Office of Research and Development, Washington, DC 20460

National Center for Environmental Assessment 

EPA/600/R-07/094F | April 2009 | www.epa.gov

Assessment of the Impacts of Global 

Change on Regional U.S. Air Quality:

A synthesis of climate change impacts

on ground-level ozone 

AN INTERIM REPORT OF THE U.S. EPA GLOBAL

CHANGE RESEARCH PROGRAM
EPA/600/R-07/094F 

April 2009 

Assessment of the Impacts of Global Change on Regional 

U.S. Air Quality: A Synthesis of Climate Change Impacts 

on Ground-Level Ozone 

An Interim Report of the U.S. EPA Global Change Research Program 

National Center for Environmental Assessment 

Office of Research and Development 

U.S. Environmental Protection Agency 

Washington, DC 20460 
DISCLAIMER 

 This document has been reviewed in accordance with U.S. Environmental Protection 

Agency policy and approved for publication. Mention of trade names or commercial products 

does not constitute endorsement or recommendation for use. 

Preferred citation: 

U.S. Environmental Protection Agency (EPA). (2009) Assessment of the impacts of global change on regional U.S. 

air quality: a synthesis of climate change impacts on ground-level ozone. An Interim Report of the U.S. EPA Global 

Change Research Program. National Center for Environmental Assessment, Washington, DC; EPA/600/R-07/094F. 

Available from the National Technical Information Service, Springfield, VA, and online at 

http://www.epa.gov/ncea. 

ii
TABLE OF CONTENTS 

LIST OF TABLES ......................................................................................................................... vi 

LIST OF FIGURES ...................................................................................................................... vii 

LIST OF ABBREVIATIONS ........................................................................................................ ix 

FOREWORD ................................................................................................................................. xi 

AUTHORS, CONTRIBUTORS, AND REVIEWERS ............................................................... xiii 

ACKNOWLEDGEMENTS ......................................................................................................... xvi 

EXECUTIVE SUMMARY ........................................................................................................ xvii 

SUMMARY OF POLICY RELEVANT FINDINGS ................................................................. xxii 

1 INTRODUCTION TO THE PROBLEM .............................................................................. 1-1 

1.1 INTRODUCTION ....................................................................................................... 1-1 

1.2 MAJOR THEMES OF THE INTERIM ASSESSMENT REPORT ........................... 1-2 

1.3 BACKGROUND ......................................................................................................... 1-4 

1.3.1 Air Pollution...................................................................................................... 1-4 

1.3.2 Climate Change and Air Quality Linkages ....................................................... 1-4 

1.3.2.1 Air Quality Impacts on Climate Change .......................................... 1-6 

1.3.2.2 Climate Change Impacts on Air Quality .......................................... 1-6 

1.4 DESIGN OF THE GLOBAL CHANGE AND AIR QUALITY ASSESSMENT ...... 1-8 

1.4.1 Scope of the Assessment Effort ........................................................................ 1-9 

1.4.2 What is Covered in this Report ....................................................................... 1-10 

1.5 THE CLIENT COMMUNITIES ............................................................................... 1-11 

1.5.1 EPA Office of Air and Radiation (OAR), State, Tribal, and Local Air 

Quality Planners .............................................................................................. 1-11 

1.5.2 U.S. Climate Change Science Program (CCSP) ............................................. 1-12 

1.5.3 Climate Change Research Community ........................................................... 1-14 

1.5.4 Air Quality Research Community .................................................................. 1-14 

1.6 CONSIDERING UNCERTAINTY IN THE ASSESSMENT EFFORT .................. 1-15 

1.7 STRUCTURE OF THIS REPORT ........................................................................... 1-16 

2 OVERVIEW OF APPROACH .............................................................................................. 2-1 

2.1 INTRODUCTION ....................................................................................................... 2-1 

2.1.1 Process for Developing the Global Change-Air Quality Assessment 

Effort ................................................................................................................. 2-2 

2.2 WORKSHOP RECOMMENDATIONS ..................................................................... 2-2 

2.2.1 Modeling ........................................................................................................... 2-2 

2.2.2 Time Horizon Selected ..................................................................................... 2-5 

2.2.3 Dual-Phase Assessment Approach ................................................................... 2-6 

2.2.4 Research Priorities to Support Phase II ............................................................ 2-7 

2.3 RESEARCH PARTNERSHIPS .................................................................................. 2-7 

iii
TABLE OF CONTENTS (continued) 

3 RESULTS AND SYNTHESIS .............................................................................................. 3-1 

3.1 INTRODUCTION ....................................................................................................... 3-1 

3.2 SUMMARY OF RESULTS FROM INDIVIDUAL GROUPS .................................. 3-1 

3.2.1 GCTM-Focused Modeling Work ...................................................................... 3-2 

3.2.1.1 Application of a Unified Aerosol-Chemistry-Climate GCM 

to Understand the Effects of Changing Climate and Global 

Anthropogenic Emissions on U.S. Air Quality: Harvard 

University ......................................................................................... 3-2 

3.2.1.2 Impacts of Climate Change and Global Emissions on U.S. 

Air Quality: Development of an Integrated Modeling 

Framework and Sensitivity Assessment: Carnegie Mellon 

University ......................................................................................... 3-4 

3.2.2 Linked Global-Regional-Focused Modeling Work .......................................... 3-6 

3.2.2.1 The Climate Impacts on Regional Air Quality (CIRAQ) 

Project: EPA .................................................................................... 3-6 

3.2.2.2 Modeling Heat and Air Quality Impacts of Changing Urban 

Land Uses and Climate: Columbia University ................................ 3-7 

3.2.2.3 Impacts of Global Climate and Emission Changes on U.S. 

Air Quality: University of Illinois ................................................... 3-8 

3.2.2.4 Impact of Climate Change on U.S. Air Quality Using 

Multi-Scale Modeling with the MM5/SMOKE/CMAQ System: 

Washington State University .......................................................... 3-10 

3.2.2.5 Guiding Future Air Quality Management in California: 

Sensitivity to Changing Climate—University of California, 

Berkeley .......................................................................................... 3-11 

3.2.2.6 Sensitivity and Uncertainty Assessment of Global Climate 

Change Impacts on Ozone and Particulate Matter: 

Examination of Direct and Indirect, Emission-Induced 

Effects: GIT-NESCAUM-MIT ..................................................... 3-12 

3.3 SYNTHESIS OF RESULTS ACROSS GROUPS .................................................... 3-13 

3.3.1 Regional Modeling Results ............................................................................. 3-15 

3.3.1.1 Modeling System Configurations, Simulations, and Evaluation ... 3-15 

3.3.1.2 Changes in O3 ................................................................................. 3-19 

3.3.1.3 Changes in Drivers ......................................................................... 3-24 

3.3.2 Global Modeling Results ................................................................................ 3-32 

3.4 CHALLENGES AND LIMITATIONS OF THE MODEL-BASED 

APPROACH .............................................................................................................. 3-37 

3.4.1 Inter-Model Variability and Model Evaluation .............................................. 3-38 

3.4.2 The Role of Downscaling ............................................................................... 3-42 

3.4.3 Uncertainties in Chemical Mechanisms .......................................................... 3-45 

3.5 SYNTHESIS CONCLUSIONS AND FUTURE RESEARCH NEEDS .................. 3-47 

iv
TABLE OF CONTENTS (continued) 

4 FUTURE DIRECTIONS ....................................................................................................... 4-1 

4.1 PHASE II OF THE GLOBAL CHANGE AND AIR QUALITY 

ASSESSMENT ........................................................................................................... 4-1 

4.2 EXTENDING THE MODELING SYSTEMS ............................................................ 4-1 

4.2.1 Exploring Modeling Uncertainties .................................................................... 4-1 

4.2.2 Additional Model Development ....................................................................... 4-2 

4.2.3 Additional Pollutants—PM ............................................................................... 4-3 

4.2.4 Additional Pollutants—Mercury ....................................................................... 4-4 

4.3 COMBINED IMPACTS OF CLIMATE AND EMISSIONS CHANGES: 

PRELIMINARY WORK ............................................................................................ 4-4 

4.4 MODELING THE DRIVERS OF AIR POLLUTANT EMISSIONS ........................ 4-5 

4.4.1 Economic Growth and Technology Choices .................................................... 4-7 

4.4.2 Land Use and Transportation ............................................................................ 4-8 

4.4.3 Emissions Changes Due to Changing Ecosystems: Biogenic VOCs............... 4-8 

4.4.4 Emissions Changes Due to Changing Ecosystems: Wildfires ......................... 4-9 

4.4.5 Taking Integrated Emissions Scenarios Through to Future U.S. 

Regional Air Quality ......................................................................................... 4-9 

REFERENCES ........................................................................................................................... R-1 

APPENDIX A: CURRENT U.S. REGIONAL AIR QUALITY, ITS SENSITIVITY 

TO METEOROLOGY AND EARLY STUDIES OF THE EFFECT 

OF CLIMATE CHANGE ON AIR QUALITY ............................................... A-1 

APPENDIX B: CHARACTERIZING AND COMMUNICATING UNCERTAINTY: 

THE NOVEMBER 2006 WORKSHOP ...........................................................B-1 

APPENDIX C: THE 2001 EPA GLOBAL CHANGE RESEARCH PROGRAM’S AIR 

QUALITY EXPERT WORKSHOP .................................................................C-1 

APPENDIX D: U.S. EPA STAR GRANT RESEARCH CONTRIBUTING TO THE 

GCAQ ASSESSMENT .................................................................................... D-1 

APPENDIX E: MODELING APPROACH FOR INTRAMURAL PROJECT ON 

CLIMATE IMPACTS ON REGIONAL AIR QUALITY................................ E-1 

APPENDIX F: USING MARKAL TO GENERATE EMISSIONS GROWTH 

PROJECTIONS FOR THE EPA GLOBAL CHANGE RESEARCH 

PROGRAM’S AIR QUALITY ASSESSMENT .............................................. F-1 

GLOSSARY OF CLIMATE AND AIR QUALITY TERMS .................................................... G-1 

v
LIST OF TABLES 

3-1 The regional modeling systems whose results are discussed in 

Sections 3.3.1 and 3.3.2 ..................................................................................... 3-16 

3-2 GCTM-only model simulations whose results are discussed in 

Section 3.3.2....................................................................................................... 3-32 

vi
LIST OF FIGURES 

1-1 Schematic representation of the multiple interactions between tropospheric 

chemical processes, biogeochemical cycles, and the climate system .................. 1-5 

2-1 Links between global and regional climate and atmospheric chemistry 

processes with anthropogenic activities governing air pollution emissions ........ 2-1 

3-1 2050s-minus-present differences in simulated summer mean MDA8 O3

concentrations (in ppb) for the (a) NERL; (b) Illinois 1; (c) Illinois 2; (d) 

WSU; and (e) GNM experiments (see Table 3-1) ............................................. 3-20 

3-2 95th percentile MDA8 O3 concentration differences for the NERL 

experiment.......................................................................................................... 3-21 

3-3 2050s-minus-present differences in simulated summer mean MDA8 O3

concentrations (in ppb); reproduced from Figure 2 in Hogrefe et al. 

(2004b) ............................................................................................................... 3-22 

3-4 Frequency of simulated summer mean MDA8 O3 values exceeding 

80 ppb in different regions from the NERL experiment; reproduced 

from Figure 11 in Nolte et al. (2008) ................................................................. 3-23 

3-5 2050s-minus-present September-October compared to June-August 

differences in simulated mean MDA8 O3 concentrations (in ppb); 

reproduced from Figure 6 in Nolte et al. (2008) ................................................ 3-24 

3-6 2050s-minus-present differences in simulated summer mean near-surface 

air T (oC) for the (a) NERL; (b) Illinois 1; (c) Illinois 2; (d) WSU; and (e) 

GNM experiments .............................................................................................. 3-27 

3-7 2050s-minus-present differences in simulated summer mean surface 

insolation (W m-2) for the (a) NERL; (b) Illinois 1; (c) Illinois 2; (d) WSU; 

and (e) GNM experiments ................................................................................. 3-28 

3-8 2050s-minus-present differences in simulated summer mean biogenic 

VOC emissions (g Carbon m-2 day-1) for the (a) NERL; (b) Illinois 1; 

(c) Illinois 2; (d) WSU; and (e) GNM experiments ........................................... 3-30 

3-9 2050s-minus-present differences in simulated summer (JJA) mean 

(a) MDA8 O3 concentration (ppb); (b) near-surface air temperature (oC); 

(c) surface insolation (W m-2); and (d) biogenic isoprene emissions 

(g Carbon m-2 day-1) for the Harvard global modeling experiment 

(see Table 3-2) ................................................................................................... 3-34 

3-10 Same as Figure 3-9 but for the CMU global modeling experiment ................... 3-35 

vii
TABLE OF CONTENTS (continued) 

3-11 The mean (top two panels) and standard deviation (bottom two panels) 

in future-minus-present MDA8 O3 concentration differences across 

(left-hand panels) all seven experiments (five regional and two global) 

shown in Figures 3-1, 3-9, and 3-10 and, for comparison purposes, 

(right-hand panels) not including the WSU experiment because it 

shows differences for July only, while the other experiments show 

JJA differences ................................................................................................... 3-49 

3-12 Averages across the subregions shown in Figure 3-13 for each of the 

simulations for (a) mean MDA8 O3 (ppb); (b) near-surface air 

temperature (oC) ................................................................................................. 3-50 

3-12 continued. Averages across the subregions shown in Figure 3-13 for each 

of the simulations for (c) surface insolation (W m-2); and (d) biogenic 

isoprene emissions (g Carbon m-2

 sec

-1) ............................................................ 3-51 

3-13 The averaging subregions used in Figure 3-12 .................................................. 3-52 

4-1 Integrated system of future climate, meteorology, and emissions scenarios ....... 4-6 

viii 
LIST OF ABBREVIATIONS 

AGCM Atmospheric General Circulation Model 

AOGCM Atmosphere-Ocean General Circulation Model 

AQ air quality 

BC boundary conditions 

BEIS Biogenic Emissions Inventory System 

CAA Clean Air Act 

CAM Community Atmosphere Model 

CACM Caltech Atmospheric Chemistry Mechanism 

CICE The Los Alamos Sea Ice Model 

CCM3 Community Climate Model version 3 

CCSM Community Climate System Model 

CSIM Community Sea Ice Model 

CLM Community Land Model 

CMAQ Community Multiscale Air Quality Model 

CMIP Coupled Model Intercomparison Project 

CTM Chemical Transport Model 

EC elemental carbon 

ENSO El Niño-Southern Oscillation 

GCM General Circulation Model 

GCTM Global Chemical Transport Model 

GISS Goddard Institute for Space Studies 

GMAO Global Modeling and Assimilation Office 

HadCM3 Hadley Centre Coupled Model 

IC initial condition 

IGSM Integrated Global System Model 

LANL Los Alamos National Laboratory 

LWC liquid water content 

MDA8 Maximum Daily 8-hour Average Ozone Concentration 

MM Mesoscale Model 

MM5 Mesoscale Model (Version 5) 

MARKAL MARKet Allocation Model 

MOSIS Meteorology Office Surface Exchange Scheme 

MPMPO Model to Predict the Multiphase Partitioning of Organics 

NAAQS National Ambient Air Quality Standard 

NCAR National Center for Atmospheric Research 

NH4

+

 ammonium ion 

NO3

-

 nitrate ion 

OC organic carbon 

O3 ozone 

OGCM Oceanic General Circulation Model 

PAN peroxyacetylnitrate 

PBL planetary boundary layer 

PCM Parallel Climate Model 

ix
LIST OF ABBREVIATIONS (continued) 

PCTM PCM/CCSM Transition Model 

POP Parallel Ocean Program 

RACT reasonably available control technology 

RCM Regional Climate Model 

RCMS Regional Climate Modeling System 

RCTM Regional Chemical Transport Model 

RH relative humidity 

RRF relative reduction factor 

PM2.5 particulate matter with aerodynamic diameter below 2.5 μm 

SIP State Implementation Plan 

SAPRC statewide air pollution research center 

SMOKE Sparse Matrix Operator Kernel Emissions 

SOA secondary organic aerosols 

SO2 sulfur dioxide 

SO4

=

 sulfate ion 

SRES special report on emissions scenarios 

SST sea surface temperature 

THC thermohaline circulation 

TKE turbulent kinetic energy 

UKMO United Kingdom Meteorology Office 

VOC volatile organic compound 

x
FOREWORD 

The Global Change Research Program (GCRP) in EPA’s Office of Research and 

Development (ORD) is an assessment-oriented program with primary focus on evaluating the 

potential consequences of global change—particularly climate change and climate variability— 

for air and water quality, aquatic ecosystems, and human health in the United States. The 

program investigates adaptation options to improve society’s ability to effectively respond to the 

risks presented by global change. The program also has begun to evaluate alternative strategies 

for reducing greenhouse gas emissions and the environmental implications of those strategies. 

This initial report, entitled Assessment of the Impacts of Global Change on Regional U.S. 

Air Quality: A Synthesis of Climate Change Impacts on Ground-Level Ozone, was prepared by 

the GCRP to provide air quality managers and scientists with timely and useful information 

about the potential effects of climate change on air quality in the United States. It represents an 

integrated, multidisciplinary research and assessment effort that includes contributions from 

multiple Laboratories and Centers in ORD, and it was done in partnership with EPA’s Office of 

Air and Radiation (OAR), which is interested in developing a foundation for considering the 

effects of climate change in the Agency’s air quality management programs. Additional 

contributors included partners in other federal agencies. I would like to thank the many people 

in ORD, OAR, the Regional Offices, the academic community, and our external review panel for 

their many contributions. 

The GCRP began an initial assessment of the implications of climate change on air 

quality in 2000, because available scientific evidence suggested that climate and air quality are 

closely coupled through atmospheric chemical, radiative, and dynamic processes. It was known 

that meteorology plays an essential role in whether or not a metropolitan area meets the National 

Ambient Air Quality Standards (NAAQS) set by EPA for pollutants considered harmful to 

public health and the environment. It was also known that a warming climate will lead to 

significant changes in regional meteorological patterns. However, it was not known how a 

changing climate will affect air quality for a given region, and how climate change will affect a 

region’s ability to meet the NAAQS. 

The GCRP’s long-term climate change/air quality assessment goals therefore are the 

following: 

1. Provide an answer to the basic question, “Is global change something we will have to 

account for when moving forward with U.S. air quality policy?” 

2. Develop research tools and a knowledge base to answer science questions about the 

potential impacts of global change on regional U.S. air quality. 

xi 
3. Deliver to the air quality policy and management community an improved understanding 

of the behavior and complexities of the global change/air quality system as well as the 

strengths and limitations of the available scientific tools and methods. 

4. Provide a foundation for applying these scientific insights and tools to help answer 

specific policy and management questions. 

It is important to ascertain whether climate change should be considered in the 

formulation of future air quality policy. To do so, we must gain an understanding of the 

importance of climate change relative to other stressors on air quality (e.g., changes in land-use) 

and the relative difficulty of coping with all stressors. However, this assessment design called 

for first providing insights about how air quality may respond to future changes in climate before 

tackling the additional complexities of incorporating potential future changes in anthropogenic 

emissions and long-range pollutant transport. This report is therefore an initial assessment that 

evaluates the effects of climate change alone on air quality across the United States. 

The assessment focuses primarily on the impact of climate change out to 2050 on groundlevel ozone, which is mainly a summertime pollutant in the United States. Possible changes in 

“biogenic” emissions (i.e., emissions from natural sources), such as emissions of volatile organic 

carbon (VOC) from vegetation, were considered. Future assessment reports will focus on other 

regulated pollutants, including particulate matter (PM) and mercury, as well as on the combined 

effects of both climate and human-caused emissions changes, to provide a more complete 

understanding of the range of possible impacts of global change on air quality. 

Caution must be exercised in interpreting the results presented here. First, this report 

does not address the question of whether regulatory standards for particular pollutants (e.g., 

ozone) should change because of climate change. Rather, the report sheds light on the question 

of whether climate change will make attainment of any standard—wherever it is set—more 

difficult. It presents scientific findings that, in combination with other information, will inform 

policymaking. Second, our understanding of the linkages between climate and air quality is still 

at an early stage. There remains considerable uncertainty in climate modeling, and our 

knowledge of certain aspects of atmospheric chemistry are still lacking. With these caveats, this 

report includes results useful to national and regional air quality planners. 

This report represents a significant advancement in our understanding of the possible 

impacts of climate change on regional air quality in the United States. It is our hope that the 

information contained in this report will enhance our ability as a nation to protect air quality and 

human health, even as our climate changes. 

Lek Kadeli 

Acting Assistant Administrator 

Office of Research and Development 

xii
AUTHORS, CONTRIBUTORS, AND REVIEWERS 

Principal Authors 

Ms. Anne Grambsch–National Center for Environmental Assessment, Office of Research and 

Development, U.S. Environmental Protection Agency, Washington, DC 20460 

Dr. Brooke L. Hemming–National Center for Environmental Assessment, Office of Research 

and Development, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711 

Dr. Christopher P. Weaver–National Center for Environmental Assessment, Office of Research 

and Development, U.S. Environmental Protection Agency, Washington, DC 20460 

Contributing Authors 

Dr. Alice Gilliland–Air Resources Laboratory, National Oceanic and Atmospheric 

Administration in partnership with the National Exposure Research Laboratory, U.S. 

Environmental Protection Agency, Research Triangle Park, NC 27711 

Mr. Doug Grano–Office of Air Quality Planning and Standards, Office of Air and Radiation, 

U.S. Environmental Protection Agency, Research Triangle Park, NC 27711 

Dr. Sherri Hunt–National Center for Environmental Research, Office or Research and 

Development, U.S. Environmental Protection Agency, Washington, DC 20460 

Dr. Tim Johnson–National Risk Management Research Laboratory, Office of Research and 

Development, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711 

Dr. Dan Loughlin–National Risk Management Research Laboratory, Office or Research and 

Development, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711 

Dr. Darrell Winner–National Center for Environmental Research, Office or Research and 

Development, U.S. Environmental Protection Agency, Washington, DC 20460 

Contributors 

Dr. William G. Benjey–Air Resources Laboratory, National Oceanic and Atmospheric 

Administration in partnership with the National Exposure Research Laboratory, U.S. 

Environmental Protection Agency, Research Triangle Park, NC 27711 

Dr. Ellen J. Cooter–Air Resources Laboratory, National Oceanic and Atmospheric 

Administration in partnership with the National Exposure Research Laboratory, U.S. 

Environmental Protection Agency, Research Triangle Park, NC 27711 

xiii
AUTHORS, CONTRIBUTORS, AND REVIEWERS (continued) 

Dr. Cynthia Gage–National Risk Management Research Laboratory, Office or Research and 

Development, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711 

Dr. Chris Nolte–Air Resources Laboratory, National Oceanic and Atmospheric Administration in 

partnership with the National Exposure Research Laboratory, U.S. Environmental Protection 

Agency, Research Triangle Park, NC 27711 

Internal Reviewers 

Mr. Bret Anderson–Region 7, U.S. Environmental Protection Agency, Kansas City, KS 66101 

Ms. Louise Camalier–Office of Air Quality Planning and Standards, Office of Air and Radiation, 

U.S. Environmental Protection Agency, Research Triangle Park, NC 27711 

Mr. Bill Cox–Office of Air Quality Planning and Standards, Office of Air and Radiation, U.S. 

Environmental Protection Agency, Research Triangle Park, NC 27711 

Mr. Ben DeAngelo–Office of Atmospheric Programs, Office of Air and Radiation, U.S. 

Environmental Protection Agency, Washington, DC 20460 

Mr. Brian Eder–Air Resources Laboratory, National Oceanic and Atmospheric Administration in 

partnership with the National Exposure Research Laboratory, U.S. Environmental Protection 

Agency, Research Triangle Park, NC 27711 

Mr. Tyler Fox–Office of Air Quality Planning and Standards, Office of Air and Radiation, U.S. 

Environmental Protection Agency, Research Triangle Park, NC 27711 

Dr. Bryan Hubbell–Office of Air Quality Planning and Standards, Office of Air and Radiation, 

U.S. Environmental Protection Agency, Research Triangle Park, NC 27711 

Dr. Carey Jang–Office of Air Quality Planning and Standards, Office of Air and Radiation, U.S. 

Environmental Protection Agency, Research Triangle Park, NC 27711 

Dr. Terry Keating–Office of Policy Analysis and Review, Office of Air and Radiation, U.S. 

Environmental Protection Agency, Washington, DC 20460 

Mr. Jim Ketcham-Colwill–Office of Policy Analysis and Review, Office of Air and Radiation, 

U.S. Environmental Protection Agency, Washington, DC 20460 

Dr. Meredith Kurpius–Air Division, Region 9, U.S. Environmental Protection Agency, San 

Francisco, CA 94105 

Ms. Anne McWilliams–Region 1, U.S. Environmental Protection Agency, Boston, MA 02114-

2023 

xiv
AUTHORS, CONTRIBUTORS, AND REVIEWERS (continued)

Dr. Andy Miller–National Risk Management Research Laboratory, Office or Research and 

Development, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711 

Ms. Madonna Narvaez–Region 10, U.S. Environmental Protection Agency, Seattle, WA 98101 

Ms. Kathryn Parker–Office of Atmospheric Programs, Office of Air and Radiation, U.S. 

Environmental Protection Agency, Washington, DC 20460 

Ms. Sharon Philips–Office of Air Quality Planning and Standards, Office of Air and Radiation, 

U.S. Environmental Protection Agency, Research Triangle Park, NC 27711 

Mr. Frank Princiotta–National Risk Management Research Laboratory, Office or Research and 

Development, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711 

Dr. ST Rao–Air Resources Laboratory, National Oceanic and Atmospheric Administration in 

partnership with the National Exposure Research Laboratory, U.S. Environmental Protection 

Agency, Research Triangle Park, NC 27711 

Mr. Randy Robinson–Region 5, U.S. Environmental Protection Agency, Chicago, IL 

60604-3507 

Mr. Jason Samenow–Office of Atmospheric Programs, Office of Air and Radiation, U.S. 

Environmental Protection Agency, Washington, DC 20460 

Dr. Ravi Srivastava–Office of Air Quality Planning and Standards, Office of Air and Radiation, 

U.S. Environmental Protection Agency, Research Triangle Park, NC 27711 

Ms. Sara Terry–Office of Air Quality Planning and Standards, Office of Air and Radiation, U.S. 

Environmental Protection Agency, Research Triangle Park, NC 27711 

xv
ACKNOWLEDGEMENTS 

This interim report is part of a larger climate change and air quality assessment that is 

being carried out by ORD’s Global Change Research Program. It was made possible because of 

the many people who helped plan and implement the assessment effort, participated in 

workshops, conducted research, and contributed ideas that shaped the final product. We are 

grateful for their support and encouragement. In particular, we wish to acknowledge the 

following: Doug McKinney, Michele Aston, Robert Gilliam, Bill Rhodes, Joseph DeCarolis, 

Carol Shay, Jenise Swall, Deb Mangis, Ruby Leung, and the Science To Achieve Results 

(STAR) grantees. Many thanks as well to Jinhong Zhu and Xin-Zhong Liang who archived 

model output from the various participating groups and helped produce many of the figures in 

this report. Furthermore, we wish to thank our external review panel, David Allen, Russ 

Dickerson, Greg Huey, Alan Robock, Drew Shindell, Yuhang Wang, and Jason West, for their 

many valuable insights and critical contributions to improving the quality of this report. Finally, 

we are very grateful to the NCEA Global Change Research Program staff for their perspectives 

and support. 

xvi
EXECUTIVE SUMMARY 

It is increasingly recognized that the science and policy communities need to explore the 

potential impact of long-term (multi-decadal), global climate change on regional air quality—

specifically the possibility that such change may complicate air quality managers’ ability to 

attain their management goals. These concerns are grounded in information derived from 

observational studies, basic atmospheric chemistry, and modeling of short-term air pollution 

episodes. For example, these analyses have established the major role that weather patterns play 

in establishing conditions conducive to ozone (O3) formation and accumulation, such as 

abundant sunshine, high temperatures, and stagnant air. It is now well understood that year-toyear variability in summer climate is strongly correlated with the number of days that exceed O3

air quality standards. 

Historically, air pollution has been studied mostly in terms of immediate local and 

regional concerns, rather than as a global change issue. In 2001, the EPA Office of Research and 

Development’s Global Change Research Program initiated an effort to increase understanding of 

the multiple complex interactions between long-term global climate change and atmospheric 

chemistry which have the potential to influence air pollution in the United States. 

The overall goal of the assessment effort is to enhance the ability of air quality policy 

makers and managers to consider global climate change in their decisions through this 

increased understanding. 

A phased approach has been used to systematically achieve progress toward this overall 

goal. Phase I focused on building coupled modeling systems capable of capturing the range of 

processes from global climate to regional air quality and applying these systems to study the 

sensitivity of U.S. air quality to climate change, with a particular emphasis on O3. Phase I 

considers the impact of climate change on air quality in isolation, without including the 

interacting effects of changes in the emissions of pollutants from human systems. Phase II, in 

progress, focuses on the combined impacts of changing climate and changing human-caused air 

pollutant emissions. It builds on Phase I by: extending the modeling systems developed under 

Phase I to explore additional pollutants and processes; investigating more comprehensively the 

key modeling uncertainties uncovered in Phase I; and integrating changes in climate with the 

changes in emissions that might result from changes in air quality regulations, population growth 

and economic development, changes in energy technologies, and land use change. 

This report provides a synthesis of the major results from Phase I of the overall 

assessment. The findings presented here are interim, as Phase II is currently ongoing, and 

future reports will update and extend our knowledge of the potential impacts of global change on 

air quality. 

xvii
Specifically, this report provides a synthesis of the EPA-funded modeling studies that 

have been carried out to date under Phase I, primarily for O3. Future interim reports will provide 

syntheses of additional topics, e.g., particulate matter. These projects have all adapted and 

combined existing tools from diverse fields, such as global climate models, global chemistry and 

transport models, regional meteorological models, and regional air quality models, into systems 

capable of carrying out numerical experiments to explore the sensitivity of U.S. air quality to 

changes in global climate. These linked modeling systems have simulated nationwide changes in 

O3 concentrations, primarily for summertime, as a result of simulated climate change a few 

decades into the future. The numerical experiments discussed in this report held human-caused 

emissions of O3 precursor pollutants constant at present-day levels, but allowed climate-sensitive 

natural emissions, like volatile organic compounds (VOCs) from vegetation, to vary in response 

to the simulated changes in climate. 

Coupling atmospheric chemical processes and the climate system presents considerable 

challenges because of the large number of physical, chemical, and biological processes involved, 

many of which are poorly understood, all interacting in complex ways. The types of modeling 

systems developed under this assessment permit the detailed exploration of the potential 

responses of air quality to climate change over the next few decades in a way that would be 

difficult or impossible with other approaches. For example, they permit the exploration of 

climate changes well outside of the envelope of historical experience. In addition, they permit 

the systematic investigation of the multiple competing climate- and weather-related drivers of air 

quality interactions on the regional scale, which produce aggregate patterns of air quality change. 

This effort represents the first systematic attempt to use linked global-to-regional climate 

and air quality modeling systems from multiple research groups to jointly investigate the 

regional dimensions of potential climate-induced air quality changes across the United States. 

The major findings from this suite of experiments are 

First, while these modeling studies cannot tell us what the future will hold, they 

demonstrate the potential for global climate change to make U.S. air quality management more 

difficult, and therefore future air quality management decisions should begin to account for the 

impacts of climate change. 

Second, the science of modeling climate and atmospheric chemistry for the purposes of 

understanding the sensitivity of regional air quality to climate change is in its early stages. This 

effort highlights a number of uncertainties that limit the information that can be provided to 

support decision-making, as well as what work is needed (some currently underway) to begin 

addressing these uncertainties. 

xviii
The synthesis of scientific information in this report supports the scientific community 

and air quality managers and policy makers by 

• Providing an improved understanding of the richness and range of behaviors of the global 

change-regional air quality system; 

• Providing an appreciation for the strengths and limitations of the scientific tools and 

methods used to develop this improved understanding; 

• Creating the foundation for a suite of collaborative activities between the scientific 

research and air quality policy and management communities to investigate specific air 

quality policy and management questions. 

The two major findings rest on a foundation of a number of more detailed conclusions 

drawn from the modeling studies. In support of the first major finding 

• For every region of the country, at least one (usually multiple) of the modeling groups 

found that simulated climate change caused increases in summertime O3 concentrations. 

• These climate-induced increases, averaged over the summer season, were in the range of 

approximately 2-8 parts per billion (ppb) for Maximum Daily 8-hour Average O3

concentration, a key metric for regulating U.S. air quality. 

• The climate sensitivity of O3 was greatest for the peak pollution episodes that tend to 

occur over a number of days each summer, resulting in substantially larger increases for 

these times than for the overall seasonal average. 

While the results from the different research groups agreed on the above points, their 

modeling systems did not necessarily simulate the same regional patterns of climate-induced O3

changes, with the individual simulations showing regions of little change, or even decreases, in 

addition to the O3 increases. This speaks to the second major finding of this report, articulated 

above, of important modeling uncertainties. Certain regions show greater agreement than others: 

for example, there is very generally more agreement on the spatial patterns of climate-induced 

increases for the eastern half of the country than for the West, though parts of the Southeast show 

some of the strongest disagreements across the modeling groups. These differences in the 

regional patterns of O3 change result, in large part, from differences in how the different 

modeling systems, composed of different combinations of climate models, chemistry models, 

greenhouse gas scenarios, and number of years modeled, simulated the competing regional 

influences of changes in key meteorological drivers of air quality, especially the amount of 

sunlight reaching the surface and near-surface air temperature. 

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In general, differences between climate simulations tend to be more pronounced at the 

regional scales considered in this report than at the global scale. This is because of differences 

across models and simulations in the representation of large-scale circulation patterns that 

strongly affect regional meteorology, like the mid-latitude storm tracks and the subtropical high 

pressure systems. In addition, there are differences between models in how they capture smallscale processes, like clouds and precipitation, which also are important for air quality. In the 

studies discussed in this report, these modeling uncertainties strongly influenced the O3

simulations, producing much of the difference in regional patterns of change between studies. 

For example, there were differences across modeling groups in the regions of the country where 

simulated increases in cloud cover, and hence decreases in the amount of sunlight reaching the 

surface, partially counteracted the effects of warming temperatures on O3 concentrations in these 

regions. This highlights current limitations in our ability to understand regional impacts of 

global climate change. 

The results from the modeling studies discussed in this report clearly show that a 

complex interplay between multiple meteorological factors drives regional O3 changes. Simply 

considering a single variable, such as temperature, may not provide a sufficient basis for 

determining future air quality risks due to climate change in every region. 

Another important impact is that climate change leads to changes in the natural emissions 

of VOCs., e.g., isoprene from vegetation. All of the modeling groups found climate-induced 

increases in these biogenic VOC emissions over most of the United States, with especially 

pronounced increases in the Southeast. However, there are large disagreements across the 

different groups as to the degree to which these increases affect O3 concentrations, with some 

simulations showing large O3 increases while others show little change. An important factor that 

helps explain these differences is the differing representation of isoprene nitrate chemistry in the 

different modeling systems, another key uncertainty in the science. 

In addition to the issues discussed above, most of the groups examined the importance of 

year-to-year variability on their results to some degree. These groups found that the climateinduced differences in O3 concentrations are roughly the same size as present-day year-to-year 

variability. This implies that simulated future O3 change can be strongly affected by the choice 

of present-day and future years to compare. It also implies that climate change has the potential 

to push O3 concentrations in extreme years beyond the envelope of current natural year-to-year 

variability. 

Finally, while this report focuses mainly on summertime results, some of the modeling 

groups also found climate-induced increases in O3 concentrations in some regions for the spring 

and fall, suggesting a possible future extension of the O3 season that would present additional 

challenges for air quality managers. 

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Moving forward, this report has highlighted key areas for improving integrated climate 

and air quality modeling systems that can deliver improved information to meet evolving climate 

policy and air quality management decision support needs. These include: 

• Using recent advances in global and regional models, parameterizations, and downscaling 

techniques to build more advanced coupled climate and air quality modeling systems; 

• Developing ensembles of multiple modeling systems over many years of simulation to 

develop more robust results of air quality sensitivity to climate change; 

• Carrying out more extensive evaluations of climate models for their ability to represent 

processes (and timescales) that strongly influence regional air quality, such as regionalscale stagnation events. 

• Carrying out more extensive evaluations of atmospheric chemistry models for their 

ability to represent certain chemical pathways that lead to O3 climate change sensitivity, 

such as the chemical fate of isoprene nitrate. 

Finally, this report provides a preview of ongoing and upcoming work under Phase II of 

the overall assessment: exploring the uncertainties discussed above; extending the modeling 

systems to investigate the climate sensitivity of additional pollutants (i.e., particulate matter and 

mercury) in greater detail; exploring feedbacks between chemistry and climate; and assessing the 

integrated effects of changes in climate and changes in emissions of pollutants by changes in 

human systems, such as population growth and migration, economic development, new 

regulations, energy use and technology, and land use. 

xxi
SUMMARY OF POLICY RELEVANT FINDINGS 

 The recent Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment 

Report (AR4) states, “Warming of the climate system is unequivocal, as is now evident from 

observations of increases in global average air and ocean temperatures, widespread melting of 

snow and ice, and rising global average sea level” (IPCC, 2007). Directly relevant to EPA’s 

mission to protect human health and the environment is the IPCC finding that, “Future climate 

change may cause significant air quality degradation by changing the dispersion rate of 

pollutants, the chemical environment for ozone and aerosol generation and the strength of 

emissions from the biosphere, fires and dust. The sign and magnitude of these effects are highly 

uncertain and will vary regionally.” Climate change impacts have not yet been explicitly 

considered in air quality program planning—accounting for them will be a critical challenge for 

the air quality management system in the coming decades. 

 In partnership with EPA’s Office of Air and Radiation (OAR) and several Regional 

offices, the EPA’s Office of Research and Development (ORD) Global Change Research 

Program began an assessment effort to increase scientific understanding of the multiple complex 

interactions between climate and atmospheric chemistry. The ultimate goal of this assessment is 

to enhance the ability of air quality managers to consider global change in their decisions 

through improved characterization of the potential impacts of global change on air quality. An 

integrated framework for the assessment was designed that leveraged the research and 

development strengths within the EPA, within other agencies, and within the academic research 

community. This framework calls for first developing insight into the range of possible air 

quality responses to future climate changes alone (Phase I) before tackling the additional 

complexities of integrating the effects of potential future changes in anthropogenic emissions and 

long-range pollutant transport with these climate-only impacts (Phase II). The core approach of 

the assessment is the development of integrated modeling systems capable of capturing these 

effects and applying them in simulations to explore the global change-air quality problem. 

 This interim report provides an update on the progress in this first phase of the 

assessment. Its primary focus is on the potential changes in U.S. regional air quality due to 

global climate change alone, including direct meteorological impacts on atmospheric chemistry 

and transport, and the effect of these meteorological changes on climate-sensitive natural 

emissions of pollutant precursors. The aim in this phase was to consider the effects of climate 

change in isolation, without accompanying changes in anthropogenic emissions of precursor 

pollutants expected to occur over the same timeframe. Future reports will explore the potential 

impacts when also considering possible changes in future air pollution emissions. 

xxii
 Two “grand challenges” have emerged in the course of developing and conducting this 

assessment. The first arises from the Global Change Research Program’s emphasis on decision 

support, namely, to provide the best possible scientific basis for understanding potential climate 

change impacts on air quality and air quality policies in a useful form and a timely manner as one 

key set of inputs to help managers develop pollution control strategies. The second “grand 

challenge” is to convey to the scientific research community the knowledge gaps that limit our 

understanding of the problem and/or create barriers to the use and interpretation of scientific 

information by decision makers. 

 The discussion below summarizes information that has emerged from the assessment to 

date. Most of the discussion centers on topics related to tropospheric ozone (O3) since our 

understanding of O3 is more complete at this time than that of particulate matter (PM). 

Preliminary findings related to PM are presented where available. Unless otherwise indicated, to 

isolate the impacts of climate change, all model results discussed are for simulations that 

assumed no future changes in the anthropogenic emissions of precursor pollutants. Also, unless 

otherwise indicated, “future” refers to the time period around 2050. 

 The organization of the rest of this Summary is as follows: In the first sub-section, what 

has been learned about possible impacts of climate change on O3 (and PM) concentrations is 

presented. With this information in hand, in the second sub-section, it is then possible to focus 

on those meteorological drivers important for air quality and highlight complexities in the 

interaction between these drivers and pollutant concentrations, such as reinforcing or competing 

effects of individual drivers. The third sub-section discusses climate change impacts on climatesensitive natural emissions of pollutant precursors. The fourth and fifth sub-sections discuss 

important modeling uncertainties, and preliminary sensitivity tests comparing the first-order 

impacts of climate and anthropogenic emissions changes, respectively, as previews of issues that 

will receive more attention in the next phase of the assessment. 

I. Summary of Impacts on O3 (and PM) Concentrations 

A. Climate change has the potential to produce significant increases in near-surface O3

concentrations throughout the United States.

1. A large number of earlier observation- and model-based studies have demonstrated 

connections between meteorological variability and O3 concentrations and 

exceedances, implying the possibility of climate change leading to increasing O3

levels in some regions. 

2. The new modeling studies discussed in this report show increases in summertime O3

concentrations over substantial regions of the country as a result of simulated 2050 

climate change. These results were obtained under the assumption of anthropogenic 

emissions of precursor pollutants held constant at present-day levels while allowing 

xxiii
for some changes in climate-sensitive natural emissions. For nearly every region of 

the country, at least one (usually multiple) of the modeling groups found that climate 

change caused increases in summertime O3 concentrations. 

3. Where these increases occur, the amount of increase in summertime average 

Maximum Daily 8-hour Average (MDA8) O3 concentrations across all the modeling 

studies tends to fall in the range 2−8 ppb, as illustrated in the figures shown in 

Section 3. 

4. The largest increases in O3 concentrations in these simulations occur during peak 

pollution events. (For example, the increases in 95th percentile of MDA8 O3 tend to 

be significantly greater than those in summertime-mean MDA8 O3.) 

5. Though in agreement on the above points, the different modeling systems did not 

necessarily simulate the same regional patterns of climate-induced O3 changes, with 

the individual simulations showing some regions of little change, or even decreases, 

in addition to the O3 increases. 

6. As will be discussed in Sections II and III below, these disagreements in the spatial 

patterns of future O3 changes can largely be attributed to the wide variations across 

simulations in the patterns of changes of key meteorological drivers (e.g., temperature 

and cloud cover), along with the differing representations of key chemical 

mechanisms in the various model systems. 

7. There is greater agreement across simulations in these O3 changes for certain regions 

than for others. For example, there is generally more agreement on the spatial 

patterns of climate-induced increases for the eastern half of the country than for the 

West, though parts of the Southeast show some of the strongest disagreements across 

the modeling groups. 

8. A subset of results also suggests that climate change effects on O3 grow continuously 

over time, with evidence for significant impacts (in the same direction as described 

above) emerging as early as the 2020s. For example, the Columbia research group 

(which simulated only the eastern half of the United States) found significant 

summertime O3 increases across broad swathes of the Midwest and Mid-Atlantic by 

the 2020s, with greater increases by the 2050s and 2080s. 

Relevance for air quality policy: These studies suggest that EPA’s Office of Air 

Quality Planning and Standards should begin to consider climate change, for example, in 

the next update of EPA's ozone modeling guidance, especially for planning horizons in 

2020 and beyond. In other words, they may need to account for a “climate penalty” that 

could influence the amount of controls needed in some locations. Conflicting results 

among simulations for certain regions of the country suggest that evaluations of the 

potential effectiveness of future controls in those regions will be particularly sensitive to 

uncertainties in the modeling systems. The findings also indicate that, where climatechange-induced increases in O3 do occur, damaging effects on ecosystems, agriculture, 

and health may be pronounced, due to increases in the frequency of extreme pollution 

events. 

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B. Climate change has the potential to push O3 concentrations in extreme years beyond 

the envelope of current natural year-to-year variability. In addition, it has the potential 

to lengthen the O3 season.

1. Interannual variability in weather conditions plays an important role in determining 

average O3 levels and exceedances in a given year. For example, statistical analyses 

of current O3 observations show that, for several U.S. cities that have not attained the 

current O3 NAAQS, weather-related interannual variability can increase or decrease 

observed mean O3 concentrations by as much as 10 ppb from the 25-year 

(1981−2006) mean. 

2. The subset of modeling groups that examined multiple simulation years for both 

present-day and future climate found that, in many regions, increases in summer O3

concentrations due to climate change were comparable in magnitude to, or even 

greater than, simulated present-day interannual variability. 

3. Similarly, a subset of the future climate simulations showed that, for parts of the 

country with a defined summertime O3 season, climate change expanded its duration 

into the fall and spring. 

Relevance for air quality policy: Multi-year simulations may be necessary to support 

the development of long-term air quality control strategies, to capture the effects of both 

natural meteorological variability and climate-induced changes. Air quality managers 

may also need to plan to extend the season over which they monitor O3 concentrations 

and be prepared to issue air quality alerts earlier in the spring and later into the fall. 

C. Climate change is expected to cause a decrease in O3 concentrations in remote areas 

with low ambient NOx levels. 

1. The global modeling studies described in this report simulate general decreases in O3

concentrations over remote areas with low NOx concentrations (e.g., oceans) as a 

result of climate change. Consistent with current understanding of O3 chemistry, this 

is due to increased O3 destruction in an atmosphere with more water vapor. 

2. This decrease is in contrast to the significant climate-related increases for many 

already-polluted areas. 

3. The relative impact of these changes in remote background O3 on simulated U.S. O3

concentrations is unclear. One potential influence pathway seen in some of the 

modeling results is an increased mixing of clean air into coastal areas, via stronger 

ocean-land flow combined with the reduced O3 concentrations over the oceans. 

Relevance for air quality policy: Changes in O3 concentrations as a result of climate 

change will depend, in part, on whether an area is clean or polluted, and/or on the degree 

of influence of air masses from adjacent clean or polluted areas. For example, under low 

NOx conditions, a reduced atmospheric lifetime for O3 in the future due to increased 

humidity may imply reductions in the quantity of O3 transported downwind. 

D. The potential impact of climate change on PM is less well understood than that on O3. 

Preliminary results from the modeling studies show a range of increases and decreases 

in PM concentrations in different regions and for different component chemical 

species in the same region. 

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1. Precipitation is a more important primary meteorological driver of PM than of O3, 

due to its role in removing PM from the atmosphere (wet deposition). Precipitation is 

particularly difficult to model and tends to show greater disagreement across 

simulations than other variables. 

2. Aerosol chemical processes, especially those concerning the formation of organic 

aerosols and aerosol/cloud interactions, are not fully understood and therefore not 

well characterized in current regional air quality models. 

3. In addition, increase in wildfire frequency associated with a warmer climate has the 

potential to increase PM levels in certain regions, but the relative importance of this 

effect is not well characterized. 

4. Preliminary simulation results suggest that the PM response reflects the combined 

climate change responses of the individual species that make up PM (e.g., sulfate, 

nitrate, ammonium, black carbon, organic carbon, etc.). Depending on the region, 

these individual responses can be in competing directions, producing either increases 

or decreases in PM (on the order of a few percent). 

Relevance for air quality policy: The more limited scientific understanding and greater 

modeling uncertainties concerning the production and loss of PM highlight the need for 

future research. Assessing the effects of a changing climate on PM on an airshed-byairshed basis may be helpful for considering the detailed chemical characteristics of local 

PM, the possible range of changes in local precipitation, and the potential influence of 

changing wildfire frequency. An upcoming EPA report that is expected to incorporate 

new research findings will address the impacts of climate change on PM in more detail. 

II. Impacts on Meteorological Variables that Directly Affect O3 Concentrations 

A. Climate change has the potential to impact a number of meteorological variables 

important for O3. Whether changes in these variables lead to increases, decreases, or 

no change in O3 concentrations in a given region depends on whether the effects of 

these individual changes on O3 act in concert or compete with each other. This 

discussion of meteorological mechanisms is intended to provide additional detail to the 

general conclusions summarized in Section I above. 

1. The simulations discussed in this report all show significant future changes in 

meteorological quantities such as temperature, cloud cover, humidity, precipitation, 

wind speed and pattern, and mixing depth. 

2. However, there is significant variability across simulations in the spatial patterns of 

these future changes. 

3. As noted above in Section I.A, these variations across simulations help explain the 

disagreements in the spatial patterns of simulated future O3 changes. Each simulation 

produces its own unique pattern of changes in these key meteorological drivers. The 

combined effects of all of these changes in individual O3 drivers in turn help create 

the unique pattern of future O3 changes across regions seen for each simulation. 

4. For example, the different simulations provide examples of regions where both 

temperature increased and surface solar radiation increased (due to a decrease in 

cloudiness). These regions tended to experience increases in future O3 concentration. 

xxvi
In contrast, regions where the changes in these variables were in opposite directions 

tended to have mixed O3 results. 

5. In general, variations in individual meteorological drivers are not independent of each 

other. This is because these variables are linked through underlying atmospheric 

processes, and thus there will tend to be consistent variations across groups of 

variables as a result of specific changes in pressure and cloud patterns. It is through 

such changes in short-term weather that the effects of long-term climate change on O3

are expressed. 

Relevance for air quality policy: It is the interrelationships between the many 

meteorological variables important for O3 that determine O3 concentrations at a particular 

time and place. Evaluating the potential influence of climate change on air quality and 

the potential effectiveness of future control strategies will require accounting for these 

sometimes complex interactions. These complexities can best be appreciated through the 

use of integrated modeling systems capable of simulating interactions among drivers in a 

realistic and self-consistent way. Current modeling uncertainties lead to disagreements 

about the spatial patterns of future changes in meteorological variables and, hence, the 

specific regional distributions of future O3 changes across the United States. 

B. Global climate change is expected to produce changes in planetary-scale circulation 

systems, thereby influencing regional weather patterns. These changes have the 

potential to strongly affect regional O3 concentrations, since O3 episodes are driven by 

synoptic meteorological variability. 

1. Observations suggest that the extratropical storm tracks have moved poleward over 

the last few decades. A number of recent modeling studies suggest that this trend 

could continue into the future (IPCC, 2007), resulting in significant changes in winds, 

precipitation, and temperature patterns in mid-latitudes, with implications for the 

simulated frequency and duration of synoptic stagnation events and resulting extreme 

O3 episodes. 

2. Some of the modeling studies discussed in this report simulate increases in the 

duration and frequency of extreme O3 events in the Midwest and Northeast that can 

be directly traced to the weaker frontal systems and decreased frequency of surface 

cyclone activity due to a poleward storm track shift. 

3. There remains some disagreement across models of the effects of climate change on 

the summertime mid-latitude storm tracks and stagnation, however, as other studies 

discussed in this report do not seem to simulate these circulation changes as strongly, 

and/or do not simulate the corresponding O3 increases. 

4. Similarly, differences in simulations of the climate response of other key large-scale 

circulation patterns, like the Bermuda High off the U.S. east coast, also can produce 

significant differences in the amount and spatial distribution of simulated future O3. 

Relevance for air quality policy: Changes in large-scale circulation systems can have a 

significant impact on O3 throughout the country. For example, understanding and 

accounting for changes in synoptic stagnation events resulting from large-scale storm 

track shifts is critical for understanding potential changes in future O3 concentrations in 

the northern portion of the United States. At present, modeling uncertainties persist, and 

xxvii
further research is needed. Consideration of historic patterns in local meteorology versus 

current observations may help determine whether and where changes in stagnation should 

be addressed in city-level air quality planning. 

III. Impacts on Climate-Sensitive Natural Emissions of O3 Precursors 

A. Climate change has the potential to increase biogenic emissions of O3 precursors, but 

significant uncertainties remain about the impact of these emissions changes on O3

concentrations in a given region. Increases in lightning NOx production may also be a 

factor in future O3 changes. It is important to note that the modeling results discussed 

in this report do not account for all climate-sensitive natural emissions of chemical 

precursors, excluding, for example, oceanic dimethyl sulfide and sea-salt, mineral dust, 

methane from wetlands, and wildfire emissions. 

1. Earlier observational studies suggest that increases in biogenic emissions of volatile 

organic compounds (VOCs) would occur in many regions as a result of the higher 

temperatures associated with expected future climate change. 

2. The modeling studies discussed in this report generally simulate increases in biogenic 

VOC emissions over most of the country as a result of climate change, with 

particularly substantial increases in certain regions, notably the Southeast. 

3. However, these biogenic emissions increases do not necessarily correspond with large 

O3 concentration increases, depending on the region and modeling system used. One 

reason for this appears to be because the response of O3 to changes in biogenic VOC 

emissions depends strongly on how isoprene chemistry is represented in the models. 

4. Globally, an increase in the rate of natural production of NOx by lightning is expected 

in a warmer and wetter climate. Some of the simulations discussed here examined 

this issue and did, in general, see future increases. As the significance of these results 

for regional U.S. O3 concentrations is unclear given the research available at this 

time, these findings are not highlighted in this report. 

Relevance for air quality policy: Resolving uncertainties in the response of O3 to 

biogenic emissions changes is important to improve the understanding of potential 

climate change impacts on O3. For example, the success of regional O3 control strategies 

in regions like the southeastern United States may be highly sensitive to this 

uncertainty—additional anthropogenic emissions controls may need to be considered to 

offset climate-induced increases in biogenic emissions, but only if these emissions 

increases will lead to large O3 increases. A better understanding of the chemical 

reactions involving isoprene nitrate is critical for resolving this issue. Regional O3

control strategies in areas where biogenic VOC emissions are projected to increase due to 

climate change are likely to continue to be NOx-limited areas and thus continue to 

respond to NOx emissions decreases with O3 concentration decreases. In addition, localand regional-scale O3 modeling does not typically consider NOx production from 

lightning. Given potential future changes in lightning NOx emissions, long-term air 

quality management strategies may need to account for growth in this source as well. 

xxviii
IV. Modeling Uncertainties 

A. Simulated future U.S. regional air quality is highly sensitive to model configuration 

choices in the integrated global-to-regional climate and air quality modeling systems 

used in this assessment. 

1. As discussed in Section II above, there are large differences across modeling groups, 

and/or across different model configurations used by the same group, in the specific 

spatial patterns of future simulated changes in meteorology that lead to differences in 

simulated future concentrations of O3. 

2. These differences in simulated meteorology can largely be traced to differences in a 

number of elements of model system configuration. Key elements include which 

global climate model (GCM) was used to simulate future global climate change, 

whether the output from this GCM was “downscaled” to much higher resolution over 

the United States with a regional climate model (RCM), and which model physical 

parameterizations were used, for example for representing cumulus convection. 

3. Sensitivities of air quality-relevant meteorology to other parameterizations (e.g., for 

turbulent mixing, radiative transfer, microphysics, and land-surface processes) may 

also be important but have yet to be examined systematically. 

4. The specific techniques used to implement the downscaling of the GCM output with 

an RCM may also significantly affect the results, but this issue is still to be examined 

systematically as well. 

5. As discussed above, there are also significant sensitivities of simulated O3

concentrations to uncertainties in the representation of key chemical processes in the 

models. 

6. The choice of future greenhouse gas scenario also affects the future GCM climate 

simulation, though in 2050, as opposed to the end of the century, the range in 

greenhouse gas forcing across the various IPCC scenarios used in this assessment is 

still relatively small. 

Relevance for air quality policy: It is important to carefully select and describe the 

GCM, RCM, model physical parameterizations, and downscaling techniques used as part 

of any model-based analysis of potential future changes in air quality. Interpretation of 

the causes of simulated air quality changes will, in general, be highly sensitive to these 

components. Additional efforts to understand and quantify the uncertainties associated 

with these components (as planned for Phase II) will aid in the interpretation of results 

produced by these modeling systems. Furthermore, work is needed on new strategies for 

incorporating information from climate models into uncertainty analysis while 

accounting for all sources of uncertainty. 

V. Combined Impacts of Climate and Anthropogenic Emissions Changes 

A. Preliminary work suggests that the impacts of climate change on future U.S. regional 

O3 concentrations remain significant when also considering possible future 

anthropogenic O3 precursor emissions changes. Several major efforts to address the 

combined impacts are underway and will be the subject of another EPA Global Change 

Research Program report in 2012.

xxix
1. A number of the modeling teams whose results are discussed in this report also 

carried out simulations with modified future air pollutant emissions constructed using 

spatially non-explicit scaling factors generally derived from the assumptions used to 

formulate the various IPCC greenhouse gas emissions scenarios. 

2. These preliminary tests found that the combined effects of climate and anthropogenic 

precursor emissions changes are highly sensitive to the assumptions about future 

emissions trajectories. 

3. For example, simple scaling of future emissions to match the gross assumptions of the 

IPCC A1B or B1 Special Report on Emissions Scenarios (SRES) scenario (IPCC, 

2000) resulted in substantial reductions of U.S. NOx emissions in 2050, which in turn 

resulted in corresponding reductions in simulated future O3 concentrations. In 

contrast, using future emissions consistent with the weaker pollutant control 

assumptions in the “dirtier” A2 or A1Fi scenarios tended to result in higher future O3

concentrations. 

4. The size of the climate change impact on air quality is highly dependent on the 

emissions levels. In other words, the effects of climate and emissions changes were 

not, in general, additive. 

5. These results highlight the need for emissions scenarios with greater regional detail, 

consistency between global and regional assumptions, and consistency between 

greenhouse gases and precursor emissions. Meeting this need is a major focus of 

Phase II of the assessment effort. 

Relevance for air quality policy: While existing air quality controls will likely continue 

to produce significant benefits, to the extent that climate change may increase O3

concentrations in some areas and therefore threaten the ability of an area to attain or 

maintain air quality standards, additional controls (i.e., a climate penalty) may be 

required. Preliminary results suggest that the magnitudes of additional controls could be 

significant in certain regions but also that they are highly dependent on detailed 

assumptions about future emissions. Exploring these assumptions and improving our 

understanding of the fundamental emissions drivers, as part of Phase II of this 

assessment, is expected to lead to the creation of improved scenarios of future emissions 

that in turn will be integrated into the climate and air quality modeling systems to 

produce more robust estimates of potential climate impacts on control policies. 

 This is an interim report, and, therefore, these findings should be considered to be 

preliminary. Future reports will update, refine, and augment the synthesis contained herein. 

 Finally, it is important to emphasize that this assessment is a science assessment, not a 

policy assessment. In other words, the primary means by which this assessment will achieve its 

ultimate goal of enhancing the ability of air quality managers to consider global change in their 

decisions is through the development of tools and a knowledge base to answer science questions 

about the potential impacts of global change on air quality. The resulting improved 

understanding of the behavior and complexities of the system can then provide a basis for a suite 

of parallel, collaborative activities between the science and policy audiences of this report. Such 

xxx
xxxi

activities would be aimed at answering specific air quality management questions and might 

include, for example, the development of new tools and models, designed with an explicit focus 

on decision support, that incorporate the new scientific and technical knowledge gained as a 

result of this assessment. The initiation of such collaborative efforts would represent a 

significant assessment outcome. 
1 INTRODUCTION TO THE PROBLEM 

1.1 INTRODUCTION 

The recent Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment 

Report (AR4) found that “Warming of the climate system is unequivocal, as is now evident from 

observations of increases in global average air and ocean temperatures, widespread melting of 

snow and ice, and rising global average sea level” (IPCC, 2007). The IPCC also found that 

“Most of the observed increase in globally averaged temperatures since the mid-20th century is 

very likely due to the observed increase in anthropogenic greenhouse gas concentrations.” 

Furthermore, of particular importance for the U.S. Environmental Protection Agency’s (EPA) 

mission to protect human health and the environment was the IPCC’s finding that “Future 

climate change may cause significant air quality degradation by changing the dispersion rate of 

pollutants, the chemical environment for ozone and aerosol generation and the strength of 

emissions from the biosphere, fires and dust. The sign and magnitude of these effects are highly 

uncertain and will vary regionally.” 

The National Research Council (NRC), in 2001, posed the question “To what extent will 

the U.S. be in control of its own air quality in the coming decades?” noting that “...changing 

climatic conditions could significantly affect the air quality in some regions of the U.S.” (NRC, 

2001). The NRC called for the expansion of weather and air quality studies to include “studies 

of how air quality is affected by long-term climatic changes.” To address this concern, the 

EPA’s Office of Research and Development (ORD) Global Change Research Program initiated a 

research effort to increase our understanding of the multiple complex interactions between 

climate and atmospheric chemistry. The ultimate goal of EPA’s air quality assessment is to 

enhance the ability of air quality managers to consider global change in their decisions through 

improved characterization of the potential impacts of global change on air quality. 

This ultimate goal will be achieved via three distinct assessment sub-goals: 

• To develop tools and a knowledge base to answer science questions about the impacts of 

global change on air quality. 

• To deliver the general benefits to the air quality policy and management community that 

derive from addressing these science questions, namely, an improved understanding of 

the behavior and complexities of the global change-air quality system, an appreciation for 

the strengths and limitations of the scientific tools and methods used to develop this 

improved understanding, and an answer to the first and most basic “policy” question, “is 

climate change something we will have to account for when moving forward with U.S. 

air quality policy?” 

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• To set the stage for determining how to apply these scientific insights and tools to help 

answer specific, detailed policy and management questions. 

This last sub-goal anticipates a separate activity, or set of activities, branching off from 

this science assessment, that will coalesce around specific air quality decision support needs. 

These activities might include, for example, developing new tools and models designed 

explicitly for decision support (rather than primarily for scientific research). 

This interim assessment report provides an update on the progress toward these three 

sub-goals. As will be discussed in more detail in Section 1.4 below, and in Section 2, the 

assessment design calls for first providing insight into possible air quality responses to future 

climate changes before tackling the additional complexities of incorporating potential future 

changes in anthropogenic emissions and long-range pollutant transport. Therefore, its primary 

focus is on the potential changes in U.S. regional air quality due to global climate change alone, 

including direct meteorological impacts on atmospheric chemistry and transport, and the effect 

of these meteorological changes on climate-sensitive natural emissions of pollutant precursors. 

As such, this interim report cannot fully address questions related to the importance of changing 

future anthropogenic emissions of air pollutants. Meeting this need is a major focus of Phase II 

of the assessment effort. 

The following sub-sections will present the major themes that run through this report, 

provide background on the potential links between climate and air quality that motivate the 

science questions underlying the assessment research, outline the structure and design of the 

overall assessment, identify the assessment stakeholders, discuss issues related to handling 

scientific uncertainty, and present a roadmap to the rest of the report. 

1.2 MAJOR THEMES OF THE INTERIM ASSESSMENT REPORT 

In the course of conducting this assessment, two “grand challenges” have emerged. The 

first stems directly from the EPA Global Change Research Program’s emphasis on decision 

support. The challenge is to provide the best possible scientific basis for understanding the 

potential range of impacts of climate change on air quality, and air quality policies, in a useful 

form and a timely manner, as one important set of information inputs to help managers develop 

appropriate pollution control strategies. Having these improved insights into the way the global 

change-air quality system works may yield new options for addressing air quality issues or 

minimize the potential for introducing policies with significant unintended consequences. At the 

same time, the complexity of the problem, and hence the data, models, and techniques used to 

address it, means that many unanswered scientific questions and unresolved uncertainties will 

exist at a given point in the decision-making timeline. These must be understood and accurately 

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conveyed to policy makers so they have a sense of the levels of confidence underlying individual 

elements of this scientific understanding. 

The second “grand challenge” is to convey to the scientific research community the key 

knowledge gaps that limit our understanding of the problem and/or create barriers to the use and 

interpretation of scientific information by decision makers. These range from the sensitivity of 

regional climate simulations to the parameterizations and methods used in downscaling to how 

the intricate details of the chemical mechanisms are represented in the models. For example, as 

will be discussed in Section 3, there are a number of meteorological metrics that are crucial for 

modeling regional air quality for which the climate modeling community has not yet 

systematically evaluated the skill of their modeling systems. Similarly, future emissions 

scenarios that are consistent across pollutants and geographic scales and that incorporate 

important processes such as fire, land use, biogenic emissions, and technological change are 

lacking, limiting the kinds of studies that can be accomplished at this time. 

It is possible to think of these challenges as informing two parallel “readings” of this 

report, one tuned to the perspective of a “science” audience and the other to that of a “policy” 

audience. While these obviously intersect and overlap, each would highlight its own distinct set 

of issues, falling broadly under two questions: “What do we know, scientifically, about the 

climate change-air quality problem?” and “What might this knowledge mean for me, as an air 

quality manager?” 

For example, for the scientific audience, this report generates additional information by 

synthesizing across the findings from multiple research groups. This synthesis improves our 

understanding of the potential for climate change to impact air quality in different regions of the 

United States and the complex interplay between air quality and its different climatic and 

meteorological drivers. It also throws into relief scientific and technical uncertainties that will be 

helpful in guiding future research efforts. 

For the policy audience, the scientific findings presented in this report begin to answer 

the question raised above: “Is climate change something we will have to account for when 

moving forward with U.S. air quality policy?” In addition, by illuminating the subtleties and 

complexities of the interactions between climate, meteorology, and air quality, these findings can 

inform thinking about policy responses. This knowledge can be carried forward into the next 

phase of the assessment, which will consider added complications such as changes in 

anthropogenic emissions drivers. Furthermore, this report provides a basis for evaluating the 

relative robustness of these scientific findings in light of the uncertainties that surround them. 

Finally, all of these general insights create a foundation for targeted efforts to solve specific air 

quality management problems. 

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1.3 BACKGROUND 

1.3.1 Air Pollution 

EPA's mission is to protect human health and the environment. To achieve this mission, 

EPA implements a variety of programs under the Clean Air Act that reduce ambient 

concentrations of air pollutants, including those that cause smog, haze, and acid rain. Pollutants 

such as ozone (O3) are not emitted directly into the atmosphere: instead they are created by 

chemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in 

the presence of heat and sunlight. VOCs are emitted from a variety of sources, including motor 

vehicles, chemical plants, refineries, factories, consumer and commercial products, other 

industries, and natural (biogenic) sources. NOx is emitted from motor vehicles, power plants, 

other sources of combustion, and natural sources including lightning and biological processes in 

the soil. EPA’s efforts have been successful: between 1980 and 2007, emissions of VOCs and 

NOx decreased by 50 and 39 percent respectively, even though gross domestic product increased 

124 percent, vehicle miles traveled increased 103 percent, and energy consumption increased 30 

percent (U.S. EPA, 2008; see also http://www.epa.gov/airtrends/sixpoll.html). 

Air pollution, however, continues to be a widespread public health and environmental 

problem in the United States. In 2007, approximately 158 million people lived in counties that 

exceeded at least one of the National Ambient Air Quality Standards (NAAQS). The health 

effects of air pollution range from increased mortality to chronic effects on respiratory and 

cardiovascular health (e.g., see Jerrett et al., 2009). Air pollution also has been associated with 

increased use of health care services, including visits to physicians and emergency rooms and 

admissions to hospitals. Other effects include reduced visibility, damage to crops and buildings, 

and acidifying deposition on soil and in water bodies, where the chemistry of the water and 

resident aquatic species are affected.1

 Moreover, there is growing concern that global change 

may make it more difficult to reach these goals. The air quality assessment effort itself does not 

address health and other effects. However, it will provide information that will be used in the 

Global Program’s climate and air quality health assessments, the first of which focuses on O3. 

1.3.2 Climate Change and Air Quality Linkages 

The NRC, in 2001, highlighted the linkages between climate and regional air quality and 

the need for a comprehensive research strategy: 

Air pollution is generally studied in terms of immediate local concerns rather than 

as a long-term 'global change' issue. In the coming decades, however, rapid 

 1

 See, for example the Ozone Criteria Document, at 

http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=149923, and the Particulate Matter Criteria Document, at 

http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=149923. 

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population growth and urbanization in many regions of the world, as well as 

changing climatic conditions, may expand the scope of air quality concerns by 

significantly altering atmospheric composition over broad regional and even 

global scales. ... Although air quality and climate are generally treated as separate 

issues, they are closely coupled through atmospheric chemical, radiative, and 

dynamical processes. ... A better understanding is needed in order to make 

accurate estimates of future changes in climate and air quality and to evaluate 

options for mitigating harmful changes. 

Coupling atmospheric chemical processes and the climate system remains a challenge to 

the science and modeling communities, however, because a large number of physical, chemical, 

and biological processes are involved (see Figure 1-1), and many of these are poorly understood. 

Figure 1-1. Schematic representation of the multiple interactions between 

tropospheric chemical processes, biogeochemical cycles, and the climate 

system. RF represents radiative forcing, UV represents ultraviolet radiation, and 

IR represents infrared radiation. The red arrows, discussed below in Section 

1.4.1, represent the scope of EPA’s assessment effort. (Adapted from IPCC, 

2007.) 

Atmospheric

Concentrations

CH4

VOCs

CO

Pollutant Emissions

• Fossil Fuel Combustion

•Biomass Burning

•Industrial Emissions

•Emissions Relating to Land‐use Change

Climate

Nitrogen Cycle

Emissions

Environmental

Concentrations

Atmospheric Concentrations

O3

Ecosystem Nutrients

Æ Biogenic Emissions

Lightning,

Precipitation,

Temperature

RF (IR)

CH4, VOC

Emissions

Rates

RF (UV + IR)

O3 Production O3 Production

Carbon

Oxidation

(via OH) Temperature,

Humidity,

Dynamics

N2O, RF

NOx, HNO3

Lifetimes

Atmospheric

Concentrations

CH4

VOCs

CO

Atmospheric

Concentrations

CH4

VOCs

CO

Atmospheric

Concentrations

CH4

VOCs

CO

Pollutant Emissions

• Fossil Fuel Combustion

•Biomass Burning

•Industrial Emissions

•Emissions Relating to Land‐use Change

Pollutant Emissions

• Fossil Fuel Combustion

•Biomass Burning

•Industrial Emissions

•Emissions Relating to Land‐use Change

Climate

Nitrogen Cycle

Emissions

Environmental

Concentrations

Nitrogen Cycle

Emissions

Environmental

Concentrations

Nitrogen Cycle

Emissions

Environmental

Concentrations

Atmospheric Concentrations

O3

Ecosystem Nutrients

Æ Biogenic Emissions

Lightning,

Precipitation,

Temperature

RF (IR)

CH4, VOC

Emissions

Rates

RF (UV + IR)

O3 Production O3 Production

Carbon

Oxidation

(via OH) Temperature,

Humidity,

Dynamics

N2O, RF

NOx, HNO3

Lifetimes

1-5
1.3.2.1 Air Quality Impacts on Climate Change 

Prior to the mid-1970s, anthropogenic climate change was largely viewed as a 

CO2-driven phenomena. This picture began to change with a series of papers on non-CO2

reactive gases including: 

• The impact of Chlorofluorocarbons (CFCs) on the greenhouse effect (e.g., Ramanathan, 

1975); 

• The impact of NOx on stratospheric O3 (a strong greenhouse gas) (e.g., Crutzen, 1972); 

• The identification of methane (CH4) and nitrous oxide (N2O) as greenhouse gases (e.g., 

Wang et al., 1976); and 

• The contribution of tropospheric O3 (and therefore CO, VOCs, and NOx) to global 

warming (e.g., Fishman et al., 1980). 

A World Meteorological report published in 1985 concluded that trace gases other than 

CO2 contributed as much anthropogenic climate forcing as CO2 since the industrial revolution 

(Ramanathan et al., 1985), and our understanding of the multiple strong and complex links 

between climate and air quality have continued to evolve (Ramanathan and Feng, 2009). 

Air pollution emissions can also affect concentrations of the hydroxyl radical (OH), the 

primary cleansing agent of the lower atmosphere, with increases in NOx tending to elevate OH 

levels, while increases in CO have the opposite effect. Changes in OH affect the lifetime and 

thus the concentrations of reactive greenhouse gases such as CH4, HFCs, and HCFCs (NRC, 

2001). The nitrogen cycle itself plays a key role in climate and atmospheric chemistry, 

contributing N2O (a greenhouse gas), NOx (an O3 precursor, with indirect effects on CH4 via 

shortening of atmospheric lifetime), and ammonia (NH3), which contributes to the formation of 

sulphate and nitrate aerosols (IPCC, 2007). 

Aerosol particles affect climate by scattering and absorbing radiation (the “direct effect”) 

and through their impact on clouds (the “indirect effect”). Aerosols interact with clouds and 

precipitation in a variety of ways: e.g., by acting as cloud condensation nuclei and/or ice nuclei; 

through effects on the albedo or reflectivity of the cloud; and by impacting cloud lifetimes. Such 

effects can change precipitation patterns as well as cloud extent and optical properties (CCSP, 

2009). 

1.3.2.2 Climate Change Impacts on Air Quality 

Concerns about the impacts of climate change on air quality are grounded in information 

derived from a wealth of observational studies, knowledge of basic atmospheric chemistry, and, 

more recently, modeling studies (see Appendix A for more details about these lines of evidence). 

1-6
For example, there have been many empirical analyses showing that weather patterns play a 

major role in establishing conditions conducive to O3 formation and accumulation, given 

sufficient levels of precursor pollutants such as nitrogen oxides (NOx) and volatile organic 

compounds (VOCs): e.g., year-to-year variability in warm-season climate is strongly correlated 

with variability in O3 exceedances. Generally speaking, meteorological conditions favorable to 

high levels of O3 include sunshine, high temperatures, and stagnant air (NRC, 1991). However, 

this NRC report also cautioned about the potential complexities of the problem arising from 

interactions between key drivers, noting that, for example, the relationship between temperature 

and O3 “cannot readily be extrapolated to a warmer climate because higher temperatures are 

often correlated empirically with sunlight and meteorology.” 

A variety of statistical methods have been successfully applied to weather, O3, and other 

data to obtain short-term air quality forecasts (U.S. EPA, 1999), estimate time trends (Thompson 

et al., 2001; Bloomfield et al., 1996; Cox and Chu, 1993; Camalier et al., 2007), and increase 

understanding of underlying mechanisms (Sillman and Samson, 1995). There are substantially 

fewer observations for particulate matter (PM), as monitoring networks have been in place for a 

much shorter time period. This should improve over time as more data become available. 

Two early modeling studies (Morris et al., 1995; U.S. EPA, 1989) of the effect of a 

warming climate on U.S. O3 levels considered a uniform 4°C increase in temperature across 

horizontal, vertical, and temporal scales.2

 The EPA study modeled specific episodes and 

simulated changes in daily 1-hour maximum O3 concentrations ranging from +3 to +20% for 

Central California and from −2.4 to +8% for the Midwest and Southeast. Morris et al. (1995) 

included the effect of warmer conditions on mobile source and biogenic emissions in their 

simulation of a 4-day episode in the Northeast, simulating O3 concentration increases of 

15−25 parts per billion by volume (ppb) in much of the modeling domain above baseline daily 

one-hour maximum concentrations of 110−120 ppb and 120−140 ppb (i.e., increases of 

10−20%). 

The results of these early studies suggested that regional air quality may be sensitive to a 

warming climate, creating an additional challenge for air quality managers. However, as noted 

by the authors, their studies were constrained by the limitations of the tools and data available at 

the time. It was recognized that the relationship between climate change and air quality was not 

a simple one of “higher temperatures equals worse air quality” (NRC, 1991; U.S. EPA, 1989). 

The number of meteorological factors, and the complex interactions between and among them 

and air pollutants (see Box 1-1), highlight the need to use sophisticated modeling tools and 

experimental designs to help understand the multiple ways that climate change can affect 

 2

 Because of the technical hurdles existing at the time in adapting climate model output to be input to a 

regional air quality model, the researchers elected to make this simplifying assumption. 

1-7
regional air quality. Fortunately, modeling capabilities have improved substantially since that 

time and continue to improve. 

Box 1-1. Climate Change Factors Important for Regional Air Quality 

Adapted from U.S. EPA (1989) 

Changes in the following affect air quality: 

• The average maximum or minimum temperature and/or changes in their spatial distribution and duration 

leading to a change in reaction rate coefficients and the solubility of gases in cloud water solution; 

• The frequency and pattern of cloud cover leading to a change in reaction rates and rates of conversion of 

SO2 to sulfate aerosols, leading to acid deposition; 

• The frequency and intensity of stagnation episodes or a change in the mixing layer leading to more or 

less mixing of polluted air with background air; 

• Background boundary layer concentrations of water vapor, hydrocarbons, NOx, and O3, leading to more 

or less dilution of polluted air in the boundary layer and altering the chemical transformation rates; 

• The vegetative and soil emissions of hydrocarbons and NOx that are sensitive to temperature and light 

levels, leading to changes in their concentrations; 

• Deposition rates to vegetative surfaces whose absorption of pollutants is a function of moisture, 

temperature, light intensity, and other factors, leading to changes in concentrations; and 

• Circulation and precipitation patterns leading to a change in the abundance of pollutants deposited 

locally versus those exported off the continent.

1.4 DESIGN OF THE GLOBAL CHANGE AND AIR QUALITY ASSESSMENT 

To address the need for an improved understanding of the potential impacts of global 

change on U.S. regional air quality, building on the scientific understanding summarized above, 

an integrated assessment framework was designed that blends the research and development 

strengths within the EPA with those of other agencies and the academic research community. 

The assessment program was designed to provide the scientific information and modeling 

capabilities to answer the following types of questions:3

• What are the effects of plausible future changes in climate, climate variability, and landuse patterns on air quality, specifically ground-level O3 and PM? 

• What is the range of potential impacts of climate change on air quality relative to the 

range of potential impacts of emissions changes due to pollution controls, technological 

development, and land-use change? 

• How might the effectiveness of air quality management be affected by climate change, 

i.e., can changes in emissions, technology, and land use offset air quality changes due to 

climate change? 

 3

 These questions were adapted from the November 2002 EPA Global Change Research Program Research 

Strategy (EPA/600/R-02/087), which can be found at: http://www.epa.gov/ncea/pdfs/glblstrtgy.pdf. 

1-8
1.4.1 Scope of the Assessment Effort 

The discussion in Section 1.3.1 is not a comprehensive description of all the potential 

linkages between climate and atmospheric chemistry. Instead, it is meant to highlight the fact 

that these linkages are complex, involve nonlinear coupling among numerous processes, and that 

many of these are not well quantified. The scientific enterprise required to elucidate all of the 

linkages exceeds the resources available to the EPA’s Global Change Research Program. 

Accordingly, the Program elected to focus its efforts on the impact of climate change on 

regional air quality (the red arrows emanating from the “Climate” box in Figure 1-1) to inform 

and support EPA’s air quality programs. The NRC (2004) identified climate change as an 

important new challenge to the air quality management (AQM) system. The report concluded 

that “The AQM system must be flexible and vigilant in the coming decades to ensure that 

pollution mitigation strategies remain effective and sufficient as our climate changes.” Focusing 

on climate effects on air quality also takes advantage of the considerable expertise within EPA in 

regional air quality modeling. Other federal agencies have active research programs 

investigating other aspects of Figure 1-1, such as the feedback effects of aerosols and 

atmospheric chemistry on the climate system. The Atmospheric Composition research element 

of the U.S. Climate Change Science Program (CCSP) coordinates research on atmospheric 

chemistry and climate system interactions across the federal government.4

The assessment addresses its questions in two phases. Phase I of the effort focuses on 

augmenting, linking, and applying existing climate and atmospheric chemistry models to 

investigate the range of current and potential future meteorological effects on air quality. It does 

not include changes in air pollutant emissions other than those that are explicitly linked to 

meteorological variables and incorporated within the models (e.g., biogenic VOC emissions, 

evaporative emissions, lightning NOx, depending on the modeling system). 

Phase II of the assessment focuses on the combined impact of changing climate and 

changing air pollutant emissions on air quality. It builds on the findings from the first phase by 

extending the linked modeling systems developed therein, and also by exploring the scientific 

uncertainties more comprehensively. Simultaneously, it integrates plausible, spatially detailed 

scenarios of U.S. criteria pollutant emissions 50 years in the future with the climate and air 

quality modeling efforts initiated in the first phase. The development of the tools to create 

plausible scenarios of technology, land use, and demographic changes needed to derive these 

emissions scenarios is a critical aspect of this phase of the assessment. 

 4

 See http://www.usgcrp.gov/usgcrp/ProgramElements/atmosphere.htm. 

1-9
1.4.2 What is Covered in this Report 

The problem and challenge of air quality is defined by its local impacts combined with its 

global dimensions and the linkages across scales and disciplines needed to address it. The 

purpose of this interim assessment report is to provide an update on our progress toward the 

development of tools and a knowledge framework that encompasses these linkages in the 

investigation of global change impacts on U.S. air quality. It is not intended to provide a 

comprehensive assessment of the literature. There have been several recent state-of-the-science 

reviews that provide such assessments (e.g., see IPCC, 2007 Chapter 7; Jacob and Winner, 2009; 

U.K. Royal Society, 2008). 

By design, the emphasis in this report is on EPA, and EPA-funded, work carried out 

under the EPA Global Change Research Program’s assessment. In Section 3, the focus is on 

results emerging from the subset of participating intramural and extramural research groups that 

are currently producing model simulations of the impacts of climate change on air quality, as part 

of Phase I of the assessment. This is a mid-course overview of the findings to date from the 

several parallel efforts to build, test, and apply individual versions of these linked climate and air 

quality modeling systems. Notably, this is the first systematic effort to apply combined global 

and regional climate and air quality models to investigations of potential climate change impacts 

on future U.S. regional air quality. Though the focus is on EPA, and EPA-funded, research, this 

body of work does in fact represent the large majority of the research to date in the area of 

applying these types of linked modeling systems to the problem of regional U.S. air quality (e.g., 

see Jacob and Winner, 2009). 

From a scientific perspective, the main goal is to assess the larger meaning of the various 

research groups’ model simulation results when examined all together. The aim is to synthesize 

the simulated air quality changes in different regions of the United States, as well as the 

dependence of these changes on different climatic drivers. By highlighting scientific and 

technical uncertainties to which these findings are sensitive, the synthesis helps identify future 

research needs. 

From a policy perspective, this synthesis across scientific findings begins to answer the 

question: “Is climate change something we will have to account for when moving forward with 

U.S. air quality policy?” In addition, by illuminating the subtleties and complexities of the 

interactions between climate, meteorology, and air quality, it helps build up intuition about the 

way the coupled system works. Section 3 also provides an extended discussion of the challenges 

and uncertainties associated with the modeling approach that underpins the assessment, to create 

an improved understanding about the level of confidence in the scientific findings, and an 

appreciation for the limits on what questions the science can answer now, and may be able to 

answer in the future. 

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Changes in anthropogenic forcing (i.e., fossil fuels, biomass burning, and land use) are 

not covered in this report. However, they will be addressed in the second phase of the 

assessment effort. Section 4 provides an overview of Phase II. Ongoing activities include 

investigation of modeling uncertainties (for example, through the use of ensemble approaches), 

additional model development (for example, the incorporation of dynamic vegetation 

sub-models), and examination of additional pollutants including PM and mercury. Preliminary 

results for PM are provided, but a more comprehensive presentation awaits future assessment 

reports focusing on these additional pollutants. Future assessment reports also will cover the 

combined impacts of changing climate and air pollutant emissions on air quality. Initial results 

from combined climate and emissions sensitivity studies and ongoing work on the drivers of 

emissions changes—e.g., technology, population growth and geographic distribution, economic 

growth, and land use—are also described in Section 4. 

1.5 THE CLIENT COMMUNITIES 

Section 1.2 referred to the two broadly defined themes, audiences, and readings of this 

report that flow from the two “grand challenges.” Though this conceptualization provides a 

useful roadmap to the major purposes of the report, it is also important to identify specific groups 

that are potential beneficiaries of the information contained herein, and that supply the audiences 

and perspectives to which the report speaks. These include air quality managers, employees of 

agencies working as part of the overall U.S. federal climate change research effort, and the 

climate change and air quality research and modeling communities. 

1.5.1 EPA Office of Air and Radiation (OAR), State, Tribal, and Local Air Quality 

Planners 

The EPA’s Global Change Research Program engages in activities that support EPA’s 

mission to protect human health and the environment. As the specific focus of this report is air 

quality, OAR is a major client for this work. Recent air quality regulations, such as the NOx

State Implementation Plan (SIP) Call,5

 Clean Air Interstate Rule (CAIR),6 Heavy Duty Highway 

Diesel Rule,7

 and Non-road Diesel Rule,8 are expected to bring many urban areas of the United 

 5

 “Finding of Significant Contribution and Rulemakings for Certain States in the Ozone Transport 

Assessment Group Region for the Purposes of Reducing Regional Transport of Ozone (“NOx SIP Call”).” U.S. 

EPA Technology Transfer Network: O3 Implementation. 6

 “Clean Air Interstate Rule.” U.S. EPA: Clean Air Rules of 2004. http://www.epa.gov/cair/. 7

 “Clean Diesel Trucks, Buses, and Fuel: Heavy-Duty Engine and Vehicle Standards and Highway Diesel 

Fuel Sulfur Control Requirements (the “2007 Heavy-Duty Highway Rule”).” U.S. EPA. 

http://www.epa.gov/otaq/highway-diesel/regs/2007-heavy-duty-highway.htm. 8

 “Clean Air Nonroad Diesel – Tier 4 Final Rule.” U.S. EPA. 

http://www.epa.gov/nonroad-diesel/2004fr.htm. 

1-11
States into attainment with current PM and O3 standards by 2015. However, as noted by the 

NRC (2004), 

The AQM system will need to ensure that pollution reduction strategies remain 

effective as the climate changes, because some forms of air pollution, such as 

ground-level ozone, might be exacerbated. In addition, emissions that contribute 

to air pollution and climate change are fostered by similar anthropogenic 

activities, that is, fossil fuel burning. Multi-pollutant approaches that include 

reducing emissions contributing to climate warming as well as air pollution may 

prove to be desirable. 

Furthermore, air quality management involves policy decisions with consequences that 

can last for decades. For example, policy guides the choices made for electricity production 

investment and the emissions and fuel efficiencies of motor vehicles. Power plant and motor 

vehicle fleet replacement involves very long lead-times (see, e.g., U.S. EPA, 1992). In this 

context, it will be important to consider the air quality impacts of global change to identify 

actions that accomplish air quality goals with the least long-term cost to society. Information 

and tools supporting the creation of holistic, robust decisions are thus very much needed. 

Similarly, information and tools supporting new and innovative approaches to existing and 

emerging issues are needed as well. As introduced in Section 1.1 above, providing a foundation 

for developing such decision support instruments that can be transferred to national, regional, 

state, and local decision-makers is a critical goal of the overall air quality assessment effort. 

1.5.2 U.S. Climate Change Science Program (CCSP) 

The CCSP integrates federal research on climate change, as sponsored by 13 federal 

agencies and overseen by the Office of Science and Technology Policy (OSTP), the Council on 

Environmental Quality (CEQ), the National Economic Council (NEC), and the Office of 

Management and Budget (OMB). The primary EPA role within the CCSP is to develop an 

understanding of the potential consequences of global change on human health, ecosystems, and 

socioeconomic systems in the United States. Currently, EPA’s ORD, within which the Global 

Change Research Program is located, is focusing on topics that include impacts on future water 

and air quality, risks to coral reefs and watersheds, and impacts on biological criteria and aquatic 

invasive species, as well as developing decision support methods and resources. 

The impact of climate change on air quality is one of the overarching questions guiding 

the Atmospheric Composition research element of the CCSP (CCSP, 2003; see Box 1-2). The 

CCSP Atmospheric Composition Interagency Working Group coordinates research that focuses 

on how the composition of the global atmosphere is altered by human activities and natural 

phenomena and how such changes influence climate, O3, PM, ultraviolet radiation, pollutant 

exposure, ecosystems, and human health. Atmospheric composition issues involving 

1-12
interactions with climate variability and change—such as the potential effects of global climate 

change on regional air quality—are important research topics. Several federal agencies, 

including the National Oceanographic and Atmospheric Administration (NOAA), the National 

Aeronautics and Space Administration (NASA), and the Department of Energy (DOE), are 

involved in research activities in this area, including satellite observations, aircraft field 

campaigns, laboratory studies, and global modeling studies. EPA contributes its expertise in 

regional air quality modeling and anthropogenic emissions, along with research support in other 

air quality-relevant topic areas. 

Box 1-2. Contributions to CCSP 

The EPA Global Change Research Program Air Quality Assessment addresses a number of CCSP research and 

development elements, as described in the CCSP strategic plan (CCSP, 2003), including 

Chapter 3. Atmospheric Composition

Question 3.3: What are the effects of regional pollution on the global atmosphere and the effects of global 

climate and chemical changes on regional air quality and atmospheric chemical inputs to ecosystems? 

Question 3.5: What are the couplings and feedback mechanisms among climate change, air pollution, and ozone 

layer depletion, and their relationship to the health of humans and ecosystems? 

Chapter 9. Human Contributions and Responses to Environmental Change 

Question 9.2: What are the current and potential future impacts of global environmental variability and change 

on human welfare, what factors influence the capacity of human societies to respond to change, and how can 

resilience be increased and vulnerability reduced? 

Question 9.4: What are the potential human health effects of global environmental change, and what climate, 

socioeconomic, and environmental information is needed to assess the cumulative risk to health from these 

effects? 

Chapter 11. Decision Support Resources Development 

Goal 11.1: Prepare scientific syntheses and assessments to support informed discussion of climate variability and 

change issues by decision-makers, stakeholders, the media, and the general public. 

Goal 11.2: Develop resources to support adaptive management and planning for responding to climate variability 

and climate change, and transition these resources from research to operational application. 

Goal 11.3: Develop and evaluate methods (scenario evaluations, integrated analyses, alternative analytical 

approaches) to support climate change policymaking and demonstrate these methods with case studies.

In addition to contributing to efforts under the Atmospheric Composition element, the 

scientific and technical accomplishments of the current assessment are enlarging the database of 

information needed to address questions under a number of other CCSP elements (see Box 1-2). 

Information from the ongoing air quality assessment is included in the CCSP Synthesis and 

Assessment Product 4.6: “Analyses of the effects of global change on human health and welfare 

and human systems” (CCSP, 2008). 

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1.5.3 Climate Change Research Community 

Understanding potential impacts of global change on U.S. air quality is a particularly 

challenging task, given the varying climate regimes contained within the continental United 

States and the 3-dimensional modeling at high spatial and temporal resolution that is required to 

capture effects of importance to policy planners. The larger climate change research community, 

including other government science agencies and academia, plays a crucial role in the EPA 

Global Change Research Program’s research and development process by assuming the task of 

advancing the capabilities of global and regional climate models and global and regional 

atmospheric chemistry models. Beyond the many challenges of understanding potential future 

global climate change itself, the problem of impacts on air quality adds additional dimensions. 

For example, the global climate modeling community has typically focused on long-term 

average meteorological parameters on continental and planetary scales, while adverse regional 

air quality events are often determined by finer-scale geographic and temporal variability. 

Successfully simulating the impact of climate change on air quality requires advances in the 

climate sciences and climate modeling, with particular attention to these spatial and temporal 

needs. The research synthesis portion of this report (Section 3) looks across the modeling studies 

conducted as part of this assessment, studies that represent an initial step toward addressing this 

challenge. 

In addition, the modeling work in this assessment provides an important test of some 

methodologies used for linking (downscaling) global and regional climate models, a key aspect 

of climate impacts work in general. Further advances in meeting the demanding requirements of 

simulating climate change impacts on U.S. air quality will improve our capabilities to assess 

other global change impacts of great importance to the environmental policy community, 

including impacts on water quality, aquatic ecosystems, water resources, agriculture, and forests, 

in addition to the quantification of air quality-related human health effects. 

1.5.4 Air Quality Research Community 

Developing coupled climate and air quality modeling systems challenges the capabilities 

of regional air quality models. Improvements in our ability to model chemistry of air pollution 

are needed in a number of areas to better understand the influence of climate change on air 

quality. For example, enhancing linkages between climate/meteorology models and air quality 

models, developing suitable initial and boundary conditions for all important chemical species, 

and producing plausible future emission scenarios are all required. Comprehensive examinations 

like this assessment effort also reveal key uncertainties in chemical mechanisms and processes 

that can be used to prioritize future modeling improvements. Notable among these is the need to 

introduce the ability to simulate two-way interactions between climate and chemistry: for 

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example, changes in the distribution of particulates as a result of climate or emissions changes 

could have important impacts on the Earth’s radiation budget, thereby further influencing 

climate. Finally, the extremely large data files involved in this assessment effort have required 

the development of automated data management and quality control tools and highlighted the 

need for new data distribution systems. 

1.6 CONSIDERING UNCERTAINTY IN THE ASSESSMENT EFFORT 

Characterization of the uncertainty in a given finding, judgment, or prediction, and 

communication of this uncertainty in clear, precise, objective language, are important 

components of scientific assessments. Large global change assessment efforts, such as those 

conducted by the IPCC and CCSP, have produced general guidance on handling uncertainty in 

assessment reports (see CCSP, 2009; IPCC, 2005). For example, a fundamental principle is that 

basic differences between descriptions of uncertainty in terms of likelihood of an outcome and 

level of confidence of the science underlying a finding must be recognized. 

Likelihood is relevant when assessing the chance of defined future occurrence or 

outcome. When the maturity of the scientific knowledge base warrants it, it is considered best 

practice to assign numerical probabilities to qualifiers such as “probable,” “possible,” “likely,” 

“unlikely,” etc., to avoid differing interpretations among people and contexts. 

Level of confidence refers to the degree of belief in the scientific community that 

available understanding, models, and analyses are accurate, expressed by the degree of 

consensus in the available evidence and its interpretation. One way to think about the level of 

confidence concept is to consider two attributes of the state of knowledge underlying a given 

finding or judgment: the amount of evidence available to support it and the degree of consensus 

within the scientific community about the interpretation of that available evidence. 

The study of climate impacts on air quality is a still-emerging field of research. In 

addition, the modeling studies discussed herein were designed to be sensitivity studies, not 

predictions. Therefore, this report does not attempt to express the findings from the scientific 

synthesis in terms of the probabilities (“likelihoods”) of particular future events. Instead, the 

report provides information to help evaluate the relative levels of confidence in the findings. 

Findings for which multiple lines of evidence are presented, and for which there is general 

agreement across these lines of evidence, should be viewed with higher confidence than findings 

for which there is a paucity of observations and/or model simulation results or for which there 

are competing interpretations of the results that are available. For example, as will be discussed 

in Section 3, there is broad agreement across the modeling studies, consistent with scientific 

understanding from theory and observations, that simulated future climate change leads to 

increases in biogenic VOC emissions in the southeastern United States, but there is significant 

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disagreement as to whether these emissions increases lead to large increases in O3 concentrations 

due to uncertainty about how to represent isoprene nitrate chemistry. 

Section 3 provides a detailed discussion of the major uncertainties associated with the 

coupled climate and air quality modeling systems upon which rests the science synthesis 

presented in this report. Moving forward into the second phase of the assessment, the 

complexity of the problem will grow when the multiple dimensions of climate and emissions 

changes are fully integrated. In anticipation of the challenges that multiple, interacting 

categories of uncertainties will present for interpretation of the assessment findings, EPA 

convened an expert workshop in November 2006 to begin the process of identifying a set of 

guiding principles to assist in evaluating uncertainty as the assessment moves forward. 

Participants included experts in global and regional climate modeling, socioeconomic modeling 

and emissions projection, atmospheric chemistry, regional air quality modeling, and uncertainty 

analysis and communication, along with key stakeholders from OAR and the EPA regions. The 

workshop findings suggested emphases on the following issues: building a healthy, 

collaborative process involving both scientists and policy makers; identifying formal uncertainty 

analysis techniques appropriate for complex, computationally expensive linked climate and air 

quality modeling systems; evaluating the potential contributions of complementary methods, 

such as expert elicitation; communications strategies; and the need for future workshops to focus 

on specific technical issues. The workshop and its findings are summarized in Appendix B. 

1.7 STRUCTURE OF THIS REPORT 

This report presents the progress made toward the overall assessment goals. It is divided 

into five sections (including this one): 

The Summary of Policy Relevant Findings, which precedes this section, seeks to draw 

some preliminary connecting lines between the scientific findings of the assessment to date and 

the issues of concern to air quality managers. Analogous to the approach taken in the IPCC 

Summary for Policymakers, OAR was substantially engaged in the writing of this section in 

order to ensure the salience of the results for air quality policy. 

Section 2 discusses in greater detail the design of the assessment effort, including the 

process used to develop this design, key decisions made by the research team, research priorities, 

and program capabilities. The focus on developing and applying linked global-to-regional 

climate and air quality modeling systems is in recognition of the complexities of the global 

change-air quality problem, including its multi-scale (i.e., from global to local; from decadal to 

diurnal) dimensions. 

Section 3 synthesizes the results emerging from the initial applications of these modeling 

systems to the simulation of U.S. air quality under potential future climate change. It highlights 

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the sensitivities in the climate-air quality system and the uncertainties associated with the 

modeling tools. 

Section 4 discusses the next phase of the assessment. It summarizes ongoing work that 

seeks to increase our understanding of key modeling issues and develop new capabilities for 

simulating future changes in anthropogenic emissions. 

Appendix A describes the meteorological variables to which U.S. air quality is known to 

be sensitive, e.g., the basis for the anticipated effects of changing climate on future air quality. 

Appendix A also discusses early research results on the role of climate in future air quality. 

Appendix B describes the 2006 workshop convened by EPA NCEA to identify the essential 

issues that must be addressed in identifying and communicating the uncertainties inherent in this 

assessment, and other complex, model-based assessments. Appendix C describes the 2001 expert 

workshop convened by EPA NCEA to evaluate the research and assessment framework 

developed by the EPA Global Change Research Program for identifying and quantifying the 

effects of global change on U.S. regional air quality. Finally, Appendices D, E, and F expand 

upon the descriptions provided in the main report of the internal EPA ORD programs 

contributing the GCRP assessment effort. A glossary has been provided to assist readers who are 

unfamiliar with the terms that are frequently used in the discussion of climate and air quality 

research and policy. 
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2 OVERVIEW OF APPROACH 

2.1 INTRODUCTION 

The NRC stated in 2001 that, “improving our understanding of linkages between climate, 

atmospheric chemistry, and air quality and our ability to assess future states of the atmosphere 

will require coupling local- and regional-scale air quality models with global-scale climate and 

chemistry models” (NRC, 2001). The EPA’s Global Change Research Program initiated a 

research program designed to meet the “grand challenges” introduced in Section 1 that is 

consistent with EPA’s traditional “place-based” regional assessment approach, and that focuses 

on spanning the breadth of issues from global-scale drivers of climate and air quality to 

developing regional-scale inputs for air quality modeling. 

In the design of this program, the EPA recognized three key linkages inherent to the 

global change and air quality issue: those across spatial scales, those across temporal scales, and 

those across disciplines. The processes linking global to regional scales, symbolized in Figure 

2-1, and the requirements for modeling them, were identified as a first step in the assessment 

design. Similarly, while air quality is defined, studied, and managed most readily on the 

synoptic timescales associated with meteorological and air quality episodes, global climate 

change is manifested on timescales of decades and longer, imposing significant research 

challenges to bridge this gap. 

Finally, given the inherently 

multi-disciplinary nature of 

the problem, it was recognized 

that merging the efforts of the 

climate change, air quality, 

emissions inventory, land use, 

energy, and transportation 

economics research 

communities would be critical 

to bring about advances 

required for this assessment. 

Developing the modeling 

tools and knowledge base to 

achieve these linkages is a 

fundamental task of the 

assessment. 

Figure 2-1. Links between global and regional climate and 

atmospheric chemistry processes with anthropogenic activities 

governing air pollution emissions. The dashed arrows represent 

feedbacks not considered as part of this assessment. 

Regional

Meteorology

Regional

Emissions

Regional

Air Quality

Regional Boundary Conditions

Meteorological Chemical

Global

Emissions

Global

Air Quality

Global

Meteorology

Global Change

Scenarios

(IPCC SRES)

Regional Change

Scenarios

•Economic Growth

•Technology

•Population Growth

and Migration

•AQ and GHG Policy

Regional

Meteorology

Regional

Emissions

Regional

Air Quality

Regional Boundary Conditions

Meteorological Chemical

Global

Emissions

Global

Air Quality

Global

Meteorology

Global Change

Scenarios

(IPCC SRES)

Regional Change

Scenarios

•Economic Growth

•Technology

•Population Growth

and Migration

•AQ and GHG Policy

Regional

Meteorology

Regional

Emissions

Regional

Air Quality

Regional Boundary Conditions

Meteorological Chemical

Regional Boundary Conditions

Meteorological Chemical

Global

Emissions

Global

Air Quality

Global

Meteorology

Global

Air Quality

Global

Meteorology

Global Change

Scenarios

(IPCC SRES)

Global Change

Scenarios

(IPCC SRES)

Regional Change

Scenarios

•Economic Growth

•Technology

•Population Growth

and Migration

•AQ and GHG Policy
2.1.1 Process for Developing the Global Change-Air Quality Assessment Effort 

In 1997, the EPA’s Global Program underwent a major redirection, including the 

development of a new Strategic Plan in 1999. As part of that effort, the global change-air quality 

assessment was designed. Specifically, a small workgroup was formed, made up of scientists 

knowledgeable about various aspects of the issue, including atmospheric and emissions 

modeling, technology, socioeconomics, climate modeling, and air quality programs. The 

workgroup included members from all of the Labs and Centers involved in the EPA’s Global 

Change Research Program, and input from several offices within OAR was also solicited to help 

guide the effort. An iterative process within the workgroup was used to define the purpose, 

goals, and issues to be addressed; to identify appropriate EPA participants and stakeholders; and 

to develop an initial conceptual framework for organizing the assessment effort, leading to a 

white paper describing the proposed framework and timeline for accomplishing key milestones. 

To review this draft framework and help EPA identify priority research needs, a 

workshop was held in Research Triangle Park, North Carolina in December 2001 that brought 

together technical experts from ORD and OAR, as well as invited international experts. The 

goal of the workshop was to identify the important processes and inputs and to discuss the design 

and implementation of the assessment. Participants included experts in climate modeling, air 

quality modeling, anthropogenic emissions inventory development, and biogenic emissions 

inventory development. The workshop agenda included presentations by a panel of experts on 

regional climate modeling, future emissions inventory development, regional air quality 

modeling, biogenic emissions and wildfires, and socioeconomic and technological change 

projection methods. The workshop participants were assembled into four groups to discuss 

specific issues related to the EPA Global Change Research Program’s objectives: (1) the 

Regional Climate Modeling Group, (2) the Emission Drivers and Anthropogenic Emissions 

Group, (3) the Biogenic Emissions and Wildfires Group, and (4) the Air Quality Modeling 

Group. Each examined charge questions about possible approaches, and each developed 

recommendations for research required to meet the needs of the assessment. Here, the key 

recommendations from the workshop that define the approach used in the assessment are 

summarized (for further details see Appendix C). 

2.2 WORKSHOP RECOMMENDATIONS 

2.2.1 Modeling 

The three key conceptual linkages introduced above, i.e., across spatial scales, temporal 

scales, and disciplines, are embodied in the foundational technical challenge of the assessment: 

linking available modeling tools to span the climate, meteorology, air quality, and human 

dimensions of the problem. As will be described in more detail below, the primary focus of this 

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2007 interim report is the potential for future climate change to impact air quality, independent 

of changes in anthropogenic emissions. The individual research communities use a number of 

different types of models, described in Box 2-1, to study the various aspects of this sub-problem. 

Box 2-1. Climate and Chemistry Modeling Tools 

General Circulation Model (GCM): Comprehensive model of Earth system, including components that 

simulate 3-D flow in atmosphere and ocean, exchange of energy and water with land and ocean surface, and 

growing and melting of ice sheets and sea ice, ultimately in response to amount of solar energy received over time 

across planet; typically operated with horizontal grid spacing of 100-500 km to examine climate variables at 

continental to global scales; most often applied in simulations of how long-term climate statistics evolve over 

years, decades, or centuries in response to past or future changes in outside forcings (e.g., variations in solar 

input, volcanic aerosols, and changes in anthropogenic greenhouse gas emissions). [Note: The use of “GCM” as 

an acronym for “Global Climate Model” and “Global Circulation Model” reflects current usage as well.] 

Global Chemistry and Transport Model (GCTM): Type of model that blends representations of chemical 

reactions and physical chemical transformations with meteorology supplied either from gridded observational 

analyses or a GCM simulation; applied to study how transport by winds, deposition onto or emissions from 

surface, and atmospheric chemistry control long-term distributions of important gases and aerosols within the 

atmosphere (e.g., O3, carbon monoxide, sulfates, and black carbon, among many others); chemistry/transport can 

also be built directly into a GCM for similar applications. 

Regional Climate Model (RCM): Similar to a high-resolution (e.g., 10-50 km) version of a GCM but only 

applied to limited area of globe (e.g., continental United States); designed to capture more accurately role of finescale forcings (e.g., topography, land-surface heterogeneity) and atmospheric processes (e.g., nonlinear dynamics 

of fronts, development of convective rainfall systems) hard to represent at coarse scales of a GCM; derived 

primarily from weather prediction models but including some additional features that allow simulations longer 

than typical several-day timescale of weather forecasts; driven at boundaries by gridded analyses of observational 

data or output from a GCM to study in greater detail how long-term, large-scale climate variability is expressed in 

weather events over shorter timescales and in particular locations. 

Regional Air Quality Model (RAQM): Developed to account for impact of meteorological transport and 

mixing, atmospheric chemistry, and surface deposition/emission of multiple chemical species, particularly 

regulated pollutants; most often applied by air quality management community to evaluate impact of control 

strategies and practices; also frequently used in research mode to develop improved understanding of chemical 

and physical interactions in atmosphere; typically operated on time and space scales characteristic of air pollution 

episodes, i.e., a metropolitan area or larger region over period of a few days.

These different modeling tools have historically been developed for distinct purposes. 

The assessment design reflects the need for bridging the gaps between these standard 

applications to move toward more comprehensive, integrated systems capable of addressing the 

breadth of the problem of potential climate change impacts on air quality. 

As such, one core recommendation that emerged from the workshop was to use these 

tools separately and in combination in multiple modeling approaches to investigate the relevant 

space and time scales and physical/chemical processes governing the connections between 

climate and air quality. These approaches are 

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• Comprehensive modeling approach: This approach uses linked global and regional 

climate and chemistry models to simulate fine regional details of present-day and future 

air quality while simultaneously accounting for global drivers like changes in 

anthropogenic emissions of greenhouse gases. Output from GCM simulations of longterm climate change is used as input into a higher-resolution RCM, which “downscales” 

the climate and meteorological variables to the scales required for input into an RAQM. 

This approach is the most computationally expensive and methodologically complex, 

with concerns such as the length of simulation required to extract a meaningful climate 

change signal from interannual climate variability. 

• Intermediate modeling approach: This approach relies primarily on GCMs and GCTMs 

to capture the broader impacts of climate change on air quality. The emphasis in this 

approach is on the potential for increases or decreases in air pollution events as the 

climate changes over a long simulation period. The results from such modeling work can 

be used to guide the comprehensive modeling approach (e.g., by guiding the selection of 

time periods for the higher-resolution simulations). 

• Sensitivity approach: This approach applies detailed, state-of-the-art RAQMs at regional 

and even urban scales. Rather than a dynamic linkage, air quality simulations are carried 

out by varying key meteorological and emissions parameters to examine the sensitivity of 

the air quality outputs over particular, identified meteorological and air quality episodes. 

The sensitivity approach might permit use of more detailed descriptions of important 

processes, i.e., aerosol processes. 

Initially, the assessment team proposed to move forward primarily with the 

Comprehensive approach. The workshop participants endorsed this plan as effective and 

reasonable, but they also suggested the other two strategies to complement the Comprehensive 

approach and add richness to the assessment. 

Another key model-related discussion was the need to address uncertainty by sampling 

over multiple GCMs, RCMs, GCTMs, RAQMs, as well as the need to examine sensitivities to 

model parameterizations and downscaling methodologies. A critical challenge is to quantify the 

uncertainty produced by the system of linked models required to simulate changes in air quality 

driven by climate change. It was also acknowledged that an important research gap was the 

evaluation of the climate models for their ability to simulate air quality-relevant variables and air 

quality-relevant weather patterns at the appropriate space and time scales. 

Finally, the assessment team was urged to consider in more detail the role of 

hemispheric-scale air pollutant transport and to support the development of appropriate initial 

and boundary conditions for regional-scale air quality modeling efforts. 

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2.2.2 Time Horizon Selected 

A key consideration is the timeframe for building future scenarios and carrying out future 

climate and air quality simulations. It was decided to focus on a time horizon of roughly 2050 in 

order to balance the following considerations: 

Natural meteorological variability versus climate change: Because meteorology varies 

from year-to-year, the signal from the changing climate needs to be relatively strong to discern 

climatically driven effects on air quality. In its Third Assessment Report (TAR) (IPCC, 2001), 

the IPCC projected that global average temperatures could increase from 1.4−5.8oC (2.5−10.4oF) 

by 2100, and that the warming is expected to be larger than the global average for land areas in 

the mid- and high latitude regions. These findings are consistent with the most updated 

projections from the IPCC AR4 (IPCC, 2007). This trend is expected to lead to intermediate 

levels of warming in the intervening decades. For example, the U.S. National Assessment 

(NAST, 2001) based their findings on average U.S. temperature increases of 0.5−2.0oF by 2025, 

1.5−4.0oF by 2050, and 3.0−9.0oF by 2100. Therefore, the longer the timeframe, the stronger the 

climate change signal captured relative to natural interannual and interdecadal variability. 

Uncertainties in GCM climate projections: The IPCC AR4 (IPCC, 2007) documents 

significantly greater divergence in the climate change projections for 2100 compared to 2050, 

largely because the various driving greenhouse gas emissions scenarios from the IPCC Special 

Report on Emissions Scenarios (SRES) (IPCC, 2000) have diverged relatively little by 2050. 

Even though the climate change signal is stronger in 2100, the spread between model projections 

created using different scenarios is not as wide. Choosing 2050 thus constrains somewhat one of 

the potential sources of uncertainty in the assessment. 

Uncertainties in the assumptions concerning long-term change in emissions drivers: The 

uncertainty in projections of economic growth, patterns of land-use and land-cover change, 

energy use, migration, transportation patterns, and technological development needed to develop 

projections of anthropogenic emissions increases significantly over longer time horizons. An 

assessment timeframe of, e.g., 2100, would likely be too speculative for practical application to 

current air quality management planning. 

Current EPA decision processes: In areas such as investment in electricity production, 

motor vehicle emissions, and power plant and fleet replacement, the EPA already makes air 

quality management decisions with long lead times of one to several decades. Therefore, a time 

horizon of the next half-century for assessing the potential consequences of climate change on air 

quality is consistent with this planning timescale. 

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2.2.3 Dual-Phase Assessment Approach 

It is well recognized that anthropogenic emissions levels are a dominant factor in 

determining air quality, as evidenced by the dramatic improvements that took place with the 

implementation of emissions controls beginning in the mid-20th Century in the United States and 

other developed countries. Understanding how changes in air quality due to changing climate 

might confound long-term management of these emissions for NAAQS attainment and 

maintenance is a critical assessment goal. To more readily achieve this understanding, a second 

core recommendation from the workshop was to investigate possible regional air quality 

responses to future climate and meteorological changes alone, before tackling the additional 

complexities of accounting for changes in other aspects of the system, such as anthropogenic 

emissions and long-range pollutant transport. 

The assessment research program was, therefore, designed in two phases. Phase I 

focuses on developing tools, capabilities, and a knowledge base, and then applying these in 

research to address the impacts of climate change on air quality with anthropogenic emissions 

held constant between present and future. Phase II builds on the insights from Phase I, by 

extending the capabilities of the modeling systems developed therein (e.g., to more 

comprehensively explore uncertainties, encompass additional pollutants, and investigate climate 

and air quality feedbacks) and by adding the effects of changing patterns of anthropogenic 

emissions (e.g., due to population, land-use, and energy and transportation technologies 

changes). In this second phase, emissions will be projected into the future, accounting for factors 

such as differential population growth and migration, economic growth, and technology change. 

As described in Section 1.4.2, the major focus of this interim assessment report is the 

progress to date under Phase I, presented in Section 3. The Phase II work will be the subject of 

follow-on reports. A summary of research efforts already ongoing to support Phase II is 

provided in Section 4. 

One of the key challenges in executing the comprehensive approach described in 

Section 2.2.1 lies as much with maintaining logical consistency in linking the many models as 

with the technical difficulties of simulating changes to 2050. The O3 simulations in Phase I, 

reflecting the climate in 2050, have been accomplished while holding air pollution emissions 

constant at present-day levels. In the strictest sense, this therefore introduces an internal 

inconsistency, i.e., between emissions of greenhouse gases and those of conventional air 

pollutants, which in reality are coupled. The model results from this first phase serve as 

sensitivity tests—to determine the potential effect on climate-induced meteorological changes on 

air quality and to better understand the characteristics of the linked modeling systems—and 

cannot be construed in any way as future predictions. 

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2.2.4 Research Priorities to Support Phase II 

Finally, we briefly summarize some key workshop recommendations on additional 

research needed to support Phase II of the assessment. 

Processes governing biogenic emissions: Algorithms will have to be developed that 

describe chemical emissions of major vegetative species response to climate change for use in 

current and biogenic emission forecasting. Projections of land-use changes will have to be 

integrated with forest physiological models to project current and future biogenic VOC 

emissions. 

Wildfires: There is a need to develop methods to define fire emissions as a function of 

fire intensity, extent, and frequency. Simultaneously, there is a need to develop methods to 

relate fire intensity, extent, and frequency to current and future land use, land management, fuel 

loading, socioeconomic conditions, and climate. 

Anthropogenic emissions projections: Plausible scenarios for future emissions need to be 

developed that account for changes in urbanization, population growth, migration, 

industrialization, fuel, technology, etc. Also needed is normalization of procedures for emissions 

calculations across regions and countries and reconciliation between global and regional 

emission inventories. Principles of downscaling socioeconomic scenarios to more detailed 

geographic scales must be applied. There is also a need to incorporate feedbacks of climate 

change on energy use, economic development, land use, and migration. 

Air quality modeling: Improvements in our ability to model the chemistry of air pollution 

in a number of areas will be required to more accurately simulate the influence of climate change 

on air quality. These areas include representations of aerosol physical and chemical processes, 

two-way linkages between climate/meteorology models and air quality models, the availability 

of suitable initial and boundary conditions for all important chemical species, and stratospheretroposphere exchange. 

2.3 RESEARCH PARTNERSHIPS 

To implement the workshop recommendations and achieve the goals of the assessment, 

the EPA’s Global Change Research Program designed a joint intramural and extramural research 

program. The goal is to harness the unique capabilities of the EPA research laboratories and the 

academic community to build a broad program. 

Within the EPA’s intramural effort, the National Exposure Research Laboratory (NERL) 

is the primary developer of the Community Multiscale Air Quality (CMAQ) model that predicts 

air quality pollutant transport and fate (Byun and Schere, 2006). CMAQ, which, as of December 

2006, has undergone three external peer reviews, is being used by the Office of Air Quality 

Planning and Standards (OAQPS) within OAR for current rulemakings, as well as by the 

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research community for a range of research applications including climate and air quality 

interactions. Via a partnership between EPA and NOAA, a team at NERL is charged under this 

assessment with leading the development of a series of regional-scale air quality simulations 

using CMAQ under current and future climate scenarios. This effort, the Climate Impacts on 

Regional Air Quality (CIRAQ) project, was initiated in 2002 following the above-mentioned 

workshop. This team provides the air quality modeling expertise to develop these simulations, to 

interpret the sensitivity of air quality to the future climate changes simulated, and to consider 

regulatory implications of potential changes in air quality. 

In addition, NERL researchers are key contributors to the development of models of 

environmentally influenced emissions from the air-surface interface for regional and global 

emissions inventories and application to air quality modeling, such as biogenic emissions (the 

Biogenic Emission Inventory System; BEIS) and wildfire emissions (based on the Blue Sky 

wildfire model). NERL was also the primary ORD collaborator in the development of the Sparse 

Matrix Operator Kernel Emission (SMOKE) modeling system. SMOKE assembles input data 

from anthropogenic emission inventories, and biogenic, mobile, and wildfire emission models 

into the hourly, gridded, speciated form required by air quality models such as CMAQ. These 

emissions models are needed for both retrospective and future air quality modeling scenarios. 

More information on aspects of the NERL effort is contained in Appendix E. 

Simultaneously, researchers in the National Risk Management Research Laboratory 

(NRMRL) are focused on evaluating the potential impact of technological evolution on futureyear air pollutant emissions, in coordination with the NERL efforts. This process involves 

characterizing future energy demands and technologies, and using this information within energy 

system models to estimate emissions over a wide range of alternative scenarios. In addition, 

NRMRL researchers have developed a suite of analytical and visualization tools for examining 

the flexibility available in meeting future emission targets and for evaluating sensitivity to 

uncertainties in model parameters and inputs. NRMRL is applying these methods and tools to 

examine the system-wide implications on fuel use and emissions of the penetration of new 

transportation and electric generation technologies. This work directly addresses the need, 

identified in the 2001 workshop, to develop realistic future emissions scenarios that are 

regionally plausible and also consistent with assumptions about global trends. Together, NERL 

and NRMRL have the expertise required to contribute crucially to both Phase I and Phase II of 

the overall assessment. For additional information, see Appendix F and Section 4. 

The assessment effort benefits substantially from a strong collaboration with the 

extramural research community. The EPA’s National Center for Environmental Research 

(NCER), through its competitive Science To Achieve Results (STAR) grants program, funded a 

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number of leading university research groups through the following Requests for Applications 

(RFAs): 

• 2000: Assessing the Consequences of Interactions between Human Activities and a 

Changing Climate

• 2002: Assessing the Consequences of Global Change for Air Quality: Sensitivity of U.S. 

air quality to climate change and future global impacts

• 2003: Consequences of Global Change for Air Quality: Spatial Patterns in Air Pollution 

Emissions

• 2004: Regional Development, Population Trend, and Technology Change Impacts on 

Future Air Pollution Emissions

• 2005: Fire, Climate and Air Quality

• 2006: Consequences of Global Change for Air Quality

These RFAs, most of which derive from the recommendations of the 2001 workshop, 

encompass roughly 25 projects, totaling over $20 million, covering topics including projection of 

population, development, and transportation trends; observations of biosphere-air quality 

interactions; coupled climate and air quality modeling; and human health effects. Many of the 

current projects involve collaboration across disciplines to link models. All of this is emblematic 

both of the breadth of the issue and EPA’s commitment to build and populate a comprehensive 

framework to address it. Further details are provided in Appendix D. 

Finally, the National Center for Environmental Assessment (NCEA) has unique expertise 

in preparing the air quality criteria documents upon which the NAAQS are based, conducting 

environmental assessments, and performing synthetic analyses of the type presented in Section 3. 

NCEA’s global change assessment team has the primary responsibility for developing the reports 

synthesizing the results of the broad inter-laboratory and extramural research effort represented 

in this assessment. 
3 RESULTS AND SYNTHESIS 

3.1 INTRODUCTION 

The goal of this section is to synthesize the EPA, and EPA-funded, climate and air 

quality modeling research that has emerged in Phase I of the assessment. The material presented 

here is intended to map onto each of the two intertwining readings introduced in Section 1, i.e., 

“science” and “policy,” that run through the report and reflect the two “grand challenges” of 

evaluating the state of the science and providing a foundation on which effective decision 

support can be built. 

Section 3.2 provides brief summaries of activities and key findings to date from each of 

the participating modeling groups. Section 3.3 attempts to assess the larger meaning of the 

groups’ results when they are examined all together, focusing on inter-group comparisons of the 

simulation outputs that are largely common to all (or most)—it provides a preliminary synthesis 

by taking a broad view across this subset of assessment results. Section 3.4 discusses the 

challenges and uncertainties associated with the modeling approach that underpins the 

assessment. 

As the EPA’s assessment activities continue, overall understanding will grow richer and 

techniques will become more refined. Thus, it will be possible to build on the foundation 

provided by this first attempt to interpret this evolving body of work. 

3.2 SUMMARY OF RESULTS FROM INDIVIDUAL GROUPS 

Results discussed throughout the rest of this section are drawn from the intramural, EPA 

work, as well as from several STAR-funded extramural initiatives. More detailed descriptions of 

the experimental designs and results of the extramural (Appendix D) and intramural (Appendix 

E) efforts are given in the appendices to this report. 

The projects highlighted here largely share similar fundamental goals and approaches and 

can be divided into two major groups: (1) those that, to date, have primarily used global climate 

and chemistry models to focus on the large-scale changes in future U.S. air quality,9

 and (2) 

those that have used nested, high-resolution, global-to-regional modeling systems to focus on the 

regional details of the potential future changes.10 All of these projects adapt existing modeling 

tools (as described in Section 2) as components for assembling their systems, including GCTMs, 

 9

 The Harvard University and Carnegie Mellon University teams. 

10 The EPA National Exposure Research Laboratory (NERL), Columbia University, University of Illinois, 

Washington State University, University of California, Berkeley, and Georgia Institute of Technology (GIT)-

Northeast States for Coordinated Air Use Management (NESCAUM)-Massachusetts Institute of Technology (MIT) 

teams. 

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GCMs, RCMs, and RAQMs, along with emissions models and a number of boundary and initial 

conditions datasets. They all apply these modeling systems in numerical experiments designed 

broadly to investigate the impacts of future global climate change on U.S. air quality for presentday and future time periods. 

It is important to consider both the global model simulations and the downscaled regional 

simulations together, because each method has its strengths and weaknesses. The global models 

simulate the whole world in an internally self-consistent way across both climate and chemistry, 

but because of computational demand must use coarse spatial resolution, thereby potentially 

missing or misrepresenting key processes. Dynamical downscaling with an RCM dramatically 

increases the resolution and process realism for the region of interest, but at the expense of 

introducing lateral boundary conditions into the simulation. Section 3.4 provides additional 

discussion of these relative advantages and trade-offs. Examining both sets of results gives us a 

more complete picture of the overall climate-air quality system. 

In addition to any similarities in approach, however, each project brings unique and 

complementary differences in emphasis to these tasks. In aggregate, these differences add 

greatly to the richness of the overall assessment. Below are brief summaries of selected key 

themes and findings from each of these research efforts as a prelude to the more focused intergroup comparisons of Section 3.3. 

3.2.1 GCTM-Focused Modeling Work 

3.2.1.1 Application of a Unified Aerosol-Chemistry-Climate GCM to Understand the 

Effects of Changing Climate and Global Anthropogenic Emissions on U.S. Air 

Quality: Harvard University 

In early work for this project, the Harvard research group examined the role of potential 

changes in atmospheric circulation by carrying out GCM simulations, using the Goddard 

Institute for Space Studies (GISS) GCM version II′, for the period 1950−2052, with tracers 

representing carbon monoxide (CO) and black carbon (BC) (Mickley et al., 2004). They based 

the concentrations of greenhouse gases for the historical past on observations, while future 

greenhouse gases followed the A1b IPCC SRES scenario. A key result from these simulations is 

a future 10% decrease in the frequency of summertime mid-latitude surface cyclones moving 

across southeastern Canada and a 20% decrease in cold surges from Canada into the Midwest. 

Since these events typically clear air pollution in the Midwest and Northeast, pollution episodes 

in these regions increase in duration (by 1−2 days) and intensity (by 5−10% in pollutant 

concentration) in the future. These simulated future circulation changes are consistent with 

findings from some other groups in the broader climate modeling community, and the Harvard 

model also successfully reproduces the observed 40% decrease in North American cyclones from 

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1950−2000. These results are supported, and expanded upon, by more recent work from this 

group, e.g., see Leibensperger et al. (2008), who found that the frequency of mid-latitudes 

cyclones tracking across eastern North America in the southern climatological storm track was a 

strong predictor of the frequency of summertime pollution episodes in the eastern United States 

for the period 1980−2006. In addition, they found a decreasing trend over this period in the 

number of cyclones in this storm track that they attributed to greenhouse warming, consistent 

with a number of other observational and modeling studies. However, as will be discussed in 

more detail below, other groups, including those participating in this assessment, do not 

necessarily find the same decrease in future mid-latitude cyclones when analyzing similar GCM 

outputs, or even the same GCM outputs downscaled using an RCM (e.g., see Leung and 

Gustafson, 2005). 

Subsequent to the initial modeling effort described in Mickley et al. (2004), the Harvard 

group applied the GEOS-Chem GCTM, driven by the GISS III GCM (Wu et al., 2007), to the 

direct simulation of 2050s O3 air quality over the United States (Wu et al., 2008a) and global 

tropospheric O3 and the policy-relevant background O3 over the United States (Wu et al., 2008b). 

For one set of simulations with this modeling system designed to isolate the impacts of climate 

change alone on air quality, anthropogenic emissions of precursor pollutants were held constant 

at present-day levels, while climate changed in response to greenhouse gas increases under the 

IPCC A1b scenario (Wu et al., 2008a). Climate-sensitive natural emissions, e.g., of biogenic 

VOCs, were allowed to vary in response to the change in climate. In these simulations, they 

found that at global scales, future O3 averaged throughout the depth of the troposphere increases, 

primarily due to increases in lightning (leading to additional NOx production), but near the 

surface increases in water vapor generally caused O3 decreases, except over polluted continental 

regions. Focusing in more detail on the United States, they found that the response of O3 to 

climate change varies by region. Their results show increases in mean summertime O3

concentrations of 2−5 ppb in the Northeast and Midwest, with little change in the Southeast. The 

Harvard group also found that peak O3 pollution episodes are far more affected by climate 

change than mean values, with effects exceeding 10 ppb in the Midwest and Northeast. 

In contrast to this regional pattern of future U.S. O3 change, the Carnegie Mellon work 

(described next) found a relatively smaller response in the Northeast and Midwest but a strong 

increase in the Southeast, using some similar models and assumptions as the Harvard project 

(although with a different IPCC greenhouse gas scenario and some key differences in the ocean 

surface boundary condition). As will be discussed in greater detail below, the explanations for 

these differences appear to reside in (1) differences in how the chemical mechanisms regulating 

the reactions and transformation of biogenic VOC emissions are represented in the two modeling 

systems and (2) possible differences in future simulated mid-latitude storm track changes. 

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In addition to these findings, this group used historically measured relationships between 

temperature and the probability of O3 concentrations above the air quality standard (e.g., see Lin 

et al., 2001), together with statistically downscaled climate projections for the Northeast United 

States from an ensemble of IPCC AR4 GCMs and scenarios, to project future O3 exceedances in 

the region (Lin et al., 2007). They found a doubling of the frequency of exceedances in the 

climate of the 2050s if anthropogenic emissions were to remain constant. As will be discussed 

further below, statistical relationships between observed O3 and temperature reflect both the 

direct impact of temperature on O3 chemistry and the often strong correlation between 

temperature and other factors conducive to high O3 concentrations, such as clear skies, stagnant 

air, and increased biogenic emissions. As such, they tend to be regionally and seasonally 

dependent. Work exploring the use of these types of statistical approaches to project O3 NAAQS 

exceedances (and PM concentrations) is ongoing. 

As a final part of this project, the Harvard group has developed, and is in the process of 

testing, a linked global-to-regional system of models (including a GCM, GCTM, RCM, and 

RAQM). This system will be applied to investigations of the effects of climate change, as well 

as future changes in pollutant emissions and long-range transport, on regional-scale O3 and PM 

concentrations and mercury (Hg) deposition. 

Additional information on the Harvard research effort can be found in Appendix D and at 

• http://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.abstractDetail/abstract/6

157/report/0

• http://www.as.harvard.edu/chemistry/trop/gcap/

3.2.1.2 Impacts of Climate Change and Global Emissions on U.S. Air Quality: 

Development of an Integrated Modeling Framework and Sensitivity Assessment: 

Carnegie Mellon University 

The Carnegie Mellon group performed global-scale simulations of atmospheric chemistry 

under present and future (2050s) climate conditions using a “unified model,” i.e., the GISS II′

model modified to incorporate tropospheric gas phase chemistry and aerosols. Ten years of both 

present and future climate were simulated, following the A2 IPCC greenhouse gas emissions 

scenario, with anthropogenic air pollution emissions held at present-day levels to isolate the 

effects of climate change. As in the Harvard project described above, the effects of changes in 

certain climate-sensitive natural emissions were also included as part of the “climate” changes 

simulated. 

They found that a majority of the atmosphere near the Earth’s surface experiences a 

decrease in average O3 concentrations under future climate with air pollution emissions held 

constant, mainly due to the increase in humidity, which lowers O3 lifetimes (Racherla and 

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Adams, 2006). Further analysis of these results on a seasonal and regional basis found that, 

while global near-surface O3 decreases, a more complex response occurs in polluted regions. 

Specifically, summertime O3 increases over Europe and North America, with larger increases for 

the latter. A second key finding is that the frequency of extreme O3 events increases in the 

simulated future climate: over the eastern half of the United States, where the largest simulated 

future O3 changes occurred, the greatest increases were at the high end of the O3 distribution, and 

there was increased episode frequency that was statistically significant with respect to 

interannual variability (Racherla and Adams, 2008). They further suggested that it is necessary 

to simulate a minimum of five present-day and future years to separate a climate change 

response from this interannual variability. These general results are broadly consistent with the 

Harvard experiments described above. However, as also mentioned, there are important regional 

differences in response between the two groups. These can largely be attributed to differences in 

the modeled chemical mechanism for isoprene oxidation in the southeastern United States, as 

well as possibly differences in the future simulation of the summertime storm track across the 

northern part of the country. These issues will be discussed in more detail in the synthesis to 

follow these summaries. 

The Carnegie Mellon team is also pursuing two complementary approaches in 

conjunction with their global modeling efforts. First, they are investigating the sensitivity of O3, 

PM, acid deposition, and visibility to individual meteorological parameters by performing a set 

of sensitivity experiments using the PM Comprehensive Air Quality Model with Extensions 

(PMCAMx) (e.g., see Dawson et al., 2007a, b). One key finding from this work is that O3

concentrations increased nearly linearly with temperature in the study region/period, and that a 

2.5oC increase in temperature led to a 30% increase in the area exceeding the EPA 8-hour 

standard. Second, they have now developed and tested a global-to-regional modeling system to 

carry out higher-resolution investigations of the impacts of climate and anthropogenic emissions 

changes on air quality (Dawson et al., 2008). 

Additional information on this research effort can be found in Appendix D and at 

• http://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.abstractDetail/abstract/6

240/report/0

• http://www.ce.cmu.edu/~adams/index.html

• http://www.cheme.cmu.edu/who/faculty/pandis.html

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3.2.2 Linked Global-Regional-Focused Modeling Work 

3.2.2.1 The Climate Impacts on Regional Air Quality (CIRAQ) Project: EPA 

In addition to the extramural projects described in this section, an intramural modeling 

study, the CIRAQ project, is being conducted at EPA NERL, as introduced in Section 2. Under 

this project, the NERL team built a coupled global-to-regional climate and chemistry modeling 

system covering the continental United States. They used the output from a global climate 

simulation with the GISS II′ model (including a tropospheric O3 chemistry model) for 

1950−2055, following the A1b IPCC SRES greenhouse gas emissions scenario for the future 

simulation years (i.e., the same simulation described in Mickley et al., 2004) as climate and 

chemical boundary conditions for the regional climate and air quality simulations. The Penn 

State/NCAR Mesoscale Model Version 5 (MM5) was used at DOE’s Pacific Northwest National 

Laboratory (PNNL) to create downscaled fields from this GCM simulation for the periods 

1996−2005 and 2045−2055 (Leung and Gustafson, 2005). The NERL group used this regionally 

downscaled meteorology to simulate air quality for 5-year-long subsets of these present and 

future time periods with the CMAQ model. Multiple years were simulated, in spite of the 

considerable computational expense, to examine the role of interannual variability in the results. 

A key element of this project was extensive evaluations of the simulated meteorological 

variables, not just for long-term climate statistics (e.g., monthly and seasonal means), but of 

synoptic-scale patterns that can be linked more directly to air quality episodes (Cooter et al., 

2005; Gilliam et al., 2006; Gustafson and Leung, 2007). One important finding was that the 

subtropical Bermuda High pressure system off the southeastern United States coast, a critical 

component of eastern United States warm season weather patterns, was not well simulated in the 

downscaled model runs, a result that is likely attributable to biases in the GCM, as will be 

discussed further below. Another key finding was that, as mentioned above in the summary of 

the Harvard project, the reduction in cyclones tracking across the northern United States found in 

Mickley et al. (2004) was not as clearly present when this global model output was downscaled 

using MM5 (Leung and Gustafson, 2005). 

The NERL team also evaluated the CMAQ results against historical O3 observations, 

finding high biases in summertime O3 related to the choice of chemical mechanism in CMAQ 

between the Carbon Bond-IV (CB-IV) vs. the Statewide Air Pollution Research Center (SAPRC) 

representations. In addition, they found O3 biases related to biases in MM5-downscaled 

meteorology. For example, the model under-predicted precipitation and over-predicted 

temperature in the areas of the Midwest and Southeast where O3 was most over predicted, 

highlighting the strong control that meteorology can exert on O3. 

In a set of future simulations with this global-to-regional climate and air quality modeling 

system, for which anthropogenic emissions of precursor pollutants were held constant while 

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climate changed, the NERL group found increases in future summertime maximum daily 8-hour 

(MDA8) O3 concentrations of roughly 2−5 ppb in some areas (e.g., Northeast, Mid-Atlantic, and 

Gulf Coast) compared to the present-day, though with strong regional variability and even 

decreases in some regions (Nolte et al., 2008). This regional variability in future O3

concentration changes was associated primarily with changes in temperature, the amount of solar 

radiation reaching the surface, and, to a lesser extent, climate-induced changes in biogenic 

emissions. The increases in peak O3 concentrations tended to be greater and cover larger areas 

than those in mean MDA8 O3. These results will be discussed in more detail in the synthesis 

below. The NERL team also found significant O3 increases in September and October over large 

portions of the country, suggesting a possible extension of the O3 season into the fall in the 

future. 

Additional information on the NERL effort can be found in Appendix E and at 

http://www.epa.gov/asmdnerl/Climate/index.html. 

3.2.2.2 Modeling Heat and Air Quality Impacts of Changing Urban Land Uses and 

Climate: Columbia University 

The Columbia group built a linked air quality modeling system based on the GISS 

Atmosphere-Ocean (AO) GCM (Russell et al., 1995) and the MM5 RCM and carried out 

simulations using two SRES greenhouse gas scenarios (A2 and B2) for 5 summers each during 

the 1990s, 2020s, 2050s, and 2080s, focusing on the eastern half of the continental United States. 

Additional simulations using higher resolution were carried out for the New York City metro 

area for particular meteorological/air quality episodes. One important feature of the Columbia 

effort is that the team carried the air quality modeling results through to an assessment of human 

health endpoints. 

A key aspect of the Columbia team’s work was the evaluation of the performance of this 

coupled modeling system. They found that (1) dynamical downscaling with MM5 reduces 

biases present in the GCM simulation, most strongly for temperature and less so for precipitation 

and (2) there is a strong sensitivity of climate to the choice of RCM parameterizations, e.g., the 

cumulus convection scheme (e.g., see Lynn et al., 2004). In addition, the downscaled results 

were often quite different from those of the driving GCM, including, for example, warmer 

summers. For O3, they found that their modeling system was able to simulate synoptic and 

interannual variability reasonably well, including the frequency and duration of extreme O3

events, but underestimated variability on shorter time scales (Hogrefe et al., 2004a). 

In future climate change simulations (with anthropogenic emissions of air pollutants held 

constant at present-day levels), the Columbia group found summertime O3 increases of 2−8 ppb 

across broad swathes of the Midwest and Mid-Atlantic (Hogrefe et al., 2004b). Significant 

3-7
effects were already seen by the 2020s, with greater increases by the 2050s and 2080s. One 

exception was certain geographic areas that experienced increases in mixed layer depths and 

convective activity in the 2080s, changes that actually ended up decreasing O3, illustrating the 

complexity of the climate-meteorology-O3 relationship. In general, the spatial correlation of O3

increases with any one meteorological variable was not particularly strong in their results. Again 

the largest future increases in O3 were for the highest-concentration O3 episodes, leading to large 

increases in hypothetical exceedances concentrated in the Ohio Valley and the Mid-Atlantic 

coast. They also found an increase in the duration of high-O3 events. The effect of climate 

change in 50 eastern U.S. cities, without considering future changes in air pollution emissions, 

was to increase the number of days exceeding the 8-hour O3 standard by 68% (Bell et al., 2007). 

These model results also showed future increases in biogenic VOC emissions in most 

places as a result of climate change, with the largest absolute increases in the southern and 

southeastern parts of the United States. While biogenic emissions changes were responsible for 

up to half of the total climate effect on O3 concentrations in some parts of the Ohio Valley and 

Mid-Atlantic further to the north, they did not produce significant O3 changes in these more 

southern areas that experienced the largest changes in these emissions. The impact of how 

biogenic emissions chemistry is represented in air quality modeling systems on simulated O3 is 

discussed in more detail in the synthesis below. 

Finally, an analysis of the effects of land-use change on O3 (and heat waves) in the 

smaller New York City metro region suggests that such changes could also have local impacts of 

comparable magnitude to the climatic, emissions, and boundary conditions factors considered 

(Civerolo et al., 2007). 

For more information on the Columbia team’s efforts, see Appendix D and 

• http://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.abstractDetail/abstract/8

12/report/0

• http://www.mailman.hs.columbia.edu/ehs/research.html

• http://www.geography.hunter.cuny.edu/luca/

• http://www.cmascenter.org/2003_workshop/session2/hogrefe_abstract.pdf

3.2.2.3 Impacts of Global Climate and Emission Changes on U.S. Air Quality: University 

of Illinois 

The University of Illinois group focused on exploring and evaluating, as comprehensively 

as possible, the capabilities and sensitivities of the tools and techniques underlying the full, 

global-to-regional model-based approach to the problem. They concentrated on building a 

system that accounts for global chemistry and climate, and regional meteorology and air quality, 

capable of simulating effects of climate changes, emissions changes, and long-range transport 

3-8
changes on regional air quality for the continental United States (Huang et al., 2007; 2008). To 

capture a wider range of sensitivities, they built different versions of this system, which 

combines multiple GCMs (PCM and the Hadley Centre Model, HadCM3), SRES scenarios 

(A1Fi, A2, B1, B2), and convective parameterizations (the Grell and Kain-Fritsch schemes) with 

the Model for OZone And Related chemical Tracers (MOZART) GCTM, a modified version of 

the MM5 RCM (referred to as CMM5), and the SARMAP11 Air Quality Model (SAQM). They 

also made considerable efforts to evaluate both climate and air quality variables with respect to 

historical observations and to understand the implications of these evaluations for simulations of 

future changes. 

Several important findings emerge from this group’s model evaluation efforts. First, they 

demonstrated that any individual GCM will likely have significant biases in temperature, 

precipitation, and circulation patterns, as a result of both parameterizations and internal model 

variability, so multi-model ensemble means will tend to be more accurate than individual models 

(Kunkel and Liang, 2005). With proper attention, RCM downscaling can improve on these 

GCM biases in climate variables over different temporal scales (e.g., diurnal, seasonal, 

interannual), due to higher resolution and more comprehensive physics, and that furthermore the 

RCM can produce future simulations of temperature and precipitation patterns that differ 

significantly from those of the driving GCM (e.g., Liang et al., 2006). They found that the 

improvements in present-day climate generally led directly to improvements in simulated air 

quality endpoints, though they also found that the performance of their modeling system tended 

to be better for monthly and seasonal average O3 concentrations than for multi-day high-O3

episodes, reflecting the primary use for which the driving climate models have been designed 

(Huang et al., 2007). In addition, they found a high sensitivity of downscaled climate (and 

downscaling skill) to the convective scheme chosen, with different parameterizations working 

better in different regions/regimes (Liang et al., 2007). This sensitivity strongly affects 

simulated air quality, for example by altering meteorology and hence also biogenic emissions 

(Tao et al., 2008). All of these findings are consistent with, and expand considerably upon, the 

results from the Columbia project described above. 

Notably, the Illinois team also found that the different patterns of GCM biases with 

respect to present-day observations in different simulations, as well as the way the RCM 

downscaling altered these biases, were consistently reflected in the future GCM and GCM-RCM 

differences as well. This suggests a strong link between the ability of a GCM or GCM-RCM 

downscaling system to accurately reproduce present-day climate and the type of future climate it 

simulates (Liang et al., 2008). 

 11 SARMAP stands for the San Joaquin Valley Air Quality Study (SJVAQS)/Atmospheric Utility 

Signatures, Predictions, and Experiments (AUSPEX) Regional Model Adaptation Project. 

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In future simulations with their coupled global-to-regional modeling system completed to 

date, based on PCM GCM simulations following both the A1Fi and B1 SRES greenhouse gas 

scenarios, the Illinois group found changes in O3 due to climate change alone (i.e., with 

anthropogenic pollutant emissions held constant at present-day levels) that were of comparable 

magnitude to those seen by the NERL and Columbia groups, though with differences in regional 

spatial patterns (Tao et al., 2007). These similarities and differences will be described in greater 

detail in the synthesis below. The larger greenhouse gas concentrations, and hence greater 

simulated climate change, associated with the A1Fi scenario generally resulted in larger future 

O3 increases than for the climate change simulation driven by the B1 scenario. 

For more information on the Illinois group’s efforts, see Appendix D and 

• http://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.abstractDetail/abstract/6

160/report/0

• http://www.sws.uiuc.edu/atmos/modeling/caqims/

3.2.2.4 Impact of Climate Change on U.S. Air Quality Using Multi-Scale Modeling with 

the MM5/SMOKE/CMAQ System: Washington State University 

Similar to the NERL, Columbia, and Illinois groups, the Washington State team 

developed a combined global and regional climate and air quality modeling system to investigate 

changes in O3 (and PM) (Chen et al., 2009; Avise et al., 2009). They used the PCM, MM5, and 

CMAQ models, and they focused on the IPCC A2 scenario for future greenhouse gases. With 

this system, the Washington State group investigated climate and air quality changes for the 

continental United States as a whole, and in addition focused in more detail on two specific 

regions: the Pacific Northwest and the northern Midwest. A key distinguishing feature of their 

effort is the attention to biogenic emissions and the consideration of land cover changes (both 

vegetation cover and urban distributions), as well as changes in the frequency of wildfires in 

their simulations. Evaluations of their coupled system against observations indicated reasonable 

agreement with observed climatology and O3 concentrations in their two focus regions. They 

also examined wet and dry deposition rates and found qualitatively similar results between 

modeled and measured rates in the Pacific Northwest. 

In five years of simulated summertime O3 under both present-day and future climate 

conditions (with constant anthropogenic precursor pollutants), the Washington State group found 

future O3 increases in certain regions, most notably in the Northeast and Southwest, with smaller 

increases or slight decreases in other regions (Avise et al., 2009). These climate change effects 

were most pronounced when considering the extreme high end of the O3 concentration 

distribution. The magnitude of the O3 increases found by the Washington State group (i.e., a few 

to several ppb) were roughly comparable to those found by the other regional modeling groups 

3-10
already discussed, though again with differences in the specific regional spatial patterns of the 

future changes, linked to differences in the spatial patterns of key O3 drivers, discussed in more 

detail in the synthesis below. 

In addition, by accounting for plausible future changes in land-use distribution, they 

simulated both net decreases and increases in biogenic emission capacity, depending on region: 

i.e., they found that reductions in forested area in the Southeast and West due to increases in 

development more than offset potential increased biogenic emissions due to climate change, 

leading to reduction in MDA8 O3 levels, while enhanced use of poplar plantations for carbon 

sequestration significantly increased isoprene emissions in the Midwest and eastern United 

States, leading to O3 increases. Finally, they found that warmer and drier conditions in their 

future simulations yielded increased occurrences of fire in the western states. 

Additional information on this group’s effort can be found in Appendix D and at 

• http://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.abstractDetail/abstract/6

229

• http://www.nwairquest.wsu.edu

3.2.2.5 Guiding Future Air Quality Management in California: Sensitivity to Changing 

Climate—University of California, Berkeley 

Distinct from that of the other groups described above, the Berkeley group’s research 

focused in detail on central California, using a combination of model and observation-based 

analyses to determine the effects on air quality of changes in temperature, humidity, atmospheric 

mixing, and biogenic and anthropogenic emissions changes. 

Specifically, the Berkeley group used CMAQ at very high resolution (4 km horizontal 

grid spacing), driven by MM5, to investigate the effects of perturbations in these drivers on O3

concentrations during a 5-day O3 episode in the state (Steiner et al., 2006). They derived 

plausible, spatially resolved future changes in summertime temperatures from two simulations 

with the Community Climate Model version 3 (CCM3) GCM downscaled to a 40 km grid 

spacing for the western United States: one with a “pre-industrial” CO2 concentration of 280 

parts per million (ppm) and one representing a hypothetical 2050 climate with a doubled CO2

concentration of 560 ppm (Snyder et al., 2002). The average August temperature difference 

between these two downscaled simulations at each point in the domain was added to the MM5 

meteorological output used to drive CMAQ. This temperature perturbation was applied in an 

uncoupled manner so as not to affect other meteorological quantities such as wind speed and 

boundary layer height, to isolate the impact of temperature changes on chemical reaction 

kinetics. This imposed temperature increase was also used to derive perturbations of humidity 

and biogenic VOC emissions for additional, separate sensitivity experiments. In addition to 

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these climate-based changes, the Berkeley group carried out simulations to investigate the 

sensitivity of O3 to changes in anthropogenic NOx and VOC emissions, as well as to the inflow 

of pollutants from outside the state. 

They found that higher temperatures increased O3 concentrations in this simulated 

pollution episode both directly (through increased reaction rates) and indirectly (through 

increases in biogenic emissions). Across all the different effects explored, they found that O3

sensitivity varied depending on proximity to the Pacific Coast (e.g., where impacts of increased 

pollution at the inflow boundary are greatest), and on preexisting NOx or VOC levels (e.g., 

NOx-saturated regions in central California appear to be most sensitive to climate-related 

changes). 

The Berkeley team also conducted an observationally based study of the temperature 

sensitivity of anthropogenic VOC emissions: the role of temperature in increasing fuel 

evaporation was highlighted in this analysis (Rubin et al., 2006). Increased evaporation was 

apparent in observed correlations between speciated VOCs and temperatures as they varied by 

time of day and from day to day, with implications for the climate sensitivity of these emissions. 

Additional information about the Berkeley project can be found in Appendix D and at 

http://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.abstractDetail/abstract/6231/rep

ort/0. 

3.2.2.6 Sensitivity and Uncertainty Assessment of Global Climate Change Impacts on 

Ozone and Particulate Matter: Examination of Direct and Indirect, EmissionInduced Effects: GIT-NESCAUM-MIT 

Similar to the NERL, Columbia, Washington State, and Illinois groups discussed above, 

the GIT-NESCAUM-MIT group constructed a linked global-to-regional climate and air quality 

modeling system to investigate the impacts of global change on regional U.S. O3 and PM 

concentrations (Tagaris et al., 2007; Liao et al., 2007). Specifically, they used CMAQ, driven by 

present-day and future climate simulations with the GISS II′ GCM downscaled using MM5 (the 

same MM5-downscaled GISS II′ GCM simulations developed for the NERL project described 

above). However, compared to these other groups, they had a unique focus on understanding the 

climate sensitivity of regional air quality in the context of expected future pollutant emissions 

under the implementation of current and future control strategies. This effort not only 

investigated O3, but also PM and its speciated components of sulfates, nitrates, ammonium, and 

organics, in detail. A strong, built-in link between the academic and regional air quality 

management communities is achieved via the inclusion of NESCAUM in the partnership. 

Their work to date attempts to determine if climate change will have significant impacts 

on the efficacy of O3 and PM emissions control strategies currently being considered in the 

3-12
United States by focusing on (1) comparing the sensitivity of future regional U.S. air quality to 

changes in emissions around present-day and projected future climate and emissions baselines 

and (2) accounting for the effects of uncertainties in future climate on simulated future air quality 

to evaluate the robustness of these results (see Liao et al., 2009). 

To address these issues, the GIT-NESCAUM-MIT team developed a detailed, spatially 

resolved U.S. future air pollutant emissions inventory to understand the relative impacts of 

climate change on future air quality in different emissions and control strategy regimes. To 

accomplish this, they used the latest projection data available for the near future (to about 2020), 

such as the EPA CAIR Inventory, and they extended point source emissions to 2050 using the 

IMAGE12 model combined with the IPCC A1b emissions scenario (the same scenario used in the 

GISS II′ future climate simulations) and mobile source emissions from Mobile Source Emission 

Factor Model version 6 (MOBILE6), projecting reductions of more than 50% in NOx and SO2

emissions (Woo et al., 2007). 

A key finding from the GIT-NESCAUM-MIT work is that, overall, existing control 

strategies should continue to be effective in an altered future climate, though with regional 

variations in relative benefit (Tagaris et al., 2007). The magnitude of the “climate change 

penalty” for controlling O3 (as defined by the Harvard group) is found to be consistent with the 

work of Wu et al. (2008a). The spatial distribution and annual variation in the contribution of 

precursors to O3 and PM formation under the combined future scenario of climate change and 

emission controls remain similar to the baseline case, implying the continued effectiveness of 

current control strategies. The findings further suggest, however, that compliance with air 

quality standards in areas at or near the NAAQS in the future would be sensitive to the amount of 

future climate change. Finally, an analysis of potential health impacts of these simulated future 

air quality changes, using the environmental Benefits Mapping and Analysis Program 

(BenMAP),13 is ongoing. 

Additional information on the GIT-NESCAUM-MIT project can be found in Appendix D 

and at 

• http://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.abstractDetail/abstract/6

238/report/0

• http://www.ce.gatech.edu/~trussell/lamda/

3.3 SYNTHESIS OF RESULTS ACROSS GROUPS 

This sub-section synthesizes findings across the global and regional modeling results 

from the groups that have just been introduced, focusing on nationwide changes in summertime 

 12 A Netherlands Environmental Assessment Agency modeling tool. 

13 See http://www.epa.gov/ttn/ecas/benmodels.html for more information. 

3-13
O3 concentrations due to simulated climate change a few decades into the future. Other 

pollutants are not addressed here. As already mentioned, the major focus is the particular subset 

of results completed to date which are largely common across groups, to facilitate a synthesis. 

Nevertheless, even limiting discussion to this subset allows us to effectively illustrate a number 

of key points to carry forward. 

Specifically, then, the focus is on inter-group comparisons of future decade (~2050s) and 

present-day simulations of summertime O3 under scenarios of climate change. The emphasis on 

summer reflects that of the participating research groups, i.e., on the primary season for O3

episodes and exceedances. All of the future simulations discussed in this sub-section held 

anthropogenic emissions of precursor pollutants constant at present-day levels, but allowed 

climate-sensitive natural emissions (e.g., of biogenic VOCs) to vary in response to the simulated 

changes in climate.14 The organization is as follows: first, the O3 results from the fully 

downscaled, high-resolution regional model simulations are presented and compared; then, 

comparisons of differences in key meteorological variables (and biogenic emissions) from these 

same simulations are provided to begin explaining these O3 results and to highlight the 

sometimes complex interactions between O3 and its drivers; and finally, some results from the 

global-model-only runs are presented to complement the regional model findings and to 

illuminate more clearly certain important issues. 

Most of the groups whose results make up this synthesis of the impacts of climate change 

on O3 have also carried out additional, in most cases highly preliminary, simulations designed to 

investigate, to first-order, the effects of changes in climate relative to changes in worldwide 

and/or U.S. anthropogenic emissions of precursor pollutants. The results from these simulations 

are not included in the synthesis below to maintain the focus on first exploring climate change 

impacts alone. However, these sensitivity studies provide useful insights that will help inform 

the more detailed treatments of future emissions planned for Phase II, highlighting key 

assumptions and uncertainties that will need to be addressed. Therefore, Section 4 contains a 

brief summary of these analyses and findings. 

Similarly, some of the groups have also completed simulations of potential future 

changes in PM (and its component chemical species), but these results are not discussed here. 

This is because the research effort and the level of scientific understanding are much more 

mature at this time for climate and O3 than for climate and PM—there are far more O3 results 

from these projects to date to draw from, along with a greater knowledge base for interpreting 

them. In addition, it is anticipated that many of the modeling-related issues revealed in the 

examination of the O3 results will likely apply to PM as well, though PM also poses unique 

 14 Differences in IPCC SRES scenarios between the different simulations thus refer only to greenhouse gas 

concentrations, and not precursor pollutants. 

3-14
3-15

challenges for coupled climate-air quality modeling. Some discussion of progress toward 

understanding climate change impacts on PM is also included in Section 4, and a future report 

focusing on PM is anticipated. 

3.3.1 Regional Modeling Results 

3.3.1.1 Modeling System Configurations, Simulations, and Evaluation 

Table 3-1 lists the regional climate and O3 modeling results discussed in this section. 

These simulations were carried out with linked systems consisting of a GCM/GCTM, dynamical 

downscaling with an RCM, and regional-scale air quality calculations with an RAQM. In 

aggregate, they cover a range of models, IPCC SRES scenarios of future greenhouse gas 

emissions, climate and meteorological model physical parameterizations, and chemical 

mechanisms. 

The principal comparison in this section is across the regional modeling experiments 

listed in Table 3-1 that have regional simulation domains covering the entire continental United 

States. These are the NERL, University of Illinois (Illinois 1 and Illinois 2), Washington State 

(WSU), and Georgia Tech-NESCAUM-MIT (GNM) sets of simulations. Results from the 

Berkeley and Columbia simulations, conducted for subsets of the country, are referred to in the 

course of the text to reinforce particular findings. Note that the NERL and GNM sets both relied 

on the same MM5-downscaled GISS III climate simulations, though GNM is for three summers 

versus five for NERL. They also differed in their development of their emissions inventories. 

Note also that Illinois 1 and Illinois 2 are identical except for the greenhouse gas emissions 

scenario used in the GCM simulation of future global climate, with Illinois 1 using the IPCC 

SRES A1Fi and Illinois 2 using B1. The many additional details of each of these sets of 

numerical experiments can be found in the references cited in Table 3-1 (and further references 

therein). 

It is important to reiterate that the differences in IPCC SRES scenarios for the simulations 

listed in Table 3-1 refer only to greenhouse gas concentrations, and not precursor pollutants. As 

emphasized previously in this report, all of the results shown here are from simulations that held 

anthropogenic emissions of precursor pollutants, as well as other relevant chemical species (e.g., 

CH4) constant at present-day levels. Climate-sensitive natural emissions, such as biogenic 

VOCs, evaporative emissions, and lightning NOx (depending on the modeling system used), 

were allowed to change in response to the simulated climate change, with the biogenic VOCs 

being the dominant impact. Land use and land cover also remained constant. Finally, potential 

impacts of changes in O3 concentrations on plant productivity and carbon uptake were not 

included (e.g., see Sitch et al., 2007).
Table 3-1. The regional modeling systems whose results are discussed in Sections 3.3.1 and 3.3.2. The regi

resolution listed for each group represents the horizontal grid spacing of the regional air quality simulation (also to the innermost nested grid of the RCM). The Illinois AQM runs use 30 km grid spacing over four sub-regions and 90 km everywhere else (their CMM5 runs use 30 km everywhere). Therefore, for the O3 results shown below

km values in the sub-regions are overlaid on the background map of 90 km values, introducing some minor contodiscrepancies at the boundaries of the sub-regions. 

 3-16 Berkeleya Columbiab NERLc Illinois 1d Illinois 2d WSUe 

Domain Cent. CA East. U.S. Cont. U.S. Cont. U.S. Cont. U.S. Cont. U.S. Simulation Period 1 August 5 JJAs 5 JJAs 1 JJA 1 JJA 5 Julys GCM CCM3 GISS AO GISS III PCM PCM PCM Global Resolution 2.8o × 2.8o 4o × 5o 4o × 5o 2.8o × 2.8o 2.8o × 2.8o 2.8o × 2.8o GHG Scenario 2 × CO2 A2 A1b A1Fi B1 A2 RCM MM5 MM5 MM5 CMM5g CMM5g MM5 Regional Resolution 

4 km 36 km 36 km 90/30 km 90/30 km 36 km 

Convection Scheme 

N/A Betts-Miller Grell Grell Grell Kain-Fritsch 

RAQM CMAQ CMAQ CMAQ AQMh AQMh CMAQ Chemical Mechanismi

SAPRC99j CB-IVk SAPRC99 RADM2l RADM2 SAPRC99 

Climate Sensitive Emissions 

BVOCs BVOCs; 

Evaporativem

BVOCs; 

Evaporativem

BVOCs; 

Evaporativem

BVOCs; 

Evaporativem

BVOCs; 

Evaporativem

aFor more details, see Steiner et al. (2006). bFor more details, see Hogrefe et al. (2004a,b)—the GISS AO model refers to the model of Russell et al. (1995). 
 3-17

cFor more details, see Leung and Gustafson (2005); Nolte et al. (2008). dFor more details, see Liang et al. (2006); Huang et al. (2007; 2008); Tao et al. (2007). eFor more details, see Chen et al. (2009); Avise et al. (2009). fFor more details, see Tagaris et al. (2007); Liao et al. (2007); Woo et al. (2007). gCMM5 is based on the standard MM5, but with modifications to the buffer zone, ocean interface, and cloud-radiation interactions. hAQM has been adapted from the SARMAP model, incorporating a faster, more accurate numerical solver for gas-phase chemistry. iNote that the SAPRC99 and RADM2 chemical mechanisms recycle isoprene nitrate, while the CB-IV mechanism does not. jFor more details, see Carter (2000). kFor more details, see Gery et al. (1989). lFor more details, see Stockwell et al. (1990). mThrough the SMOKE emissions modeling system, e.g., see Houyoux et al. (2000). 

Cent. = Central; East. = Eastern; Cont. = Continental; U.S. = United States.
All of these modeling systems have been evaluated to some degree with respect to 

historical observations of both climate and chemistry. Each of the individual modeling 

components making up the coupled system is well established in their respective research 

communities, and has undergone extensive testing and evaluation, though not necessarily for the 

particular variables, and statistics, most appropriate for coupled climate and air quality research. 

For example, the CMAQ model has been extensively evaluated against observations for 

operational air quality forecasting evaluations (e.g., see Eder and Yu, 2006; Eder et al., 2006), as 

well as for the purposes of examining issues such as the sensitivity of simulated O3 concentration 

to nudging of meteorological fields and subsequent impact on O3 biases (e.g., see Otte, 2008). 

Similar claims may be made for the other global and regional climate and chemistry modeling 

components. 

Evaluation of the coupled modeling systems, built out of these individual components, is 

at an early stage. Each of the modeling teams has performed a number of evaluations of their 

coupled climate and air quality systems using station observations of meteorological variables 

and ozone concentrations (e.g., from EPA’s Air Quality System database15) for various historical 

time periods. Details of these evaluations can be found in the references cited above, and 

additional references therein for the individual modeling components. For example, the NERL 

group compared their combined GCM-RCM-RAQM MDA8 O3 distributions with AQS 

observations nationally, finding reasonable agreement comparable to that found for uncoupled 

CMAQ simulations. In general, they found the smallest biases in the northeastearn United 

States, and at the high end of the O3 distribution. They attributed these biases to both 

meteorological and chemical mechanism factors. 

Beyond providing insight into the performance and biases of the modeling systems, these 

evaluation studies also provide a number of important insights that complement the simulations 

of climate change impacts on O3 that will be discussed shortly, e.g., on the role of meteorological 

drivers or alternative chemical mechanisms in O3 variability. For example, Nolte et al. (2008) 

attribute a portion of the O3 biases over the eastern United States that they observe in their 

coupled system to the biases in temperature and precipitation present in the MM5 regional 

climate used to drive their ozone simulation (see also Leung and Gustafson, 2005). They also 

found, in sensitivity studies, differences in simulated O3 using the SAPRC vs. the CB-IV 

chemical mechanism in CMAQ (see also Faraji et al., 2008). Similarly, Huang et al. (2007) 

showed how low or high biases in simulated temperature over the Northeast and Midwest lead to 

O3 concentration biases in the same directions. 

 15 http://www.epa.gov/ttn/airs/airsaqs/. 

 3-18 
3.3.1.2 Changes in O3

Figure 3-1 shows summertime mean MDA8 O3 concentration differences between 

simulated future and present-day climates for the regional modeling experiments listed in 

Table 3-1 that have model domains covering the entire continental United States. These are the 

NERL, Illinois 1, Illinois 2, WSU, and GNM simulations. Results from the Berkeley and 

Columbia simulations, conducted for subsets of the country, are referred to in the course of the 

text to reinforce particular findings. MDA8 O3 is selected because of its direct relevance to U.S. 

air quality standards. All plots discussed here show future minus present differences. All O3

values are in ppb. 

Key similarities between the results from the different groups emerge: 

• For all the present/future simulation pairs, some substantial regions of the country show 

future increases in O3 concentrations of roughly 2−8 ppb under a future climate. 

• Other regions show little change in O3 concentrations, or even decreases, though the 

decreases tend to be less pronounced than the increases. 

• These patterns of O3 differences are accentuated in the 95th percentile MDA8 O3 (shown 

in Figure 3-2 for the NERL experiment, as one example of this result) compared to the 

mean MDA8 O3. 

The basic result of larger climate sensitivity of O3 concentrations for high-O3 conditions 

(e.g., 95th percentile MDA8 O3) is one of the most robust findings of this synthesis—it holds 

across all the modeling groups and appears in many different analyses carried out by these 

groups. These more detailed results can be found in the papers cited in Table 3-1. This is 

significant, because these high-O3 episodes are of particular concern for air quality managers. 

Some pronounced differences in the broad spatial patterns of change across experiments 

emerge as well. For example, the NERL and GNM simulations show increases in O3

concentration in the Mid-Atlantic and parts of the Northeast, Gulf Coast, and parts of the West. 

They also show decreases in the upper Midwest and Northwest and little change elsewhere, 

including the Southeast. By contrast, the Illinois 1 experiment shows the strongest increases in 

the Southeast, the Northwest, and the Mississippi Valley (as well as the Gulf Coast, in agreement 

with NERL), with weaker increases in the upper Midwest. In addition, these changes tend to be 

larger than those from the NERL experiment. The WSU experiment shows the largest increases 

 3-19 
Figure 3-1. 2050s-minus-present differences in simulated summer mean 

MDA8 O3 concentrations (in ppb) for the (a) NERL; (b) Illinois 1; (c) Illinois 

2; (d) WSU; and (e) GNM experiments (see Table 3-1). 

 3-20 
Figure 3-2. 95th percentile MDA8 O3 concentration differences for the NERL 

experiment. 

in the Northeast, parts of the Midwest, and desert Southwest, with decreases in some parts of the 

West, the Southeast, the Northwest, the Plains states, and the Gulf Coast. As is to be expected, 

the NERL and GNM patterns are quite similar, with differences primarily reflecting the 

averaging over five vs. three summers, respectively. This highlights the potential importance of 

interannual variability in driving differences between modeling groups, as will be discussed 

further below. 

Certain regions show greater agreement across experiments than others. Figure 3-1 

illustrates that a loosely bounded area, encompassing parts of the Mid-Atlantic, Northeast, and 

lower Midwest, tends to show at least some O3 increase across all the simulations. By contrast, 

the West and the Southeast/Gulf Coast are areas of greater disagreement, hinting at some of the 

complexities underlying the interactions between climate and O3. Even for these regions, 

however, at least some of the models (here and in Section 3.3.2) show substantial climateinduced O3 increases. Changes in drivers that help explain these agreements and disagreements, 

and help illustrate these complexities, will be presented and discussed shortly. 

All of these findings are generally consistent with results from the earlier Columbia study 

(see Hogrefe et al., 2004b). Figure 3-3 shows future-minus-present climate summertime mean 

 3-21 
MDA8 O3 concentration difference for their modeling domain, covering the eastern half of the 

United States. 

Figure 3-3. 2050s-minus-present differences in simulated summer mean 

MDA8 O3 concentrations (in ppb); reproduced from Figure 2 in Hogrefe et 

al. (2004b). 

Note from Table 3-1 that there are differences in the number of years of simulation 

completed by the different groups so far. As introduced in Section 1, and discussed further 

below, it is well recognized that interannual meteorological variability drives large year-to-year 

changes in O3 (e.g., see White et al., 2007; Leibensperger et al., 2008; Jacob and Winner, 2009). 

All of the modeling groups eventually aim to analyze interannual variability in their simulations. 

In this context, Figure 3-4 (reproduced from Nolte et al., 2008) illustrates two points. First, for 

some regions, the average change in O3 from the present to the 2050s as a result of climate 

change is just as large as (and on top of) the year-to-year O3 variability that is of concern today. 

In other words, climate change has the potential to push O3 concentrations in extreme years 

beyond the envelope of natural interannual variability. Second, it highlights the need for 

simulating multiple years to increase the robustness of findings about present-to-future changes. 

These results are consistent with those presented in Racherla and Adams (2008) (based on their 

GCTM runs), who found that the magnitude of simulated future changes in O3 concentrations 

over the eastern United States tended to be greater than the magnitude of present-day interannual 

 3-22 
O3 variability, and that at least 5 years of simulation were needed to fully separate the effects of 

climate change and interannual variability. 

presents 2050s climate and decreases in anthropogenic 

3 precursor emissions. 

und 

g 

e extension of the O3 season 

for som regions of the United States under future climate change. 

Figure 3-4. Frequency of simulated summer mean MDA8 O3 values 

exceeding 80 ppb in different regions from the NERL experiment; 

reproduced from Figure 11 in Nolte et al. (2008). Each bar represents 1 year. 

The leftmost group of bars corresponds to present-day climate, the center group to

2050s climate with anthropogenic emissions held constant at present-day values, 

and the rightmost group re

O

Finally, while this analysis focuses on summertime results, a few of the groups also fo

strong increases in O3 concentrations in their future compared to present climate simulations

over certain regions of the country (e.g., Nolte et al., 2008; Avise et al., 2009; Racherla and 

Adams, 2008). Figure 3-5 (reproduced from Nolte et al., 2008) illustrates this point, showin

September-October O3 increases in a band stretching from the Southwest, across the Plains 

states, and into the Upper Midwest. These results suggest a possibl

e

 3-23 
Figure 3-5. 2050s-minus-present September-October compared to JuneAugust differences in simulated mean MDA8 O3 concentrations (in ppb); 

reproduced from Figure 6 in Nolte et al. (2008). 

3.3.1.3 Changes in Drivers 

There is already a great deal of regional variability in near-surface O3 under current 

climate conditions. For example, as introduced in Section 1, a large body of observational and 

empirical work has helped us understand that concentrations tend to be especially great where 

the emissions of precursor chemical species like VOCs and NOx are also large, and that, 

furthermore, these pollutants tend to drive up O3 even more during the times when 

meteorological conditions most favor strong net photochemical production—persistent high 

pressure, stagnant air, lack of convection, clear skies, and warm temperatures—and vice versa. 

It is for these reasons that the O3 NAAQS are most often exceeded during summertime hot spells 

in places with large natural or anthropogenic precursor emissions (e.g., cities). To the extent that 

climate change may alter weather patterns, and, hence, the frequency, duration, and intensity of 

these episodes, for example, O3 concentrations could be significantly affected. 

However, the causal chain linking (a) long-term global climate change, (b) changes in the 

aspects of (often) short-term meteorological variability that most directly drive near-surface O3

concentration changes of concern to air quality managers, and (c) any O3 changes that ultimately 

result from the interaction of these meteorological changes with the pollutants present in the 

environment (which may themselves be sensitive to meteorology and climate) may not be 

straightforward. Changes in the O3 distribution of a given region due to climate change will 

reflect a balance among competing changes in multiple factors. 

For example, a number of meteorological variables have been identified as potentially 

important, including 

• Near-surface temperature 

• Near-surface humidity 

 3-24 
• Precipitation 

• Cloud cover 

• Planetary Boundary Layer (PBL) height 

• Near-surface wind speed and direction 

• Ventilation and mixing due to convective events 

• Ventilation and mixing due to synoptic-scale cyclones 

• Ventilation and mixing due to coastal onshore flow. 

These variables are not, in general, independent of each other. Instead, they vary, 

together or separately in different combinations, at different locations over different timescales, 

in ways that may favor either increases or decreases in O3. For example, all other factors being 

equal, increases in temperature at a given time and place might lead to increases in O3

concentration, but if these temperature increases are accompanied by increases in cloudiness, the 

net result might be a decrease in O3 concentration. Box 3-1 provides a discussion of how one’s 

perception of the relationship between O3 and its meteorological drivers can vary depending on 

the timescale considered, using the temperature-O3 relationship as an example. This provides 

some additional context for interpreting these next modeling results to be presented. This issue 

is revisited in Section 3.4 below, where the implications for interpreting long-term mean climate 

change-air quality modeling results are discussed. 

The advantage of the type of model-based approach that is the focus of this section, i.e., 

the strategy of linking climate, meteorology, and air quality models, is that such integrated 

modeling systems are capable of capturing these complexities by representing the reinforcing 

and competing interactions between variables in an internally self-consistent way. As such, they 

help illuminate potentially non-obvious impacts of climate change on O3 that result from 

synergistic interactions between the changes in key drivers. 

Figures 3-6 and 3-7 display the average future-minus-present differences in near-surface 

air temperature and surface incoming solar radiation (typically referred to as “insolation”), which 

are two of the most critical meteorological drivers of ground-level O3. The insolation changes 

largely reflect changes in cloud cover. Other variables besides the two shown in Figures 3-6 and 

3-7 were also examined, including average daily maximum temperature, precipitation, number of 

rainy days, and PBL height. However, none of these additional comparisons are shown here 

because, at least at this level of analysis, they do not seem to add a great deal to the explanatory 

power of temperature and surface insolation (and, as will be discussed below, biogenic VOC 

emissions). This is likely due to the strong correlations among these variables already been 

discussed. 

 3-25 
Box 3-1. The Temperature-O3 Relationship 

As seen through the lenses of different meteorological/climatic timescales 

Episode: The severity of a particular O3 episode lasting one or a few days can depend strongly on temperature. 

For example, Aw and Kleeman (2003) found that, by increasing temperature (but without modifying the other 

meteorological variables) in an air quality model simulation of a southern California O3 episode, they 

significantly increased daily peak O3 concentrations. Temperature affects the kinetics of the O3-forming and 

destroying chemical reactions. For example, in polluted environments, increasing temperature will tend to lead to 

more NOx, and hence more O3, via a decrease in peroxyacetylnitrate (PAN) production. The new results from the 

Berkeley and Carnegie Mellon groups described in Section 3.2 have yielded similar insights. Steiner et al. 

(2006), in their very high-resolution simulations of a 5-day O3 episode over California, found that temperature 

perturbations consistent with plausible 2050s climate change led to increases in afternoon O3 concentrations of 1-

5 ppb across the state. Dawson et al. (2007b) found similar effects of temperature modification when using the 

PMCAMx model to simulate O3 concentrations during a week-long period over the eastern U.S. 

Season: From the perspective of an entire season, however, mean O3 concentration and the number of O3

exceedances will likely depend at least as much on how many of these meteorological episodes that promote O3

formation occur, and how long they last, as on how hot it is during them. In other words, how often in a given 

summer that cool, cloudy, rainy, and windy conditions give way to spells of hot, clear, dry, and stagnant 

conditions will play a large role in determining whether it was a “high-O3” or “low-O3” summer. At this 

timescale, temperature and O3 will also be positively correlated, but here the “temperature-O3” relationship exists 

at least partly because temperature itself is highly correlated with these other meteorological conditions, like more 

sunlight and less ventilation, that also favor increased O3 concentrations. 

Long-Term Climate Change: On the multi-decadal timescales of global climate change, however, the 

relationship between temperature and these other meteorological drivers may or may not play out in the same way 

that is characteristic of seasonal timescales. In some regions, climate change may indeed have the effect of 

producing long-term average associations between higher temperatures, less cloudiness, and weaker mixing that 

in aggregate would be likely to lead to O3 concentration increases. This would be true, for example, in the 

regions most at risk for increases in the frequency, duration, and intensity of summertime heat waves (e.g., see 

Meehl and Tebaldi, 2004; IPCC, 2007). In other regions, however, climate change may lead to changes in these 

other variables that do not favor increases in O3 concentrations. For example, a warmer world is likely, on 

average, to be a wetter world. Both the Harvard and Carnegie Mellon GCTM results summarized earlier showed 

how increases in humidity in their future simulations led to decreases in near-surface O3 in less-polluted regions 

(Wu et al., 2008a; Racherla and Adams, 2006). Similarly, regions that experience increases in cloudiness (and 

hence decreases in sunlight and O3 photo-production) in an altered future climate might have net O3 concentration 

decreases, in spite of increased temperatures.

Combined with the O3 results shown above in Figure 3-1, Figures 3-6, and 3-7 reveal 

some key similarities in the relationships between O3 and meteorological drivers among the 

different model studies: 

• First, in many regions the O3 concentration changes (Figure 3-1) seem to correspond 

relatively well with combined changes in mean temperature (Figure 3-6) and mean 

surface insolation (Figure 3-7). For example, the NERL results show the O3 increases 

corresponding with temperature and insolation increases in the Mid-Atlantic and Gulf 

Coast and O3 decreases associated with the insolation decreases and the local minimum 

in temperature increases in the upper Midwest and the northern Plains. 

 3-26 
Figure 3-6. 2050s-minus-present differences in simulated summer 

mean near-surface air T (oC) for the (a) NERL; (b) Illinois 1; (c) 

Illinois 2; (d) WSU; and (e) GNM experiments. 

 3-27 
Figure 3-7. 2050s-minus-present differences in simulated summer 

mean surface insolation (W m-2) for the (a) NERL; (b) Illinois 1; (c) 

Illinois 2; (d) WSU; and (e) GNM experiments. 

• In other regions, temperature and insolation vary in opposite directions, with mixed 

impacts on O3 concentrations. For example, in the Illinois 1 simulations, in spite of 

insolation decreases over much of the Northwest, the large increase in temperature there 

seems to drive O3 increases. 

In a small number of regions across the simulations, there is no strong correspondence 

between O3 concentrations and either insolation or temperature (e.g., the areas around Oklahoma 

 3-28 
in the Illinois 1 experiment and Nevada/Utah/Idaho in the Illinois 2 experiment), suggesting that 

other forcing factors may be important, and/or that a correspondence might exist, but only for 

different averaging periods and statistics of these variables. 

The differences between the NERL and GNM results are consistent with this last bullet. 

For example, in the Plains states, GNM shows greater O3 decreases, consistent with the 

difference in temperature and insolation trends resulting from the difference in the number of 

summers simulated. 

Again, as discussed above and in Box 3-1, when interpreting these monthly- or seasonalmean results it is important to recognize they encompass not just changes in the meteorological 

conditions most related to O3 episodes, but the whole spectrum of changes in regional 

climatology arising from global climate change. 

Considering the results from the Columbia group, Hogrefe et al. (2004b) do not report 

any single clear relationship across their study region between the spatial patterns of futureminus-present O3 concentrations and a number of meteorological variables (e.g., temperature, 

wind speed, and mixed layer height), as mentioned in the summary in Section 3.2. This is 

consistent with the potential for different competing effects in different regions illustrated by the 

results shown here. They do note a strong sensitivity of future O3 changes to changes in 

convective activity in certain areas, which may reflect the dependence on insolation found by the 

other groups. 

Figure 3-8 shows the patterns of changes in mean biogenic VOC emissions across the 

simulations. As documented in earlier work (e.g., Chameides et al., 1988; Roselle et al., 1991; 

Guenther et al., 1994; Pierce et al., 1998; Fuentes et al., 2000; Purves et al., 2004; among others), 

the emissions of these important natural O3 precursors are themselves sensitive to meteorology, 

including sunlight and temperature. Therefore, in conjunction with the direct forcing exerted on 

O3 processes by changes in meteorology, climate-induced changes in biogenic emissions levels 

can lead to changes in O3 concentrations as well (see also Zhang et al., 2008). As will be 

discussed again below, in the context of the global modeling results, this impact depends in part 

on the relative amounts of NOx and VOCs in the environment. For example, Steiner et al. (2006) 

found significant O3 concentration increases in the high-NOx San Francisco Bay area due to 

increases in biogenic VOC emissions, whereas even larger increases in biogenic emissions over 

the Sierras actually produced slight O3 decreases. 

The climate-induced biogenic emissions changes shown in Figure 3-8 seem to contribute 

to the O3 concentration changes, but only in some regions, and not wholly consistently across 

model studies. For example, temperature-driven increases in biogenic emissions may contribute 

to the above-mentioned O3 increases in the Northwest in the Illinois 1 experiment, the 

Mid-Atlantic in the NERL and GNM experiments, the Northeast in the Illinois 2 experiment, and 

 3-29 
Figure 3-8. 2050s-minus-present differences in simulated summer mean 

biogenic VOC emissions (g Carbon m-2 day-1) for the (a) NERL; (b) Illinois 1; 

(c) Illinois 2; (d) WSU; and (e) GNM experiments. 

the Southeast in the Illinois 1 experiment. Contrastingly, in parts of the Southeast and Mountain 

West in the NERL and GNM experiments, emissions increase significantly but O3 concentrations 

do not change. Notably, the WSU simulation shows large decreases in O3 in some of the parts of 

the Southeast and Gulf Coast where increases in VOC emissions are the strongest, a result that is 

partially attributed to increases in precipitation, and hence reduced photo-production. Where 

there are strong correlations between biogenic emissions changes and O3 concentration changes, 

often there are similarly strong changes in insolation and/or temperature, so separating the 

different effects is not always straightforward. The earlier work by Hogrefe et al. (2004b) found 

 3-30 
the strongest increases in emissions in the Southeast, similar to the results from the NERL and 

Illinois 1 and 2 experiments, but found that the largest O3 concentration changes that could be 

attributed to biogenic emissions changes occurred instead in parts of the Ohio Valley and coastal 

Mid-Atlantic. 

Discerning the precise chemical pathways whereby O3 responds to changes in biogenic 

emissions, and how they vary as a function of region and climatic conditions, is an area of 

ongoing scientific inquiry. Different air quality models employ different representations of these 

pathways in their code. As such, differences between the simulated O3 response to changes in 

simulated biogenic emissions from different modeling systems is at this time a key source of 

uncertainty in climate change impacts on future air quality, particularly in certain regions where 

the effect of increasing VOC concentrations is highly dependent on NOx levels. This issue will 

be highlighted further in Section 3.3.2 below, in the intercomparison of the results from the 

global modeling experiments. 

One way to summarize the aggregate results presented in Figures 3-1 and 3-6 to 3-8 is to 

say that O3 responds to the meteorological/emissions drivers in a qualitatively consistent manner 

across the simulations, but the regional patterns of relative changes in these drivers is highly 

variable across these same simulations. 

In other words, there are important differences in the simulated future regional climate 

changes across groups that seem to drive the differences in the regional patterns of O3 increases 

(and decreases). The differences in modeling systems among the groups, as documented in 

Table 3-1, provide some indication of a number of possible contributing factors that might be 

responsible for these differences in simulated future regional climate patterns, including 

• Differences in the driving GCM 

• Differences in the SRES greenhouse gas scenario 

• Differences in the RCM (and/or model physical parameterizations) used to simulate 

regional meteorology 

• Differences in the RAQM (and/or chemical mechanisms) 

• Differences in the amount of interannual variability captured 

These issues of inter-group differences, and the sensitivity of simulation results to 

modeling methodology, are discussed in greater detail in Section 3.4 below, to provide additional 

guidance on interpreting the findings and evaluating their robustness in the context of the 

existing scientific uncertainties. 

 3-31 
The findings presented here, in Sections 3.3.1.2 and 3.3.1.3, are generally consistent with 

the limited number of regional climate and air quality modeling experiments recently carried out 

for Europe. For example, Forkel and Knoche (2006) simulated changes in near-surface O3

concentrations between the 1990s and the 2030s over Southern Germany under climate change 

but no change in anthropogenic emissions. They found a 10 percent increase in average daily 

maximum O3 during summer (approximately 2−6 ppb, depending on location in the model 

domain). Langner et al. (2005), in a set of regional modeling experiments, found climate 

change-induced increases in April-September O3 concentrations during the mid-21st century 

compared to the present over Southern and Central Europe, with decreases over Northern 

Europe, and that these changes were significant with respect to interannual variability. Meleux 

et al. (2007) found higher summertime O3 concentrations under future climate conditions over 

Europe, due primarily to increased temperatures, decreased cloudiness and precipitation, and 

increases in biogenic VOC emissions. They also found large regional variability in these O3

changes. Finally, Szopa and Hauglustaine (2007) found worsening O3 conditions over Europe as 

a result of anticipated climate change in 2030, though this was sensitive to the choice of global 

and regional emissions change scenarios. 

3.3.2 Global Modeling Results 

 Table 3-2 lists the groups that have results from GCTM simulations available at the time 

of developing this report. 

Table 3-2. GCTM-only model simulations whose results are discussed in 

Section 3.3.2. CMU stands for Carnegie Mellon University. The two Harvard 

runs use different GCMs with the same SRES greenhouse gas scenario. The two 

Illinois runs have identical setups but are driven with different SRES scenarios. 

As with the regional modeling system results discussed above, anthropogenic 

emissions of precursor pollutants were held constant across present-day and future 

simulations, while natural climate-sensitive emissions were allowed to change. 

Harvard 1a Harvard 2b CMUc Illinois 1d Illinois 2d

Simulation Period 5 summer/falls 5 summers 10 summers/falls 5 summers 5 summers 

GCM GISS III GISS II′ GISS II′ PCM PCM 

Resolution 4o × 5o 4o × 5o 4o × 5o 2.8o × 2.8o 2.8o × 2.8o 

GHG Scenario A1b A1b A2 A1Fi B1 

GCTM GEOS-Chem GISS II′ e GISS II′ e MOZART v.4 MOZART v.4 

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Chemical 

Mechanism 

GEOS-Chemf Harvard Trop 

Chem Modelg

Harvard Trop 

Chem Model 

MOZART v.4h MOZART v.4 

Climate Sensitive 

Emissions 

BVOCs; 

Lightning and 

soil NOx

BVOCs; 

Lightning and 

soil NOx

BVOCs; 

Lightning and 

soil NOx

BVOCs; 

Lightning and 

soil NOx

BVOCs; 

Lightning and 

soil NOx

a

For more details, see Wu et al. (2007); Wu et al. (2008a; 2008b) 

b

For more details, see Mickley et al. (2004) 

c

For more details, see Racherla and Adams (2006; 2008) 

d

For more details, see Tao et al. (2007); Lin et al. (2008); Huang et al. (2008) e

The GISS II′ model was coupled to the Harvard tropospheric O3-NOx-hydrocarbon chemical model 

(Mickley et al., 1999) 

f

For more details, see http://homepages.see.leeds.ac.uk/~lecmje/GEOS-CHEM/GEOS-CHEM_Chemistry.htm. g

For more details, see Mickley et al. (1999) 

h

For more details, see Horowitz et al. (2003) and http://gctm.acd.ucar.edu/mozart/models/m4/index.shtml. 

All of these GCM/GCTM simulations are also associated with regional downscaling and 

air quality modeling efforts. The Illinois GCM/GCTM runs are the same ones used to provide 

climatic and chemical boundary conditions for the Illinois 1 and 2 regional simulations listed in 

Table 3-1 and described above (see also Lin et al., 2008), and the Harvard 2 run is the same one 

used to drive the NERL regional simulations (see also Mickley et al., 2004). The Harvard 1 and 

CMU simulations will similarly eventually be used to drive RCM and RAQM models—these 

groups have developed and tested full global-to-regional systems, with results expected in the 

near future. Here, a somewhat more limited inter-group comparison than for the regional 

modeling results is presented, with the goal of illustrating a few specific points. 

In a global context, the results from these simulations are generally consistent with other 

GCTM climate change experiments (e.g., see Murazaki and Hess, 2006; Stevenson et al., 2006; 

Zeng et al., 2008): e.g., decreases in background O3 concentrations in clean environments (e.g., 

the oceans), due to increased water vapor concentrations, and increases regionally over the 

polluted continents. 

A comparison of results across all of these simulations for the United States in particular 

(not shown) supports the most general conclusions from the regional modeling studies: i.e., 

large regions of the country show future O3 concentration increases of a few to several ppb, and 

there can be significant differences in the spatial patterns of these changes between different 

modeling experiments. The purpose of this sub-section is to highlight a comparison between two 

of these simulation sets—Harvard 1 (see also Wu et al., 2008a) and CMU (see also Racherla and 

Adams, 2006)—because these results illustrate particularly well two critical insights: the 

potential importance for simulated future O3 of large-scale circulation changes, and the potential 

importance of how isoprene chemistry is represented in the modeling systems. 

 3-33 
Figure 3-9 shows the mean MDA8 O3 changes from the Harvard 1 experiment, along 

with accompanying changes in temperature, insolation, and biogenic emissions. In these results, 

the largest O3 increases are mostly in a sweeping pattern from the central United States, across 

the Plains states and the Midwest, and extending into the Northeast. In contrast to the regional 

model results shown above, there is not as obvious a spatial correlation between the changes in 

O3 and those of any one of the driver variables. The insolation increase in the Midwest matches, 

to some degree, the pattern of O3 increase there, but the largest temperature, insolation, and 

biogenic emissions increases occur in the southern part of the country, where there are much 

smaller changes in O3. This weak relationship also holds for a number of other variables 

considered but not shown (e.g., precipitation, PBL height, etc.). 

In Figure 3-10, which shows the same quantities for the CMU experiment, a different 

regional pattern of change emerges. Here, the major increases in future O3 concentrations are 

instead centered on the Gulf Coast and eastern seaboard, with minimal O3 changes in the upper 

Midwest and northern Plains states. 

Figure 3-9. 2050s-minus-present differences in simulated summer 

(JJA) mean (a) MDA8 O3 concentration (ppb); (b) near-surface air 

temperature (oC); (c) surface insolation (W m-2); and (d) biogenic 

isoprene emissions (g Carbon m-2 day-1) for the Harvard global 

modeling experiment (see Table 3-2). 

 3-34 
Figure 3-10. Same as Figure 3-9 but for the CMU global modeling 

experiment. 

The differences between these two sets of results can seemingly mostly be explained by 

two factors: (1) differences in the future simulation of the summertime storm track across the 

northern part of the country and (2) differences in the response of O3 to changes in biogenic 

VOC emissions in the southeastern United States. 

As explained in Wu et al. (2008a), there are two distinct dynamical shifts from the 

present to the future climate in the Harvard 1 experiment: a decrease in summertime cyclones 

tracking across the upper part of the United States, resulting in a decrease in cloudiness and 

precipitation over the upper Midwest (as reflected in the insolation changes shown in 

Figure 3-9), and a northward shift of the Bermuda High, resulting in a decrease in convective 

activity over the Gulf Coast and the southern Great Plains. All other factors being equal, both 

shifts might be expected to contribute to O3 concentration increases in their respective regions. 

In this context, the spatial pattern of O3 concentration increases in Figure 3-9a is certainly 

consistent with the decrease in cyclones in the north in the Harvard 1 experiment, as suggested in 

Wu et al. (2008a) and originally posited in Mickley et al. (2004), i.e., that the decrease in cold 

surges in the simulated future climate leads to a decrease in the clearing of pollutants from the 

 3-35 
boundary layer (see also Murazaki and Hess, 2006). Racherla and Adams (2008), on the other 

hand, examined the distribution of sea-level pressure anomalies in the present-day and future 

CMU simulations and found only relatively small changes in these regions. These results 

suggest that storm track activity does not decrease in the future in this CMU model simulation, 

but a more detailed analysis of the storm tracks in this model may be needed (Leibensperger et 

al., 2008). 

Acknowledging this qualification, it seems plausible that differences in simulated future 

large-scale circulation patterns explain the differences in future O3 changes simulated by the two 

groups for the northern part of the country. What is the explanation for the even larger 

difference in simulated future O3 changes in the southern half? 

The difference in simulated future O3 changes in the southern half of the country likely 

arises because of differences in how O3 responds to the climate-induced changes in biogenic 

VOC emissions in modeling systems used in the Harvard 1 and CMU experiments. The spatial 

patterns of future-minus-present changes in isoprene emissions shown in Figures 3-9d and 3-10d 

are qualitatively similar, with the largest increases centered on the Southeast and Gulf Coast 

regions for both groups. Examining the CMU results in Figure 3-10, it appears that increases in 

temperature and decreases in cloud cover (and hence increases in insolation) have combined to 

lead to increases in both isoprene emissions and O3 concentrations in this region. An additional 

CMU simulation with future meteorology but scaled-back isoprene emissions has confirmed that 

the enhanced O3 chemical production resulting from these enhanced emissions are largely 

responsible for the simulated future O3 increases (Racherla and Adams, 2008). 

Contrast this with the Harvard 1 results, which show only weak changes in O3

concentrations over the Southeast and Gulf Coast, in spite of the large increase in future biogenic 

VOC emissions. Even the especially large increases in temperature and insolation that 

accompany these biogenic emissions changes in the Gulf Coast region do not seem to increase 

appreciably future O3 concentrations. 

One factor to which this striking difference between the two sets of results might be 

traced is the modeled isoprene nitrate chemistry. While increased emissions of biogenic VOCs 

are often associated with increases in O3 concentrations, these increased emissions can also lead 

to decreases in O3 concentrations via different pathways. For example, high concentrations of 

isoprene can reduce O3 amounts through direct ozonolysis and can also suppress O3 production 

in NOx-limited regimes (e.g., rural areas) by sequestering NOx in isoprene nitrates (e.g., see 

Fiore et al., 2005). In the modeling system used for the Harvard 1 simulations, it is plausible that 

increasing isoprene emissions results in little change, or even decreases in O3 amounts, largely 

because the model chemistry represents these isoprene nitrates as a “terminal” sink for NOx. In 

the absence of additional NOx, the small change in O3 concentrations in the Gulf Coast, in spite 

 3-36 
of the strongly favorable climate changes there, could be explained by this suppressing effect of 

isoprene. By contrast, in the CMU modeling system, the isoprene nitrates are assumed to react 

rapidly with OH and O3 and “recycle” NOx back to the atmosphere with 100% efficiency. This 

NOx then becomes available to help create O3 again, tending to favor greater O3 concentrations 

in regions of greater biogenic VOC emissions, and dominating the impact of climate change on 

O3 in the CMU results. 

This comparison strongly illustrates the importance of understanding the underlying 

details of the chemical mechanism of O3 formation. Constraining the precise pathways whereby 

isoprene, NOx, and O3 are linked is the subject of ongoing research (e.g., see Horowitz et al., 

2007), and as such remains an important source of uncertainty in the modeling systems. 

However, there are a number of other important uncertainties associated with the choice of 

chemical mechanisms, as will be discussed further in Section 3.4. 

Finally, in the Harvard 1 simulations, enhanced ventilation and mixing also plays a role 

in partially offsetting expected climate-induced O3 concentration increases in some near-coastal 

regions. This results from the combination of the humidity-driven decreases in O3 over the 

oceans reported in Wu et al. (2008b) (and also Racherla and Adams, 2006), and perhaps also 

stronger onshore flow due to an increase in the summertime land-ocean heating contrast. Lin et 

al. (2008) report similar effects in their simulations of future O3 over United States and China. 

Before concluding with a summary of the synthesis points that have emerged, the 

following sub-section provides some additional discussion of outstanding issues related to 

modeling the linked climate-air quality system and the complexities and scientific uncertainties 

inherent therein. 

3.4 CHALLENGES AND LIMITATIONS OF THE MODEL-BASED APPROACH 

All of the results shown in this section are model-based. This emphasis on model studies 

has been built, from the beginning, into the framework and implementation of the assessment. 

This sub-section spends some time outlining the challenges, limitations, and areas of uncertainty 

associated with this model-based approach to provide context for a meaningful interpretation of 

this synthesis. This discussion helps delineate areas of needed future research to build on our 

understanding of the climate change-air quality problem, and it aims to convey how the findings 

presented above might be sensitive to the various modeling uncertainties. 

The central concern of this section is the use of linked systems of global and regional 

climate and air quality models to investigate potential future changes in O3 that may occur due to 

climate change. These complex modeling systems are extremely valuable scientific tools, as 

they allow for the exploration of nonlinearities, feedbacks, threshold effects, and in general 

surprising behaviors that only emerge when the various components are linked together. They 

 3-37 
also, to a degree, encapsulate current scientific understanding of how a wide range of chemical, 

physical, and dynamical processes interact with each other; i.e., they provide a useful snapshot of 

the state of the science. 

Because of the complexity of the system they mean to mirror, however, at any moment 

they necessarily embody only an incomplete representation. This results from technical 

challenges, such as limitations on computing power, as well as from a fundamental lack of 

understanding of certain processes. 

Furthermore, different versions of these modeling systems, for example as developed by 

different groups, will sample different parts of the space of possible representations. The current 

assessment effort shows the distribution of results across multiple groups and linked modeling 

system. Therefore, it is possible to consider different combinations over a range of models, 

scenarios, and parameterizations, as summarized in Tables 3-1 and 3-2. It is also important to 

emphasize, however, that, because of the enormous computational burden of these modeling 

systems as applied to this problem, at this point it is only a very small subset of the available 

range that has been sampled here (e.g., a few GCMs and SRES scenarios, essentially one RCM, 

three regional model convection schemes, etc.). Expanding the scope to include additional 

models, scenarios, and parameterizations, along with multiple combinations of each, might 

further broaden the distribution of projected regional O3 changes. Alternatively, such new 

results might reinforce previous findings. 

Therefore, any synthesis conclusions are subject to revisions pending results from future 

investigations. However, this preliminary synthesis makes it possible to identify some of the key 

modeling-related sensitivities that are likely to determine our ability to accurately simulate 

climate change-driven O3 changes, as summarized in the following questions: 

• What kinds of differences do different GCMs (under different greenhouse gas emissions 

scenarios) simulate in the climate, and especially in the weather patterns that matter most 

for air quality? 

• How do RCMs translate these climate and meteorological changes down to the regional 

scales that are desired? 

• How are important chemical mechanisms represented in the climate-air quality modeling 

systems? 

3.4.1 Inter-Model Variability and Model Evaluation 

The IPCC AR4 (IPCC, 2007) summarizes current understanding of variations in future 

global climate simulations. The spread across models, groups, and scenarios is the result of 

differences in exogenous forcings, like natural volcanic or solar changes or changes in 

 3-38 
anthropogenic emissions of greenhouse gases and aerosols. This spread also results from 

internal model variability and nonlinear behavior that reflect the inherently chaotic nature of the 

atmospheric and oceanic circulations. Finally, it arises from model configuration differences due 

to different choices for dealing with resolution constraints, numerical approximations, and lack 

of perfect understanding of processes or perfect observations of key parameters. The impact of 

these factors is reflected in the range of average climates, and regional spatial distributions of 

climate characteristics, simulated by the different GCMs that are featured here. 

The significance of these inter-model/scenario differences varies depending on the lens 

provided by the particular problem of interest. For air quality in general, and O3 specifically, a 

critical question is “What kind of changes do models simulate in the weather patterns that matter 

most for air quality?” The results shown in Figures 3-9 and 3-10 illustrate some of the 

uncertainties associated with this question. Physical and dynamical arguments suggest that 

future decreases in the equator-to-pole temperature gradient should drive poleward shifts in the 

mid-latitude storm tracks, and that this may lead to decreases in the frequency of cyclone 

ventilation of pollutants in the Northeast and Midwest. The results from the Harvard 1 

experiment show this clearly, while those from the CMU experiment do not seem to. Taking a 

broader perspective across many models and groups, the IPCC AR4 states 

Central and northern regions of North America are under the influence of 

mid-latitude cyclones. Projections by AOGCMs [Atmosphere-Ocean Global 

Circulation Models] generally indicate a slight poleward shift in storm tracks, an 

increase in the number of strong cyclones but a reduction in medium-strength 

cyclones over Canada and poleward of 70°N (IPCC, 2007). 

 However, the agreement across groups is by no means absolute. Furthermore, the IPCC 

report states 

Results from a systematic analysis of AMIP-2 simulations (Hodges, 2004; 

Stratton and Pope, 2004) indicate that models run with observed SSTs are capable 

of producing storm tracks located in about the right locations, but nearly all show 

some deficiency in the distribution and level of cyclone activity (IPCC, 2007). 

Recent increases in model resolution and other improvements have led to improvements 

in simulations of present-day storm tracks, and may eventually lead to a stronger consensus on 

the likely magnitude and direction of future climate-induced changes over the United States. At 

this time, however, current levels of uncertainty probably do not allow us to say much more than 

(1) the number and intensity of summertime cyclones passing over the northern United States is 

 3-39 
a key factor in determining air quality there and (2) the occurrence of fewer and weaker cyclones 

is a plausible consequence of global climate change. 

This discussion about cyclones suggests a broader question: how should the scientific 

community evaluate the performance of these modeling systems for the task at hand? It is not 

possible to answer this question comprehensively here, but it is possible to place some general 

issues with which the climate modeling community continuously struggles in the context of the 

specific problem of climate change impacts on air quality. 

First, all groups carry out evaluations of their modeling systems compared to historical 

observations. The key is to conduct these evaluations for the variables, and statistics of those 

variables, that are most relevant for the problem of interest. As discussed in various places in 

this report, “air quality,” from a health, environmental, and regulatory perspective in the United 

States, has been largely determined by episodes that occur during specific, sporadic weather 

events. Therefore, what is most important to know is how well available modeling tools 

simulate these events and how well they can predict future changes. At present, however, the 

focus of the climate modeling community is still largely on long-term mean values of variables 

like temperature, precipitation, and cloud cover. These quantities can be important in situ drivers 

of air quality on short timescales, but more effort is needed to understand how changes in 

atmospheric flow patterns are reflected in the changes in these long-term means. There is a need 

to address questions like “Did a simulated temperature change in a given region result from an 

across-the-board change in baseline temperature during all weather regimes, or instead from a 

change in the frequency of occurrence of one particular weather pattern (e.g., the afternoon sea 

breeze, synoptic-scale anticyclones, or mesoscale convective systems)?” Climatological 

averages of variables like temperature will only have explanatory power for air quality to the 

extent that they reflect the changes in the most relevant circulation patterns, as opposed to being 

obscured by “noise” that is less related to air quality (e.g., increases in nighttime average 

temperature). 

The current situation reflects the relatively youthful state of coupled climate and air 

quality science. The application of climate models to air quality represents a significant 

challenge for the climate modeling community. One path forward is to make it standard practice 

to conduct in-depth evaluations of global and regional climate models for additional variables 

and metrics more relevant for air quality. As Gustafson and Leung (2007) state, 

Our ability to address these questions relies critically on the ability of climate 

models in simulating the meteorological conditions needed to realistically 

simulate air quality. Because of the nonlinear nature of atmospheric chemistry 

and its dependence on difficult to model variables, such as precipitation and the 

planetary boundary layer (PBL) height, biases in variables considered acceptable 

 3-40 
for other downscaling applications may not be appropriate for this new 

application. An additional challenge in air quality assessment is the required 

knowledge of the three dimensional structures of the atmosphere, which are not 

needed for most other assessments. 

New efforts carried out under the auspices of this assessment, as summarized in Leung 

and Gustafson (2005), Gilliam et al. (2006), and Gustafson and Leung (2007) represent 

significant advances in this area and provide useful insights moving forward. 

Second, it is important to remember that, for the problem under consideration here, 

accurately reproducing present-day conditions is not interesting in and of itself, but is interesting 

for what it might imply for simulating and understanding future changes. The connection 

between the two is not necessarily straightforward. Again from the IPCC AR4: “What does the 

accuracy of a climate model’s simulation of past or contemporary climate say about the accuracy 

of its projections of climate change? This question is just beginning to be addressed...” (IPCC, 

2007: Ch. 8). 

Given a particular variable, and statistic of that variable, to be evaluated, there are two 

sources of error in any future-minus-present comparison: the bias in the present-day simulation, 

and some (hypothetical) bias in simulating the future conditions. The modeling community 

typically makes two implicit assumptions about these sources, but these assumptions are 

potentially contradictory. First, there is the assumption that these two errors are correlated, i.e., 

the better the modeling system is at reproducing present-day observations, the better it will be at 

reproducing future climate shifts. This could lead logically to the conclusion that a model 

system that does a poor job of simulating the present will likely be even worse at getting the 

“correct” future-minus-present changes. However, it is often simultaneously asserted that 

looking at differences between simulated future and present results will yield accurate insights, 

i.e., that the biases should be similar in the present and future simulations and thus will cancel. 

Barring improbable coincidences, these two assumptions can only be reconciled if a third 

assumption also holds: namely, that most of the biases in the present-day simulation come from 

error sources that will not impact the model’s ability to capture the future changes, i.e., the 

present-day biases will simply be carried along to the future. The validity of this assumption for 

a highly nonlinear system like climate must be tested. Again, research carried out for this 

assessment is contributing to this need. For example, Liang et al. (2008) showed how GCM (and 

downscaled RCM) biases with respect to historical observations are consistently propagated into 

future simulations, empirically linking the ability of a modeling system to accurately reproduce 

present-day climate to the types of future climate changes it predicts. 

 3-41 
3.4.2 The Role of Downscaling 

As described in Section 2, this assessment has been built, in part, around dynamical 

downscaling, i.e., the use of an RCM to derive higher-resolution meteorology from a GCM 

simulation for a particular sub-region of the globe. This is in recognition of the dual need to be 

regionally explicit, so as to connect more closely with the priorities of policy makers, while at 

the same time capturing the inherently global scale of the climate drivers. As noted, this is really 

the first systematic attempt to apply these techniques to air quality impacts work, and valuable 

lessons are being learned. 

The fundamental task of dynamical downscaling is to maximize the “value retained” 

from the GCM and the “value added” by the RCM. In other words, successful downscaling will 

take advantage of the things the RCM does well in simulating weather and climate, by virtue of 

its high resolution, without sacrificing too much of what the GCM does well, by virtue of its 

global extent. From the results presented above, it is clear that changes in both large-scale 

circulation patterns and local-scale forcings are crucial drivers of O3 changes. A given modeling 

system will be able to accurately simulate changes in O3 only to the extent that it can accurately 

capture both. 

Because of its higher resolution, the RCM develops small-scale features that the GCM 

cannot. These features develop for three primary reasons (see, e.g., Denis et al., 2002): 

• finer-scale representations of surface characteristics, like topography, water bodies, 

vegetation, soil moisture, and land use, that lead to local-scale circulation systems like 

sea and lake breezes and mountain-valley flows; 

• nonlinearities in the fluid dynamics equations that lead to the development of fronts and 

other mesoscale features; 

• hydrodynamic instabilities arising from shear or buoyancy forcing that create turbulent 

eddies and convection and are more accurately represented with higher resolution. 

RCMs therefore add the most value by more accurately simulating near-surface 

meteorological fields, as well as extreme conditions (e.g., cyclone low pressure, intense 

precipitation, high winds). These advantages make it possible to significantly improve on 

regional biases in temperature and precipitation present in GCM simulations (e.g., see Liang et 

al., 2006), and these improvements can lead directly to improved simulations of O3. 

RCM performance is highly sensitive, however, to the physical parameterizations used, 

as already summarized above. For example, Liang et al. (2006; 2004a,b) and Lynn et al. (2004; 

2007) found strong sensitivities of temperature and precipitation to the convection scheme 

chosen. These meteorological sensitivities drive corresponding sensitivities in simulated air 

 3-42 
quality (e.g., Kunkel et al., 2007; Tao et al., 2008). In addition, sensitivities of air quality to 

PBL, radiation, microphysics, and land-surface schemes may also be important, but these have 

yet to be examined as systematically in this assessment. 

Along with the physical parameterizations, the other major sensitivity of the RCM is the 

application of the large-scale boundary conditions from the GCM, i.e., the actual 

“implementation” of the dynamical downscaling that links the GCM with the RCM. By itself, an 

RCM cannot simulate the large-scale circulation of the atmosphere because the drivers are 

planetary in scale (e.g., the difference in net radiation between equator and poles), necessitating a 

global domain. So, for example, an RCM cannot generate dynamical systems like the 

mid-latitude storm tracks, which instead must be supplied by a GCM. It is in the context of this 

GCM-provided large-scale circulation that the smaller-scale features described above evolve. 

This leads to the basic question of dynamical downscaling: how best to close the system? In 

other words, what is the optimal method for importing information from the GCM into the RCM 

so as to preserve any desired features of the large-scale circulation patterns without 

compromising the ability of the RCM to develop realistic smaller scales? 

The most common practice has been to assimilate the GCM fields into a narrow strip at 

the lateral boundaries of the RCM domain. This technique is commonly referred to as “lateral 

nudging,” and follows Davies (1976). Everywhere else in the domain, the RCM develops its 

own solution, which it is hoped will evolve consistently within the envelope defined by the GCM 

flow at the boundaries. This approach is widely used and has yielded valuable results in a 

number of different applications across the field of regional climate modeling. It is the approach 

that is used in all the downscaling work contributing to this report. The major perceived 

advantage of this approach is that it allows for the possibility of the RCM correcting biases not 

only in the relatively fine-scale, near-surface temperature, and precipitation features, but also in 

continental-scale circulation patterns. For example, Gustafson and Leung (2007) illustrate how a 

better representation of the Rockies leads to improvements in the overall flow patterns over the 

United States when MM5 is used to downscale the GISS II′ GCM simulation. 

Recent work (see Rockel et al., 2008; Miguez-Macho et al., 2004, 2005; Castro et al., 

2005; von Storch et al., 2000), however, suggests that this lateral nudging approach can be 

problematic and introduce additional biases of its own. Specifically, if the RCM captures the 

energy of the large-scale flow only through assimilation at its lateral boundaries, two problems 

can arise. First, the energy of the large-scale circulation can be progressively lost as a result of 

several factors as it makes its way into the domain from the RCM boundaries. This lost energy 

cannot be re-supplied by the RCM, since, as already noted, the drivers are planetary in scale. A 

potential consequence, then, is weaker large-scale circulation features in the RCM compared to 

the GCM. Second, the large-scale flow field can be modified significantly as it makes its way 

 3-43 
across the RCM domain. This can cause problems at the RCM boundaries that, in turn, can 

introduce artificial flow features back in the main body of the model domain. For example, the 

jet stream entering the western boundary of the RCM domain will encounter the steeper (because 

higher-resolution) Rockies and be deflected, so that by the time it reaches the eastern boundary, 

it will not be consistent with the GCM boundary condition there. Both of these problems are 

more pronounced with larger RCM domains and coarser RCM resolution.16

One method for handling these problems is so-called “spectral nudging,” i.e., nudging 

applied not at the lateral boundaries at all spatial scales, but instead applied at all locations in the 

RCM domain (above the PBL at least) but only for the longest waves that are resolved in the 

GCM (see Miguez-Macho et al., 2004 and von Storch et al., 2000 for descriptions of the 

technique). At this time, whether lateral nudging or spectral nudging is preferable is just 

becoming an active research question: does one take the large-scale flow field of the GCM as 

“truth” and force the RCM to conform to it as closely as possible, or does one instead allow the 

RCM to evolve a more independent circulation? Therefore, the implications for simulating air 

quality are as yet unclear, since the downscaled simulations carried out to date for this 

assessment have all used the lateral nudging approach. 

Given what we do know at this time about dynamical downscaling, however, the 

following should be considerations when interpreting the regional air quality results presented in 

this section: 

• The RCM may not faithfully capture important features of the large-scale circulation 

patterns present in the driving GCM. In particular, the large-scale flow might be too 

weak in the RCM, leading to a proportionally too-strong influence of more local-scale 

forcing, like convection. Alternatively, there might be artificial flow features introduced 

by discrepancies between the RCM and GCM at the boundaries. 

• Even if the RCM reproduces the GCM’s large-scale circulation very closely, it may still 

simulate different air quality patterns because of differences in the way it simulates 

convective clouds and rainfall, or other fine-scale processes, embedded within this largescale flow. 

Either or both of these considerations may help explain why, as mentioned previously, 

the influence of a shift in the storm track present in the Mickley et al. (2004) GCM experiment 

does not show up as clearly when this same GCM simulation is downscaled using MM5 (Nolte 

et al., 2008; Leung and Gustafson, 2005). Precisely attributing these differences between the 

downscaled results and the driving global simulation remains a key task in the furthering of our 

 16 To date, these two potential pitfalls of lateral nudging have mostly been investigated for RCM 

simulations driven by global reanalysis data and not GCM output, and there may be differences between the two in 

the impact on the downscaled fields. 

 3-44 
understanding of the impacts of global climate change on regional air quality, and it remains the 

subject of ongoing investigation. 

In any case, the strong influence of the GCM-simulated climate on the downscaled results 

is inescapable, regardless of the methodological details. Gustafson and Leung (2007) emphasize 

that the GCM chosen will strongly impact any downstream regional air quality findings. Nolte et 

al. (2008) show clearly that a large portion of the bias in the NERL group’s regional simulations 

for the eastern United States can be traced directly to an incorrect northeastward displacement of 

the Bermuda High in the driving GISS II′ GCM simulation. This and similar results, then, 

underscore again the discussion from above: quantifying the biases and characteristics of the 

individual global model simulations being relied upon for representing future climate change is 

of critical importance for the problem of global change impacts on air quality. 

3.4.3 Uncertainties in Chemical Mechanisms 

The differences in simulated O3 as a function of isoprene chemistry, as discussed in 

Section 3.3.2, highlight the importance of the choice of regional air pollution modeling 

mechanisms in investigating the potential for climate-induced changes in air quality. Differences 

in simulated O3 concentrations among modeling studies may be attributed, in part, to differences 

in the choice of photochemical mechanism. Each mechanism, in turn, will have characteristic 

uncertainties as well as biases in simulated O3 concentrations, due to uncertain information about 

the chemical reactions represented by the model mechanism, and the simplifying assumptions 

used to optimize computational speed. 

For example, Atkinson (2000) listed several sources of scientific uncertainty in air quality 

photochemical modeling mechanisms. Note that most of these uncertainties relate to the 

oxidation of biogenic compounds: 

• Quantitative knowledge of the rate constants and mechanism of the reactions of organic 

peroxy (RO2) radicals with NO, HO2 radicals, and other RO2 and NO3 radicals 

• Additional data concerning the organic nitrates yields from the reactions of organic 

peroxy radicals with NO as a function of temperature and pressure 

• Knowledge of reaction rates of alkoxy radicals for decomposition, isomerization and 

reaction with O2, especially alkoxy radicals other than those formed from alkanes and 

alkenes (for example, from hydroxyl-compounds, ethers, glycol ethers, and esters) 

• The detailed mechanisms of the reactions of O3 with alkenes and VOCs containing 

>C=C< bonds (this involves understanding the reactions of the initially energy-rich 

biradicals and of the thermalized biradicals formed in these reactions) 

 3-45 
• Studies of the thermal decompositions of other atmospherically-important reactions of the 

higher PANs, including, for example, CH2=C(CH3)C(O)OONO2 formed in the 

atmospheric photo-oxidation of isoprene 

• Further understanding of the products and mechanisms of the reactions of monoterpenes 

and oxygenated VOCs (including 2-methyl-3-buten-2-ol) emitted from vegetation with 

OH radicals, NO3 radicals and O3

• Improved knowledge of the mechanisms and products of the reactions of OH-aromatic 

adducts with O2 and NO2

• An improved understanding of the tropospheric chemistry of many oxygenated VOCs 

formed as first-generation products of VOC photooxidations, including but not limited to 

carbonyls (including unsaturated dicarbonyls), di-unsaturated dicarbonyls, and 

unsaturated epoxy-carbonyls), hydroperoxides, and esters 

• A quantitative understanding of the reaction sequences leading to products which 

gas/particle partition and lead to secondary aerosol formation 

Compounding the uncertainties created by incomplete information about the chemical 

pathways, and their associated rate constants, is the practical necessity for abbreviating the 

overall chemical reaction scheme to improve the computational speed of the air quality model. 

A photochemical oxidation mechanism that explicitly treated all of the known atmospheric 

reactions would have to include more than 20,000 reactions and several thousand organic 

reactants and products (Dodge, 2000). Reaction schemes like those used by the research teams 

participating in the EPA assessment program have been streamlined to minimize the number of 

reaction steps, either by lumping several relevant organic compounds into classes that are given 

“average” reaction rates, or by ignoring reaction pathways that appear to be unimportant in 

determining the concentrations of the targeted pollutant. These design choices are made through 

a process of evaluation against observational data. Mechanisms may also be fine-tuned to 

produce output that better fits the ambient data by adjusting the reaction rate constants within the 

laboratory-established experimental uncertainty range. Furthermore, differences exist amongst 

models in the size of the time steps used for calculating pollutant concentrations. Given the 

complex, nonlinear, nature of O3 production, these differences tend to result in differences in 

predicted O3 concentrations among models. See also Fine et al. (2003) for additional discussion 

of many of these issues. 

A number of intercomparison studies of photochemical mechanisms have been reported 

in the air quality literature: e.g., see Russell and Dennis (2000), Jimenez et al. (2003), and Faraji 

et al. (2008), among many others. For example, Jimenez et al. (2008) compared box model 

calculations, with identical inputs and boundary conditions, for several state-of-the-art 

photochemical mechanisms. They found that the calculated average and maximum O3

 3-46 
concentrations, along with the concentrations for nine related chemical species, varied widely 

between these mechanisms. Gilliland et al. (2008) evaluated the performance of the CB-IV and 

SAPRC99 chemical mechanisms in a study of model response to NOx reductions associated with 

the implementation of the NOx SIP Call. They found that CB-IV significantly underestimates 

the contribution of O3 (and its precursors) from long-range transport, and it is less successful 

than the SAPRC99 mechanism at capturing the effects of meteorological changes on O3

concentrations. 

Variability in simulated O3 concentrations among photochemical mechanisms does not 

necessarily imply that any one mechanism is incorrect: rather, that each may have been 

optimized for different local or regional conditions. Given the necessity of simplifying 

photochemical mechanisms for the sake of computational efficiency, future studies of climate 

change-induced air quality change might reasonably include photochemical mechanisms that 

have been tailored to perform best under a range of well defined conditions consistent with the 

emissions, meteorological, and land-use conditions under consideration. 

Additional detailed discussions of both well understood and highly uncertain O3

photochemistry can be found in the U.S. EPA Air Quality Criteria Document for O3 (U.S. EPA, 

2006). 

3.5 SYNTHESIS CONCLUSIONS AND FUTURE RESEARCH NEEDS 

This section concludes by collecting and summarizing the major points that have 

emerged from the scientific synthesis. These help address the goals of this report by addressing 

questions like “What new findings are emerging from the body of work that EPA has made 

possible?” and “What have we learned about our ability to simulate potential future changes in 

U.S. regional air quality due to climate change?” Specifically, 

• Across all of the modeling experiments, global and regional, carried out by the different 

groups, simulated global climate change causes increases in summertime O3

concentrations over substantial regions of the country. For nearly every region of the 

country, at least one (usually multiple) of the modeling groups found that climate change 

caused increases in summertime O3 concentrations. 

• For summertime-mean MDA8 O3, the increases are in the 2−8 ppb range. 

• The largest increases in O3 concentrations in these simulations occur during peak 

pollution events. For example, the increases in 95th percentile MDA8 O3 tend to be 

significantly greater than those for summertime-mean MDA8 O3. 

• Though in agreement on the above points, the different modeling systems did not 

necessarily simulate the same regional patterns of climate-induced O3 changes, with the 

individual simulations showing some regions of little change, or even decreases, in 

addition to the O3 increases. 

 3-47 
• These differences in the regional patterns of O3 changes result from variations across the 

simulations in the patterns of mean changes in key meteorological drivers, such as 

temperature and surface insolation. The modeling experiments provide examples of 

regions where simulated future changes in meteorological variables either have 

reinforcing or competing effects on O3 concentrations. Figure 3-11 shows the mean and 

standard deviation in future-minus-present MDA8 O3 differences across all seven sets of 

simulation results displayed in Section 3.3. 

• For example, regions where the changes in simulated temperature and insolation are in 

the same direction tend to experience O3 concentration changes in a similar direction, 

while temperature and insolation varying in opposite directions tends to correspond with 

mixed O3 changes. Figure 3-12 shows regional averages across MDA8 O3 concentration 

differences and the differences in these drivers for the sub-regions shown in Figure 3-13. 

• Large-scale circulation patterns play an important role in modifying these local 

meteorological drivers. For example, how a given modeling system simulates changes in 

key circulation features, like the mid-latitude storm track or the Bermuda High, has a 

strong impact on the simulated future O3 concentrations. 

• Other factors to which the patterns in the simulated meteorological variables appear to be 

highly sensitive include the choice of convection scheme and whether or not the global 

model outputs are dynamically downscaled with an RCM. 

• Certain regions show greater agreement than others. For example, there is very generally 

more agreement on the spatial patterns of climate-induced increases for the eastern half 

of the country than for the West, though parts of the Southeast show some of the 

strongest disagreements across the modeling groups. Even for these regions, however, at 

least some of the models show substantial climate-induced O3 increases. 

• Across nearly all simulations, climate change is associated with simulated increases in 

biogenic VOC emissions over most of the United States, with especially pronounced 

increases in the Southeast. 

• These biogenic emissions increases do not necessarily correspond with O3 concentration 

increases, however, depending on the region and modeling system. 

• One factor in this, as highlighted by the global modeling results, is that the response of O3

to changes in biogenic emissions may depend sensitively on how isoprene chemistry is 

represented in the model. Models that recycle isoprene nitrates back to NOx may tend to 

simulate greater O3 concentration increases in regions with biogenic emissions increases 

than models for which isoprene nitrate is a terminal sink for NOx. 

• Interannual variability plays a critical role in determining seasonal-average O3 levels in a 

given year. Some of the modeling groups found that, in some regions of the United 

States, the average increase in MDA8 O3 concentrations from the present to the 2050s as 

a result of climate change was as large as the present-day year-to-year variability. In 

other words, climate change has the potential to push O3 concentrations in extreme years 

beyond the envelope of natural interannual variability. 

 3-48 
 3-49 

Figure 3-11. The mean (top two panels) and standard deviation (bottom two 

panels) in future-minus-present MDA8 O3 concentration differences across 

(left-hand panels) all seven experiments (five regional and two global) shown 

in Figures 3-1, 3-9, and 3-10 and, for comparison purposes, (right-hand 

panels) not including the WSU experiment because it shows differences for 

July only, while the other experiments show JJA differences. 

Mean MDA8 O3 Mean MDA8 O (ppb) 3 (ppb)

Standard Deviation in MDA8 O3 (ppb) Standard Deviation in MDA8 O3 (ppb)

40N

30N

40N

30N

40N

30N

40N

30N

110W 100W 90W 80W 110W 100W 90W 80W

0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.8

110W 100W 90W 80W 110W 100W 90W 80W

-4.5 -3.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5

Mean MDA8 O3 Mean MDA8 O (ppb) 3 (ppb)

Standard Deviation in MDA8 O3 (ppb) Standard Deviation in MDA8 O3 (ppb)

40N

30N

40N

30N

40N

30N

40N

30N

110W 100W 90W 80W 110W 100W 90W 80W

0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.8

110W 100W 90W 80W 110W 100W 90W 80W

-4.5 -3.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5
MDA8

-9

-6

-3

0

3

6

NE MW CA TX SE

Harvard.A1B CMU.A2 PGR.B1 NERL.A1B GATECH.A1B WSU.A2 PGR.A1Fi

SAT

-2

-1

0

1

2

3

4

NE MW CA TX SE

Harvard.A1B CMU.A2 PGR.B1 NERL.A1B GATECH.A1B WSU.A2 PGR.A1Fi

Figure 3-12. Averages across the subregions shown in Figure 3-13 for 

each of the simulations for (a) mean MDA8 O3 (ppb); (b) near-surface 

air temperature (oC). 

 3-50 
SWDOWN

-20

-15

-10

-5

0

5

10

15

20

25

NE MW CA TX SE

Harvard.A1B CMU.A2 PGR.B1 NERL.A1B GATECH.A1B WSU.A2 PGR.A1Fi

VOC

-12

-8

-4

0

4

8

12

16

20

NE MW CA TX SE

Harvard.A1B CMU.A2 PGR.B1 NERL.A1B GATECH.A1B WSU.A2 PGR.A1Fi

Figure 3-12 continued. Averages across the subregions shown in Figure 3-13 

for each of the simulations for (c) surface insolation (W m-2); and (d) biogenic 

isoprene emissions (g Carbon m-2 sec-1). 

 3-51 
Figure 3-13. The averaging subregions used in Figure 3-12. 

• It also highlights the fact that the amount of future-minus-present change in O3

concentration simulated will likely depend strongly on the choice of present and future 

simulated years to compare, and that multi-year simulations are desirable for producing 

findings that are more robust. 

• In addition, while this analysis focuses on summertime results, some of the groups also 

found increases in O3 concentrations in some regions in the spring and fall, suggesting 

the possibility of an extension of the O3 season under climate change. 

These findings should be interpreted as speaking to the question, “How does the system 

work?” rather than the question, “What will happen in the future?” They provide insight into the 

subtleties and complexities of the interactions between climate, meteorology, and air quality, 

thereby helping to build intuition about the richness, and range of behaviors, of the climate-air 

quality system. They also illustrate how valuable the modeling systems developed for this 

assessment can be for exploring this problem. 

This improved system understanding, combined with a clear appreciation of the 

important uncertainties, opens the doors to a wide range of future applications based on this 

knowledge and these tools. For example, the results of modeling experiments have the potential 

 3-52 
to provide guidance as to whether, for example, statistical relationships based on historical 

observations of O3 and temperature will serve as accurate approximations of the effects of 

climate change in a given region. Other applications might include evaluating the potential for 

unintended consequences of a particular policy choice, e.g., whether tree plantations for carbon 

sequestration might harm air quality in a given region in the face of future climate change. 

In addition, these findings highlight a number of areas where further research is needed: 

1. First, as has been emphasized throughout, an improved understanding of how well 

models simulate the large-scale circulation patterns that are important for air quality is 

needed. This issue was being considered at least as early as 1991, when the NRC pointed 

out that whether a GCM simulated a persistent high or low pressure pattern over a given 

region had the potential to counteract any increase in O3 associated with warmer 

temperatures, through changes in other meteorological drivers (NRC, 1991). The NRC 

also pointed out in this report that GCMs do not in general simulate the same shifts in 

pressure patterns in response to increases in greenhouse gases. As discussed above in 

Section 3.4, these kinds of disagreements among models persist today. 

2. As a related point, there is a need for an improved understanding of how well RCMs can 

downscale changes in these GCM-simulated circulation patterns, as well as a need for 

more insight into the sensitivity of these downscaled regional simulations to model 

parameterizations, including convection schemes, but also expanding to PBL, radiative 

transfer, microphysics, and land-surface schemes. 

3. Recalling the discussion surrounding Box 3-1, a critical component of addressing points 

1 and 2 above will be extending efforts, initiated in this first phase of the assessment, to 

evaluate the GCM- and RCM-based systems for the meteorological variables, and 

especially the temporal statistics of the meteorology, most appropriate for air quality: for 

example, long-term average changes in the frequency, duration, and intensity of 

stagnation episodes driven by synoptic-scale variability. This will need to include 

outputting and analyzing the required quantities, at the required temporal frequency, from 

the models, as well as further analyses of historical observational data. 

4. Development and refinement of techniques for systematically exploring the effects of the 

modeling uncertainties are also needed, including ensemble methods, techniques for 

blending ensemble approaches with dynamical downscaling, and reduced form models. 

5. An issue raised in a small subset of the results discussed in this section is whether or not 

the possible future extension of the O3 season into the spring and fall is robust across 

more simulations. Additional simulations that go beyond summertime are needed to 

address this. 

6. Another issue arising from a small subset of the results is the question of interannual 

variability. Particularly in the regional modeling results, to date there is disparity in the 

number of years simulated across the different groups. Moving forward, more precise 

quantification of the magnitude of mean future O3 changes relative to interannual 

variability, as well as the potential for future increases or decreases in interannual 

variability itself, is needed. 

 3-53 
Moving beyond meteorology, the results to date also suggest important gaps in our 

understanding of issues related to chemistry and emissions: 

1. More research is needed into the links between climate, biogenic emissions, and O3. The 

results presented here highlight the importance of correctly representing isoprene nitrate 

chemistry in models to accurately capture the response of O3 to changes in emissions. In 

addition, there are other uncertainties in chemical mechanisms with the potential to 

influence climate change-air quality impacts that require further study. 

2. Improving biogenic emissions inventories and process models of the response of biogenic 

emissions to climate and atmospheric composition changes should also be a priority. 

3. Changes in deposition velocity as a function of the impact of changing CO2

concentrations on stomatal conductance could also be incorporated into the modeling 

systems. 

4. An overarching issue that has not been fully addressed to date is whether or not the 

overall O3 chemical regime change as a function of climate change, and/or global 

atmospheric composition change (e.g., as a function of changing concentrations of CH4

and other species). 

5. As already discussed, while some of the groups have also carried out simulations of PM, 

in addition to O3, the focus in this section is only on the O3 results. Our understanding of 

how to represent PM chemistry in modeling systems is more limited, and there are a 

number of additional complexities surrounding PM, including the fact that it consists of 

multiple species, and that precipitation is a more important primary meteorological driver 

for PM than for O3, an issue because the uncertainties in modeling precipitation are much 

greater than in modeling, for example, temperature. Much additional research is needed 

on simulating the potential impacts of climate change on PM. Brief summaries of the 

ongoing work on PM under this assessment, as well as on emissions and chemistry 

issues, is provided next, in Section 4. 

Furthermore, there are a wide range of issues related to anthropogenic emissions of 

precursor pollutants that will become important as the assessment moves into its next phase. 

These include the impacts of changes, on future emissions in the United States (and worldwide), 

in: 

• Energy use 

• Land use 

• Agricultural practices 

• Transportation patterns 

• Demographics 

• Technology 

 3-54 
 3-55 

Building on the modeling experiments discussed here, one major consideration is that 

much additional work is needed to construct emissions scenarios that are realistic and internally 

self-consistent across both greenhouse gases and precursor pollutants. These and other issues 

will feature prominently in Phase II of the assessment, and they are previewed in Section 4. 

Finally, there are a number of issues for the air quality management community to 

consider, related to the potential for scientific research to provide improved decision support. 

These include how best to inform the scientific community about the specific air quality metrics 

to focus on in research that would best inform management activities, as well as how best to 

address mismatches between the timescales of air quality management and long-term global 

climate change. 
4 FUTURE DIRECTIONS 

4.1 PHASE II OF THE GLOBAL CHANGE AND AIR QUALITY ASSESSMENT 

As outlined in Section 2, Phase II of the assessment program requires a transition from 

climate-only studies to an evaluation of the integrated effects of changes in climate and changes 

in anthropogenic air pollutant emissions. Simplistic assumptions about future U.S. emissions are 

of limited usefulness for evaluating the possible range of climate change impacts on air quality at 

scales that are of interest for planning and management. Therefore, EPA ORD has initiated 

several projects that are developing new methods and modeling tools for creating regional-scale 

emissions projections for the United States. These projects recognize that the important drivers 

of future changes in air pollutant emissions are linked. For example, economic factors influence 

population migration which, in turn, affects land use, thereby affecting air pollutant emissions 

via choices in transportation modalities. To realistically represent the feedbacks among the 

drivers of air pollutant emissions, modeling systems must be developed that capture these links 

between underlying processes. 

Phase II of the air quality assessment will also build upon the insights gained in Phase I 

from the efforts of the contributing research teams in producing climate change-only air quality 

simulations, including the effects of particular modeling choices. This section, therefore, begins 

by highlighting efforts underway to improve the climate-air quality modeling systems, and 

planned efforts to develop efficient approaches for evaluating the impact of uncertainties on 

model outputs. An overview of the projects focused on devising modeling tools to capture the 

processes governing the underlying drivers of air pollutant emissions, and the links between 

them, follows. Air pollutant emissions scenarios will eventually be shared with the climate-air 

quality modeling teams, who will, in turn, simulate the integrated effects of climate and 

emissions changes on regional U.S. air quality. 

4.2 EXTENDING THE MODELING SYSTEMS 

 Section 3 concluded with a discussion of modeling uncertainties and research needs to be 

addressed. Ongoing and upcoming activities designed to achieve these improvements and 

needed advances in modeling capability are discussed in the following subsections. 

4.2.1 Exploring Modeling Uncertainties 

Ensemble modeling techniques are being applied to more fully explore the effects on 

model outputs of uncertainties in the global-to-regional climate and air quality modeling 

systems. This involves blending multiple alternative GCMs, RCMs, and RAQMs with multiple 

4-1
emissions scenarios and model physical parameterizations (including both PBL and convection 

schemes). In addition, some of the work will explore the use of Bayesian weighting of ensemble 

members based on their skill in representing both observed climate and air quality, as a means of 

reducing the number of ensemble members required for capturing the probable range of future 

climate changes. Adding new GCMs, RCMs, and RAQMs to the suite used in Phase I is also an 

important element of this work. 

Several modeling teams plan to evaluate potential changes in the length and timing of 

annual O3 seasons under a changed climate. To better capture and characterize changes in 

interannual variability in different climate regimes, simulations of additional present-day and 

future years with the global-to-regional modeling systems are also planned. 

Finally, the groups discussed in Section 3 that carried out global scale-only simulations 

are in the process of conducting comparable studies using downscaled global-to-regional 

modeling systems. The application of these new systems to simulations of future regional 

climate and air quality will also expand the range of models, scenarios, and methodologies in the 

assessment. Added to the results obtained to date, these new simulations have the potential to 

increase the level of confidence in, and/or add nuance to, key conclusions made in this report. 

4.2.2 Additional Model Development 

Substantial uncertainty remains in the modeling of current biogenic VOC emissions. 

EPA ORD is currently supporting studies to better define the processes governing biogenic 

emissions to improve their representation in regional air quality modeling systems. These 

studies include work to identify and quantify species-dependent emissions sensitivities to 

temperature and other meteorological variables, to changes in forest composition in response to 

changing climate, and to changes in ambient CO2 concentrations, based on observations and 

biochemical modeling. 

The accumulating body of new scientific insights is being used to design biogenic 

emissions models with greater process realism. These models are also being extended to include 

complementary capabilities, such as dynamic vegetation sub-models to capture the two-way 

coupling between land cover and climate. These improvements will assist in increasing our 

understanding of the potential role of biogenic emissions changes in global change-related 

impacts on air quality. 

The importance of feedbacks between climate change and regional air quality is not 

presently well understood. Should climate change produce significant changes in aerosol 

chemistry and composition, or substantial changes in tropospheric O3, those perturbations could 

feed back onto the Earth’s radiation budget, possibly driving further changes in climate. Other 

research efforts within the assessment program include an investigation of the importance of 

4-2
these two-way feedbacks between climate change and air quality. To explore this question, 

NERL is expanding the pollutant chemistry represented in the Weather Research and Forecast 

Model with Chemistry (WRF/Chem). Simultaneously, an extramural effort funded by the STAR 

program is directly linking WRF with the CMAQ model in a combined WRF-CMAQ system. 

Both will be applied in studies of future climate and air quality. Downscaling GCM simulations 

of future climate using WRF/Chem and WRF-CMAQ will allow for the assessment of possible 

long-term impacts of global change on regional air quality while accounting for feedbacks 

between meteorology, air quality, and radiation in a unified modeling framework. 

4.2.3 Additional Pollutants—PM 

 Some of the groups whose O3 results are featured in Section 3 have also carried out 

simulations of PM. Because of the additional complexities and uncertainties associated with PM 

and its response to climate change, these results were not incorporated into the synthesis. 

However, a few preliminary results suggest that 

• Globally, PM generally decreases as a result of simulated climate change (with 

anthropogenic emissions held constant), due to increased atmospheric humidity and/or 

increased precipitation; 

• Regionally, simulated climate change produces both increases and decreases in PM (on 

the order of a few percent) in 2050, depending on the region of the United States, with 

the largest increases in the Midwest and Northeast; 

• The responses of the individual species that make up net PM (e.g., sulfate, nitrate, 

ammonium, black carbon, organic carbon, etc.) to climate change are highly variable, 

depending on the chemistry and transport characteristics of each species; 

• Key uncertainties to which simulated PM is sensitive include model precipitation, model 

aerosol chemistry, aerosol-cloud interactions, volatilization of semi-volatile PM species, 

such as nitrate and secondary organic aerosol (SOA), and assumed future air pollution 

emissions. 

 Building on these findings, work underway, both within EPA and funded through the 

STAR program, is continuing to explore the impacts of climate and emissions changes on PM in 

coupled climate and air quality modeling systems. Efforts to improve the relevant aerosol 

chemistry in these models, as well as to introduce the capability of two-way coupling between 

chemistry and meteorology (as noted above) are also underway. In addition, substantial work is 

being done outside the EPA sphere that is expected to contribute knowledge and techniques as 

the assessment moves forward. 

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4.2.4 Additional Pollutants—Mercury 

Some of the modeling groups already highlighted in this report, in conjunction with 

several new groups, will also be extending our understanding of the impact of global change on 

air pollution to mercury (Hg). Climate change can potentially impact a number of atmospheric 

processes that help determine the fate of Hg, including heterogeneous oxidation of gas-phase Hg, 

dry deposition of elemental, reactive gas-phase and particulate Hg, and Hg chemistry in the 

presence of fog, clouds, and photochemical smog. 

These groups will use both models and observational datasets to explore Hg chemistry 

and transport as a function of climate and emissions changes. The focus will be on present and 

future Hg distribution for the United States as a whole, as well as for particular regions, e.g., the 

Great Lakes, Florida. In addition, this work will be aimed at improving the Hg chemistry in the 

linked climate and air quality modeling systems by incorporating additional reactions and 

refining existing representations. 

4.3 COMBINED IMPACTS OF CLIMATE AND EMISSIONS CHANGES: 

PRELIMINARY WORK 

Several of the modeling teams that produced the simulations discussed in Section 3 also 

conducted preliminary evaluations of the combined effects of changes in anthropogenic air 

pollutant precursor emissions and changes in climate on regional U.S. quality. The general 

approach taken was to assume that, rather than remaining constant at the NEI 1999−2000 levels, 

future U.S. emissions of pollutant precursors, i.e., NOx, SO2, VOCs, and CO, scaled in ways that 

were consistent with the IPCC SRES scenarios. 

The major findings that emerged from these sensitivity studies are as follows: First, that 

the combined effects of climate and anthropogenic precursor emissions changes are much more 

sensitive to the assumptions about future emissions trajectories than differences in simulated 

climate across models and groups. For example, simple scaling of future emissions to match the 

gross assumptions of the IPCC A1b or B1 SRES scenario resulted in substantial reductions in 

NOx emissions, with corresponding reductions in simulated future O3 that dominated any 

increases associated with climate change. In contrast, using future emissions consistent with the 

weaker pollutant control assumptions in the “dirtier” A2 or A1Fi scenarios tended to result in 

climate and emissions producing changes of comparable magnitudes. Second, the effects of 

climate and emissions changes are not, in general, additive. In other words, the degree of 

“climate penalty” on air quality is itself highly dependent on the emissions levels. 

Therefore, these results highlight the need for additional work to develop more 

sophisticated, regionally detailed scenarios of U.S. anthropogenic precursor pollutants that 

account for population, economic, energy, and transportation changes, along with work to 

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improve the representation of natural emissions sensitive to climate and land-use changes. These 

efforts are highlighted in the next sub-section. 

4.4 MODELING THE DRIVERS OF AIR POLLUTANT EMISSIONS 

Human activities, such as population growth and migration, economic growth, land use, 

and technology change are key drivers affecting emissions. Changes in human activity patterns 

impact pollutant emissions across the globe, and, combined with global scale circulation 

patterns, influence the long-range transport of air pollution into the United States. 

There is a gap in our understanding of how these factors will interact to influence air 

quality at urban and regional scales in the United States. In addition, while human activities 

generate the largest share of the U.S. air pollutant emissions burden, biogenic and wildfire 

emissions also contribute to the degradation of regional-scale air quality. The vegetation 

composition and biomass density of forest ecosystems help determine both the emissions of 

biogenic VOCs and the intensity and frequency of wildfires. These properties are sensitive, to 

varying degrees, to changing climate and to local and regional development. Future progress 

will require integrating population growth and land-use models with economic forecasts, 

technology models, travel demand models, mobile source models, and forest composition and 

wildfire process models to create emissions modeling systems that can be used to blend 

comprehensive scenarios of future air pollution emissions with those of future climate and 

meteorology changes (Figure 4-1). 

As described in Section 2, evaluating the combined air quality impacts of changing 

anthropogenic emissions levels, changing biogenic and wildfire emissions levels, and changing 

climate is a critical goal of Phase II of the air quality assessment effort. To accomplish this, the 

assessment program has undertaken a significant research effort to develop and/or apply the 

necessary emissions projection tools. The following sub-sections highlight efforts underway to 

investigate the critical processes leading to pollutant emissions changes and to incorporate this 

information into modeling tools capable of realistically simulating long-term emissions changes. 

A growing U.S. population can be expected to lead to increased energy and transportation 

service demands, potentially leading to increased pollutant emissions, depending on control 

strategies implemented. In addition, internal migration of the U.S. population could redistribute 

pollutant emissions geographically. 

The Cohort-Component methodology17 is being used to develop a range of scenarios of 

future U.S. population. These scenarios build on the Census Bureau’s population projections, 

systematically incorporating assumptions to express the differences captured in the IPCC SRES 

 17 For example, see http://www.census.gov/population/www/projections/aboutproj.html. 

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Figure 4-1. Integrated system of future climate, meteorology, and emissions 

scenarios. Population growth, migration, and land use. The dashed blue 

lines represent feedbacks. 

storylines. The migration component of the demographic model uses a regression-based 

“gravity” model that depends on the functional connectivity of each county to all others and 

amenity values to estimate production and attraction values for domestic migration. This effort 

is exploring the wide range of assumptions at national, state, and local scales in the United States 

that are consistent with the general SRES storylines. 

Future development patterns will result in changes in both the quantity and location of 

pollutant emissions. The demographic-migration model described above is being coupled with a 

spatial allocation-type land-use model to develop urban and exurban growth projections 

consistent with the SRES storylines. The potential of these land-use scenarios for spatially 

allocating emission sources is under investigation. 

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4.4.1 Economic Growth and Technology Choices 

Absent additional air pollution controls and/or improvements in technologies, economic 

growth would be expected to increase emissions. Other trends, like further transformation from 

a manufacturing-based to a service-based economy, can also lead to changes in domestic 

emissions. A range of plausible economic scenarios to capture these factors is needed as part of 

an integrated evaluation of human-driven change in future emissions. Several models have been 

employed by OAR in policymaking, and the EPA’s Global Change Research Program is 

planning to evaluate them (and others) for application in the Phase II assessment effort. 

Changes in future anthropogenic emissions cannot be understood apart from the 

development, deployment, and use of energy and transportation technologies. To assist in 

defining those relationships, a Market Allocation (MARKAL) energy-systems modeling 

framework has been developed to examine the most emission-intensive sectors of the U.S. 

economy: transportation and electric power production. MARKAL maps the energy economy 

from primary energy sources, through their refining and transformation processes, to the point at 

which a variety of technologies (e.g., classes of light-duty personal vehicles, heat pumps, or gas 

furnaces) service end-use energy demands (e.g., projected vehicle miles traveled, space heating). 

A large linear programming model, MARKAL determines the least-cost pattern of technology 

investment and use required to meet specified demands, and then calculates the resulting criteria 

pollutant and greenhouse gas emissions. Preliminary scenarios of potential future emissions and 

emissions growth factors for energy system technologies, such as combustion technologies in the 

electricity generation, transportation, industrial, residential, and commercial sectors, have been 

generated for the United States. Particular attention has been paid to alternative-fuel vehicles 

(e.g., ethanol-gasoline, plug-in gasoline-electric hybrids, hydrogen fuel cell) and analyses to date 

show that different technology development and penetration scenarios can have greatly differing 

emissions consequences. 

Research has also been conducted on the response of electricity consumption to warming 

from climate change, capacity siting and dispatch decisions, and characterization of emerging 

energy generation technologies in terms of cost and cost projections and learning parameters. 

This modeling system has been used to analyze the effect of climate change upon the temporal 

and spatial distributions of NOx emissions in the Mid-Atlantic and Midwest power markets. An 

additional study investigates air quality consequences from the broad adoption of ethanolgasoline, plug-in gasoline-electric hybrids, and wind-electrolysis-hydrogen fuel-cell vehicles. 

The consequence of this technology shift will be explored for Los Angeles, the Central Valley, 

and Atlanta over the next 50 years. 

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4.4.2 Land Use and Transportation 

A critical and previously unexplored dimension in projecting air quality in response to 

human factors is the spatial distribution of the emissions projected to result from land-use and 

transportation choices. Several studies of the connection between socioeconomic forces, landuse planning and development patterns, policy design, and future air quality are underway as part 

of the assessment’s research program. Specific studies include 

• In Washington DC, development and application of a flexible modeling framework to 

estimate long-term mobile sources emissions; 

• In Chicago, an examination of the consequences of continued deindustrialization of U.S. 

manufacturing and its impact on the city’s manufacturing-heavy metro area; 

• In the Upper Midwest, a study of the air quality changes associated with a “smart 

growth” land-use and development policy over the next 25 to 50 years; 

• In the San Joaquin Valley, CA, investigation of the effect on emissions from combined 

changes in economics, land-use, water constraints, transportation, and stationary sources; 

• In the Charlotte, NC metro area, an examination of the influence of development patterns 

(e.g., transit oriented development, dense mixed-use development, development 

supportive of non-motorized transportation modes for non-work trips, neo-traditional 

suburbs, new urban core development, and redevelopment) on the spatial characteristics 

and quantity of emissions; 

• In Austin, TX, a comparison of emissions, air quality, and exposures from an integrated 

transportation-land-use model with four urban growth scenarios developed through a 

regional “visioning” initiative known as Envision Central Texas; 

• In the Puget Sound region, a project to integrate an activity-based travel model 

component and a network assignment component into a land-use model (UrbanSim) and 

to tightly couple this system to air emissions models. 

4.4.3 Emissions Changes Due to Changing Ecosystems: Biogenic VOCs 

Changing amounts and distributions of biogenic emissions due to land-use and climate 

changes is potentially a key factor for future air quality, as discussed throughout this report. Past 

studies have shown that emissions of VOCs from forest ecosystems can cause increases in 

pollution in near-urban and suburban areas. In one example, VOC emissions from forests near 

Atlanta entirely offset the effects of the policies put in place to reduce mobile-source emissions. 

As described above, substantial uncertainty remains in modeling biogenic emissions. As 

part of the assessment effort, EPA is supporting studies on the VOC-emitting species in the 

current climate. Fundamental scientific questions are being addressed concerning the chemical 

and physical properties of primary and secondary organic aerosols (POAs, SOAs), the identity of 

4-8
the biogenic VOCs that form SOAs, and the sensitivity of VOCs, POAs, and SOAs emission and 

formation rates to changes in environmental conditions. In addition, much research is being 

done outside the EPA sphere that is expected to contribute new findings to the assessment as it 

moves forward. 

4.4.4 Emissions Changes Due to Changing Ecosystems: Wildfires 

Fires, both natural and anthropogenic, have significant impacts on U.S. air quality, 

especially on PM concentrations. Recent studies show that fires in North America can have 

important effects on U.S. visibility and air quality on an episodic basis. Climate variability 

influences the extent and intensity of fires, e.g., moist years followed by dry years produce very 

favorable conditions for wildfires. Climate change, which is very likely to increase the 

frequency of precipitation in some areas, drought in other areas, and produce higher temperatures 

in general, may enhance future fire frequency, extent, and intensity regionally. 

Therefore, along with better model representations of the effects of climate change on 

biogenic VOC emissions, simulations of the effects of climate on air quality should also consider 

changing levels in wildfire-generated O3 and PM precursor emissions. Three modeling studies 

are underway that integrate the complex interactions of fire, climate, and air quality and are 

exploring important uncertainties. Two groups are focusing on the U.S. Southeast as a test case, 

with the third working to evaluate wildfire changes across the continental United States as a 

whole. All three teams are working to develop integrated models that account for fire-related 

changes in ecosystems in a warming climate, such as the extent of vegetative cover and fuel 

characteristics. State-level fire statistics, along with ground and satellite observations, will be 

used to evaluate the performance of the modeling systems. In addition, the continental-scale 

study will develop a climatology of plume heights from forest fires since 2000, and will relate 

plume heights to area burned for use in the climate change scenarios. 

4.4.5 Taking Integrated Emissions Scenarios Through to Future U.S. Regional Air 

Quality 

 As shown in Figure 4-1, Phase II of the assessment will involve integrating these 

demographic, land-use, economics, transportation and energy models to produce a series of 

future emissions scenarios as input for the integrated climate and regional air quality models 

developed in Phase I of the program. Building on the improved understanding from the work 

already accomplished, and the new insights that will emerge in the near future, an important task 

will be to identify a subset of emission scenarios that capture the range of desired assumptions 

and outcomes to explore the critical questions of interest in the integrated climate and emissions 

modeling efforts. Conducting a series of sensitivity test simulations over shorter time periods, so 

4-9
4-10

that a wider range of emissions scenarios can be tested, will likely be a key aspect of the research 

design. The results from these sensitivity tests will provide guidance on which set of scenarios 

offers sufficient representation of the range of plausible emissions changes for the future. 
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