

Americans are noticing changes all around them. Summers are longer and hotter, and extended 

periods of unusual heat last longer than any living American has ever experienced. Winters are 

generally shorter and warmer. Rain comes in heavier downpours. People are seeing changes in the 

length and severity of seasonal allergies, the plant varieties that thrive in their gardens, and the 

kinds of birds they see in any particular month in their neighborhoods. 

Other changes are even more dramatic. Residents of some coastal cities see their streets flood 

more regularly during storms and high tides. Inland cities near large rivers also experience more 

flooding, especially in the Midwest and Northeast. Insurance rates are rising in some vulnerable 

locations, and insurance is no longer available in others. Hotter and drier weather and earlier snow 

melt mean that wildfires in the West start earlier in the spring, last later into the fall, and burn 

more acreage. In Arctic Alaska, the summer sea ice that once protected the coasts has receded, 

and autumn storms now cause more erosion, threatening many communities with relocation. 

Scientists who study climate change confirm that these observations are consistent with significant 

changes in Earth’s climatic trends. Long-term, independent records from weather stations, 

satellites, ocean buoys, tide gauges, and many other data sources all confirm that our nation, 

like the rest of the world, is warming. Precipitation patterns are changing, sea level is rising, the 

oceans are becoming more acidic, and the frequency and intensity of some extreme weather 

events are increasing. Many lines of independent evidence demonstrate that the rapid warming of 

the past half-century is due primarily to human activities. 

The observed warming and other climatic changes are triggering wide-ranging impacts in every 

region of our country and throughout our economy. Some of these changes can be beneficial over 

the short run, such as a longer growing season in some regions and a longer shipping season on 

the Great Lakes. But many more are detrimental, largely because our 

society and its infrastructure were designed for the climate that we have 

had, not the rapidly changing climate we now have and can expect in the 

future. In addition, climate change does not occur in isolation. Rather, 

it is superimposed on other stresses, which combine to create new 

challenges. 

This National Climate Assessment collects, integrates, and assesses 

observations and research from around the country, helping us to see 

what is actually happening and understand what it means for our lives, 

our livelihoods, and our future. The report includes analyses of impacts 

on seven sectors – human health, water, energy, transportation, 

agriculture, forests, and ecosystems – and the interactions among 

sectors at the national level. The report also assesses key impacts on 

all U.S. regions: Northeast, Southeast and Caribbean, Midwest, Great 

Plains, Southwest, Northwest, Alaska, Hawai`i and Pacific Islands, as 

well as the country’s coastal areas, oceans, and marine resources. 

Over recent decades, climate science has advanced significantly. 

Increased scrutiny has led to increased certainty that we are now 

seeing impacts associated with human-induced climate change. With 

each passing year, the accumulating evidence further expands our 

understanding and extends the record of observed trends in temperature, precipitation, sea level, 

ice mass, and many other variables recorded by a variety of measuring systems and analyzed by 

independent research groups from around the world. It is notable that as these data records have 

grown longer and climate models have become more comprehensive, earlier predictions have 

largely been confirmed. The only real surprises have been that some changes, such as sea level 

rise and Arctic sea ice decline, have outpaced earlier projections. 

What is new over the last decade is that we know with increasing certainty that climate change 

is happening now. While scientists continue to refine projections of the future, observations 

unequivocally show that climate is changing and that the warming of the past 50 years is primarily 

due to human-induced emissions of heat-trapping gases. These emissions come mainly from 

burning coal, oil, and gas, with additional contributions from forest clearing and some agricultural 

practices. 

Global climate is projected to continue to change over this century and beyond, but there is still 

time to act to limit the amount of change and the extent of damaging impacts. 

This report documents the changes 

already observed and those projected for 

the future. 

It is important that these findings and 

response options be shared broadly to 

inform citizens and communities across 

our nation. Climate change presents a 

major challenge for society. This report 

advances our understanding of that 

challenge and the need for the American 

people to prepare for and respond to its 

far-reaching implications. 

THE AMERICAN PEOPLE
6 U.S. GLOBAL CHANGE RESEARCH PROGRAM OVERVIEW of Climate Change Impacts in the United States 7

Climate change is already affecting the American people 

in far-reaching ways. Certain types of extreme weather 

events with links to climate change have become more 

frequent and/or intense, including prolonged periods 

of heat, heavy downpours, and, in some regions, floods 

and droughts. In addition, warming is causing sea level 

to rise and glaciers and Arctic sea ice to melt, and oceans 

are becoming more acidic as they absorb carbon dioxide. 

These and other aspects of climate change are disrupting 

people’s lives and damaging some sectors of our economy. 

Climate Change: 

Present and Future

Evidence for climate change abounds, from the top of 

the atmosphere to the depths of the oceans. Scientists 

and engineers from around the world have meticulously 

collected this evidence, using satellites and networks 

of weather balloons, thermometers, buoys, and other 

observing systems. Evidence of climate change is also 

visible in the observed and measured changes in location 

and behavior of species and functioning of ecosystems. 

Taken together, this evidence tells an unambiguous story: 

the planet is warming, and over the last half century, this 

warming has been driven primarily by human activity.

Multiple lines of independent evidence confirm that human activities are the primary cause of the global warming of the past 50 years. The burning of coal, oil, and gas, 

and clearing of forests have increased the concentration 

These are just some of the indicators measured globally over many decades that show that the 

Earth’s climate is warming. White arrows indicate increasing trends; black arrows indicate decreasing 

trends. All the indicators expected to increase in a warming world are increasing, and all those 

expected to decrease in a warming world are decreasing. (Figure source: NOAA NCDC, based on 

data updated from Kennedy et al. 2010a).

Ten Indicators of a Warming World

of carbon dioxide in the atmosphere by more than 40% 

since the Industrial Revolution, and it has been known for 

almost two centuries that this carbon dioxide traps heat. 

Methane and nitrous oxide emissions from agriculture and 

other human activities add to the atmospheric burden of 

heat-trapping gases. Data show that natural factors like 

the sun and volcanoes cannot have caused the warming 

observed over the past 50 years. Sensors on satellites have 

measured the sun’s output with great accuracy and found 

no overall increase during the past half century. Large volcanic eruptions during this period, such as Mount Pinatubo 

in 1991, have exerted a short-term cooling influence. In 

fact, if not for human activities, global climate would actually have cooled slightly over 

the past 50 years. The pattern of temperature change 

through the layers of the atmosphere, with warming near 

the surface and cooling higher 

up in the stratosphere, further 

confirms that it is the buildup 

of heat-trapping gases (also 

known as “greenhouse gases”) 

that has caused most of the 

Earth’s warming over the past 

half century. 

Because human-induced 

warming is superimposed 

on a background of natural 

variations in climate, warming is not uniform over time. 

Short-term fluctuations in the 

long-term upward trend are 

thus natural and expected. 

For example, a recent slowing in the rate of surface 

air temperature rise appears to be related to cyclic 

changes in the oceans and in the sun’s energy output, as well as a series of small volcanic eruptions 

and other factors. Nonetheless, global temperatures are still on the rise and are expected to rise 

further.

U.S. average temperature has increased by 1.3°F 

to 1.9°F since 1895, and most of this increase has 

occurred since 1970. The most recent decade was 

the nation’s and the world’s hottest on record, and 

2012 was the hottest year on record in the continental United States. All U.S. regions have experienced warming in recent decades, but the extent 

of warming has not been uniform. In general, 

temperatures are rising more quickly in the north. 

Alaskans have experienced some of the largest 

increases in temperature between 1970 and the 

present. People living in the Southeast have experienced some of the smallest temperature increases 

over this period.

Temperatures are projected to rise another 2°F to 

4°F in most areas of the United States over the next few decades. Reductions in some short-lived human-induced emissions that contribute to warming, such as black carbon (soot) and methane, could reduce some of the projected warming 

over the next couple of decades, because, unlike carbon dioxide, these gases and particles have relatively short atmospheric lifetimes.

The amount of warming projected beyond the next few decades is directly linked to the cumulative global emissions of 

heat-trapping gases and particles. By the end of this century, a roughly 3°F to 5°F rise is projected under a lower emissions scenario, which would require substantial reductions in emissions (referred to as the “B1 scenario”), and a 5°F to 

10°F rise for a higher emissions scenario assuming continued increases in emissions, predominantly from fossil fuel combustion (referred to as the “A2 scenario”). These projections are based 

on results from 16 climate models 

that used the two emissions scenarios 

in a formal inter-model comparison 

study. The range of model projections 

for each emissions scenario is the result of the differences in the ways the 

models represent key factors such as 

water vapor, ice and snow reflectivity, 

and clouds, which can either dampen 

or amplify the initial effect of human 

influences on temperature. The net 

effect of these feedbacks is expected 

to amplify warming. More information about the models and scenarios 

used in this report can be found in 

Appendix 5 of the full report.1

Different amounts of heat-trapping gases 

released into the atmosphere by human activities produce different projected increases in 

Earth’s temperature. The lines on the graph 

represent a central estimate of global average 

temperature rise (relative to the 1901-1960 

average) for the two main scenarios used in 

this report. A2 assumes continued increases 

in emissions throughout this century, and B1 

assumes significant emissions reductions, 

though not due explicitly to climate change 

policies. Shading indicates the range (5th to 

95th percentile) of results from a suite of climate models. In both cases, temperatures are 

expected to rise, although the difference between lower and higher emissions pathways 

is substantial. (Figure source: NOAA NCDC / 

CICS-NC).

Projected Global Temperature Change

The green band shows how global average temperature would have changed 

over the last century due to natural forces alone, as simulated by climate 

models. The blue band shows model simulations of the effects of human and 

natural forces (including solar and volcanic activity) combined. The black line 

shows the actual observed global average temperatures. Only with the inclusion of human influences can models reproduce the observed temperature 

changes. (Figure source: adapted from Huber and Knutti 2012b). 

Separating Human and Natural 

Influences on Climate

Coal-fired power plants emit heat-trapping carbon dioxide to 

the atmosphere.

OVERVIEW
8 U.S. GLOBAL CHANGE RESEARCH PROGRAM OVERVIEW of Climate Change Impacts in the United States 9

Prolonged periods of high temperatures and the persistence of high nighttime temperatures have increased 

in many locations (especially in urban areas) over the past 

half century. High nighttime temperatures have widespread impacts because people, livestock, and wildlife 

get no respite from the heat. In some regions, prolonged 

periods of high temperatures associated with droughts 

contribute to conditions that lead to larger wildfires and 

longer fire seasons. As expected in a warming climate, 

recent trends show that extreme heat is becoming more 

common, while extreme cold is becoming less common. 

Evidence indicates that the human influence on climate 

has already roughly doubled the probability of extreme 

heat events such as the record-breaking summer heat 

experienced in 2011 in Texas and Oklahoma. The incidence 

of record-breaking high temperatures is projected to rise.2

Human-induced climate change means much more than 

just hotter weather. Increases in ocean and freshwater 

temperatures, frost-free days, and heavy downpours 

have all been documented. Global sea level has risen, and 

there have been large reductions in snow-cover extent, 

glaciers, and sea ice. These changes and other climatic 

changes have affected and will continue 

to affect human health, water supply, 

agriculture, transportation, energy, 

coastal areas, and many other sectors 

of society, with increasingly adverse 

impacts on the American economy and 

quality of life.3

Some of the changes discussed in this 

report are common to many regions. 

For example, large increases in heavy 

precipitation have occurred in the 

Northeast, Midwest, and Great Plains, 

where heavy downpours have frequently led to runoff that exceeded the 

capacity of storm drains and levees, 

and caused flooding events and accelerated erosion. Other impacts, such as 

those associated with the rapid thawing 

of permafrost in Alaska, are unique to 

a particular U.S. region. Permafrost 

thawing is causing extensive damage 

to infrastructure in our nation’s largest 

state.4

Some impacts that occur in one region 

ripple beyond that region. For example, 

Percent changes in the amount of precipitation falling in very heavy events (the heaviest 1%) 

from 1958 to 2012 for each region. There is a clear national trend toward a greater amount 

of precipitation being concentrated in very heavy events, particularly in the Northeast and 

Midwest. (Figure source: updated from Karl et al. 2009c ).

Observed Change in Very Heavy Precipitation

coastal communities, especially along the 

Gulf Coast, the Atlantic seaboard, and in 

Alaska. Coastal infrastructure, including 

roads, rail lines, energy infrastructure, 

airports, port facilities, and military bases, 

are increasingly at risk from sea level rise 

and damaging storm surges. Sea level is 

projected to rise by another 1 to 4 feet in 

this century, although the rise in sea level 

in specific regions is expected to vary from 

this global average for a number of reasons. 

A wider range of scenarios, from 8 inches to 

more than 6 feet by 2100, has been used in 

risk-based analyses in this report. In general, 

higher emissions scenarios that lead to 

more warming would be expected to lead to 

higher amounts of sea level rise. The stakes 

are high, as nearly five million Americans 

and hundreds of billions of dollars of property are located 

in areas that are less than four feet above the local hightide level.6

In addition to causing changes in climate, increasing levels 

of carbon dioxide from the burning of fossil fuels and 

other human activities have a direct effect on the world’s 

oceans. Carbon dioxide interacts with ocean water to 

form carbonic acid, increasing the ocean’s acidity. Ocean 

surface waters have become 30% more acidic over the last 

250 years as they have absorbed large amounts of carbon 

dioxide from the atmosphere. This ocean acidification 

makes water more corrosive, reducing the capacity of 

marine organisms with shells or skeletons made of calcium 

carbonate (such as corals, krill, oysters, clams, and crabs) 

to survive, grow, and reproduce, which in turn will affect 

the marine food chain.7

Widespread Impacts

Impacts related to climate change are 

already evident in many regions and sectors 

and are expected to become increasingly 

disruptive across the nation throughout 

this century and beyond. Climate changes 

interact with other environmental and 

societal factors in ways that can either 

moderate or intensify these impacts.

Some climate changes currently have 

beneficial effects for specific sectors or 

regions. For example, current benefits of 

warming include longer growing seasons for 

agriculture and longer ice-free periods for 

shipping on the Great Lakes. At the same 

time, however, longer growing seasons, 

along with higher temperatures and 

carbon dioxide levels, can increase pollen 

production, intensifying and lengthening 

the allergy season. Longer ice-free periods 

on the Great Lakes can result in more lakeeffect snowfalls.

the dramatic decline of summer sea ice in the Arctic – a 

loss of ice cover roughly equal to half the area of the 

continental United States – exacerbates global warming by 

reducing the reflectivity of Earth’s surface and increasing 

the amount of heat absorbed. Similarly, smoke from 

wildfires in one location can contribute to poor air quality 

in faraway regions, and evidence suggests that particulate 

matter can affect atmospheric properties and therefore 

weather patterns. Major storms and the higher storm 

surges exacerbated by sea level rise that hit the Gulf Coast 

affect the entire country through their cascading effects 

on oil and gas production and distribution.5

Water expands as it warms, causing global sea levels to 

rise; melting of land-based ice also raises sea level by 

adding water to the oceans. Over the past century, global 

average sea level has risen by about 8 inches. Since 1992, 

the rate of global sea level rise measured by satellites 

has been roughly twice the rate observed over the last 

century, providing evidence of acceleration. Sea level 

rise, combined with coastal storms, has increased the 

risk of erosion, storm surge damage, and flooding for 

The correlation between rising levels of carbon dioxide in the atmosphere (red) with 

rising carbon dioxide levels (blue) and falling pH in the ocean (green). As carbon 

dioxide accumulates in the ocean, the water becomes more acidic (the pH declines). 

(Figure source: modified from Feely et al. 2009d).

As Oceans Absorb CO2

They Become More Acidic

Pteropods, or “sea butterflies,” are eaten by a variety of marine species ranging from 

tiny krill to salmon to whales. The photos show what happens to a pteropod’s shell in 

seawater that is too acidic. On the left is a shell from a live pteropod from a region in the 

Southern Ocean where acidity is not too high. The shell on the right is from a pteropod 

in a region where the water is more acidic. (Figure source: (left) Bednaršek et al. 2012e

(right) Nina Bednaršek).

Shells Dissolve in Acidified Ocean Water
10 U.S. GLOBAL CHANGE RESEARCH PROGRAM OVERVIEW of Climate Change Impacts in the United States 11

Northeast Communities are affected by heat waves, more extreme precipitation events, and 

coastal flooding due to sea level rise and storm surge.

Southeast 

and 

Caribbean

Decreased water availability, exacerbated by population growth and land-use change, 

causes increased competition for water. There are increased risks associated with 

extreme events such as hurricanes.

Midwest

Longer growing seasons and rising carbon dioxide levels increase yields of some crops, 

although these benefits have already been offset in some instances by occurrence of 

extreme events such as heat waves, droughts, and floods.

Great Plains Rising temperatures lead to increased demand for water and energy and impacts on 

agricultural practices.

Southwest Drought and increased warming foster wildfires and increased competition for scarce 

water resources for people and ecosystems.

Northwest

Changes in the timing of streamflow related to earlier snowmelt reduce the supply of 

water in summer, causing far-reaching ecological and socioeconomic consequences.

Alaska

Rapidly receding summer sea ice, shrinking glaciers, and thawing permafrost cause 

damage to infrastructure and major changes to ecosystems. Impacts to Alaska Native 

communities increase.

Hawai‘i 

and Pacific 

Islands

Increasingly constrained freshwater supplies, coupled with increased temperatures, 

stress both people and ecosystems and decrease food and water security.

Coasts

Coastal lifelines, such as water supply infrastructure and evacuation routes, are 

increasingly vulnerable to higher sea levels and storm surges, inland flooding, and 

other climate-related changes.

Oceans

The oceans are currently absorbing about a quarter of human-caused carbon dioxide 

emissions to the atmosphere and over 90% of the heat associated with global 

warming, leading to ocean acidification and the alteration of marine ecosystems.

Observed and projected climate change impacts vary across the regions of the United States. Selected impacts 

emphasized in the regional chapters are shown below, and many more are explored in detail in this report. 

Sectors affected by climate changes include agriculture, 

water, human health, energy, transportation, forests, 

and ecosystems. Climate change poses a major challenge 

to U.S. agriculture because of the critical dependence 

of agricultural systems on climate. Climate change has 

the potential to both positively and negatively affect the 

location, timing, and productivity of crop, livestock, and 

fishery systems at local, national, and global scales. The 

United States produces nearly $330 billion per year in 

agricultural commodities. This productivity is vulnerable 

to direct impacts on crops and livestock from changing 

climate conditions and extreme weather events and indirect impacts through increasing pressures from pests and 

pathogens. Climate change will also alter the stability of 

food supplies and create new food security challenges for 

the United States as the world seeks to feed nine billion 

people by 2050. While the agriculture sector has proven 

to be adaptable to a range of stresses, as evidenced by 

continued growth in production and 

efficiency across the United States, 

climate change poses a new set of 

challenges.8

Water quality and quantity are being 

affected by climate change. Changes 

in precipitation and runoff, combined 

with changes in consumption and 

withdrawal, have reduced surface 

and groundwater supplies in many areas. These trends 

are expected to continue, increasing the likelihood of 

water shortages for many uses. Water quality is also 

diminishing in many areas, particularly due to sediment 

and contaminant concentrations after heavy downpours. 

Sea level rise, storms and storm surges, and changes 

in surface and groundwater use patterns are expected 

to compromise the sustainability of coastal freshwater 

aquifers and wetlands. In most U.S. regions, water 

resources managers and planners will 

encounter new risks, vulnerabilities, 

and opportunities that may not be 

properly managed with existing 

practices.9

Climate change affects human 

health in many ways. For example, 

increasingly frequent and intense 

heat events lead to more heatrelated illnesses and deaths and, over time, worsen 

drought and wildfire risks, and intensify air pollution. 

Increasingly frequent extreme precipitation and associated 

flooding can lead to injuries and increases in waterborne 

disease. Rising sea surface temperatures have been linked 

with increasing levels and ranges of diseases. Rising sea 

levels intensify coastal flooding and storm surge, and 

thus exacerbate threats to public safety during storms. 

Certain groups of people are more vulnerable to the 

range of climate change related health impacts, including 

the elderly, children, the poor, and the sick. Others are 

vulnerable because of where they live, including those 

in floodplains, coastal zones, and some urban areas. 

Improving and properly supporting the public health 

infrastructure will be critical to managing the potential 

health impacts of climate change.10

Climate change also affects the living world, including 

people, through changes in ecosystems and biodiversity. 

Ecosystems provide a rich array of benefits and services to 

humanity, including habitat for fish and wildlife, drinking 

water storage and filtration, fertile soils for growing crops, 

Certain groups of people are 

more vulnerable to the range 

of climate change related 

health impacts, including the 

elderly, children, the poor, 

and the sick.

Climate change can exacerbate respiratory and asthma-related conditions through increases in pollen, ground-level ozone, 

and wildfire smoke.

Increasing air and water temperatures, more intense precipitation and runoff, and intensifying droughts can decrease water 

quality in many ways. Here, middle school students in Colorado 

test water quality.
12 U.S. GLOBAL CHANGE RESEARCH PROGRAM OVERVIEW of Climate Change Impacts in the United States 13

The amount of future climate 

change will still largely be 

determined by choices society 

makes about emissions. 

buffering against a range of stressors including climate 

change impacts, and aesthetic and cultural values. These 

benefits are not always easy to quantify, but they support 

jobs, economic growth, health, and human well-being. 

Climate change driven disruptions to ecosystems have 

direct and indirect human impacts, including reduced water 

supply and quality, the loss of iconic species and landscapes, 

effects on food chains and the timing and success of species 

migrations, and the potential for extreme weather and climate events to destroy or degrade the ability of ecosystems 

to provide societal benefits.11

Human modifications of ecosystems and landscapes often 

increase their vulnerability to damage from extreme 

weather events, while simultaneously reducing their natural capacity to moderate the impacts of such events. For 

example, salt marshes, reefs, mangrove forests, and barrier 

islands defend coastal ecosystems 

and infrastructure, such as roads and 

buildings, against storm surges. The 

loss of these natural buffers due to 

coastal development, erosion, and 

sea level rise increases the risk of 

catastrophic damage during or after 

extreme weather events. Although 

floodplain wetlands are greatly reduced from their historical extent, those that remain still absorb floodwaters 

and reduce the effects of high flows on river-margin lands. 

Extreme weather events that produce sudden increases 

in water flow, often carrying debris and pollutants, can 

decrease the natural capacity of ecosystems to cleanse 

contaminants.12

The climate change impacts being felt in the regions and 

sectors of the United States are affected by global trends 

and economic decisions. In an increasingly interconnected world, U.S. vulnerability is linked to impacts in other 

nations. It is thus difficult to fully evaluate the impacts of 

climate change on the United States without considering 

consequences of climate change elsewhere.

Response Options

As the impacts of climate change are becoming more 

prevalent, Americans face choices. Especially because of 

past emissions of long-lived heat-trapping gases, some 

additional climate change and related impacts are now 

unavoidable. This is due to the long-lived nature of many 

of these gases, as well as the amount of heat absorbed 

and retained by the oceans and other responses within 

the climate system. The amount of future climate change, 

however, will still largely be determined by choices society 

makes about emissions. Lower emissions of heat-trapping 

gases and particles mean less future warming and lesssevere impacts; higher emissions mean more warming and 

more severe impacts. Efforts to limit emissions or increase 

carbon uptake fall into a category of response options 

known as “mitigation,” which refers to reducing the amount 

and speed of future climate change by reducing emissions 

of heat-trapping gases or removing carbon dioxide from the 

atmosphere.13

The other major category of response options is known 

as “adaptation,” and refers to actions to prepare for and 

adjust to new conditions, thereby reducing harm or taking 

advantage of new opportunities. Mitigation and adaptation 

actions are linked in multiple ways, including that effective 

mitigation reduces the need for adaptation in the future. 

Both are essential parts of a comprehensive climate change response 

strategy. The threat of irreversible impacts makes the timing of mitigation 

efforts particularly critical. This report 

includes chapters on Mitigation, 

Adaptation, and Decision Support that 

offer an overview of the options and 

activities being planned or implemented around the country as local, state, federal, and tribal governments, as well as 

businesses, organizations, and individuals begin to respond 

to climate change. These chapters conclude that while response actions are under development, current implementation efforts are insufficient to avoid increasingly negative 

social, environmental, and economic consequences.14

Large reductions in global emissions of heat-trapping gases, 

similar to the lower emissions scenario (B1) analyzed in 

this assessment, would reduce the risks of some of the 

worst impacts of climate change. Some targets called for 

in international climate negotiations to date would require 

even larger reductions than those outlined in the B1 scenario. Meanwhile, global emissions are still rising and are on 

a path to be even higher than the high emissions scenario 

(A2) analyzed in this report. The recent U.S. contribution to 

annual global emissions is about 18%, but the U.S. contribution to cumulative global emissions over the last century 

is much higher. Carbon dioxide lasts for a long time in the 

atmosphere, and it is the cumulative carbon emissions 

that determine the amount of global climate change. After 

decades of increases, U.S. CO2 emissions from energy use 

(which account for 97% of total U.S. emissions) declined by 

around 9% between 2008 and 2012, largely due to a shift 

from coal to less CO2-intensive natural gas for electricity 

production. Governmental actions in city, state, regional, 

and federal programs to promote energy efficiency have 

also contributed to reducing U.S. carbon emissions. Many, 

if not most of these programs are motivated by other policy 

objectives, but some are directed specifically at greenhouse 

gas emissions. These U.S. actions and others that might be 

undertaken in the future are described in the Mitigation 

chapter of this report. Over the remainder of this century, 

aggressive and sustained greenhouse gas emission reductions by the United States and by other nations would be 

needed to reduce global emissions to a level consistent with 

the lower scenario (B1) analyzed in this assessment.15

With regard to adaptation, the pace and magnitude of 

observed and projected changes emphasize the need to 

be prepared for a wide variety and intensity of impacts. 

Because of the growing influence of human activities, the 

climate of the past is not a good basis for future planning. 

For example, building codes and landscaping ordinances 

could be updated to improve energy efficiency, conserve 

water supplies, protect against insects that spread disease 

(such as dengue fever), reduce susceptibility to heat stress, 

and improve protection against extreme events. The fact 

that climate change impacts are increasing points to the 

urgent need to develop and refine approaches that enable 

decision-making and increase flexibility and resilience in 

the face of ongoing and future impacts. Reducing non-climate-related stresses that contribute to existing vulnerabilities can also be an effective approach to climate change 

adaptation.16

Adaptation can involve considering local, state, regional, 

national, and international jurisdictional objectives. For 

example, in managing water supplies to adapt to a changing 

climate, the implications of international treaties should 

be considered in the context of managing the Great Lakes, 

the Columbia River, and the Colorado River to deal with 

increased drought risk. Both “bottom up” community planning and “top down” national strategies may help regions 

deal with impacts such as increases in electrical brownouts, 

heat stress, floods, and wildfires.17

Proactively preparing for climate change can reduce 

impacts while also facilitating a more rapid and efficient 

response to changes as they happen. Such efforts are 

beginning at the federal, regional, state, tribal, and local 

levels, and in the corporate and non-governmental sectors, 

to build adaptive capacity and resilience to climate change 

impacts. Using scientific information to prepare for climate 

changes in advance can provide economic opportunities, 

and proactively managing the risks can reduce impacts and 

costs over time.18

There are a number of areas where improved scientific 

information or understanding would enhance the capacity 

to estimate future climate change impacts. For example, 

knowledge of the mechanisms controlling the rate of 

ice loss in Greenland and Antarctica is limited, making it 

difficult for scientists to narrow the range of expected 

future sea level rise. Improved understanding of ecological 

and social responses to climate change is needed, as is 

understanding of how ecological and social responses will 

interact.19

A sustained climate assessment process could more efficiently collect and synthesize the rapidly evolving science 

and help supply timely and relevant information to decision-makers. Results from all of these efforts could continue 

to deepen our understanding of the interactions of human 

and natural systems in the context of a changing climate, 

enabling society to effectively respond and prepare for our 

future.20

The cumulative weight of the scientific evidence contained 

in this report confirms that climate change is affecting the 

American people now, and that choices we make will affect 

our future and that of future generations. 

Cities providing transportation options including bike lanes, buildings designed with energy saving features such as green roofs, and 

houses elevated to allow storm surges to pass underneath are among the many response options being pursued around the country.
14 U.S. GLOBAL CHANGE RESEARCH PROGRAM OVERVIEW of Climate Change Impacts in the United States 15

CLIMATE TRENDS

Extreme Weather

There have been changes in some 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. There have 

been regional trends in floods and droughts. Droughts in the Southwest and 

heat waves everywhere are projected to become more intense, and cold 

waves less intense everywhere.

Hurricanes

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 

are still uncertain. Hurricane-associated storm 

intensity and rainfall rates are projected to 

increase as the climate continues to warm. 

Severe Storms

Winter storms have increased in frequency and 

intensity since the 1950s, and their tracks have 

shifted northward over the United States. Other 

trends in severe storms, including the intensity 

and frequency of tornadoes, hail, and damaging 

thunderstorm winds, are uncertain and are being 

studied intensively.

Temperature

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 most recent decade was the nation’s warmest 

on record. Temperatures in the United States are expected to continue to rise. Because humaninduced warming is superimposed on a naturally varying climate, the temperature rise has not 

been, and will not be, uniform or smooth across the country or over time.

Ocean Acidification

The oceans are currently absorbing about a 

quarter of the carbon dioxide emitted to the 

atmosphere annually and are becoming more 

acidic as a result, leading to concerns about 

intensifying impacts on marine ecosystems. 

FPO

Global climate is changing and this change is apparent across a wide range of observations. 

The global warming of the past 50 years is primarily due to human activities. Global climate 

is projected to continue to change over this century and beyond. The magnitude of climate 

change beyond the next few decades depends primarily on the amount of heat-trapping gases 

emitted globally, and how sensitive the Earth’s climate is to those emissions. Heavy Downpours

Heavy downpours are increasing nationally, especially over the last three to five 

decades. Largest increases are in the Midwest and Northeast. Increases in the 

frequency and intensity of extreme precipitation events are projected for all U.S. 

regions. 

Frost-free Season

The length of the frost-free season (and the corresponding growing season) has been 

increasing nationally since the 1980s, with the largest increases occurring in the western 

United States, affecting ecosystems and agriculture. Across the United States, the growing 

season is projected to continue to lengthen.

Sea Level

Global sea level has risen by about 8 inches 

since reliable record keeping began in 1880. 

It is projected to rise another 1 to 4 feet by 

2100.

 Ice Melt

Rising temperatures are reducing ice volume and surface 

extent on land, lakes, and sea. This loss of ice is expected 

to continue. The Arctic Ocean is expected to become 

essentially ice free in summer before mid-century.

Precipitation

Average U.S. precipitation has increased since 1900, but some areas have had increases 

greater than the national average, and some areas have had decreases. More winter 

and spring precipitation is projected for the northern United States, and less for the 

Southwest, over this century. 

FPO

These two pages present the Key Messages from the “Our Changing Climate” chapter of the full report. They 

pertain to Report Findings 1, 2, and 3.
16 U.S. GLOBAL CHANGE RESEARCH PROGRAM OVERVIEW of Climate Change Impacts in the United States 17

These findings distill important results that arise from this National Climate Assessment. They do not represent a 

full summary of all of the chapters’ findings, but rather a synthesis of particularly noteworthy conclusions.

1. Global climate is changing and this is apparent across the United States in a 

wide range of observations. The global warming of the past 50 years is primarily 

due to human activities, predominantly the burning of fossil fuels.

Many independent lines of evidence confirm that human activities are affecting climate in 

unprecedented ways. 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 most recent 

decade was the warmest on record. Because human-induced warming is superimposed on a 

naturally varying climate, rising temperatures are not evenly distributed across the country or 

over time.21

4. Impacts related to climate change are already evident in many sectors and 

are expected to become increasingly disruptive across the nation throughout this 

century and beyond.

Climate change is already affecting societies and the natural world. Climate change interacts 

with other environmental and societal factors in ways that can either moderate or intensify 

these impacts. The types and magnitudes of impacts vary across the nation and through 

time. Children, the elderly, the sick, and the poor are especially vulnerable. There is 

mounting evidence that harm to the nation will increase substantially in the future unless 

global emissions of heat-trapping gases are greatly reduced.24 

3. Human-induced climate change is projected to continue, and it will accelerate 

significantly if global emissions of heat-trapping gases continue to increase.

Heat-trapping gases already in the atmosphere have committed us to a hotter future with 

more climate-related impacts over the next few decades. The magnitude of climate change 

beyond the next few decades depends primarily on the amount of heat-trapping gases that 

human activities emit globally, now and in the future.23

2. Some extreme weather and climate events have increased in recent decades, 

and new and stronger evidence confirms that some of these increases are related 

to human activities.

Changes in extreme weather events are the primary way that most people experience climate 

change. Human-induced climate change has already increased the number and strength of 

some of these extreme events. Over the last 50 years, much of the United States has seen an 

increase in prolonged periods of excessively high temperatures, more heavy downpours, and 

in some regions, more severe droughts.22

REPORT FINDINGS 5. Climate change threatens human health and well-being in many ways, including 

through more extreme weather events and wildfire, decreased air quality, and 

diseases transmitted by insects, food, and water.

Climate change is increasing the risks of heat stress, respiratory stress from poor air quality, 

and the spread of waterborne diseases. Extreme weather events often lead to fatalities and 

a variety of health impacts on vulnerable populations, including impacts on mental health, 

such as anxiety and post-traumatic stress disorder. Large-scale changes in the environment 

due to climate change and extreme weather events are increasing the risk of the emergence 

or reemergence of health threats that are currently uncommon in the United States, such as 

dengue fever.25

8. Climate disruptions to agriculture have been increasing and are projected to 

become more severe over this century. 

Some areas are already experiencing climate-related disruptions, particularly due to extreme 

weather events. While some U.S. regions and some types of agricultural production will be 

relatively resilient to climate change over the next 25 years or so, others will increasingly suffer 

from stresses due to extreme heat, drought, disease, and heavy downpours. From mid-century 

on, climate change is projected to have more negative impacts on crops and livestock across 

the country – a trend that could diminish the security of our food supply.28 

7. Water quality and water supply reliability are jeopardized by climate change in 

a variety of ways that affect ecosystems and livelihoods.

Surface and groundwater supplies in some regions are already stressed by increasing demand 

for water as well as declining runoff and groundwater recharge. In some regions, particularly 

the southern part of the country and the Caribbean and Pacific Islands, climate change is 

increasing the likelihood of water shortages and competition for water among its many 

uses. Water quality is diminishing in many areas, particularly due to increasing sediment and 

contaminant concentrations after heavy downpours.27

6. Infrastructure is being damaged by sea level rise, heavy downpours, and 

extreme heat; damages are projected to increase with continued climate change. 

Sea level rise, storm surge, and heavy downpours, in combination with the pattern of continued 

development in coastal areas, are increasing damage to U.S. infrastructure including roads, 

buildings, and industrial facilities, and are also increasing risks to ports and coastal military 

installations. Flooding along rivers, lakes, and in cities following heavy downpours, prolonged 

rains, and rapid melting of snowpack is exceeding the limits of flood protection infrastructure 

designed for historical conditions. Extreme heat is damaging transportation infrastructure such 

as roads, rail lines, and airport runways.26
18 U.S. GLOBAL CHANGE RESEARCH PROGRAM OVERVIEW of Climate Change Impacts in the United States 19

9. Climate change poses particular threats to Indigenous Peoples’ health, wellbeing, and ways of life. 

Chronic stresses such as extreme poverty are being exacerbated by climate change impacts 

such as reduced access to traditional foods, decreased water quality, and increasing exposure 

to health and safety hazards. In parts of Alaska, Louisiana, the Pacific Islands, and other 

coastal locations, climate change impacts (through erosion and inundation) are so severe that 

some communities are already relocating from historical homelands to which their traditions 

and cultural identities are tied. Particularly in Alaska, the rapid pace of temperature rise, ice 

and snow melt, and permafrost thaw are significantly affecting critical infrastructure and 

traditional livelihoods.29 

12. Planning for adaptation (to address and prepare for impacts) and mitigation 

(to reduce future climate change, for example by cutting emissions) is becoming 

more widespread, but current implementation efforts are insufficient to avoid 

increasingly negative social, environmental, and economic consequences.

Actions to reduce emissions, increase carbon uptake, adapt to a changing climate, and 

increase resilience to impacts that are unavoidable can improve public health, economic 

development, ecosystem protection, and quality of life.32 

11. Ocean waters are becoming warmer and more acidic, broadly affecting ocean 

circulation, chemistry, ecosystems, and marine life. 

More acidic waters inhibit the formation of shells, skeletons, and coral reefs. Warmer waters 

harm coral reefs and alter the distribution, abundance, and productivity of many marine 

species. The rising temperature and changing chemistry of ocean water combine with other 

stresses, such as overfishing and coastal and marine pollution, to alter marine-based food 

production and harm fishing communities.31

10. Ecosystems and the benefits they provide to society are being affected by 

climate change. The capacity of ecosystems to buffer the impacts of extreme 

events like fires, floods, and severe storms is being overwhelmed.

Climate change impacts on biodiversity are already being observed in alteration of the timing 

of critical biological events such as spring bud burst and substantial range shifts of many 

species. In the longer term, there is an increased risk of species extinction. These changes 

have social, cultural, and economic effects. Events such as droughts, floods, wildfires, and 

pest outbreaks associated with climate change (for example, bark beetles in the West) are 

already disrupting ecosystems. These changes limit the capacity of ecosystems, such as 

forests, barrier beaches, and wetlands, to continue to play important roles in reducing the 

impacts of these extreme events on infrastructure, human communities, and other valued 

resources.30

REPORT FINDINGS

Chair

Jerry Melillo, Marine Biological Laboratory

Vice-Chairs

Terese (T.C.) Richmond, Van Ness Feldman, LLP

Gary Yohe, Wesleyan University

Committee Members

Daniel Abbasi, GameChange Capital, LLC

E. Virginia Armbrust, University of Washington

Timothy (Bull) Bennett, Kiksapa Consulting, LLC

Rosina Bierbaum, University of Michigan and PCAST

Maria Blair, Independent

James Buizer, University of Arizona

Lynne M. Carter, Louisiana State University

F. Stuart Chapin III, University of Alaska

Camille Coley, Florida Atlantic University

Jan Dell, ConocoPhillips

Placido dos Santos, WestLand Resources, Inc.

Paul Fleming, Seattle Public Utilities

Guido Franco, California Energy Commission

Mary Gade, Gade Environmental Group 

Aris Georgakakos, Georgia Institute of Technology

David Gustafson, Monsanto Company

David Hales, Second Nature 

Sharon Hays, Computer Sciences Corporation

Mark Howden, CSIRO

Anthony Janetos, Boston University

Peter Kareiva, The Nature Conservancy

Rattan Lal, Ohio State University

Arthur Lee, Chevron Corporation

Jo-Ann Leong, Hawai‘i Institute of Marine Biology

Diana Liverman, University of Arizona and Oxford University

Rezaul Mahmood, Western Kentucky University

Edward Maibach, George Mason University

Michael McGeehin, RTI International

Susanne C. Moser, Susanne Moser Research & Consulting and 

Stanford University

Richard Moss, University of Maryland and PNNL

Philip Mote, Oregon State University

Jayantha Obeysekera, South Florida Water Management District

Marie O’Neill, University of Michigan

Lindene Patton, Zurich Financial Services

John Posey, East-West Gateway Council of Governments

Sara Pryor, Indiana University

Andrew Rosenberg, University of New Hampshire and Union of 

Concerned Scientists

Richard Schmalensee, Massachusetts Institute of Technology

Henry Schwartz, HGS Consultants, LLC

Joel Smith, Stratus Consulting

Donald Wuebbles, University of Illinois

Ex Officio Committee Members

Ko Barrett, U.S. Department of Commerce 

Katharine Batten, U.S. Agency for International Development 

Virginia Burkett, U.S. Department of the Interior 

Patricia Cogswell, U.S. Department of Homeland Security 

Gerald Geernaert, U.S. Department of Energy 

John Hall, U.S. Department of Defense 

Leonard Hirsch, Smithsonian Institution 

William Hohenstein, U.S. Department of Agriculture 

Patricia Jacobberger-Jellison, National Aeronautics and Space 

 Administration 

Thomas R. Karl, Subcommittee on Global Change Research, U.S. 

Department of Commerce

George Luber, U.S. Department of Health and Human Services 

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

Robert O’Connor, National Science Foundation 

Susan Ruffo, White House Council on Environmental Quality 

Arthur Rypinski, U.S. Department of Transportation

Trigg Talley, U.S. Department of State 

Federal National Climate Assessment and Development Advisory Committee (NCADAC) 

John Holdren, Assistant to the President for Science and Technology 

and Director, White House Office of Science and Technology Policy

Katharine Jacobs, Director, National Climate Assessment, White House 

Office of Science and Technology Policy (through December 2013)

Thomas Armstrong, Director, U.S. Global Change Research Program 

National Coordination Office, White House Office of Science and 

Technology Policy

Thomas R. Karl, Chair, Subcommittee on Global Change Research, 

U.S. Department of Commerce

Tamara Dickinson, Principal Assistant Director for Environment and 

Energy, White House Office of Science and Technology Policy

Fabien Laurier, Director, Third National Climate Assessment, White 

House Office of Science and Technology Policy

Glynis C. Lough, NCA Chief of Staff, U.S. Global Change Research 

Program

David Easterling, NCA Technical Support Unit Director, NOAA NCDC

Federal Executive Team

Highlights and Report Production Team

Susan Joy Hassol, Senior Science Writer

Brooke Stewart, Science Editor/Production Coordinator

Tom Maycock, Technical Editor

Daniel Glick, Editor

Sara W. Veasey, Creative Director

Jessicca Griffin, Lead Graphic Designer

Report Authors and Additional Staff, see nca2014.globalchange.gov

Assessment Support Staff

Fredric Lipschultz, Senior Scientist, Regional Coordinator 

Susan Aragon-Long, Senior Scientist, Sector Coordinator

Emily Therese Cloyd, Public Participation/Engagement Coordinator

Ilya Fischhoff, Program Coordinator

Bryce Golden-Chen, Program Coordinator

Julie Maldonado, Engagement Assistant, Tribal Coordinator 

Alison Delgado, Scientist, Sector Coordinator
20 U.S. GLOBAL CHANGE RESEARCH PROGRAM

U.S. Global Change Research Program

1717 Pennsylvania Avenue, NW • Suite 250 • Washington, DC 20006 USA

http://www.globalchange.gov

This report summarizes the impacts of climate change 

 on the United States, now and in the future. 

U.S. National Climate Assessment