# U.S. Climate Change Science Program 

# Synthesis and Assessment Product 3.2

September 2008 

# The First State of the Carbon Cycle Report The North American Carbon Budget and Implications for the Global Carbon Cycle Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

## FEDERAL EXECUTIVE TEAM
Director, Climate Change Science Program

............................................ William J. Brennan
Director, Climate Change Science Program Office

................................. Peter A. Schultz
Lead Agency Principal Representative to CCSP; Deputy Under Secretary of Commerce for Oceans and Atmosphere, National Oceanic and Atmospheric Administration

................................ Mary M. Glackin
Product Lead, Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration

................................ Hiram Levy II
Synthesis and Assessment Product Advisory Group Chair; Associate Director, EPA National Center for Environmental Assessment

..................................................... Michael W. Slimak
Synthesis and Assessment Product Coordinator, Climate Change Science Program Office

................................................ Fabien J.G. Laurier
Special Advisor, National Oceanic and Atmospheric Administration

............................................................. Chad A. McNutt



## EDITORIAL AND PRODUCTION TEAM

Chair

......................................................................................................... Hiram Levy II, NOAA/GFDL Scientific Editor

........................................................................................ Jessica Blunden, STG, Inc. Scientific Editor

........................................................................................ Anne M. Waple, STG, Inc. Scientific Editor

........................................................................................ Christian Zamarra, STG, Inc. Technical Advisor

..................................................................................... David J. Dokken, USGCRP Graphic Design Lead

............................................................................... Sara W. Veasey, NOAA Graphic Design Co-Lead

......................................................................... Deborah B. Riddle, NOAA Designer

................................................................................................... Brandon Farrar, STG, Inc. Designer

................................................................................................... Glenn M. Hyatt, NOAA Designer

................................................................................................... Deborah Misch, STG, Inc. Copy Editor

.............................................................................................. Anne Markel, STG, Inc. Copy Editor

.............................................................................................. Lesley Morgan, STG, Inc. Copy Editor

.............................................................................................. Susan Osborne, STG, Inc. Copy Editor

.............................................................................................. Susanne Skok, STG, Inc. Copy Editor

.............................................................................................. Mara Sprain, STG, Inc. Copy Editor

.............................................................................................. Brooke Stewart, STG, Inc. Technical Support.............................................................................................. Jesse Enloe, STG, Inc.
This Synthesis and Assessment Product, described in the U.S. Climate Change Science Program (CCSP) Strategic Plan, was prepared in accordance with Section 515 of the Treasury and General Government Appropriations Act for Fiscal Year 2001 (Public Law 106-554) and the information quality act guidelines issued by the Department of Commerce and NOAA pursuant to Section 515 <http://www.noaanews.noaa.gov/stories/iq.htm>. The CCSP Interagency Committee relies on Department of Commerce and NOAA certifications regarding compliance with Section 515 and Department guidelines as the basis for determining that this product conforms with Section 515. For purposes of compliance with Section 515, this CCSP Synthesis and Assessment Product is an “interpreted product” as that term is used in NOAA guidelines and is classified as “highly influential”. This document does not express any regulatory policies of the United States or any of its agencies, or provide recommendations for regulatory action.

# The First State of the Carbon Cycle Report The North American Carbon Budget and Implications for the Global Carbon Cycle

Synthesis and Assessment Product 3.2 Report by the U.S. Climate Change Science Program

and the Subcommittee on Global Change Research

EDITED BY: Hiram Levy II, Drew Shindell, Alice Gilliland, Larry W. Horowitz, and M. Daniel Schwarzkopf

SCIENCE EDITOR: Anne M. Waple 

# Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

### August, 2008

Members of Congress:
On behalf of the National Science and Technology Council, the U.S. Climate Change Science Program (CCSP) is pleased to transmit to the President and the Congress this Synthesis and Assessment Product (SAP), Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols . This is part of a series of 21 SAPs produced by the CCSP aimed at provid

ing current assessments of climate change science to inform public debate, policy, and operational decisions. These reports are also intended to help the CCSP develop future program research priorities.
The CCSP’s guiding vision is to provide the Nation and the global community with the science-based knowledge needed to manage the risks and capture the opportunities associated with climate and related environmental changes. The SAPs are important steps toward achieving that vision and help to translate the CCSP’s extensive observational and research database into informational tools that directly address key questions being asked of the research community.
This SAP assesses climate projections based on emissions scenarios for long-lived and short-lived radiatively active gases and aerosols. It was developed with broad scientific input and in accordance with the Guidelines for Producing CCSP SAPs, the Information Quality Act (Section 515 of the Trea

sury and General Government Appropriations Act for Fiscal Year 2001 [Public Law 106-554]), and the guidelines issued by the Department of Commerce and the National Oceanic and Atmospheric Admin

istration pursuant to Section 515.
We commend the report’s authors for both the thorough nature of their work and their adherence to an inclusive review process.

Sincerely,

Carlos M. Gutierrez Secretary of Commerce Chair, Committee on Climate Change Science and Technology Integration Samuel W. Bodman Secretary of Energy Vice Chair, Committee on Climate

Change Science and Technology Integration John H. Marburger Director, Office of Science and Technology Policy Executive Director, Committee on Climate Change Science and Technology IntegrationTABLE OF CONTENTS Synopsis

................................................................................................................................................ V

Preface

................................................................................................................................................ IX
Executive Summary

........................................................................................................................... 1



## CHAPTER 

# 1 Introduction

.............................................................................................................................. 7 Prologue

................................................................................................................................................ 7 1.1 Historical Overview

.................................................................................................................... 8 1.2 Goals and Rationale

.................................................................................................................... 9 1.3 Limitations

.................................................................................................................................. 10 1.4 Methodology

............................................................................................................................... 11 1.5 Terms and Definitions

.............................................................................................................. 11 

# 2

......................................................................................................................................................... 15 Climate Projections from Well-Mixed Greenhouse Gas Stabilization Emission Scenarios Questions and Answers

................................................................................................................. 15 2.1 Introduction

................................................................................................................................ 17 2.2 Well-Mixed Greenhouse Gas Emission Scenarios From SAP 2.1a

............................... 19 2.3 Simplified Global Climate Model (MAGICC)

.................................................................... 20 2.4 Long-Lived Greenhouse Gas Concentrations and Radiative Forcings

....................... 21 2.5 Short-Lived Gases and Particles and Total Radiative Forcing

...................................... 22 2.6 Surface Temperature: MAGICC AND IPCC Comparisons

.......................................... 23 2.7 Climate Projections for SAP 2.1a Scenarios

...................................................................... 23 

# 3

......................................................................................................................................................... 27 Climate Change from Short-Lived Emissions Due to Human Activities Questions and Answers

................................................................................................................. 27 3.1 Introduction

............................................................................................................................... 28 3.2 Emissions Scenarios and Composition Model Descriptions

......................................... 29

3.2.1 Emissions Scenarios

............................................................................................................ 29 3.2.2 Composition Models

............................................................................................................ 31 3.2.3 Tropospheric Burden

............................................................................................................ 31 3.2.4 Aerosol Optical Depth

........................................................................................................ 34 3.3 Climate Studies

......................................................................................................................... 36

3.3.1 Experimental Design

........................................................................................................... 36 3.3.2 Climate Models

................................................................................................................... 36 3.3.3 Radiative Forcing Calculations

........................................................................................... 37 3.3.4 Climate Model Simulations 2000 to 2050

...................................................................... 44 3.3.5 Climate Simulations Extended to 2100

............................................................................ 513.4 Regional Emission Sector Perturbations and Regional Models

..................................... 53 3.4.1 Introduction to Regional Emission Sector Studies

........................................................... 53 3.4.2 Global Models

...................................................................................................................... 53 3.4.3 Impact of Emission Sectors on Short-Lived Gases and Particles

.................................. 53 3.4.4 Regional Downscaling Climate Simulations

...................................................................... 58 

# 4

......................................................................................................................................................... 61 Findings, Issues, Opportunities, and Recommendations 4.1 Introduction

............................................................................................................................... 61 4.2 Key Findings

............................................................................................................................... 61 4.3 Issues Raised

.............................................................................................................................. 62 4.3.1 Emission Projections

............................................................................................................ 62 4.3.2 Particles (Indirect Effect, Direct Effect, Mixing, Water Uptake)

.................................. 63 4.3.3 Climate and Air Quality Policy Interdependence

............................................................. 64 4.4 Research Opportunities and Recommendations

............................................................. 65 4.4.1 Emission Scenario Development

........................................................................................ 65 4.4.2 Particle Studies (Direct Effect, Indirect Effect, Mixing, Water Uptake)

...................... 66 4.4.3 Improvements in Transport, Deposition, and Chemistr y
............................................... 66 4.4.4 Recommendations for Regional Downscaling

.................................................................. 67 4.4.5 Expanded Analysis and Sensitivity Studies

....................................................................... 67 4.5 Conclusion

................................................................................................................................. 68

Appendix A

....................................................................................................................................... 69 IPCC Fourth Assessment Report Climate Projections (Supplemental to Chapter 2) A.1 Mean Temperature

.................................................................................................................. 69 A.2 Temperature Extremes

.......................................................................................................... 69 A.3 Mean Precipitation

.................................................................................................................. 69 A.4 Precipitation Extremes and Droughts

................................................................................ 70 A.5 Snow and Ice

............................................................................................................................. 70 A.6 Carbon Cycle

............................................................................................................................ 70 A.7 Ocean Acidification

................................................................................................................. 70 A.8 Sea Level

.................................................................................................................................... 70 A.9 Ocean Circulation

................................................................................................................... 70 A.10 Monsoons

................................................................................................................................ 70 A.11 Tropical Cyclones (Hurricanes and Typhoons)

............................................................... 71 A.12 Midlatitude Storms

................................................................................................................. 71 A.13 Radiative Forcing

..................................................................................................................... 71 A.14 Climate Change Commitment (Temperature and Sea Level)

..................................... 71TABLE OF CONTENTSAppendix B

........................................................................................................................................ 73 MAGICC Model Description (Supplemental to Chapter 2)

Appendix C

....................................................................................................................................... 75 Composition Models (Supplemental to Chapter 3) C.1 Geophysical Fluid Dynamics Laboratory

........................................................................... 75 C.2 Goddard Institute for Space Studies

.................................................................................. 76 C.3 National Center for Atmospheric Research

.................................................................... 77

Appendix D

....................................................................................................................................... 79 Climate Models (Supplemental to Chapter 3) D.1 Geophysical Fluid Dynamics Laboratory

........................................................................... 79 D.2 Goddard Institute for Space Studies

.................................................................................. 79 D.3 National Center for Atmospheric Research

.................................................................... 80

Appendix E

........................................................................................................................................ 81 Scenarios (Supplemental to Chapter 4) E.1 The Emissions Scenarios of the IPCC Special Report on Emission Scenarios

......... 81 E.2 Radiative Forcing Stabilization levels and Approximate Carbon Dioxide Concentrations from the CCSP SAP 2.1a Scenarios

...................... 82

Glossary and Acronyms

................................................................................................................. 83

References

......................................................................................................................................... 85The U.S. Climate Change Science Program Preface

IV 

## AUTHOR TEAM FOR THIS REPORT

Preface Authors: Hiram Levy II, NOAA/GFDL; Drew T. Shindell, NASA/GISS; Alice Gilliland, NOAA/ARL; M. Daniel Schwarzkopf, NOAA/GFDL; Larry W. Horowitz, NOAA/GFDL Contributing Author: Anne Waple, STG Inc.
Executive Summary Lead Authors: Hiram Levy II, NOAA/GFDL; Drew T. Shindell, NASA/GISS; Alice Gilliland, NOAA/ARL; M. Daniel Schwarzkopf, NOAA/GFDL; Larry W. Horowitz, NOAA/GFDL Contributing Author: Anne Waple, STG Inc.
Chapter 1 Lead Authors: Hiram Levy II, NOAA/GFDL; Drew T. Shindell, NASA/GISS; Alice Gilliland, NOAA/ARL, M. Daniel Schwarzkopf, NOAA/GFDL; Larry W. Horowitz, NOAA/GFDL Contributing Authors: Anne Waple, STG Inc.; Ronald J Stouffer, NOAA/GFDL
Chapter 2 Lead Authors: Hiram Levy II, NOAA/GFDL; Drew T. Shindell, NASA/GISS Contributing Author: Tom Wigley, NCAR
Chapter 3 Lead Authors: Drew T. Shindell, NASA/GISS; Hiram Levy II, NOAA/GFDL; Alice Gilliland, NOAA/ARL; M. Daniel Schwarzkopf, NOAA/GFDL; Larry W. Horowitz, NOAA/GFDL Contributing Author: Jean-Francois Lamarque, NCAR; Yi Ming, UCAR
Chapter 4 Lead Authors: Hiram Levy II, NOAA/GFDL; Drew T. Shindell, NASA/GISS; Alice Gilliland, NOAA/ARL
Appendix A Lead Author: Hiram Levy II, NOAA/GFDL Contributing Author: Tom Wigley, NCAR
Appendix B Lead Author: Hiram Levy II, NOAA/GFDL Contributing Author: Tom Wigley, NCAR
Appendix C Lead Authors: Larry W. Horowitz, NOAA/GFDL; M. Daniel Schwarzkopf, NOAA/GFDL; Drew T. Shindell, NASA/GISS Contributing Author: Jean-Francois Lamarque, NCAR; Tom Wigley, NCAR
Appendix D Lead Authors: Drew T. Shindell, NASA/GISS; M. Daniel Schwarzkopf, NOAA/ GFDL; Hiram Levy II, NOAA/GFDL; Larry W. Horowitz, NOAA/GFDL Contributing Author: Jean-Francois Lamarque, NCAR
Appendix E Lead Authors: Drew T. Shindell, NASA/GISS; Hiram Levy II, NOAA/GFDL Contributing Author: Anne Waple, STG Inc.V Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols



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VSYNOPSIS



# T he influence of greenhouse gases and particle pollution on our present and future climate has been widely examined and most recently reported in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report. While both long-lived ( e.g. , carbon dioxide) and short-lived ( e.g., soot) gases and particles affect the climate, previous projections of future climate, such as the IPCC reports, have focused largely on the long-lived gases. This U.S. Climate Change Science Program (CCSP) Synthesis and Assessment Product provides a different emphasis.
We first examine the effect of long-lived greenhouse gases on the global climate based on updated emissions scenarios produced by another CCSP Synthesis and Assessment Product (SAP 2.1a). In these scenarios, atmospheric concentrations of the long-lived greenhouse gases leveled off, or stabilized, at predetermined levels by the end of the twenty-first century (unlike in the IPCC scenarios). However, the projected future tem

perature changes, based on these stabilization emissions scenarios, fall within the same range as those projected for the latest IPCC report. We confirm the robust future warming signature and other associated changes in the climate.
We next explicitly assess the effects of short-lived gases and particles. Their influence is found to be global in nature, substantial when compared with long-lived greenhouse gases, and potentially extending to the end of this century. They can significantly change the regional surface temperature, and by the year 2100 short-lived gases and particles may account for as much as 40 percent of the warming over the summertime continental United States. It is noteworthy that the simulated climate response to these pollutants is not confined to the geographical area where they are released. This implies a strong linkage between regional air quality control strategies and global climate change. We identify specific emissions reductions that would lead to benefits for both air quality and climate change mitigation, including North American surface transportation and Asian domestic fuel burning. The results reveal the necessity for explicit and consistent inclusion of the short-lived pollutants in assessments of future climate.The U.S. Climate Change Science Program Preface

VI 

### RECOMMENDED CITATIONS
For the Report as a whole: CCSP, 2008: Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols . A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. H. Levy II, D.T. Shindell, A. Gilliland, M.D. Schwarzkopf, L.W. Horowitz, (eds.). Department of Commerce, NOAA’s National Climatic Data Center, Washington, D.C., USA, 100 pp.

For the Preface: Levy II, H., D.T. Shindell, A. Gilliland, M.D. Schwarzkopf, L.W. Horowitz, 2008: Preface in Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols . H. Levy II, D.T. Shin

dell, A. Gilliland, M.D. Schwarzkopf, L.W. Horowitz, (eds.). A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research, Washington, D.C.
For the Executive Summary: Levy II, H., D.T. Shindell, A. Gilliland, M.D. Schwarzkopf, L.W. Horowitz, 2008: Executive Summary in Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols . H. Levy II, D.T. Shindell, A. Gilliland, M.D. Schwarzkopf, L.W. Horowitz, (eds.). A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research, Washington, D.C.

. For Chapter 1: Levy II, H., D.T. Shindell, A. Gilliland, M.D. Schwarzkopf, L.W. Horowitz, 2008: Introduction in Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols . H. Levy II, D.T. Shindell, A. Gilliland, M.D. Schwarzkopf, L.W. Horowitz, (eds.). A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research, Washington, D.C.
For Chapter 2: Levy II, H., D. T. Shindell, T. Wigley, 2008: Climate Projections From Well-Mixed Greenhouse Gas Stabilization Sce

narios in Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols . H. Levy II, D.T. Shindell, A. Gilliland, M.D. Schwarzkopf, L.W. Horowitz, (eds.). A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research, Washington, D.C.
For Chapter 3: Shindell, D.T., H. Levy II, A. Gilliland, M.D. Schwarzkopf, L.W. Horowitz, 2008: Climate Change From Short-Lived Emissions Due to Human Activities in Climate Projections Based on Emissions Scenarios for Long-Lived and ShortLived Radiatively Active Gases and Aerosols . H. Levy II, D.T. Shindell, A. Gilliland, M.D. Schwarzkopf, L.W. Horow

itz, (eds.). A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research, Washington, D.C.
For Chapter 4: Levy II, H., D.T. Shindell, A. Gilliland, 2008: Findings, Issues, Opportunities, and Recommendations in Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols . H. Levy II, D.T. Shindell, A. Gilliland, M.D. Schwarzkopf, L.W. Horowitz, (eds.). A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research, Washington, D.C.
For Appendix A: Levy II, H., T. Wigley, 2008: IPCC 4th Assessment Climate Projections in Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols . H. Levy II, D.T. Shindell, A. Gil

liland, M.D. Schwarzkopf, L.W. Horowitz, (eds.). A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research, Washington, D.C.VII Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols For Appendix B: Levy II, H., T. Wigley, 2008: MAGICC Model Description in Climate Projections Based on Emissions Scenarios for LongLived and Short-Lived Radiatively Active Gases and Aerosols . H. Levy II, D.T. Shindell, A. Gilliland, M.D. Schwarzkopf, L.W. Horowitz, (eds.). A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research, Washington, D.C.
For Appendix C: Shindell, D.T., L.W. Horowitz, M.D. Schwarzkopf, 2008: Composition Models in Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols . H. Levy II, D.T. Shindell, A. Gilliland, M.D. Schwarzkopf, L.W. Horowitz, (eds.). A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research, Washington, D.C.
For Appendix D: Shindell, D.T., M.D. Schwarzkopf, H. Levy II, L.W. Horowitz, 2008: Climate Models in Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols . H. Levy II, D.T. Shindell, A. Gilliland, M.D. Schwarzkopf, L.W. Horowitz, (eds.). A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research, Washington, D.C.
For Appendix E: Levy II, H., D.T. Shindell, 2008: Scenarios in Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols . H. Levy II, D.T. Shindell, A. Gilliland, M.D. Schwarzkopf, L.W. Horowitz, (eds.). A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Re

search, Washington, D.C.The U.S. Climate Change Science Program Preface

VIIIIX Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and AerosolsPREFACE 

# Report Motivation and Guidance for Using this Synthesis and Assessment Product
Authors: Hiram Levy II, NOAA/GFDL; Drew T. Shindell, NASA/GISS; Alice Gilliland, NOAA/ARL; M. Daniel Schwarzkopf, NOAA/GFDL; Larry W. Horowitz, NOAA/GFDL Contributing Author: Anne Waple, STG Inc.



### I ntrod u ction
The U.S. Climate Change Science Program (CCSP) was established in 2002 to coordinate climate and global change research conducted in the United States. Building upon and incorporat

ing the U.S. Global Change Research Program of the previous decade, the program integrates federal research on climate and global change, as sponsored by 13 federal agencies and overseen by the Office of Science and Technology Policy, the Council on Environmental Quality, the National Economic Council, and the Office of Manage

ment and Budget.
A primary objective of the U. S. CCSP is to pro

vide the best possible scientific information to support public discussion and government andprivate sector decision making on key climaterelated issues. To help meet this objective, the CCSP has identified an initial set of 21 synthesis and assessment products that address its highestpriority research, observation, and decisionsupport needs.
The CCSP is conducting 21 such activities, cov

ering topics such as the North American carbon budget and implications for the global carboncycle, coastal elevation and sensitivity to sealevel rise, trends in emissions of ozone-depleting substances and ozone recovery and implications for ultraviolet radiation exposure, and use of observational and model data in decision support and decision making. The stated purpose for this report, Synthesis and Assessment Product (SAP) 3.2, is to provide information to those who use climate model outputs to assess the potential ef

fects of human activities on climate, air quality, and ecosystem behavior. In an examination of the U.S. CCSP Strategic Plan, the National Research Council (NRC) recommended that syn

thesis and assessment products should be produced, with independent oversight and review from the wider scientific and stakeholder communities. To meet this goal, NOAA requested an independent review of SAP 3.2 by the NRC. The NRC appointed an ad hoc committee composed of eight members who provided their review findings, and recommendations, suggestions, and options for the authors to consider in revising the first draft of SAP 3.2. The revised second draft was then posted for public comment for 45 days. This third draft is in response to those public comments.



### B ac k gro u nd and G oals
The initial mandate for Synthesis and Assessment Product 3.2 (SAP 3.2), which is still listed on the official CCSPwebsite <http://www.climatescience.gov/Library/sap/sapsummary.php>, was to provide “Climate Projections for Research and Assessment Based on Emissions Scenarios Developed Through the Climate Change Technology Pro

gram”. With the development of long-lived greenhouse gas scenarios by another Synthesis and Assessment Product (SAP 2.1a; Clarke et al. , 2007), our mandate evolved to “Climate Projections for SAP 2.1a Emissions Scenarios of Greenhouse Gases”. These emissions scenarios 1 were for the long-lived 2 , and therefore globally well-mixed, radiatively active gases (greenhouse gases), and were constrained by the requirement that carbon dioxide concentrations stabilize within 100 to 200 years at specified levels of roughly 450, 550, 650 and 750 parts per million (ppm). See Box P.1 for additional details.

1 Emissions scenarios represent future emissions based on a coher

ent and internally consistent set of assumptions about the driving forces ( e.g., population change, socioeconomic development, and technological change) and their key relationships.
2 Long-lived radiatively active gases of interest have atmospheric life

times that range from ten years for methane to more than 100 years for nitrous oxide. While carbon dioxide’s lifetime is more complex, we think of it as being more than 100 years in the climate system. Due to their long atmospheric lifetime, they are well-mixed and evenly distributed throughout the lower atmosphere. Global atmospheric lifetime is the mass of a gas or an particle in the atmosphere divided by the mass that is removed from the atmosphere each year.The U.S. Climate Change Science Program Preface

X The SAP 2.1a scenarios (Clarke et al. , 2007) did not explic

itly address the direct influence of short-lived 3 drivers of climate: carbon and sulfate particles and lower atmospheric ozone. Therefore, we expanded our mandate to include “Short-Lived Radiatively Active 4 Gases and Aerosols 5 ”. These short-lived gases and aerosols (particles) are largelyof human-caused origin, are important contributors to largescale changes in atmospheric temperature and climate in general, and are primarily controlled for reasons of local and regional air quality. Therefore, this added portion of the report is a critical first step in examining the climate impact of future actions taken to reduce air pollution.
The Prospectus for Synthesis and Assessment Product 3.2 contained two charges to the authors of this Product:

3 Short-lived radiatively active gases and particles of interest in the

lower atmosphere have lifetimes of about a day for nitrogen oxides, a day to a week for most particles, and a week to a month for ozone. Their concentrations are highly variable and concentrated in the lowest part of the atmosphere, primarily near their sources.
4 Radiatively active gases and particles absorb, scatter, and re-emit

energy, thus changing the temperature of the atmosphere. 1. Develop climate projections for a series of scenarios for long-lived greenhouse gases provided by Synthesis and Assessment Product 2.1a, “Scenarios of Greenhouse Gas Emissions and Atmospheric Concentrations and Review of Integrated Scenario Development and Application”. 2. Investigate the contributions of four short-lived pollu

tants in the lower atmosphere: ozone and three types of particles (soot/elemental carbon, organic carbon, and sulfate), usually identified in scientific terms as aero

sols 5

. Short-lived greenhouse gases and particles have received less attention than long-lived greenhouse gases in previous international assessments and were not explicitly treated in Synthesis and Assessment Product 2.1a (Clarke et al. , 2007) but, as this report describes, they may affect the future climate in a substantial manner. Although sources of these pollutants tend to be localized, their impact is felt globally. This is of direct relevance to policy decisions regarding pollution, air quality, and climate change.

5 Aerosols are very small airborne solid or liquid particles that reside

in the atmosphere for at least several hours, with the smallest remain

ing airborne for days. 

### BOX P.1: Stabilization Emission Scenarios and Background From CCSP SAP 2.1a
Synthesis and Assessment Product (SAP) 2.1 (Clarke et al. , 2007) is an important precursor to this Product. It explores different scenarios that lead to greenhouse gas concentrations stabilizing at different (higher) levels in the future. Scenario analysis is a widely used tool for decision making in complex and uncertain situations. Scenarios are “what ifs”—sketches of future conditions (or alternative sets of future condi

tions) used as inputs to exercises of decision making or analysis. Scenarios have been applied extensively in the climate change context. Examples include greenhouse gas emissions scenarios, climate scenarios, and technology scenarios.
The scenarios in SAP 2.1a are called “stabilization emissions scenarios” because they are constrained so that the atmospheric concentrations of the long-lived greenhouse gases level off, or stabilize, at predetermined levels by the end of the twenty-first century. They explicitly treat the economic and technological drivers needed to generate each level of greenhouse gases. Further discussion is found in Box 1.2 of Chapter 1.
Preindustrial levels of carbon dioxide were approximately 280 parts per million (ppm), and are currently around 380 ppm—a third higher than prior to the industrial era and higher than at any other time in at least the last 420,000 years (CCSP SAP 2.2). The four stabilization levels for SAP 2.1a were constructed so that the carbon dioxide concentrations resulting from stabilization are roughly 450, 550, 650, and 750 ppm. While the Intergovernmental Panel on Climate Change (IPCC) has also examined greenhouse gas emission scenarios, and those provided by SAP 2.1a are generally within the envelope of the IPCC scenarios, SAP 2.1a is an alternative approach to developing a consistent set of long-lived greenhouse gas concentrations.
This Product (SAP 3.2) explores the climate implications of such greenhouse gas “stabilization emissions scenarios” via several different computer simulations. The results of these projections are presented in Chapter 2 of this Product.XI Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols


### R ea DER’S G U IDE TO SYNTHESIS AND ASSESSMENT PROD U CT 3.2

This Product includes an Executive Summary and four Chapters.

The Executive Summary presents the key results and find

ings, and recommends four critical areas of future research. It is written in non-technical language and is intended to be accessible to all audiences.
Chapter 1 provides an Introduction to this study, and is intended to provide all audiences with a general overview. It is written in non-technical language, which should be accessible to all readers with an interest in climate change. It includes background material, discusses the scope of and motivation for this study, addresses its goals and objectives, and identifies the issues that are not addressed. It also con

tains two Boxes, one providing non-technical definitions of important terms, and the other containing a clear and concise description of the computer models employed in this study.
Chapter 2 focuses on the long-lived greenhouse gases and a set of scenarios provided by Synthesis and Assessment Product 2.1a. The Statement of Findings and the Introductory Section 2.1 are written in non-technical language and are intended for the general reader. The remainder of Chapter 2 provides detailed technical information about specific computer models, the resulting climate simulations, and a detailed interpretation of the results. It is intended primarily for the scientific community.
A simplified global climate model, MAGICC 6 , is used to simulate globally-averaged surface temperature increases for the stabilization emission scenarios, and the results are assessed in the context of the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) Working Group 1 (IPCC, 2007). These comparisons are used to answer the first four questions posed in our Prospectus:

Q1. Do SAP 2.1a emissions scenarios differ significantly from IPCC emission scenarios? Q2. If the SAP 2.1a emissions scenarios do fall within the envelope of emissions scenarios previously considered by the IPCC, can the existing IPCC

6 MAGICC is a two-component numerical model consisting of a

highly simplified representation of a climate model coupled with an equally simplified representation of the atmospheric composition of radiatively active gases and particles. This model is adjusted, based on the results of more complex climate models, to make representa

tive predictions of global mean surface temperature and sea-level rise. climate simulations be used to estimate 50- to 100-year climate responses for the CCSP 2.1a carbon dioxide emission scenarios? Q3. What would be the changes to the climate system under the scenarios being put forward by SAP 2.1a? Q4. For the next 50 to 100 years, can the climate projections using the emissions from SAP 2.1a be distinguished from one another or from the scenarios recently studied by the IPCC?
Chapter 3 attempts to assess the direction, magnitude, and duration of future changes in climate due to changing levels of short-lived radiatively active gases and particles of human-caused origin. This is an area of research that is still at the initial stages of exploration and which the IPCC Fourth Assessment Report, as well as previous IPCC reports, investigated only superficially.
First, the stabilization emissions scenarios and models used to generate them are discussed. Next, the chemical compo

sition models 7 used to produce the global distributions of short-lived gases and particles that help to drive the climate models are introduced. Twenty-first century climate is then simulated with three state-of-the-art comprehensive climate models 8 , and the results are then used to address the four questions raised in the second section of our Prospectus:

Q5. What are the impacts of the radiatively active shortlived gases and particles not explicitly the subject of SAP 2.1a? Q6. How do the impacts of short-lived species (gases and particles) compare with those of the well-mixed greenhouse gases as a function of the time horizon examined? Q7. How do the regional impacts of short-lived species (gases and particles) compare with those of long-lived gases in or near polluted areas? Q8. What might be the climate impacts of mitigation actions taken to reduce the atmospheric levels of short-lived species (gases and particles) to address air quality issues? The Statement of Findings and the Introductory Section 3.1 are written in non-technical language and are intended for the general reader. The remainder of the chapter provides detailed technical information about the models, the result

7 Chemical composition models are state-of-the-art numerical models

that use the emission of gases and particles as inputs and simulate their chemical interactions, global transport by the winds, and removal by rain, snow, and deposition to the earth’s surface. The resulting outputs are global three-dimensional distributions of the initial gases and particles and their products. 8 Comprehensive climate models are a numerical representation of

the climate based on the physical properties of its components, their interactions, and feedback processes. Coupled Atmosphere-Ocean (-sea ice) General Circulation Models (AOGCMs) represent our current state-of-the-art.The U.S. Climate Change Science Program Preface

XII ing climate simulations, and our interpretation of the results. It is intended primarily for the scientific community.
Chapter 4 provides a summary of the key findings, identi

fies a number of scientific issues and questions that arise from our study, and identifies new opportunities for future research. The five most critical areas identified by this study are:

1. The projection of future human-caused emissions for the short-lived gases and particles; 2. The indirect and direct effects of particles and mixing between particle types; 3. Transport, deposition, and chemistry of the short-lived gases and particles; 4. Regional climate forcing vs. regional climate response; 5. Sensitivity studies of climate responses to short-lived gases and particles.
We have written Chapter 4, as much as is possible, in nontechnical language, and it is intended for all audiences.EXECUTIVE SUMMARY

1 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

Lead Authors: Hiram Levy II, NOAA/GFDL; Drew T. Shindell, NASA/GISS; Alice Gilliland, NOAA/ARL; M. Daniel Schwarzkopf, NOAA/GFDL; Larry W. Horowitz, NOAA/GFDL
Contributing Author: Anne Waple, STG Inc.

SYNOPSIS
The influence of greenhouse gases and particle pol

lution on our present and future climate has been widely examined and most recently reported in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report. While both long-lived 1 ( e.g. , carbon dioxide) and short-lived 2 ( e.g. , soot) gases and particles affect the climate, previous projections of future climate, such as the IPCC reports, have focused largely on the long-lived gases. This U.S. Climate Change Science Program Synthesis and Assessment Product provides a different emphasis.
We first examine the effect of long-lived greenhouse gases on the global climate based on updated emissions scenarios produced by another CCSP Synthesis and Assessment Product (SAP 2.1a). In these scenarios, atmospheric concentrations of the long-lived greenhouse gases leveled off, or stabilized, at predetermined levels by the end of the twenty-first century (unlike in the IPCC scenarios). However, the projected future temperature changes, based on these stabilization emissions scenarios, fall within the same range as those projected for the latest IPCC report. We confirm the robust future warming signature and other associated changes in the climate.
We next explicitly assess the effects of short-lived gases and particles. Their influence is found to be global in nature, substantial when compared with long-lived greenhouse gases, and po

tentially extending to the end of this century. They can significantly change the regional surface temperature, and by the year 2100 short-lived gases and particles may account for as much as 40 percent of the warming over the summertime continental United States. It is noteworthy that the simulated climate response to these pollutants is not confined to the geographical area where they are released. This implies a strong linkage between regional air quality control strategies and global climate change. We identify specific emissions reductions that would lead to benefits for both air quality and climate change mitigation, including North American surface transportation and Asian domestic fuel burning. The results reveal the necessity for explicit and consistent inclusion of the short-lived pollutants in assessments of future climate.

1 Atmospheric lifetimes for the long-lived radiatively active gases of interest range from ten years for methane to more than 100 years for nitrous oxide. While carbon dioxide’s lifetime is more complex, we can think of it as being more than 100 years in the climate system. As a result of their long atmospheric lifetimes, they are well-mixed and evenly distributed throughout the lower atmosphere. Global atmospheric lifetime is the mass of a gas or a particle in the atmosphere divided by the mass that is removed from the atmosphere each year. 2 Atmospheric lifetimes for the short-lived radiatively active gases and particles of interest in the lower at

mosphere are about a day for nitrogen oxides, a day to a week for most particles, and a week to a month for ozone. As a result of their short lifetimes, their concentrations are highly variable in space and time and they are concentrated in the lowest part of the atmosphere, primarily near their sources.The U.S. Climate Change Science Program Executive Summary

2 

### N ewR es u lts and F indings

Changes in pollutant levels, primarily over Asia, may significantly increase surface temperature and reduce rainfall over the summertime continental United States. These results constitute important improvements in our understanding of the influence of both long-lived gases and short-lived gases and particles. The Fourth Assessment Report of the IPCC recognized that most of the global-scale warming since the middle of last century was very likely due to the increase in greenhouse gas concentrations, and also that the warming has been partially damped by increasing levels of short-lived particle pollutants. However, while the IPCC models were coordinated in their use of greenhouse gas emissions scenarios, the short-lived pollutants were widely varying in the emissions scenarios used, and their future impacts were not isolated from those of the long-lived gases.
This Synthesis and Assessment Product provides a more comprehensive and updated assess

ment of the relative future contributions of long and short-lived gases and particles, with special, explicit focus on the short-lived component. This study encompasses a realistic time frame over which available technological solutions can be employed, and this study, in particular, focuses on those gases and particles whose future atmospheric levels are also subject to reduction due to air pollution control.
Our results suggest that changes in short-lived gases and particles (pollutants) may sig

1. nificantly influence the climate, in the twenty-first century. By 2050, projected changes in short-lived pollutant concentrations in two of the three studies are responsible for approximately 20 percent of the simulated global-mean annual average warming (see Sec

tion 3.3.4 and Figure 3.5). As shown in Figure ES.1, projected changes in pollutant levels, primarily over Asia, may significantly increase surface temperature and reduce rainfall over the summertime continental United States throughout the second half of the twenty-first century (see Section 1.4 for details of the calculations). The geographic patterns of factors that drive climate change due to short-lived gases and 2. particles and the patterns of the resulting surface temperature responses are quite dif

ferent. This is clearly seen in Figure ES.2, in which the largest fractional contribution to summertime radiative forcing from changes in short-lived pollutants in the last part of the twenty first century is primarily located over Asia, while the strongest warming response is located over the central United States. Regional emissions con

trol strategies for short-lived pollutants will thus have global impacts on climate. The geo

graphic disconnect between this driver of climate change and the surface temperature response is already apparent by 2050, as discussed in Section 3.3.4 and demonstrated in Figure 3.8.
Reductions of short-lived pol

3. lutants from the domestic fuel burning sector in Asia, whose climate impacts in this study (Section 3.4) are dominated by black carbon (soot), appear to offer the greatest potential for substantial, simultaneous ES.1 K EY FINDINGS

Figure ES.1 Calculation of twenty-first century temperature and precipitation change over the United States in summer (June through August) due to changes in short-lived gases and particles (see Section 1.4 for details). Temperature change in red is shown as the difference, in degrees Centigrade, from the 2001 value. Precipitation change in green is shown as the differ

ence, in centimeters, from the 2001 value, and represents the sum of daily changes over the three months. The plotted data are 11 year moving averages.3 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

improvement in local air quality and re

duction of global warming. Reduction in emissions from surface transportation in North America would have a similar impact.
The three comprehensive climate models 4. 3 , their associated chemical composition models 4 and their differing projections of short-lived emissions all lead to a wide rangeof projected changes in climate due to shortlived gases and particles. Each of the three studies in this report represents a thoughtful, but incomplete characterization of the driv

ing forces and processes that are believed to be important to the climate or to the global distributions of the short-lived gases and particles. Much work remains to be done to characterize the sources of the differences and their range. The two most important uncertainties are found to be the projection of future emissions and the determination of the indirect effect 5 of particles on clouds. The fundamental difference between uncertainties in projecting future emissions and uncertain

ties in processes, such as the indirect effects of particles, is discussed in Section 4.3.
The range of plausible short-lived emis

5. sions projections is very large, even for a single well-defined global emission sce

nario (see Figures and discussion in Section 3.2 for details). Figure ES.3 clearly dem

onstrates this situation for the different emission projections of black carbon par

ticles (soot) used by the three research groups. This currently limits our ability to provide definitive statements on their contribution to future climate change.
Natural particles such as dust and sea 6. salt also play an important role in climate and their emissions and interactions dif

fer significantly among the models, with consequences to the role of short-lived

3 A comprehensive climate model is a state-of-the-art numerical representation of the climate based on the

physical, chemical, and biological properties of its components, their interactions, and feedback processes that account for many of the climate’s known properties. Coupled Atmosphere-Ocean (-sea ice) General Circulation Models (AOGCMs) provide a comprehensive representation of the physical climate system. 4 Chemical composition models are state-of-the-art numerical models that use the emission of gases and

particles as inputs and stimulate their chemical interactions, global transport by winds, and removal by rain, snow, and deposition to the earth’s surface. The resulting outputs are global three-dimensional distribu

tions of the initial gases and particles and their products. 5 Apart from the direct effects of particles absorbing and scattering radiation, particles produce an indirect

forcing of the climate system through their aiding in the formation of cloud droplets or by modifying the optical properties and lifetime of clouds (see Box 3.1 for a more detailed discussion). Figure ES.3 The three plausible but very different emis

sions trends projected for black carbon particles (soot). Each of the three groups in this study used a different trend. The units are million metric tons of carbon per year. Figure ES.2 The fraction of summertime (June-August) ra

diative forcing* due to changing levels of short-lived gases and particles and the resulting summertime surface temperature change (degrees Centigrade) for year 2100.*Radiative forcing is a measure of how the energy balance of the Earthatmosphere system is influenced when factors that affect climate, such as atmospheric composition or surface reflectivity, are altered. When radiative forcing is positive, the energy of the Earth-atmosphere system will ultimately increase, leading to a warming of the system. In contrast, for a negative radiative forcing, the energy will ultimately decrease, lead

ing to a cooling of the system. For technical details, see Box 3.2.The U.S. Climate Change Science Program Executive Summary

4 

### Regional emissions

control strategies for short-lived pollutants will have large-scale impacts on climate. pollutants. This inconsistency among models should be addressed in future stud

ies. This is discussed in Section 3.2.4 and demonstrated in Figure 3.2 and Table 3.5.
Climate projections based on new emissions scenarios where atmospheric concentrations 7. of the long-lived greenhouse gases level off, or stabilize 6 , at pre-determined levels (from CCSP SAP 2.1a) generally fall within the range of IPCC climate projections for the standard scenarios considered in the Fourth Assessment Report. The lower bound stabilization emissions scenarios, which have a carbon dioxide stabilization level of approximately 450 parts per million (by volume), result in global surface temperatures below those calculated for the lower bound IPCC scenario used in the Fourth Assessment, particularly beyond 2050. Nonetheless, all of them unequivocally cause warming across the range of possible emissions scenarios (see Section 2.6 for details).

6 Stabilization emissions scenarios are a representation of the future emissions of a set of substances based

on a coherent and internally consistent set of assumptions about the driving forces (such as population, socioeconomic development, and technological change) and their key relationships. These emissions are constrained so that the resulting atmospheric concentrations of the substance, or at least their net effect, level off at a predetermined value in the future.5 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols



### ES.2 R ecommendations for Fu t u re R esearch

The five most critical areas for future research identified in this Report are:
The projection of future human-caused 1. emissions for the short-lived gases and particles; Indirect and direct effects of particles and 2. mixing between particle types; Transport, deposition, and chemistry of the 3. short-lived gases and particles. Regional climate forcing 4. vs . regional cli

mate response. Sensitivity studies of climate responses to 5. short-lived gases and particles.

1. Plausible emissions scenarios for the second half of the twenty-first century show signifi

cant climate impacts, yet the range of plausible scenarios is currently large and an increase in confidence in these scenarios is necessary.Short-lived gases and particles, unlike the wellmixed greenhouse gases, do not accumulate in the atmosphere. Therefore, combined with a large range of possible emissions scenarios, the climate impact of the short-lived gases and par

ticles is currently extremely difficult to predict. Improvements in our ability to predict social, economic and technological developments af

fecting future emissions are needed. However, uncertainties in future emissions will always be with us. What we can do is develop a set of internally consistent emissions scenarios that include all of the important radiatively active gases and particles and bracket the full range of possible future outcomes.
2. The particle indirect effect (see Box 3.1 for a technical discussion), which is very poorly understood, is probably the process in most critical need of research. The cli

mate modeling community as a whole can

not yet produce a credible characterization of the climate response to particle/cloud interactions. All models (including those participating in this study) are currently ei

ther ignoring it, or strongly constraining the model response. Attempts have been made using satellite and ground-based observa

tions to improve the characterization of the indirect effect, but major limitations remain and additional observations are required.
3. The three global composition models in this study all employed different treatments of mixing in the lowest layers of the atmosphere, transport and mixing by turbulence and clouds, removal of gases and particles by rain, snow and contact with the Earth’s surface, and dif

ferent approximate treatments of the very large collection of chemical reactions that we do not yet fully understand. Further coordinated model intercomparisons, evaluation of models against existing observations, and additional observations are all needed to achieve a better understanding of these processes.
4. The major unfinished analysis question in this study is the relative contribution of a model’s regional climate response, as opposed to the contribution from the regional pattern of radiative forcing, to the simulated regional change in seasonal climate. Specific modeling studies are needed to answer questions such as: Is there a model-independent regional climate response? What are the actual physical mecha

nisms driving the regional surface temperature patterns that we observe? This appears to be a very important area of study, particularly given the strong climate response projected for the summertime central United States.
5. Analyses of surface temperature response to changes in short-lived gases and particles need to be strengthened by additional sensitivity studies that should help to clarify causes and mechanisms. There are also a wide range of 

### Partly as a result of the large range of possible future emissions scenarios,

the climate impact

of the short-lived gases and particles is currently extremely difficult to predict.The U.S. Climate Change Science Program Executive Summary

6 climate-chemistry feedbacks and controls that should be explored. Both the response of the climate system to controls on short-lived gases and particles and the possible feedbacks, and the possible impacts of climate changes on levels of short-lived gases and particles are all fertile areas for future research.



### ES.3 Gu ide FOR R eaders

For those readers who would like to learn more about the research behind the Key Results and Findings and the Recommendations for Future Research, we provide the following guide to reading the four chapters. Chapter 1 provides an introduction to this study and relevant findings from previous climate research, introduces the goals and methodology, and provides Box 1.1 and Box 1.2 with relatively non-technical de

scriptions of the modeling tools and definitions of terms. It is written in a non-technical manner and is intended to provide all audiences with a general overview. Chapters 2 and 3 provide detailed technical information about specific models, model runs and projected trends and are intended primarily for the scientific community, though the key findings and the introduction to each chapter are written in non-technical lan

guage and intended for all audiences. Chapter 4 is intended for all audiences. It provides a summary of the major findings and identifies new opportunities for future research.

# I ntroductionCHAPTER 

# 1

7 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

Lead Authors : Hiram Levy II, NOAA/GFDL; Drew T. Shindell, NASA/GISS; Alice Gilliland, NOAA/ARL


### Contributing Authors : Tom Wigley, NCAR; Ronald J. Stouffer, NOAA/GFDL

PROLOG U E

Comprehensive climate models 1 have become the essential tool for understanding past climates and making projections of future climate resulting from both natural and human causes. Projections of future climate require estimates ( e.g., scenarios) of future emissions of long-lived 2 greenhouse gases and short-lived 3 ra diatively active 4 gases and particles. A number of standard emissions scenarios 5 have been developed for the Intergovernmental Panel on Climate Change (IPCC) assessment process, and the future impacts of these have been discussed extensively in the Fourth Assessment Report (IPCC, 2007).
As part of the Climate Change Science Program (CCSP) process, scenarios of long-lived greenhouse gas emissions, with the added requirement that their resulting atmospheric concentrations level off at specified values sometime after 2100 ( e.g., stabilization), were developed by the Synthesis and Assessment Product 2.1a team (Clarke et al. , 2007) and served as the basis for Synthesis and Assessment Product 3.2, for which the National Oceanic and Atmospheric Administration (NOAA) is the lead agency. NOAA’s stated purpose for Synthesis and Assessment Product 3.2 is to provide information to those who use climate model outputs to assess the potential effects of human activities on climate, air quality, and ecosystem behavior. This report comprises two components that first assess the climate projections resulting from Synthesis and Assessment Product 2.1a scenarios in the context of existing IPCC climate projections, and then isolate and assess the future impacts on climate resulting from future emissions of short-lived gases and particles.
This second component explores the impact of short-lived radiatively active gases and particles on future climate, a critical issue that has recently become an active area of research in the reviewed literature ( e.g.,

1 Comprehensive climate models are a numerical representation of the climate based on the physical properties of its com

ponents, their interactions, and feedback processes. Coupled Atmosphere-Ocean (-sea ice) General Circulation Models (AOGCMs) represent our current state-of-the-art. However, they contain assumptions that may constrain or affect the accuracy and uncertainty of model output or results.
2 Long-lived gases of interest have atmospheric lifetimes that range from ten years for methane to more than 100 years for

nitrous oxide and carbon dioxide. Due to their long atmospheric lifetimes, they are well-mixed and evenly distributed throughout the lower atmosphere. Global atmospheric lifetime is the mass of a gas or particle in the atmosphere divided by the mass that is removed from the atmosphere each year.
3 Short-lived radiatively active gases and particles of interest in the lower atmosphere have lifetimes of about a day for

nitrogen oxides, a day to a week for most particles, and a week to a month for ozone. Their concentrations are highly variable and concentrated in the lowest part of the atmosphere, primarily near their sources.
4 Radiatively active gases and particles absorb, scatter, and re-emit energy, thus changing the temperature of the atmosphere.

They are commonly called greenhouse gases and particles.
5 Emissions scenarios represent future emissions based on a coherent and internally consistent set of assumptions about the

driving forces ( e.g., population change, socioeconomic development, technological change) and their key relationships.The U.S. Climate Change Science Program Chapter 1

8 Climate models

used for projections of the future have evolved substantially during the past several decades. Hansen et al. , 2000; Brasseur and Roekner, 2005; Delworth et al. , 2005). The existing state-of-the-art models used in this study represent incomplete characterizations of the driving forces and processes that are believed to be important to the climate responses and global distributions of the short-lived gases and particles. Moreover, these incomplete treat

ments are not consistent across the models. However, despite these challenges, this Report shows that short-lived gases and particles have a significant impact on climate, potentially throughout the twenty-first century.



### 1.1 H istorical Ov er v iew
The climate models and the representation of the agents driving climate change used for projections of the future have both evolved substantially during the past several decades. In 1967, Manabe and Wetherald published the first model-based projection of future climate change. Using a simple model representing the global atmosphere as a single column, they projected a 2oC global surface air temperature change for a doubling of the atmospheric con

centration of carbon dioxide. Model develop

ment continued on a wide range of numerical models, especially in the increasing sophistica

tion of the ocean model.
In 1979, Manabe and Stouffer developed a global model at NOAA’s Geophysical Fluid Dynamics Laboratory (GFDL) useful for es

timating the climate sensitivity. They called this model an atmosphere-mixed layer ocean model, which is sometimes called a slab model. A slab model consists of global atmospheric, land, and sea ice component models, coupled to a static 50-meter deep layer of seawater. By construction, this type of model assumes no changes in the oceanic heat transports as the climate changes. It is used to estimate only equilibrium climate changes. In 1984, Hansen et al. used the NASA Goddard Institute of Space Studies (GISS) model in the first climate studies in which ocean heat transports were included in the climate calculation, although these were prescribed (fixed). The two models discussed above, as well as one developed in part at the National Center for Atmospheric Research (NCAR), all played an important part in the first Intergovernmental Panel on Climate Change (IPCC) Assessment Report 6 (IPCC, 1990).
In the late 1980s, Washington and Meehl (1989) at NCAR and Stouffer et al. (1989) at GFDL developed the first comprehensive climate mod

els (Atmosphere-Ocean General Circulation Models [AOGCMs]) useful for investigating climate change over multi-decadal and longer time periods. These models consisted of global atmosphere, ocean, land surface, and sea ice components. Both groups used an idealized radiative forcing to drive their models. Stouffer et al. used a one percent per year increase in the carbon dioxide concentration (compounded), where its atmospheric concentration doubles in 70 years.
By the time of the IPCC Second Assessment Report (IPCC, 1996), all three United States modeling centers were running comprehensive

6 It should be noted that the IPCC does not directly per

form any research. Rather, its reports are intended to be reviews of current research. However, it must also be noted that the IPCC is, in fact, a powerful driver of research and setter of research agendas in climate science. Moreover, only the latest report (Fourth As

sessment Report) strictly enforced the requirement that all results discussed in it be previously published in the reviewed literature.9 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

In the most recent

IPCC report, the Fourth Assessment Report of Working

Group I, used 24

comprehensive climate models. climate models. In addition, representation of the climate forcing was improving. Mitchell et al. (1995) in the United Kingdom (U.K.) devel

oped a scheme for crudely incorporating the impact of sulfate particles on climate. Similarly, actual concentrations of long-lived greenhouse gases were used for the past, allowing more realistic climate simulations of the historical time period (1860 to present day). Using emis

sions scenarios developed by the IPCC in 1992 (IPCC, 1992), the U.K. group also made future projections of climate change through the year 2100. Their results were very important in the Second Assessment Report of the IPCC.
By the time of the Third IPCC Assessment Report in 2001 (IPCC, 2001), 12 comprehensive climate models were used to project climate out to the year 2100. They used the emissions scenarios produced by the Special Report on Emission Scenarios (Nakićenović and Swart, 2000), with most groups using a high (A2) and low (B2) emissions scenario. Some of the models included components to predict atmospheric particle concentrations, but most of the 12 models used variants of the Mitchell et al. (1995) method to include their impact on climate. While particle changes were included in the historical simulations, most of the future projections did not include any changes in them or tropospheric ozone.
In the most recent IPCC report, the Fourth As

sessment Report of Working Group I (IPCC, 2007), 24 comprehensive climate models par

ticipated. The component models continued to become more sophisticated and included more physical processes. The new components al

lowed the inclusion of more radiatively active agents such as dust, black carbon and organic carbon particles, and land use in the scenarios. Again, most models included all or nearly all these climate forcing agents in their historical simulations, but many did not do so for the future. Most groups used the three standard scenarios developed for the IPCC by the Special Report on Emission Scenarios (Nakićenović and Swart, 2000) (B1, A1B and A2) 7 to make

7 B1: emissions increase very slowly for a few more

decades, then level off and decline; A2: emissions continue to increase rapidly and steadily throughout the twenty-first century; A1B: emissions increase very rapidly until 2030, continue to increase until 2050, and then decline. their future climate projections. These are the same three scenarios represented in Figures 2.1 through 2.4 in Chapter 2.



### 1.2 G oals and R ationale
As described in the Prospectus outlining the purpose of this report, Synthesis and Assess

ment Product 3.2 has two primary goals:
Produce climate projections for research 1. and assessment based on the stabilization emission scenarios of long-lived greenhouse gas emissions developed by Synthesis and Assessment Product 2.1a. Assess the sign, magnitude, and duration 2. of future climate changes due to changing levels of short-lived gases and particles that are radiatively active and that may be subject to future mitigation actions to address air quality issues.
The eight key questions which address the above goals, and which were also listed in the Prospectus for this report, are:

Q1. Do SAP 2.1a emissions scenarios differ significantly from IPCC emissions sce-

narios? Q2. If the SAP 2.1a emissions scenarios do fall within the envelope of emissions sce- narios previously considered by the IPCC, can the existing IPCC climate simulations be used to estimate 50-to 100-year climate responses for the SAP 2.1a

carbon dioxide (CO 2 ) emissions scenarios?The U.S. Climate Change Science Program Chapter 1

10

This Report focuses on the directeffect of shortlived pollutants on climate. Q3. What would be the changes to the climate system under the scenarios being put for ward by SAP 2.1a? Q4. For the next 50 to 100 years, can the time- varying behavior of the climate projections using the emissions scenarios from SAP 2.1a be distinguished from one another or from the scenarios currently being studied by the IPCC? Q5. What are the impacts of the radiatively ac tive short-lived species (gases and par ticles) not being reported in SAP 2.1a? Q6. How do the impacts of short-lived species (gases and particles) compare with those of the well-mixed greenhouse gases as a func

tion of the time horizon examined? Q7. How do the regional impacts of short-lived species (gases and particles) compare with those of long-lived gases in or near pol luted areas? Q8 . What might be the climate impacts of mitigation actions taken to reduce the atmospheric levels of short-lived species (gases and particles) to address air quality issues? The answers to these questions are discussed in detail in Chapters 2 and 3.
Synthesis and Assessment Product 3.2 is in

tended to provide information to those who use climate model outputs to assess the potential ef

fects of human activities on climate, air quality, and ecosystem behavior. Since neither the IPCC nor SAP 2.1a explicitly addressed the direct influence of changing emissions of short-lived pollutants (carbon and sulfate particles and lower atmospheric ozone) on climate change, their role became a major focus of this report. This study encompasses a realistic time frame over which available technological solutions can be employed, and focuses on those gases and particles whose future atmospheric levels are also subject to mitigation via air pollution con

trol. Thus Synthesis and Assessment Product 3.2 can be very beneficial to all stakeholders of climate change science. The intended audiences include those engaged in scientific research, the media, policy makers, and members of the public. Policy and decision makers in the public sector ( e.g. , congressional staff) need to understand the implications of these scenarios and the climates that they force, in contrast to the research science community, who may be more interested in the physical basis for the behavior.



### 1.3 L imitations
The first goal, assessing the climate projections for the SAP 2.1a stabilization emissions sce

narios for long-lived greenhouse gases, is rela

tively narrowly defined and so treated. While the second goal, assessing the impact on climate of changing emissions of short-lived radiatively active gases and particles, could be viewed much more broadly, we do not. Our focus is primarily on the direct effect 8 of these shortlived pollutants on climate. Only in the case of methane do we explore any of the potential interactions of chemical sources, reactions, and removal resulting from a changing climate.
We do not examine any of the indirect effects 9 of pollutant particles on climate, nor do we ad

dress other potentially important impacts such as land use change, reactive nitrogen deposition and ecosystem responses, changing natural hydrocarbon emissions, changing oxidant levels and changing particle formation, or a wide range of other processes that can interact with climate, such as changes in ice clouds and vegetation burning. The resources were also not available for extensive sensitivity studies that might help explore more deeply the causes and mechanisms behind the potentially significant

8 The direct effect refers to the influence of gases and

particles on climate through scattering and absorbing radiation.
9 Particles may lead to an indirect radiative forcing of

the climate system by acting as cloud condensation nuclei or modifying the optical properties and lifetime of clouds.11 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

Each of the three models was used to simulate future climate under two different scenarios:one in which humancaused short-lived gases and particles

were allowed to change in the future, and one in which

these gases and particles were heldconstant at presentday concentrations. impact of short-lived pollutant levels on future climate. The above issues and many others are potential topics for future research, but were beyond the scope of this study. We will only address the impact on climate of direct radia

tive forcing by long- and short-lived greenhouse gases and particles.



### 1.4 M ethodology
In addressing the questions posed above, we rely on several different types of computer mod

els to project the climate changes that would result from the scenarios of emissions of green

house gases and particles. Projections of future climate first require estimates ( e.g., scenarios) of future emissions of long-lived greenhouse gases and radiatively active short-lived gases and particles. Next, global composition models (computer models of atmospheric transport and chemistry) employ the emissions scenarios to generate global distributions of the concentra

tions of short-lived radiatively active gases and particles. Then comprehensive climate models (computer models of the coupled atmosphere, land-surface, ocean, sea-ice system) employ global distributions of both the long-lived and short-lived radiatively active gases and particles to simulate past climates and make projections of future climates resulting from natural and anthropogenic changes affecting the climate system. This whole modeling process is dis

cussed in more detail in Box 1.1.

A number of standard scenarios have been developed for the IPCC assessment process, and the future impacts of these have been explored. As part of the Climate Change Science Program (CCSP) process, updated scenarios of long-lived greenhouse gases and their atmospheric concentrations were developed by the Synthesis and Assessment Product 2.1a team (Clarke et al. , 2007), and served as a basis for this Product. In address

ing the first four questions, we examine the 12 scenarios for long-lived greenhouse gases developed by SAP 2.1a. We simulate the global surface temperature increases and sea-level rise (due only to thermal expansion of water, not melting ice caps) resulting from these scenarios using a simplified global climate computer model, MAGICC.
In addressing the latter four questions listedin section 1.2, we focus on the effects of shortlived radiatively active gases and particles, and use three different state-of-the-art comprehen

sive climate models. Intercomparison studies, including the latest IPCC assessment, have shown that the performance of these models is comparable to other state-of-the-art compre

hensive climate models (AOGCMs). Each of the three models was used to simulate future climate under two different scenarios: one in which human-caused short-lived gases and particles were allowed to change in the future, and one in which these gases and particles were held constant at present-day concentrations. The differences between the simulated climates for the two scenarios is attributed to the projected changes in the emissions of short-lived gases and particles.



### 1.5 T erms and D efinitions

A number of technical terms are defined and briefly discussed for the benefit of those nontechnical readers who wish to proceed to Chapters 2 and 3. The definitions are collected in Box 1.2.The U.S. Climate Change Science Program Chapter 1

12 “Storyline emissions scenario,” and “stabili

zation emissions scenario,” are two different approaches to estimating future emissions. The standard emissions scenarios used to provide the climate projections for the last two IPCC Assessment Reports (Third and Fourth) were storyline emissions scenarios. A set of economic development paths and rates of 

### BOX 1.1: Model Descriptions (modified from IPCC Fourth Assessment Report)

Integrated assessment models combine key elements of physical, chemical, biological, and economic systems into a decision-making framework with various levels of detail for the different components. These models differ in their use of monetary values, their integration of uncertainty, and their formu

lation of policy with regard to optimization, evaluation, and projections. For our study, their product was a set of stabilization emissions scenarios.
Chemical composition models are used to estimate the concentrations and distributions of trace gases and particles in the atmosphere that result from a given emissions scenario. These models, known technically as chemical transport models, are driven by winds, temperatures, and other meteorological properties that are either compiled from observations or supplied by climate models. Once the gas and particle emissions from human-induced and natural sources are supplied to the chemical composition model, they can be transported through the atmosphere, converted by chemical reactions, and removed from the atmosphere by rain, snow, and contact with the surface. These models provide concentrations of radiatively active gases and particles that vary in space and time for use in climate models.

A climate model is a numerical representation of the climate system based on the physical, chemi

cal, and biological properties of its components, their interactions, and their feedback processes. The climate system can be represented by models of varying complexity. For any one component or combination of components, a hierarchy of models can be identified that differ in the number of spatial dimensions represented, the extent to which physical, chemical or biological processes are explicitly represented, or the level at which empirical parameterizations are involved.
Simple climate models estimate the change in global mean temperature and sea-level rise due to thermal expansion. They represent the ocean-atmosphere system as a set of global or hemispheric boxes, and predict global surface temperature using an energy balance equation, a prescribed value of climate sensitivity, and a basic representation of ocean heat uptake. Such models can also be coupled with simplified models of biogeochemical cycles, and allow rapid estimation of the climate response to a wide range of emissions scenarios. MAGICC (for details, see Appendix B) is such a coupled model.

State-of-the-art comprehensive climate models (generally referred to as AOGCMs, for Atmos

pere-Ocean General Circulation Models) include interacting components describing atmospheric, oce

anic, and land surface processes, as well as sea ice. Although the large-scale dynamics of these models are treated exactly, approximations are still used to represent smaller, but still critical, processes such as the formation of clouds and precipitation, ocean mixing due to waves, and the mixing of air, heat, and moisture near the earth’s surface. Uncertainties in these approximations are the primary reason for climate projections differing among different comprehensive climate models. Furthermore, the global models are generally unable to capture the small-scale features of climate in many regions. In such cases, the output from the global models can be used to drive regional climate models that have the same comprehensive treatment of interacting components, but, as they are only applied to part of the globe, are able to represent a region’s climate in much greater detail.

technological innovation, population growth, and social-political development were specified and Integrated Assessment Models (Box 1.1) were asked to solve for the greenhouse gas and particle emissions that were consistent with the specified conditions.13 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

Synthesis and Assessment Product 2.1a (Clarke et al. , 2007) took quite a different approach. They effectively established a set of targets for long-lived greenhouse gas concentrations, and then had their three integrated assessment models determine emissions pathways to those targets by applying economic principles to the relationships existing among economic devel

opment paths and rates of technological inno

vation, population growth, and social-political development. Each group used somewhat dif

ferent approaches to determine the economic pathway to stabilization. Technically, only one of the models used the “least cost” approach in its strictest economic sense. However, as we show in Chapter 2, the resulting emissions and concentrations of the long-lived greenhouse gases over the twenty-first century are similar among models for a given target. Furthermore, all of the stabilization scenarios, with the excep

tion of those for the lower bound target (only 18 percent increase in carbon dioxide over the next 100 years), fall within the range of the principal storyline scenarios used for the last two IPCC assessments. While the two approaches to constructing the emissions scenarios are differ

ent, the resulting concentrations of greenhouse gases, and their impacts on climate, are quite similar. 

### Storyline emissions scenarios are plausible representations of future emissions based on a coherent and internally consistent set of assumptions about the driving forces ( e.g., population change, socioeconomic development, technological change) and their key relationships.

Stabilization emissions scenarios represent future emissions based on a coherent and internally consistent set of assumptions where, additionally, these emissions are constrained so that the resulting atmospheric concentration levels off at a predetermined value in the future.
Radiative forcing is a measure of how the energy balance of the Earth-atmosphere system is influenced when factors that affect climate are altered. The word radiative arises because these factors change the balance between incoming solar radiation and outgoing infrared radiation within the Earth’s atmosphere. This radiative balance controls the Earth’s surface temperature. The term “forcing” is used to indicate that Earth’s radiative balance is being pushed away from its normal state. When radiative forcing from a factor or group of factors is evaluated as posi

tive, the energy of the Earth-atmosphere system will ultimately increase, leading to a warming of the system. In contrast, for a negative radiative forcing, the energy will ultimately decrease, leading to a cooling of the system.
Global atmospheric lifetime is the mass of a gas or particle in the atmosphere divided by the mass that is removed from the atmosphere each year.
Long-lived gases of interest have atmospheric lifetimes that range from ten years for methane to more than 100 years for nitrous oxide. While carbon dioxide’s lifetime is more complex, we can think of it as being more than 100 years in the climate system. As a result of their long atmospheric lifetimes, long-lived gases are well mixed and evenly distributed throughout the lower atmosphere. Their concentrations also change slowly with time.

Short-lived gases and particles of interest to climate studies have lifetimes of about a day for nitrogen oxides, a day to a week for most particles, and a week to a month for ozone. As a result of their short lifetimes, their concentrations are highly variable in space and time and are concentrated in the lowest part of the atmosphere, primarily near their sources. BOX 1.2: U seful DefinitionsThe U.S. Climate Change Science Program Chapter 1

14

The regional patterns of climatechange due to longand short-lived gases and particles are similar, despite their different concentrations and lifetimes. Radiative forcing is an important quantity that is frequently used when discussing the impact of radiatively active gases and particles. A technical definition is provided in Chapter 3, Box 3.2. We provide a relatively non-technical explanation in Box 1.2 of this chapter. It will be useful in the following discussion of long- and short-lived gases and particles.
The long-lived greenhouse gases have atmo

spheric lifetimes ranging from a decade to more than a century. As a result, they are uniformly mixed and their radiative forcing is also rela

tively uniformly distributed, both in space and time, throughout the lower atmosphere. On the other hand, the short-lived gases and particles have atmospheric lifetimes ranging from a day to weeks. Their concentrations are highly variable in space and time, and they are con

centrated in the lowest part of the atmosphere, primarily near their sources. As a result their radiative forcing is also highly localized and can vary significantly in time. However, one of our Key Findings is that, while radiative forcing patterns for long- and short-term gases and particles are quite different, the regional patterns of climate change due to long- and short-lived radiatively active gases and particles are similar.
For those wishing to read further, we provide a brief reader’s guide. Chapters 2 and 3 provide detailed technical information about specific models, model runs, and trends, and are in

tended primarily for the scientific community, though the Questions and Answers and the Introduction sections of each chapter are writ

ten in nontechnical language and are intended for all audiences. Chapter 4 is intended for all audiences. It provides a summary of the major findings, and identifies new opportunities for future research.

# C limate Projections from Well-Mixed Greenhouse Gas Stabilization Emission ScenariosCHAPTER 

# 2

15 

### i Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

Lead Authors: Hiram Levy II, NOAA/GFDL; Drew T. Shindell,

NASA/GISS; Alice Gilliland, NOAA/ARL
Contributing Author: Tom Wigley , NCAR

Q U ESTIONS AND ANSWERS This chapter focuses on climate projections for the long-lived greenhouse gas stabilization emissions scenarios for the time period 2000 to 2100 that were produced under the U.S. Climate Change Science Program by an earlier Synthesis and Assessment Product, 2.1a (Clarke et al. , 2007). Those scenarios 1 are called “stabilization emissions scenarios” because they are constrained so that the atmospheric concentra

tions of the long-lived greenhouse gases level off, or stabilize, at predetermined levels by the end of the twenty-first century. Our overall goal in this Chapter is to assess these “stabilization emissions scenarios” and the climates they would project for the twenty-first century in the context of the most recent In

tergovernmental Panel on Climate Change Report, the Fourth Assessment Report of Working Group I (IPCC, 2007a). The major conclusions are summarized below as the answers to the first four questions in our Prospectus, and then receive more detailed attention in the remainder of the Chapter:

Q1. Do the stabilization emissions scenarios produced by Synthesis and Assessment Product (SAP) 2.1a differ significantly from those used in the Fourth Assessment Report of the IPCC? A1. While different in concept and method of derivation (stabilization vs . “storyline”—see Box 1.2 for de- tails) the long-lived greenhouse gas stabilization emissions scenarios outlined in Synthesis and Assessment Product 2.1a fall among the principal storyline emissions scenarios studied in the Fourth Assessment Report of the IPCC. While each individual stabilization emissions scenario differs somewhat from the individual IPCC scenarios, they are generally encompassed by the IPCC envelope of estimated future emissions.
Q2. If the Synthesis and Assessment Product 2.1a emissions scenarios do fall within the envelope of emis- sions scenarios previously considered by the IPCC, can the existing IPCC climate simulations be used to estimate 50 to 100 year climate responses for the SAP 2.1a carbon dioxide emissions scenarios? A2. Given the close agreement between the ranges of emissions scenarios, time evolution of global con- centrations and associated radiative forcings 2 , and global mean temperature responses in the two assess- ments, we conclude that the key global and regional climate features noted in the IPCC reports can indeed be used to estimate the 50 to 100 year climate responses for the SAP 2.1a scenarios.

1 Scenarios are representations of the future development of emissions of a substance based on a coherent and internally

consistent set of assumptions about the driving forces (such as population, socioeconomic development, technological change) and their key relationships.
2 Radiative forcing is a measure of how the energy balance of the Earth-atmosphere system is influenced when factors that

affect climate, such as atmospheric composition or surface reflectivity, are altered. When radiative forcing is positive, the energy of the Earth-atmosphere system will ultimately increase, leading to a warming of the system. In contrast, for a negative radiative forcing, the energy will ultimately decrease, leading to a cooling of the system. For technical details, see Box 3.2.The U.S. Climate Change Science Program Chapter 2

16 Q3. What would be the changes to the climate system under the scenarios being put forward by SAP 2.1a? A3. The key climate changes resulting from the “stabilization emissions scenarios” should be quite similar to the key findings from Chapters 10 (Meehl et al. , 2007) and 11 (Christensen et al. , 2007) of the Fourth Assessment Report of the IPCC, which are listed in Box 2.1 in Section 2.7 and discussed in more detail in Appendix A. The simulations by the simple climate model used in this Chapter, as well as the comprehensive climate model 3 simulations in Chapter 10 of the Fourth Assessment Report of the IPCC all find increases in global-average surface air temperature throughout the twenty-first century; with the warming increasing roughly proportional to the increasing concentrations of long-lived greenhouse gases.
Q4. For the next 50 to 100 years, can the climate projections using the emissions scenarios from SAP 2.1a be distinguished from one another or from the scenarios recently studied by the IPCC? A4. For the first 30 years there is little difference in the predicted global-average climate among either the principal IPCC scenarios or the SAP 2.1a stabilization emissions scenarios for the

long-lived greenhouse gases. For the second half of the twenty-first century, global mean and certain robust regional properties predicted for the different IPCC emission scenarios and applicable to the SAP 2.1a scenarios are distinguishable from each other in magnitude (the greater the concentration of long-lived greenhouse gases, the greater the magnitude) though not in their qualitative features.

3 A comprehensive climate model is a numerical representation of the climate based on the physical, chemical,

and biological properties of its components and their interactions and feedback processes, which account for many of its known properties. Coupled Atmosphere-Ocean (-sea ice) General Circulation Models (AOGCMs) provide the current state-of-the-art representation of the physical climate system.17 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols



### Preindustrial levels of carbon dioxide were approximately 280 parts per million, and current levels are around 380 parts per million. The stabilization emissions scenarios used here were constructed to be more or less equivalent to 450, 550, 650, and 750 parts per million of carbon dioxide. 2.1 I ntrod u ction
Chapter 2 is focused on climate projections for the four long-lived greenhouse gas scenarios developed by an earlier report, Synthesis and Assessment Product 2.1a (SAP 2.1a) (Clarke et al. , 2007). Our work in this chapter involves two different types of models:
Three integrated assessment models 1. 4 that were used in Synthesis and Assessment Product 2.1a to produce stabilization emis

sions scenarios for long-lived greenhouse gases; A simplified global climate model, Model 2. for the Assessment of Greenhouse-gas Induced Climate Change (MAGICC) 5 that was used to simulate global levels of carbon dioxide, global-average radiative forcings for a variety of radiatively active 6 gases and particles, global-average surface temperature increases and global-average sea-level rise (due only to thermal expan

sion of water, not melting ice caps) for the four stabilization emissions scenarios.
The second section, 2.2, introduces the stabi

lization emissions scenarios and the models that were used to generate them in Synthesis and Assessment Product 2.1a. The stabilization levels were defined in terms of the combined radiative forcing for carbon dioxide (CO 2 ) and the other long-lived greenhouse gases that are potentially controlled under the Kyoto Protocol (methane, nitrous oxide, a suite of halocarbons, and sulfur hexafluoride [SF 6 ]). These radiative forcing levels were chosen to be more or less equivalent to 450, 550, 650, and 750 parts per

4 Integrated assessment models are a framework of

models, currently quite simplified, from the physi

cal, biological, economic, and social sciences that interact among themselves in a consistent manner and can evaluate the status and the consequences of environmental change and the policy responses to it.
5 MAGICC is a two-component numerical model

consisting of a highly simplified representation of a climate model coupled with an equally simplified representation of the atmospheric composition of radiatively active gases and particles. This model is adjusted, based on the results of more complex climate models, to make representative predictions of global mean surface temperature and sea-level rise.
6 “Radiatively active” indicates the ability of a sub

stance to either absorb or emit sunlight or infrared radiation, thus changing the temperature of the atmo

sphere. million (ppm) of carbon dioxide, and attainment was required within 100 to 200 years. For refer

ence, preindustrial levels were approximately 280 ppm, and current levels of carbon dioxide are around 380 ppm.
Each integrated assessment model produced its own reference scenario, which is considered a “business as usual” or no-climate-policy sce

nario, as well as four stabilization emissions scenarios for long-lived greenhouse gas emis

sions that required a range of policy choices. The scenarios generated by each integrated as

sessment model were internally consistent, and each modeling group made independent choices in determining both their reference emissions, and their multi-gas policies required to achieve the specified stabilization levels. “All of the groups developed pathways to stabilization targets designed around economic principles. However, each group used somewhat different approaches to stabilization emissions scenario construction”.
The third section, 2.3, introduces the simpli

fied global climate model, MAGICC, which is used to generate the projections of carbon dioxide concentrations, radiative forcings due to the long-lived greenhouse gases, and global surface temperature increases for the four sta

bilization emissions scenarios introduced in the previous section 2.2. While the three integrated assessment models used in Synthesis and As

sessment Product 2.1a each treated the cycling of carbon dioxide between the land, ocean and atmosphere in their own ways, in this study we use the carbon cycling treatment employed by MAGICC for all of the stabilization emissions scenarios. This provides a level playing field for all of the scenarios (see Wigley et al. , 2008 for a detailed discussion of this issue). We find that there is little difference between the two approaches.
MAGICC has four atmosphere boxes, one each over land and sea in each hemisphere, and two ocean boxes, one for each hemisphere. It consists of two highly simplified components: a climate component that has been adjusted to produce a global-average temperature change when the carbon dioxide concentration is doubled that is similar to the comprehensive climate models used in the IPCC Fourth As

-The U.S. Climate Change Science Program Chapter 2

18 sessment Report, and a greenhouse gas and particle component that has also been adjusted to reproduce the global-average surface temper

ature and sea-level rise simulated by the same set of complex climate models for the various storyline emissions scenarios analyzed in the Fourth Assessment Report of the IPCC. A more detailed description of MAGICC is provided for the technical audience in Appendix B.
The fourth section, 2.4, shows that the con

centrations of carbon dioxide projected by MAGICC for the twelve stabilization emissions scenarios (three models, four stabilization levels each) from Synthesis and Assessment Product 2.1a fall among earlier projections of carbon dioxide concentrations for the three primary storyline emissions scenarios employed in the Fourth Assessment Report of the IPCC (IPCC, 2007a). Next, it is shown that the 12 time series of radiative forcings for the long-lived green

house gases potentially regulated by the Kyoto Protocol, again calculated by MAGICC, fall among the time series of radiative forcings for the twenty-first century previously calculated for the same gases with the three time series of principal storyline emissions scenarios used in the IPCC Fourth Assessment Report.
The fifth section, 2.5, deals with the contri

bution of the short-lived pollutants (ozone, elemental and organic carbon particles and sulfate particles) to radiative forcing calcula

tions by MAGICC for the stabilization emis

sions scenarios. While short-lived pollutants were not explicitly included in determining the stabilization emissions scenarios for the long-lived greenhouse gases, two of the three integrated assessment models did produce emissions scenarios for the short-lived pollut

ants that were consistent with the energy and policy decisions required for stabilization of the long-lived greenhouse gas concentrations. To assign a full radiative forcing to the scenarios calculated for the third model, an intermediate IPCC emissions scenario for the short-lived pollutants was added to its stabilization emis

sions scenario for long-lived gases. Again we find that the total radiative forcing (short-lived and long-lived radiatively active gases and particles) calculated by MAGICC for the 12 stabilization emissions scenarios fall among the total radiative forcings calculated by MAGICC for the principal storyline emissions scenarios employed in the Fourth Assessment Report of the IPCC.
The sixth section, 2.6, compares two sets of global-average surface temperature time se

ries: an average of those calculated by a broad collection of comprehensive global climate models for the three principal IPCC emissions scenarios and reported in Chapter 10 of the IPCC’s Fourth Assessment Report (Meehl et al. , 2007), and those calculated by MAGICC for the 12 SAP 2.1a stabilization emissions scenarios and reported here. As was found for the carbon dioxide concentration and radiative forcing time series discussed previously, the global-average surface temperatures calculated for the 12 stabilization emissions scenarios by MAGICC are generally contained within those calculated for the three IPCC scenarios by comprehensive global climate models. The exceptions are for the lower bound stabilization emissions scenario that would require carbon dioxide not to exceed 450 ppm by year 2100 (remember that current levels of carbon dioxide already exceed 380 ppm). The global-average surface temperatures tend to fall below those for the lowest IPCC scenario, particularly in the second half of the twenty-first century.
The seventh and final section, 2.7, addresses the primary objective of Chapter 2, “Climate Projections for SAP 2.1a Scenarios.” While the stabilization emissions scenarios used in this report were derived in a fundamentally different manner from the storyline emissions scenarios used in the IPCC Fourth Assessment Report, they are generally contained within 

### Projected warmingin the twentyfirst centuryshows scenarioindependent geographical patterns similar to those observed over the past several decades.19 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols
the storyline emissions scenarios and show a similar evolution with time. Moreover, the same is true for the resulting radiative forcings and global-average surface temperatures that are calculated with a simple global climate model. Drawing on the conclusion from the latest IPCC Summary for Policy Makers (IPCC, 2007b) that “Projected warming in the twenty-first century shows scenario-independent geographical pat

terns similar to those observed over the past several decades,” we conclude that the robust conclusions arrived at in the latest IPCC report apply equally well to the climate responses expected for the four stabilization emissions scenarios provided by Synthesis and Assess

ment Product 2.1a.



### 2.2 W ell -M ixed G reenho u se G as E mission S S cenarios F rom SAP 2.1 a
The three integrated assessment models used in SAP 2.1a were EPPA (Paltsev et al. , 2005), MiniCAM (Kim et al. , 2006) and MERGE (Richels et al. , 2007). These models have dif

ferent levels of complexity in their modeling of socioeconomic, energy, industry, transport, and land-use systems. With respect to emissions, EPPA and MiniCAM are similarly compre

hensive, and produce output for emissions of the following: all the major greenhouse gases (carbon dioxide [CO 2 ], methane [CH 4 ], nitrous oxide [N 2 O], and a suite of halocarbons and sulfur hexafluoride [SF 6 ]); sulfur dioxide (SO 2 ), black carbon (BC) and organic carbon (OC) particles and their precursors; and the reactive gases carbon monoxide (CO), nitrogen oxides (NOx) and volatile organic compounds (VOCs), which are important determinants of tropo

spheric ozone change. MERGE produces emis

sions output for the major greenhouse gases and idealized short-lived and long-lived halocarbons (characterized by HFC-134a and SF 6 ), but not for any other short-lived radiatively active gases and particles and their precursors.

The stabilization levels were defined in terms of the combined radiative forcing for CO 2 and for the other gases that are potentially controlled under the Kyoto Protocol (CH 4 , N 2 O, halocar

bons, and SF 6 ). All of the groups developed pathways to stabilization targets designed around economic principles. However, each group used somewhat different approaches to stabilization emission scenario construction. (Reilly et al. , 1999; Manne and Richels, 2001; Sarofim et al. , 2005).

Consistent time series for the emissions of short-lived radiatively active gases and par

ticles, carbon (both elemental and organic) and the precursors of sulfate particles and tropo

spheric ozone, were produced by the integrated assessment models to varying degrees, but the resulting radiative forcings were not part of the scenario definitions, nor were they considered as contributing to the radiative forcing targets. The stabilization levels for radiative forcing were constructed by determining the CO 2 - only forcing associated with concentrations of 450, 550, 650, and 750 ppm and then adding additional radiative forcing to account for the other Kyoto Gases (0.8, 1.0, 1.2 and 1.4 W per

m 2 respectively). The four stabilization levels are referred to as Level 1, Level 2, Level 3, and Level 4, where Level 1 requires the largest reduction in radiative forcing and is associated with CO 2 stabilization at roughly 450 ppm.
As SAP 2.1a (Clarke et al. , 2007) notes, “The three models display essentially the same re

lationship between greenhouse gas concentra

tions and radiative forcing, so the three refer

ence scenarios also all exhibit higher radiative forcing, growing from roughly 2.2 W per m 2 above preindustrial in 2000 for the Kyoto Gases to between 6.4 W per m 2 and 8.6 W per m 2 in 2100”. These differences arise primarily from differences in the assumptions underlying the reference scenarios, which lead to different reference emissions across the models.
The three models incorporate carbon cycles of different complexity, ranging from MERGE’s neutral biosphere assumption to EPPA’s coarse 3-D ocean. MiniCAM uses MAGICC to repre

sent its carbon cycle. However, SAP 2.1a notes that the concentration of gases that reside in the atmosphere for long periods of time—decades to millennia—is more closely related to cumu

lative emissions than to annual emissions. In particular, this is true for CO 2 , the gas respon

sible for the largest contribution to radiative forcing. This relationship can be seen for CO 2 in Figure 3.21 in SAP 2.1a (Clarke et al. , 2007), where cumulative emissions over the period 

### The robust conclusions arrived at in the latest IPCC report apply equally well to the climate responses

expected for the four stabilization emission scenarios used here.The U.S. Climate Change Science Program Chapter 2

20 2000 to 2100, from the three reference scenarios and the 12 stabilization emission scenarios, are plotted against the CO 2 concentration in the year 2100. The plots for all three models lie on essentially the same line, indicating that despite considerable differences in representation of the processes that govern CO 2 uptake, the aggregate response to increased emissions is very similar. This basic linear relationship also holds for other long-lived gases, such as N 2 O, SF 6 , and the halocarbons.”
The remainder of this chapter starts with the emissions scenarios generated by the three integrated assessment models in SAP 2.1a and examines their atmospheric composition, radia

tive forcing, and global-mean temperature. In the SAP 2.1a results, differences arise due to inter-model differences in the emissions for any given scenario, and differences between the models in their gas-cycle and climate compo

nents. Here we eliminate the second factor by using a single coupled gas-cycle/climate model to assess the scenarios—the MAGICC model as used in the IPCC Third Assessment Report (Cubasch and Meehl, 2001; Wigley and Raper, 2001). Many of the results given here have also been produced by the integrated assessment models, and some are described in SAP 2.1a. Using a single gas-cycle/climate model allows us to isolate differences arising from emissions scenario differences. Moreover, the MAGICC model was used previously to generate the carbon dioxide concentrations, Kyoto Gas 7 radiative forcing, and total radiative forcing as

7 “Kyoto Gases” refers to those long-lived greenhouse

gases covered by the Kyoto Protocol (carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluo

rocarbons, and sulfur hexafluoride). sociated with the IPCC scenarios B1, A1B, and A2 (described in Appendix A) that we compare with the current MAGICC calculations for the SAP 2.1a scenarios (Wigley et al. , 2008).



### 2.3 S implified G lobal C limate M odel (MAGICC)
MAGICC is a coupled gas-cycle/climate model that was used in the Third Assessment Report (Cubasch and Meehl, 2001; Wigley and Raper, 2001). A critical assessment focused on its skill in predicting global average sea-level rise is found in Chapter 10, Appendix 1 of the Working Group I contribution to the Fourth Assessment Report of the IPCC (Meehl et al . 2007).
The climate component is an energy-balancemodel with a one-dimensional, upwellingdiffusion ocean. For further details of models of this type, see Hoffert et al. (1980) and Harvey et al. (1997). In MAGICC, the globe is divided into land and ocean “boxes” in both hemispheres in order to account for different thermal inertias and climate sensitivities over land and ocean, and hemispheric and land/ocean differences in forcing for short-lived gases and particles such as tropospheric ozone and sulfate particles.
The climate model is coupled interactively with a series of gas-cycle models for CO 2 , CH 4 , N 2 O, a suite of halocarbons, and SF 6 . The carbon cycle model includes both CO 2 fertilization and temperature feedbacks, with model parameters tuned to give results consistent with the other carbon cycle models used in the Third Assess

ment Report (Kheshgi and Jain, 2003) and the Bern model (Joos et al. , 2001). For sulfate particles, both direct and indirect forcings are included using forcing/emissions relationships developed in Wigley (1989, 1991), with central estimates for 1990 forcing values.
The standard inputs to MAGICC are emis

sions of the various radiatively important gases and various climate model parameters. These parameters were tuned so that MAGICC was able to emulate results from a range of complexglobal climate models called AtmosphereOcean General Circulation Models (AOGCMs) in the Third Assessment Report (see Cubasch and Meehl, 2001). We use a value of 2.6°C equilibrium global-mean warming for a CO 221 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols
doubling, the median of values for the above set of AOGCMs (see Appendix B for additional details).



### 2.4 L ong -L i v ed G reenho u se G as C oncentrations and R adiati v e F orcings
Figure 2.1 compares the concentrations of the primary greenhouse gas, CO 2 , calculated by MAGICC for the 12 SAP 2.1a stabilization emission scenarios with earlier calculations of CO 2 concentrations for B1, A1B and A2, the principal storyline emission scenarios reported in Appendix II of the IPCC’s Third Assessment Report (IPCC, 2001). For the first 20 years there is little difference among the 12 SAP 2.1a scenarios due to the long CO 2 lifetime, although the extreme Level 1 (L1) scenarios start to separate noticeably by 2030. By year 2100, CO 2 concentrations for the MiniCAM and EPPA Level 1 scenarios have converged on values close to 450 ppm. For MERGE, the 2100 value is lower. CO 2 concentrations for Levels 2 through 4 (L2 through L4) start to spread in the second half of the twenty-first century, but remain approximately bound between B1 and A1B all the way to 2100. EPPA now has the low

est CO 2 for Levels 2 through 4. The CO 2 levels for the lower bound Level 1 scenario, which requires immediate reductions in CO 2 emis

sions followed by ever increasing reductions (see SAP 2.1a for details), remain substantially below those for B1.
Next, Figure 2.2 considers where the radiative forcing due to increasing Kyoto greenhouse gases in the 12 SAP 2.1a stabilization emission scenarios, again calculated by MAGICC, are plotted with the Kyoto Gas radiative forcing values taken from Appendix II in the Third As

sessment Report (IPCC, 2001) for the B1, A1B, and A2 storyline emission scenarios. The evolu

tion of the 12 radiative forcing time series over the twenty-first century is very similar to that of CO 2 , in Figure 2.1, which should not be sur

prising. However, there are some differences. The EPPA values undershoot the stabilization target for Levels 2 through 4 because they are on a trajectory where radiative forcing stabilizes some time after 2100, although emissions were calculated only to 2100 (Clarke et al. , 2007)

. For the Level 2, 3 and 4 stabilization cases, it is not possible to stabilize as early as 2100 (Wigley et al. , 1996). As we saw for carbon dioxide, the Kyoto Gas radiative forcing time series for stabilization Levels 2 through 4 are contained within the radiative forcings calculated for the IPCC scenarios, A1B and B1.
It should be noted that in general the three in

tegrated assessment models hit their radiative forcing targets when they employ their own carbon cycle and atmosphere models. Thus, failure to hit these same radiative forcing tar

gets when all three long-lived gases are run in MAGICC would seem to reflect the underlying uncertainties in the carbon cycles of the three integrated assessment models, which are known to be substantial. Figure 2.1 CO 2 concentrations (ppm) calculated by MAGICC for the 12 SAP 2.1a stabilization emission scenarios (Clarke et al. , 2007) plotted with calculations of CO 2 concentrations for the principal scenarios (B1, A1B and A2) reported in Appendix II of the Third Assessment Report (IPCC, 2001b).

Figure 2.2 Kyoto Gas radiative forcing (W per m 2 ) for the SAP 2.1a scenarios (Clarke et al. , 2007), calculated by MAGICC, plotted with the Kyoto Gas Radiative Forcing values taken from Appendix II in the Third Assessment Report (IPCC, 2001) for the B1, A1B, and A2 SRES scenarios. 

### For the first 20 years, there is little

difference among the 12 Synthesized Assessment Product 2.1a scenarios, due to the long CO 2 

### lifetime.The U.S. Climate Change Science Program Chapter 2

22 

### 2.5 S hort -L i v ed G ases and PA rticles and T otal R adiati v e F orcing
While EPPA and MiniCAM produce emissions of sulfur dioxide (SO 2 ), elemental or black carbon and organic carbon particles and their precursors, and the key precursors of tropo

spheric ozone (CO, NOx and VOCs) as part of their model’s climate projections, MERGE does not. To complete the MERGE scenarios, all four of its stabilization levels use the IPCC’s B2 scenario of emissions for sulfur dioxide (Nakićenović and Swart, 2000) and assume that ozone precursor emissions remain constant. For all of the models, rather than use emis

sions for the elemental and organic particles, it is assumed that the elemental and organic particle radiative forcings track the sulfur di

oxide emissions in each integrated assessment model’s four stabilization emission scenarios. Therefore, while carbon dioxide emissions tend to track the IPCC scenarios, the emissions of short-lived gases and particles may be different, with the exception of sulfur dioxide emissions in MERGE.
Figure 2.3 compares the total radiative forcing calculated by MAGICC for the 12 SAP 2.1a scenarios, i.e. , the sum of Kyoto Gas forcings (Figure 2.2) plus forcings due to particles, tro

pospheric ozone, halocarbons controlled under the Montreal Protocol, and stratospheric ozone (Wigley et al. , 2008 and supplementary mate

rial referenced therein) with the total radiative forcing calculated by MAGICC for the B1, A1B, and A2 scenarios used in the IPCC Fourth As

sessment Report (IPCC, 2007a). Again, just as for CO 2 and Kyoto Gas radiative forcing, the 12 total radiative forcing time series do not begin to separate noticeably before 2030.
Because of the assumptions made about the short-lived gases and particles, the MERGE Kyoto Gas and total forcings differ least. MiniCAM shows the largest differences with total forcings now significantly exceeding the stabilization targets for all four levels, primarily due to sharp decreases in sulfur dioxide emis

sions, which produce significant increases in total radiative forcing by 2100 (approximately 1 W per m 2 ). In the EPPA stabilization emis

sion scenarios the changes in sulfur dioxide emissions are small, and most of the short-lived forcing comes from increased nitrogen oxide emissions that drive increases in tropospheric ozone and its positive radiative forcing (Wigley et al. , 2008). Remember that in SAP 2.1a, the stabilization targets were met using only the long-lived greenhouse gases.
The spread of stabilization forcings is signifi

cantly less for the Kyoto Gas forcings (which were used to define the stabilization targets) than for total forcing. Again the Level 1 total radiative forcings are generally below those of the B1 scenario, while the other Levels are bounded by B1 and A1B. However, in this case the Level 2 through 4 scenarios appear to track the B1 total radiative forcing out to 2060 to 2070 before the Level 3 and 4 scenarios start moving up to A1B. The differences between the radiative forcing time evolution for the Kyoto Gases in Figure 2.2 and for all radiatively active gases and particles in Figure 2.3 are the result of differences among treatments of short-lived gases and particles. The changes in global average surface temperatures that are driven by the total radiative forcing in Figure 2.3 are examined in the next section. We will continue to explore the potential impact of short-lived gases and particles on future global warming in considerable detail in Chapter 3.

Figure 2.3 Total radiative forcing (W per m 2 ) calculated by MAGICC for the 12 SAP 2.1a scenarios (Clarke et al. , 2007) plotted with the total calculated by MAGICC for the B1, A1B, and A2 scenarios (IPCC, 2001). 

### The differences between the radiative forcing time evolution for the Kyoto Gases in Figure 2.2 and for all radiatively active gases and particles in Figure 2.3 are the result of differences among treatments of short-lived gases and particles.23 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols



### 2.6 Su rface T emperat u re : MAGICC AND IPCC C omparisons
Figure 2.4 compares multi-model global-mean surface temperature changes reported in Chap

ter 10 of the IPCC’s Fourth Assessment Report for the standard storyline emission scenarios, B1, A1B and A2, with global-mean surface temperature changes calculated by MAGICC for the 12 SAP 2.1a stabilization emission sce

narios (Clarke et al ., 2007). As we might expect, the general behavior is quite similar to that ob

served for total radiative forcing. All scenarios are close through 2020. Levels 2 through 4 stay in close agreement out to around 2050. The Level 1 scenarios are lower than B1, except for MiniCAM, where there is enhanced warming out to 2050 due to the rapid reduction in SO 2 emissions (Wigley, 1991). The other three levels follow B1 closely out to 2050 and then remain between B1 and A1B out to 2100.
For Level 1 and Level 2 temperatures, the rate of increase has begun to slow appreciably by 2100, which suggests that global-mean tem

perature could be stabilized if the emissions scenarios produced by the three integrated assessment models for these two stabilization cases (corresponding to 450 and 550 ppm CO 2, but also including the assumed or modeled levels of short-lived gases and particles) were followed. This in turn depends on the economic and technological feasibility of the Level 1 and 2 scenarios for both the long-lived greenhouse gases and the short-lived gases and particles. However, the temperatures for the less extreme Level 3 and 4 stabilization emission scenarios (corresponding to 650 and 750 ppm CO 2 ) are still growing, particularly Level 4 MiniCAM. It should also be noted that their upper bound, the A1B model-mean surface temperature, is also still growing at 2100. The global mean surface temperature projections for the 12 SAP 2.1a sta

bilization emission scenarios are well bounded by the comprehensive climate model simula

tions for the A1B scenario reported in Chapter 10 of the IPCC Fourth Assessment Report.
Table 2.1 displays the radiative forcings and temperature changes for the year 2100 for the 12 stabilization emissions scenarios from SAP 2.1a and for the three storyline emissions scenarios (A1B, A2 and B1) taken from the IPCC Fourth Assessment Report (IPCC, 2007a). These values were compiled from Figures 2.1 through 2.4.



### 2.7 C limate P ro j ections for SAP 2.1 a S cenarios

The 2.1a stabilization emissions scenarios (Clarke et al ., 2007) are derived in a fundamen

tally different manner from the development of the storyline emissions scenarios used in Fourth Assessment Report of the IPCC (IPCC, 2007a). However, we have shown in Section 2.4 that the 12 (three integrated assessment models, four stabilization emissions scenarios each) stabilization emissions scenarios reported in SAP 2.1a are contained within the range of the three principal storyline emissions scenarios used in the IPCC Assessment Report and show a similar evolution with time. The Kyoto Gases and total radiative forcings for those 12 emis

sions scenarios are generally constrained within the three principal scenarios used to make the climate projections discussed in Chapter 10 of the IPCC Fourth Assessment Report (Meehl et al. , 2007).
Section 2.6 shows that the global surface tem

peratures predicted for the SAP 2.1a scenarios over the twenty-first century by a simple coupled gas-cycle/climate model, MAGICC, fall within the range of the multi-model mean temperatures

Figure 2.4 Multi-Atmosphere-Ocean General Circulation Model global-mean sur

face temperature changes (°C) reported in Chapter 10 of IPCC Fourth Assessment Report (Meehl et al. , 2007) for the standard storyline emissions scenarios B1, A1B, and A2 plotted with global-mean surface temperatures calculated by MAGICC for the 12 SAP 2.1a stabilization emissions scenarios (Clarke et al. , 2007). 

### Three integrated
assessment models produced four stabilization emission scenarios each in Synthesized Assessment Product 2.1a. These 12 scenarios are generally contained within the range of the three principal storyline emission

scenarios used in the IPCC Fourth Assessment Report

and show a similar evolution with time.The U.S. Climate Change Science Program Chapter 2

24 calculated with state-of-the-art comprehensive climate models for the three principal IPCC scenarios and reported in Chapter 10 (Meehl et al ., 2007). In fact, the global average surface temperatures for Levels 2 through 4 scenarios all track the values reported by the IPCC for B1 out to 2050. The primary exceptions are all of the Level 1 scenarios beyond year 2050 which are significantly below B1. We also draw on the conclusion in the Summary for Policy Makers in the IPCC Fourth Assessment Report (IPCC, 2007b): “Projected warming in the twenty-first century shows scenario-independent geographi

cal patterns similar to those observed over the past several decades.” Figure 10.8 in Chapter 10 of the Fourth Assessment Report (Meehl et al. , 2007) also clearly shows that the geographical pattern of the robust climate features are pre

served across scenarios employed in the IPCC projections for the twenty-first century climate, while the magnitude of the warming increases with the magnitude of the radiative forcing andwith increases in the concentration of the longlived greenhouse gases.

Scenario CO2 (ppm) (Figure 2.1) K yoto Gases Radiative Forcing (W per m 2 ) (Figure 2.2) Total (W per m 2 ) (Figure 2.3) Temperature Change (degrees C) (Figure 2.4)

A2 836 5.75 6.74 3.40 A1B 703 4.02 4.72 2.60 B1 540 2.34 2.86 1.60

L1 MiniCAM 454 1.17 2.04 1.32 L1 Merge 432 1.14 1.36 0.93 L1 EPPA 453 1.28 1.75 1.16

L2 MiniCAM 559 2.33 3.10 1.83 L2 Merge 553 2.56 2.71 1.61 L2 EPPA 551 2.12 2.58 1.56

L3 MiniCAM 651 3.23 4.09 2.27 L3 Merge 650 3.67 3.81 2.09 L3 EPPA 601 2.98 3.36 1.92

L4 MiniCAM 712 3.83 4.73 2.50 L4 Merge 708 4.30 4.45 2.33 L4 EPPA 668 3.63 3.97 2.18 Table 2.1 Year 2100 values from Figures 2.1, 2.2, 2.3, and 2.4. We conclude that the robust conclusions ar

rived at in Chapter 10 of the Fourth Assessment Report (Meehl et al ., 2007) regarding the pre

dicted climate response to the three scenarios studied in the most detail in that Report, (B1, A1, and A1B) apply equally well to the climate responses expected for the four long-lived greenhouse gas stabilization emission scenarios (three realizations of each) provided by SAP 2.1a (Clarke et al ., 2007). These robust conclu

sions are highlighted in Box 2.1 below and further discussed in Appendix A.
At this time, we also introduce in Box 2.2 our general approach to treating uncertainty in this document. Since much of this report deals with ranges of projections of radiative forcing and surface temperature rather than explicit predictions, we do not generally assign uncer

tainty values. We do quote the IPCC explicit uncertainty values in Box 2.1. Later in Chapter 3 we present Box 3.3 that addresses the deter

mination of statistical significance and our use of it in a more technical manner. 

### We conclude that the robust conclusions arrived at in Chapter 10 of the Fourth Assessment Report regarding the predicted climate response to the three scenarios studied in the most detail in that report apply equally well to the climate responses expectedfor the four longlived greenhouse gas stabilization emission scenarios provided by SAP 2.1a.25 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

Surface Air Temperatures show their greatest increases over land (roughly twice the • global average temperature increase), over wintertime high northern latitudes, and over the summertime United States and southern Europe, and show less warming over the southern oceans and North Atlantic. These patterns are similar across the B1, A1B, and A2 scenarios with increasing magnitude with increasing radiative forcing. It is very likely that heat waves will be more intense, more frequent, and longer lasting • in a future warmer climate. By 2100, global-mean sea level is projected across the 3 SRES scenarios to rise by 0.28m • to 0.37m for the three multi-model averages with an overall 5-95 percent range of 0.19 to 0.50 m. Thermal expansion contributes 60-70 percent of the central estimate for all scenarios. There is, however, a large uncertainty in the contribution from ice sheet melt, which is poorly represented in current models. Globally averaged mean atmospheric water vapor content, evaporation rate, and • precipitation rate are projected to increase. While, in general, wet areas get wetter and dry areas get dryer, the geographical patterns of precipitation change during the twenty-first century are not as consistent across the complex climate model simula

tions and across scenarios as they are for surface temperature. Multi-model projections based on SRES scenarios give reductions in ocean pH of be

• tween 0.14 and 0.35 units over the twenty-first century, adding to the present decrease of 0.1 units from preindustrial times. There is no consistent change in El Niño-Southern Oscillation (ENSO) for those • complex climate models that are able to reproduce ENSO-like processes. Those models with a realistic Atlantic Meridional Overturning Circulation (MOC) • predict that it is very likely that the MOC will slow by 2100, but will not shut down. The Fourth Assessment Report Summary for Policymakers finds it “Likely that intense • hurricanes and typhoons will increase through the twenty-first century”.
There are also important robust conclusions for North America from Chapter 11 of the Fourth Assessment Report (Christensen et al. , 2007): “All of North America is very likely to warm during this century, and the annual mean • warming is likely to exceed the global-mean warming in most areas.” “Annual-mean precipitation is very likely to increase in Canada and the U. S. Northeast, • and likely to decrease in the U.S. Southwest”. “Snow season length and snow depth are very likely to decrease in most of North • America, except in the northernmost part of Canada where maximum snow depth is likely to increase”.
NOTE: The terms “very likely” and “likely” have specific statistical meanings defined by the IPCC. Very likely greater than 90 percent chance of occurring Likely greater than 67 percent chance of occurring 

### BOX 2.1: Robust conclusions for global climate from Chapter 10 of the Fourth Assessment Report (Meehl et al. , 2007):The U.S. Climate Change Science Program Chapter 2

26 In doing any assessment, it is helpful to precisely convey the degree of certainty of various findings and projections. There are numerous choices for categories of likelihood and appropriate wording to define these categories. In Chapter 2 of this report, since many of the findings of this Report are comparable to those discussed in the Fourth Assessment Report of the IPCC, we have chosen to be consistent with the IPCC lexicon of uncertainty:

Lexicon Probability of Occurrence Virtually certain > 99 percent
Extremely likely > 95 percent
Very Likely > 90 percent
Likely > 66 percent
More likely than not > 50 percent
Unlikely < 33 percent
Very unlikely < 10 percent
Extremely unlikely < 5 percent

Elsewhere in the report, we are projecting climate, based on model simulations that use, as a foundation, scenarios of short-lived gases and particles, which are themselves plausible, but highly uncertain. For this reason, we have largely avoided assigning uncertainty values. However, where they do occur, we have condensed the IPCC ranges of uncertainty to fewer categories because we are unable to be as precise as in the IPCC assessments, which consider primarily the long-lived greenhouse gases. This lexicon is also consistent with other CCSP reports, such as SAP 3.3 and SAP 4.1.

Figure Box 2.2 Language in this Synthesis and Assessment Product (Chapters 3 and 4) used to express the team’s expert judgment of likelihood, when such a judgment is appropriate. 

### BOX 2.2: U ncertaintyCHAPTER 

# 3

27 

# C limate Change from Short-Lived Emissions Due to Human Activities Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

Q U ESTIONS AND ANSWERS Lead Authors: Drew T. Shindell, NASA/GISS; Hiram Levy II, NOAA/GFDL; Alice Gilliland, NOAA/ARL; M. Daniel Schwarzkopf, NOAA/GFDL; Larry W. Horowitz, NOAA/GFDL
Contributing Author: Jean-Francois Lamarque, NCAR

This chapter addresses the four questions regarding short-lived gases and particles that were posed in the Prospectus for this report:
Q5 . What are the impacts of the radiatively active short-lived species (gases and particles) not explicitly the subject of prior CCSP assessments (SAP 2.1a: Scenarios of Greenhouse Gas Emissions and Atmospheric Concentrations)? A5 . Uncertainties in emissions projections for short-lived gases and particles are very large, even for a particular storyline. For particles, these uncertainties are usually dominant, while for tropospheric ozone, uncertainties in physical pro- cesses are more important. Differences among modeled future atmospheric burdens and radiative forcing for particles are dominated by divergent assumptions about emissions from South and East Asia. Particle mixing, particle indirect effects, the influence of ecosystem-chemistry interactions on methane, and stratosphere-troposphere exchange all contribute to large uncertainties separate from the emissions projections.
Q6 . How do the impacts on climate of short-lived gases and particles compare with those of the well-mixed greenhouse gases as a function of the time horizon examined? A6 . By 2050 in two of the three models, changes in short-lived gas and particle concentrations (which are not reported in SAP 2.1a stabilization emission scenarios [Clarke et al., 2007]) contribute approximately 20 percent of global-mean annual-average warming, while one model shows virtually no effect. To a large extent, the inter-model differences are related to differences in emissions. Changes in the levels of short-lived gases and particles may play a substantial role out to 2100. One model finds that they can contribute 40 percent of the total projected summertime warming in the central United States over the second half of the 2100 century.
Q7 . How do the regional impacts of short-lived species (gases and particles) compare with those of long-lived gases in or near polluted areas? A7 . The spatial distribution of radiative forcing is generally less important than the spatial distribution of climate response in predicting the impact on climate. Thus, both short-lived and long-lived gases and particles appear to cause enhanced climate responses in the same regions, rather than short-lived gases and particles having an enhanced effect primarily in or near polluted areas.
Q8 . What might be the climate impacts of mitigation actions taken to reduce the atmospheric levels of short-lived species (gases and particles) to address air quality issues? A8 . Regional air quality emissions control strategies for short-lived pollutants have the potential to substantially affect climate globally. In one study, emissions reductions in the domestic energy/power sector in developing Asia, and to a lesser extent in the surface transportation sector in North America, appear to offer the greatest potential for substan tial, simultaneous improvement in local air quality and mitigation of global warming.The U.S. Climate Change Science Program Chapter 3

28

Uncertainties in emissions projections for short-lived gases and particles are very large, even for a particular storyline

. 

### 3.1 I ntrod u ction In this chapter, we describe results from numerical simulations of twenty-first century climate, with a major focus on the effects of short-lived gases and particles. The calculations incorporate results from three different types of models:

1. Integrated assessment models that produce emissions scenarios for particles and for ozone and particle precursors. 2. Global chemical composition models, which employ these emissions scenarios to generate concentrations for the short-lived radiatively active gases and particles. 3. Global comprehensive climate models, which calculate the climate response to theprojected concentrations of both the shortlived and long-lived gases and particles. Box 1.1 outlines this sequence in detail.
The second part of Chapter 3, Section 3.2, is a discussion of the emissions scenarios and the models used to generate them, and the chemical composition models (sometimes called chemi

cal transport models) used to produce the global distributions of short-lived gases and particles that help to drive the comprehensive climate models. Section 3.2 shows that, beginning with a single socioeconomic scenario for the time evolution of long-lived (well-mixed) greenhouse gases, different assumptions about the evolu

tion of the particles and precursor gases lead to very different estimates of particle and ozone concentrations for the twenty-first century. We conclude that uncertainties in emissions projec

tions for short-lived gases and particles are very large, even for a particular storyline. For par

ticles, these uncertainties are usually dominant, while for tropospheric ozone, uncertainties in physical processes are more important.
The third part of Chapter 3, Section 3.3, dis

cusses the three global comprehensive climate models (Geophysical Fluid Dynamics Labora

tory [GFDL]; Goddard Institute for Space Stud

ies [GISS]; Community Climate System Model [CCSM] developed in part at the National Cen

ter for Atmospheric Research [NCAR]) that have been used to calculate the impact of the short- and long-lived gases and particles 1 on the climate, focusing on the changes in surface temperature and precipitation. Supplementing the climate model results are calculations of the changes in radiative forcing 2 of the earthatmosphere system. We find that by 2050, two of the three climate models show that approximately 20 percent of the global-mean annual-average warming is due to changing levels of radiatively active short-lived gases and particles. One model shows virtually no effect from short-lived gases and particles. To a large extent, the inter-model differences are related to differences in emissions. An exten

sive discussion and comparison of the projected distributions of short-lived species and resulting radiative forcings employed by the three groups and the resulting climate projections from the three comprehensive climate models can be found in Shindell et al. (2008).

One of the models has been extended to 2100. In that model, short-lived gases and particles play a substantial role, relative to the well-mixed greenhouse gases, in the surface temperature evolution out to 2100 and are responsible for 40 percent of the projected 2100 summertime warming in the central United States (Levy et al. , 2008).
The fourth part of Chapter 3, Section 3.4, dis

cusses the effects of changes in regional particle and ozone and particle precursor emissions, us

ing models that separate emissions by economic sector. The results show that regional air quality emissions control strategies for short-lived pol

lutants have the potential to substantially affect climate at large-scales. Emissions reductions from domestic sources in Asia, and to a lesser extent from surface transportation in North

1 We distinguish here between short-lived gases and

particles (which have atmospheric lifetimes less than one month and are non-uniformly distributed) and long-lived gases (which have lifetimes of a decade or more and are generally well mixed in the atmo

sphere).
2 Radiative forcing is a measure of how the energy

balance of the Earth-atmosphere system is influenced when factors that affect climate, such as atmospheric composition or surface reflectivity, are altered. Whenradiative forcing is positive, the energy of the Earthatmosphere system will ultimately increase, leading to a warming of the system. In contrast, for a negative radiative forcing, the energy will ultimately decrease, leading to a cooling of the system. For technical de

tails, see Box 3.2.29 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

Though the three groups in this study all prescribed future emissions following a specific socioeconomic scenario, the scenario has multiple

interpretations, and hence they used different emissions trends for the short-lived gases and particles. America, appear to offer the greatest potential for substantial, simultaneous improvement in local air quality and mitigation of global climate change

.


### 3.2 E mission S S cenarios and C omposition M odel D escriptions

3.2.1 Emissions Scenarios The long-lived (well-mixed) greenhouse gases included in this study were carbon dioxide (CO 2 ), nitrous oxide (N 2 O), methane (CH 4 ), and the minor gases (chlorofluorocarbons, sulfur hexafluoride). Projected global mean values were prescribed following the A1B “marker” storyline emissions scenario for all three mod

eling groups. Emissions for anthropogenic sources of particles and precursor gases and particles for all three composition model cal

culations were based on an international emis

sions inventory maintained in the Netherlands (Olivier and Berdowski, 2001).
Though the three groups in this study all pre

scribed future emissions following a specific socioeconomic scenario (A1B) that was highly studied in the Fourth Assessment by the Inter

governmental Panel on Climate Change (IPCC, 2007), they used different emissions trends for the short-lived gases and particles. There are several reasons for the differences. For one, the A1B emissions projections only provide estimates of anthropogenic emissions, and each model used its own natural emissions (though these were largely held constant). Secondly, integrated assessment models, while using the same socio-economic storyline (A1B), provided a range of emissions results (Nakićenović and Swart, 2000).
Two groups, GFDL and NCAR, used output from the AIM integrated assessment model (integrated assessment models are defined in Chapter 2, Section 2.1) while GISS used results from the IMAGE model. Though the emissions output generated from AIM was denoted the “marker” scenario by the IPCC, it was noted that it did not represent the average, best, or median result, and that all integrated assessment model results should be treated equally. Finally, emissions for some gases and particles, such as carbonaceous particles, were not provided. This last issue motivated the GISS choice of the IMAGE model output, as it provided suf

ficient regional detail to allow carbonaceous particle emissions to be estimated consistently with the other gases and particles. Another complexity was the treatment of biomass burning emissions, which are partly natural and partly anthropogenic. In the GFDL model, biomass-burning emissions were assumed to be half anthropogenic and half natural. The GISS model instead used biomass burning emissions projections from another inventory (Streets et al. , 2004).
The result is a substantial divergence in the projected trends among the three models (Fig

ure 3.1, Table 3.1). For sulfur dioxide (SO 2 ), the precursor to sulfate particle, the emissions fol

low reasonably similar trajectories, with glob

ally averaged increases until 2030 followed by decreases to 2050 and even further decreases to 2100. However, the percentage increase is roughly double for GISS and CCSM as com

pared with GFDL. Thus even two composition models using anthropogenic emissions projec

tions from the same integrated assessment model show large differences in the evolution of their total emissions, presumably owing to dif

ferences in the present-day emissions invento

ries. At 2050, the GFDL model has substantially reduced emissions compared with 2000, while the other models show enhanced emissions rela

tive to 2000. A similar divergence in projected sulfur-dioxide trends is present in the SAP 2.1a stabilization emissions scenarios (Clarke et al. , 2007) discussed in Chapter 2, with emis

sions decreasing dramatically (~70 percent) byThe U.S. Climate Change Science Program Chapter 3

30 2050 in one integrat

ed assessment model (MINICAM) while decreasing only mod

erately (~20 percent) in the two others, and even beginning to increase again after about 2040 in one of those two.

Differences are even more striking for carbonaceous par

ticle emissions, which were not provided by any of the integrated assessment models. We focus on black carbon (BC) as the more im

portant radiative perturbation. For this particle (and for organic carbon [OC]), the GFDL com

position model uses the IPCC recommendation to scale carbonaceous particle emissions to carbon monoxide emissions, leading to sub

stantial increases with time (Figure 3.1, Table 3.1). However, many of the sources of carbon monoxide emissions are different from those of carbonaceous particles. The NCAR group did not simulate the future composition of black and organic carbon based on emissions projections, but instead scaled their present-day distribution by the global factors derived for sulfur dioxide. The time evolution of black and organic carbon emissions in the CCSM model thus follows the same trajectory as that of SO 2 . On the other hand, the GISS group used emissions projec

tions from Streets et al. (2004) based on energy and fuel usage trends from the IMAGE model (as for other gases and particles) and including expected changes in technology. This led to a substantial reduction in future emissions of carbonaceous particles.
For precursors of tropospheric ozone, there was again divergence among the models. The pri

mary precursor in most regions, NO x (nitrogen oxides = NO + NO 2 ), increased steadily in the projections used by GISS, while it peaked at 2030 and decreased slightly thereafter in the projections used at GFDL (Table 3.1). Hydrocar

bons and carbon monoxide show analogous dif

ferences. Methane was prescribed according to the A1B “marker” scenario values for all three composition models. Thus ozone, in addition Species Model 2000 2030 2050 2100
NO x (Tg N per year) GFDL 40 57 (43%) 54 (35%) 48 (20%) GISS 50.5 67.0 (33%) 77.5 (53%) NA BC (Tg C per year) GFDL 10.9 14.0 (28%) 15.3 (40%) 19.9 (83%) GISS 8.6 6.8 (-21%) 6.0 (-30%) NA OC (Tg C per year) GFDL 51.5 61.9 (20%) 66.5 (29%) 84.3 (%) GISS 69.5 57.0 (-18%) 58.3 (-16%) NA SO 2 (Tg SO 2 per year) GFDL 147 187 (27%) 118 (-20%) 56 (-62%) GISS 130 202 (55%) 164 (26%) NA CCSM 125 190 (52%) 148 (18%) NA Dust (Tg per year) GFDL 2471 2471 2471 2471 GISS 1580 1580 1580 NA Table 3.1 Global emissions. Emissions (in Teragrams, Tg = 1 x 10 12 g) include both natural and anthropogenic sources. V alues in parentheses are changes relative to 2000.

Figure 3.1 A1B emissions trends used in the three models for SO 2 (top) and black carbon (bottom). Note that in the CCSM model, the present day black carbon distribution was scaled in the future rather than calculated from black carbon emissions. Scaling was chosen to mimic the global sulfur dioxide emissions, a 40 percent increase over 2000 at 2030, and 10 percent at 2050. The CCSM 2000 black carbon global emission is set at the average of the GISS and GFDL 2000 values, and follows this scaling in the future, for illustrative purposes

.31 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

to the particles, was modeled in substantially different ways at the three centers.
The three models included projected changes in the same gases and particles, with the excep

tion of nitrate, which only varied in the GISS model. As its contribution to total particle and total particle radiative forcing is small, at least in this GISS model, this particular difference was not significant in our results.
3.2.2 Composition Models The chemical composition models used to pro

duce short-lived gas and particle concentrations for the GFDL, GISS, and CCSM climate models were driven by the emissions projections dis

cussed in Section 3.2.1. While the three models did not use identical present-day emissions, their anthropogenic emissions were based on the same international inventory (Olivier and Berdowski, 2001). The chemical composition simulations were run for one or two years, with the three-dimensional monthly mean con

centrations and optical properties archived for use as off-line concentration fields to drive the climate model simulations discussed in Section 3.3. These simulations were all performed with present-day meteorology (values for tempera

ture, moisture, and wind). Further details about the chemical composition models are provided in Appendix C.

3.2.2.1 G eophysi ca l F luid D y na mi c s L a bo ra to r y (GF DL ) Composition changes for the short-lived gases and particles in the GFDL experiments were calculated using the global chemical transport model MOZART-2 (Model for OZone And Re

lated chemical Tracers, version 2.4), which has been described in detail previously (Horowitz et al. , 2003; Horowitz, 2006; and references therein). This model was used to generate the monthly average distributions of tropospheric ozone, sulfate, and black and organic carbon as a function of latitude, longitude, altitude, and time for the emissions scenarios discussed above. Simulated ozone concentrations agree well with present-day observations and recent trends (Horowitz, 2006). Overall, the predicted concentrations of particle are within a factor of two of the observed values and have a tendency to be overestimated (Ginoux et al. , 2006). Fur

ther details on the MOZART model are found in Appendix C, in the section on Geophysical Fluid Dynamics Laboratory.

3.2.2.2 G odd ar d I n stitute f o r S p ac e S tudies (G ISS ) The configuration of the GISS composition model used here has been described in detail in (Shindell et al. , 2007). In brief, the composi

tion model PUCCINI (Physical Understand

ing of Composition-Climate INteractions and Impacts) includes ozone and oxidant photochemistry in both the troposphere and stratosphere (Shindell et al. , 2006b), sulfate, carbonaceous and sea-salt particles (Koch et al. , 2006, 2007), nitrate particles (Bauer et al. , 2007), and mineral dust (Miller et al. , 2006a). Present-day composition results in the model are generally similar to those in the underlying chemistry and particle models. Further details on the PUCCINI model resolution, composi

tion, and performance are found in Appendix C, in the section on Goddard Institute for Space Studies.

3.2.2.3 c ommu n ity c lim a te system model (CC SM ) For the climate simulations described in this section, present-day tropospheric ozone was taken from Lamarque et al. ( 2005a); beyond 2000, tropospheric ozone was calculated by T. Wigley using the MAGICC composition model <http://www.cru.uea.ac.uk/~mikeh/software/magicc.htm> forced by the timevarying emissions of NO x , methane and volatile organic compounds (VOCs) and these averageglobal values were used to scale the presentday distribution. Future carbonaceous particles are scaled from their present-day distribution (Collins et al. , 2001) by a globally uniform factor whose time evolution follows the global evolution of SO 2 emissions. Stratospheric ozone changes are prescribed following the study by (Kiehl et al. , 1999). Further details on the composition models used by NCAR are found in Appendix C in the section on National Center for Atmospheric Research.
3.2.3 Tropospheric BurdenThe composition models each calculate timevarying three-dimensional distributions of the short-lived gases and particles (except for CCSM where 2030 and 2050 ozone, black carbon, and organic carbon were scaled based While the three models used in this

study did not useidentical presentday emissions, their human-caused emissions were based on the same international inventory.The U.S. Climate Change Science Program Chapter 3

32 on their 2000 distributions). We compare these using the simple metric of the global mean an

nual average tropospheric burden ( i.e. , the total mass in the troposphere). As was the case with emissions, the differences between the outputs of the composition models are substantial (Table 3.2). The GFDL model has a 67 percent greater present-day burden of sulfate than the GISS model, for example. As the GFDL sulfur diox

ide emissions were only 13 percent greater, this suggests that either sulfate stays in the air longer in the GFDL model than in the GISS model or sulfur dioxide is converted more efficiently to sulfate in the GFDL model.
This can be tested by analyzing the atmospheric residence times of the respective models (Table 3.3). The residence time of sulfate is within ~10 percent in the two models, and in fact is slightly less in the GFDL model. This indicates that the conversion of sulfur dioxide (SO 2 ) to sulfate must be much more efficient in the GFDL model for it to have a sulfate burden so much larger than the GISS model. This is clearly seen in the ratio between sulfate burden and SO 2 emis

sions (Table 3.4). This ratio can be analyzed in terms of the total sulfur dioxide burden (in Tg [teragrams]) per SO 2 emission (in Tg per yr); the change in SO 2 burden per SO 2 emis

sion change, or alternatively in the percentage change in each. The latter is probably the most useful evaluation, as the fractional change will reduce differences between the starting points of the two models. We note that this metric is affected by both production and removal rates in the models. Table 3.4 shows clearly that the production of sulfate per Tg of sulfur emitted is much greater in the GFDL model than in the GISS model, either because of differences in other sources of sulfate ( e.g. , from dimethyl sulfide [DMS]) or differences in the chemical conversion efficiency of SO 2 to sulfate (versus physical removal of SO 2 by deposition).
The residence times of black and organic carbon (BC and OC) are also fairly similar in these two models (Table 3.3). The concentrations of sulfate and carbonaceous particles are all influenced by differences in how the models simulate removal by the hydrologic cycle, ac

counting for at least some of the 10 to 15 percent difference in residence times. Sulfate produc

tion can vary even more from model to model, as its production from the emitted sulfur di

Species Model 2000 2030 2050 2100 BC (Tg C) GFDL 0.28 0.36 (29%) 0.39 (39%) 0.51 GISS ** 0.26 0.19 (-27%) 0.15 (-42%) NA CCSM (40%) (10%) OC * (Tg C) GFDL 1.35 1.59 (18%) 1.70 (26%) 2.15 GISS 1.65 1.33 (-19%) 1.27 (-23%) NA CCSM (40%) (10%) Sulfate (Tg SO 4 = ) GFDL 2.52 3.21 (27%) 2.48 (-2%) 1.50 (-40%) GISS 1.51 2.01 (33%) 1.76 (17%) NA CCSM Dust (Tg) GFDL 22.31 22.31 22.31 22.31 GISS 34.84 34.84 34.84 NA CCSM Tropospheric Ozone (DU) GFDL 34.0 38.4 (13%) 39.3 (16%) 38.2 (12%)
GISS 31.6 41.5 (31%) 47.8 (51%) NA CCSM 28.0 41.5 (48%) 43.0 (54%) NA Table 3.2 Global burdens. V alues in parentheses are changes relative to 2000.

* The organic carbon (OC) burdens include primary OC particles (with emissions as in Table 3.1) plus secondary OC particles (SOA). In the GFDL model, the global burden of SOA is 0.07 Tg C in this inventory. In the GISS model, organic carbon from SOA makes up ~24% of present-day OC emissions. Burdens are given in units of teragrams (1Tg = 1012 g) for particles and in units of Dobson units (1DU globally averaged = 10.9 Tg O3) for ozone.

** GISS sulfate burdens include sulfate on dust surfaces, which makes up as much as one-half the total burden.

The concentrations of sulfate and carbonaceous particles are all influenced by differences in how the models simulate removal by the hydrologic cycle, accounting for at least some of the 10 to 15 percent difference in residence times in the atmosphere.33 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

oxide involves chemical oxidation, which can differ substantially between models. Removal of sulfur dioxide prior to conversion to sulfate may also be more efficient in the GISS model. In contrast, BC and OC are emitted directly, and hence any differences in how these are represented in the models would be apparent in their residence times.
The particle residence times are relatively stable in time in the GISS and GFDL models. The carbonaceous particle residence times do decrease with time in the GISS model (and to a lesser extent in the GFDL model for OC), prob

ably owing to the shift with time from mid- to tropical latitudes, where wet and dry removal rates are different (more rapid net removal). The sulfate residence time is fairly stable over the 2000 to 2050 period. The ratio of sulfate burden to SO 2 emissions is the same for the present-day and the 2030 to 2000 changes in the GFDL model. For the 2100 to 2000 change in that model (not shown), the ratio drops from 1.00 to 0.65. As the total emissions of SO 2 decrease, a larger fraction of the sulfate production comes from DMS oxida

tion rather than from emitted SO 2 . The conversion efficiency from SO 2 to sulfate also varies over time in the GISS model, decreas

ing to 2030 and increasing there

after (inversely related to total sulf u r dioxide emissions). This may reflect both non-linearities in production (via oxidation chemistry) and the changing spatial pattern of emissions.
After comparison of the inter-model varia

tions in particle residence times and chemical conversion efficiencies with the variations in emissions trends, it is clear that the differences in the projected changes in particle burdens in the GISS and GFDL simulations are primarily attributable to the underlying differences in emissions. This is especially true for carbona

ceous particles, for which the residence times are quite similar in the models. Even though there is a greater difference in sulfate burdens due to the variations in chemical conversion efficiency between the models, the emissions trends at 2050 relative to 2000 are of opposite sign in the two models and thus dominate the difference in the burden change. Thus, the GISS model projects a greater sulfate burden at 2050 than at 2000, but substantially reduced burdens of carbonaceous particles, while the GFDL model projects the opposite, both because of the underlying emissions projections. It is clear that the differences in the projected changes in particle burdens simulated in the two models are primarily

attributable to the underlying differences in emissions. Species Model 2000 2030 2050

BC GFDL 9.4 9.4 9.3
GISS 11.0 10.2 9.1
OC GFDL 9.6 9.4 9.3
GISS 8.7 8.5 8.0
Sulfate GFDL 8.0 8.2 8.1
GISS 8.8 8.8 9.0 Table 3.3 Global mean annual average particle residence times (days).

Species Model 2000 Tg burden/ Tg emission

per year 2030 vs. 2000 Tg burden/ Tg emission

per year 2030 vs. 2000 % burden/ % emission 2050 vs. 2000 % burden/ % emission

Sulfate GFDL 0.017 0.017 1.00 0.08 *

GISS 0.012 0.007 0.60 0.65

Ozone GFDL 7.19 2.24 0.32 0.44

GISS 6.82 6.54 0.94 0.96 Ratios for sulfate are in Tg sulfate divided by Tg SO 2 per year emitted as SO 2 . Ozone ratios are in Tg ozone divided by Tg N per year emitted as NO x . Ozone values in Table 3.2 are converted to burden assuming 1 DU globally averaged = 10.9 Tg ozone. Burdens are given in units of Teragrams (Tg = 1 x 10 12 g).

* The burden change was only 2 percent in this case, making the calculation unreliable. Table 3.4 Ratio of sulfate and ozone burdens to precursor emissions, global mean annual average.The U.S. Climate Change Science Program Chapter 3

34 The results for tropospheric ozone tell a dif

ferent story. The ozone burden increases in the future in all three models, but the percentage increase relative to 2000 differs by more than a factor of three at 2030 (Table 3.2). Exam

ining the ozone changes relative to the NO x emissions changes, there are very large dif

ferences between the GFDL and GISS models (Table 3.4). This may reflect the influence of processes such as stratospheric ozone influx which is independent of NO x emissions, as well as the roles of precursors such as carbon monoxide (CO) and hydrocarbons that also influence tropospheric ozone. In particular, the GISS model computed a large increase in the flux of ozone into the troposphere as the stratospheric ozone layer recovered, while the composition model used at GFDL held stratospheric ozone fixed and hence did not simulate similar large increases. In addition, there are well-known non-linearities in O 3 - NO x chemistry (Stewart et al. , 1977), and it has been shown that the ozone production ef

ficiency can vary substantially with time (La

marque et al. , 2005a; Shindell et al. , 2006a). Thus for tropospheric ozone, the differences in modeled changes of nearly a factor of three (13 vs . 33 percent increase) are much larger than the differences in the NO x precursor emissions (33 vs . 43 percent increase).
3.2.4 Aerosol Optical Depth The global mean present-day all-sky aerosol optical depth (AOD) 3 in the three models ranges from 0.12 to 0.20 (Table 3.5). This dif

ference of almost a factor of two suggests that particles are contributing quite differently to the Earth’s energy balance with space in these models. Observational constraints on the all-sky value are not readily available, as most of the extant measurement techniques are reliable only in clear-sky (cloud-free) conditions. Sampling clear-sky areas only, the GISS model’s global total aerosol opti

cal depth is 0.12 for 2000 (0.13 Northern Hemisphere, 0.10 Southern Hemisphere). This includes contributions from sulfate, carbonaceous, nitrate, dust, and sea-salt particles. The clear-sky observations give global mean values of ~0.135 (ground-based AERONET) or ~0.15 (satellite composites, including AVHRR or MODIS observations), though these have substantial limitations in their spatial and temporal coverage. The CCSM

3 Aerosol optical depth is a measure of the fraction of

radiation at a given wavelength absorbed or scattered by particles while passing through the atmosphere. The ozone burden in the lower atmosphere increases in the future in all three models

. Table 3.5 All-sky aerosol optical depth (550nm extinction).

Region Particle Type Model 2000 2030 2050 2100

Global BC GFDL .0076 .0096 .0105 .0138 GISS .0045 .0034 .0028 NA Sulfate GFDL .1018 .1227 .0906 .0591 GISS .0250 .0312 .0278 NA CCSM .048 .062 .052 NA Sea salt GFDL .0236 .0236 .0236 .0236 GISS .1065 .1080 .1050 NA CCSM .018 .018 .018 NA Dust GFDL .0281 .0281 .0281 .0281 GISS .0372 .0389 .0387 NA CCSM .0275 .0275 .0275 NA OC GFDL .0104 .0122 .0131 .0166 GISS .0166 .0135 .0130 NA Nitrate GISS .0054 .0057 .0060 NA Total GFDL .1715 .1964 .1660 .1411 GISS .1959 .2007 .1934 NA CCSM .116 .1392 .1206 NA

Northern Hemisphere BC GFDL .0109 .0147 .0161 .0209 GISS .0062 .0043 .0032 NA Sulfate GFDL .1509 .1766 .1038 .0694 GISS .0352 .0449 .0388 NA CCSM .078 ** .097 ** .073 ** NA Dust GISS .0600 .0642 .0615 NA GFDL .0491 .0491 .0491 .0491 Sea salt GISS .0630 .0619 .0647 NA GFDL .0181 .0181 .0181 .0181 Total GFDL .2430 .2756 .2056 .1807 GISS .1910 .1985 .1907 NA CCSM .1538 .1827 .1502 NA

Southern Hemisphere BC GFDL .0042 .0046 .0049 .0066 GISS .0029 .0026 .0023 NA Sulfate GFDL .0526 .0689 .0774 .0487 GISS .0148 .0175 .0170 NA CCSM .052 ** .062 ** .075 ** NA Dust GISS .0144 .0137 .0159 NA GFDL .0071 .0071 .0071 .0071 Sea salt GISS .1502 .1541 .1453 NA GFDL .0291 .0291 .0291 .0291 Total GFDL .1000 .1171 .1263 .1015 GISS .1997 .2030 .1962 NA CCSM .0779 .0957 .0910 NA

** Total for sulfate plus sea salt.35 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

and GFDL models did not calculate clear-sky aerosol optical depth. Given that the all-sky values are larger, and substantially so in the GISS model (though this will depend upon the water uptake of particles), it seems clear that the values for CCSM would be too small compared with observations since even their all-sky values are lower than the estimate from observations. This may be related to CCSM’s use of AVHRR data in assimilation of aerosol optical depth to create the CCSM climatology (Collins et al. , 2001, 2006), as that data appears to be low relative to MODIS observations, for example.
For all three models, there are large differences in the contributions of the various particles (Figure 3.2; Table 3.5). This is true even for GFDL and GISS models, with relatively simi

lar all-sky global mean aerosol optical depths. More than half the aerosol optical depth in the GFDL model comes from sulfate, while this particle contributes only about one-eighth the aerosol optical depth in the GISS model. In

stead, the GISS model’s aerosol optical depth is dominated by the largely natural sea salt and dust particles, which together contribute 0.14 to the aerosol optical depth. These two particles contribute a much smaller aerosol optical depth in the CCSM and GFDL models, ~0.06 or less, with the differences with respect to GISS pre

dominantly due to sea salt. The relative contri

bution from sulfate in the CCSM model looks similar to the GFDL model, with nearly half its aerosol optical depth coming from sulfate, but the magnitude is much smaller. It seems clear that the GFDL model’s direct sulfate contribu

tion is biased high (Ginoux et al. , 2006), while the GISS model’s sulfate is biased low in this model version (Shindell et al. , 2007). However, the relative importance of the different particles is not well understood at present (Kinne et al. , 2006).
Large differences in the relative aerosol opti

cal depth in the Northern Hemisphere and Southern Hemisphere are also apparent in the models (Table 3.5). The ratios of the present-day Northern Hemisphere to Southern Hemisphere total aerosol optical depths in the three models differ widely, with values of 2.43, 1.97, and 0.96 in the GFDL, CCSM, and GISS models, respectively. This clearly reflects the dominant contribution of sulfate to optical depth in the GFDL and CCSM models, as it has large anthro

pogenic Northern Hemisphere sources, and the dominance of sea salt in the GISS model, with its largest source being the Southern Ocean. While composite satellite data shows clearly greater aerosol optical depths in the Northern Hemisphere than the Southern Hemisphere, most satellite instruments lose coverage near the northern edge of the Southern Ocean (Kinne et al. , 2006). Unfortunately, quality-controlled networks such as AERONET provide virtually no ground-based data poleward of 45°S. Thus while it seems unlikely that the aerosol optical depth is larger in the Southern Hemisphere than the Northern Hemisphere, as in the GISS model, presently available data are not adequate to fully characterize this ratio, as aerosol optical depths over the Southern Ocean are poorly known.

Figure 3.2 Present-day contributions from individual particles to global mean all-sky aerosol optical depth (550 nm). Neither GFDL nor CCSM include nitrate.The U.S. Climate Change Science Program Chapter 3

36 

### 3.3 C limate S t u dies
3.3.1 Experimental Design The climate studies discussed here consist of transient climate simulations that were designed to isolate the climate effects of projected changes in the short-lived gases and particles and calculate their importance relative to that of the long-lived well-mixed greenhouse gases. The simulations from the GFDL, GISS, and NCAR groups each employed ensembles (multiple simulations differing only in their initial conditions) in order to reduce the unforced variability in the chaotic climate system. One three-member ensemble included the evolution of short- and long-lived gases and particles following the A1B storyline emission scenario, while the second ensemble included only the evolution of long-lived gases with the short-lived gases and particles fixed at present values. While all three groups used the same values for the long-lived gases, each had its own version of an A1B scenario for short-lived gases and particles, as discussed previously in Section 3.2.

The global three-dimensional distributions of short-lived gases and particles were modeled using each group’s chemistry-particle composi

tion model. For the first ensemble, the GFDL simulations used particle and ozone distribu

tions computed each decade out to 2100, while the GISS and CCSM simulations employed values computed for 2000, 2030, and 2050. Either seasonally varying or monthly-average three-dimensional distributions were saved. Short-lived gas and particle concentrations for intermediate years were linearly interpolated between the values for computed years. In bothsets of simulations, the concentrations of longlived gases varied with time. In practice, NCAR performed only a single pair of simulations out to 2050, while GISS performed all three pairs out to 2050, and GFDL extended all three pairs out to 2100.

3.3.2 Climate Models 3.3.2.1 G eophysi ca l F luid D y na mi c s L a bo ra to r y (GF DL ) Climate simulations at GFDL used the com

prehensive climate model (Atmosphere-Ocean General Circulation Model [AOGCM] [Box 1.1]) recently developed at NOAA’s Geophysical Fluid Dynamics Laboratory, which is described in detail in Delworth et al. (2006). The control simulation of this AOGCM (using present-day values of radiatively active gases and particles) has a stable, realistic climate when integrated over multiple centuries. The model is able to capture the main features of the global evolu

tion of observed surface temperature for the twentieth century as well as many continen

tal-scale features (Knutson et al. , 2006). Its equilibrium climate sensitivity to a doubling of CO 2 is 3.4°C 4 (Stouffer et al. , 2006). Themodel includes the radiative effects of wellmixed gases and ozone on the climate as well as the direct effects of particles, but does not include the indirect particle effects (Box 3.1). Further details on the model resolution, model physics, and model performance are included in Appendix D in the section on Geophysical Fluid Dynamics Laboratory.

3.3.2.2 G odd ar d I n stitute f o r S p ac e S tudies (G ISS ) The GISS climate simulations were performed using GISS ModelE (Schmidt et al. , 2006). This model has been extensively evaluated against observations (Schmidt et al. , 2006), and has a climate sensitivity in accord with values inferred from paleoclimate data and similar to that of mainstream General Circulation Models; the equilibrium climate sensitivity for doubled CO 2 is 2.6°C. The radiatively active compounds in the model include well-mixed gases, ozone, and particles. The model includes a simple parameterization for the particle indirect effect (Menon et al. , 2002) (Box 3.1). Further details on the model resolution and model physics are included in Appendix D in the section on God

dard Institute for Space Studies.

3.3.2.3 c ommu n ity c lim a te system model (CC SM ) The transient climate simulations use the CCSM Community Climate System Model CCSM3 (Collins et al. , 2006). The equilibrium climate sensitivity of this model to doubled CO 2 is 2.7°C. Further details on the model resolution

4 Equilibrium climate sensitivity is defined here as

the global-mean, annual-mean surface temperature change of a climate model in response to a doubling of atmospheric carbon dioxide from preindustrial levels, when the model has fully adjusted to the change in carbon dioxide. The model experiments used here are designed to isolate the climate effects of projected changes in the short-lived gases and particles and calculate their importance relative to that of the long-lived well-mixed greenhouse gases.37 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

and construction are found in Appendix D in the section on National Center for Atmospheric Research.
3.3.3 Radiative Forcing Calculations The radiative forcing at the tropopause provides a useful, though limited, indicator of the climate response to perturbations (Hansen et al. , 2005) (Box 3.2). 3.3.3.1 G lob a l an d H emisphe r i c A ve rag e V

a lues : GF DL an d G ISS Radiative forcing calculations were performed by GFDL (adjusted forcing) and GISS (instan

taneous forcing), but were not performed for the CCSM model. The annual-average global-mean radiative forcing (RF) from short-lived gases and particles at 2030 relative to 2000 is small in both the GFDL and GISS models (Figure 3.3; BOX 3.1: Radiative Effects of Particles

The direct effects of particles refer to their scattering and absorption of both incoming solar and outgoing terrestrial radiation. By reflecting incoming radiation back to space, most particles have a negative radiative forcing (cooling effect). For reflective particles (sulfate, organic carbon, nitrate, dust and sea salt), this effect dominates over their absorption of outgoing radiation (the greenhouse effect) on the global scale. The balance varies both geographically and seasonally as a function of solar radiation and the ground temperature. In contrast, absorbing particles such as black carbon have a positive radiative forcing (warming effect) as they absorb incoming and outgoing radiation, reducing the overall fraction of the sun’s irradiance that it reflected back to space. They can also absorb outgoing radiation from the Earth (the greenhouse effect).
In addition to their direct radiative effects, particles may also lead to an indirect radiative forcing of the climate system through their effect on clouds. Two particle indirect effects are identified: The first indirect effect (also known as the cloud albedo effect) occurs when an increase in particles causes an increase in cloud droplet concentration and a decrease in droplet size for fixed liquid water content (Twomey, 1974). Having more, smaller drops increases the cloud albedo (reflectivity). The second indirect effect (also known as the cloud lifetime effect) occurs when the reduction in cloud droplet size affects the precipitation efficiency, tending to increase the liquid water content, the cloud lifetime (Albrecht, 1989), and the cloud thickness (Pincus and Baker, 1994). As the clouds last longer, this leads to an increase in cloud cover. It has been argued that empirical data suggest that the second indirect effect is the dominant process (Hansen et al. , 2005).
Satellite- and ground-based observations have been used to estimate particle indirect effects and to evaluate their treatment in climate models ( e.g. , Kaufmann et al. , 2005; Lohmann and Lesins, 2002). The recently available CALIPSO and CloudSat measurements, which provide vertical distributions of particles and clouds, may be particularly useful for these purposes. However, the difficulty of separating the influence of particle indirect effects from dynamical and meteorological effects remains a major problem for such observational studies (Lohmann et al. , 2006).
The direct effects of particles are relatively well-represented in climate models such as those described in Section 3.3.2 and used in this study, though substantial uncertainties exist regarding the optical properties of some particle types and especially of particle mixtures. Because of the inherent complexity of the particle indirect effect, climate model studies dealing with its quantification necessarily include an important level of simplification. While this represents a legitimate approach, it should be clear that the climate model estimates of the particle indirect effect are very uncertain.
The studies discussed in Chapter 3 of this Report include the direct effects of particles in all three models (though nitrate is only included in the GISS model). The indirect effect is only included in the GISS model, which uses a highly simplified representation of the second indirect effect.The U.S. Climate Change Science Program Chapter 3

38 Table 3.6). However, this is for quite different reasons. In the GFDL model, a large increase in sulfate optical depth leads to a negative forc

ing that is largely balanced by positive forcings from increased black carbon particle and ozone. In the GISS model, increased sulfate and re

duced black carbon both lead to relatively small negative forcings that largely offset a substantial positive forcing from increased ozone. Moving to 2050, the models now diverge in their net values as well as the individual contributions. The GFDL model finds a positive radiative forc

ing due in nearly equal parts to increased black carbon and ozone. In contrast, 2050 radiative forcing in the GISS model again reflects an offset between positive forcing from ozone and negative particle forcing, with the largest contri

bution to the latter from reduced levels of black carbon. Both models show a partial cancellation of the black carbon forcing by an opposing forc

ing from organic carbon. Thus, the two models show somewhat consistent results for ozone, but differ dramatically for black carbon and sulfate particle. By 2100, the GFDL model has a large positive radiative forcing relative to 2000, due to the continued increase in black carbon as well as the decrease in sulfate.
Inter-model differences in radiative forcing are predominantly due to differences in modeled burdens rather than to differences in the calcula

tion of radiative properties in the models. This can be seen clearly by examining the radiative forcing-to-burden ratio, which we term the ra

diative efficiency (Table 3.7). This shows fairly similar values for GFDL and GISS. The largest differences are seen for black carbon, which may reflect differences in the geographic loca

tion of projected black carbon changes as well as differing treatments of the radiative proper

ties of black carbon. Additionally, the vertical BOX 3.2: Radiative Forcing

Radiative forcing is defined as the change in net (down minus up) irradiance (solar plus longwave, in W per m 2 ) at the tropopause due to a perturbation after allowing for stratospheric temperatures to adjust to radiative equilibrium, but with surface and tropospheric temperatures and state held fixed at the unperturbed values (IPCC, 2007; Ramaswamy et al. , 2001). This quantity is also sometimes termed adjusted radiative forcing. If the stratospheric temperatures are not allowed to adjust, the irradiance change is termed instantaneous radiative forcing.
The utility of the radiative forcing concept is that, to first order, the equilibrium global-mean, annual-mean surface temperature change is proportional to the radiative forcing, for a wide range of radiative perturbations. The proportionality constant (often denoted as the climate sensitivity parameter, 

### λ ) is approximately the same (to within 25 percent) for most drivers of climate change (IPCC, 2007), with a typical value of ~0.5-0.7 for most models. This enables a readily calculable and comparable measure of the climate response to radiative perturbations, such as those discussed in this Chapter.

Figure 3.3 Global mean annual average radiative forcing (in W per m 2 ) from short-lived gases and particles at 2030 and 2050 relative to 2000. Values from the GFDL model are shown as solid bars; values from the GISS model have di

agonal hatching. (Note that instantaneous forcing values from the GFDL model are shown in this figure, not the adjusted forcings shown in Table 3.6.)39 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

distribution of the black carbon changes will affect the radiative forcing, as will their loca

tion relative to clouds. Variations in modeling the particle uptake of water, which can have a substantial impact on the aerosol optical depth, do not seem to play a very large role in the global mean radiative forcing judging from the fairly close agreement in the two models’ sulfate radiative efficiencies (Table 3.7). They may contribute to about 20 percent difference in the radiative forcing-to-burden ratios for sulfate, however. Examination of the radiative forcing-to-aerosol optical depth change (Table 3.7) shows that given a particular aerosol optical depth change, the models are in good agree

ment as to the resulting radiative forcing. We caution that this result contrasts with a wider model study that found larger differences in this ratio (Schulz et al. , 2006), though the variation in radiative forcing-to-aerosol optical depth across models was still less than the variation in aerosol optical depth itself. This suggests a possible further source of model differences that could exist were different models to be used in a study such as this. Both the GFDL and GISS models show a positive forcing from ozone that stems partially from increased tropospheric ozone concentra

tions (Table 3.2) due to increased NO x emissions (Table 3.1) and partially from the recovery of stratospheric ozone due to reductions in emis

Table 3.6 Global mean radiative forcing for short-lived gases and particles in Watts per square meter ( W per m 2 ).

Values are annual average radiative forcings at the tropopause (meteorological tropopause in the GISS model, “linear” tropopause in the GFDL model). “Particles” is the total of sulfate, black carbon (BC), and organic carbon (OC) (plus nitrate for GISS) particles. GISS values do not include particle indirect effects that were present in that model.

GFDL values are for adjusted radiative forcing; GISS values are for instantaneous radiative forcing (Box 3.2). The GFDL values are from Levy et al. (2008). Model 2030 2050 2100
Total GFDL .04 .48 1.17
GISS .00 .02 NA
Particles GFDL -.15 .24 .98
GISS -.13 -.17 NA
Sulfate GFDL -.32 .01 .51
GISS -.10 -.06 NA
BC GFDL .21 .30 .63
GISS -.09 -.16 NA
OC GFDL -.04 -.06 -.15
GISS .06 .06 NA
Ozone GFDL .19 .23 .19
GISS .13 .19 NA

Species Model (W per m 2 )/ Tg (W per m 2 )/ AOD
BC GFDL 2.8 104
GISS 1.5 94
OC GFDL -.18 NA
GISS -.16 NA
Sulfate GFDL -.47 -16
GISS -.59 -16 Table 3.7 Radiative efficiency.

Values are given for the radiative efficiency in terms of the radiative forcing-toburden ratio and the radiative forcing-to-aerosol optical depth (AOD) ratio. All values are global-mean, annual-mean averages. Values for radiative forcing and burden or aerosol optical depth changes are for 2050 versus 2000 for black carbon (BC) and organic carbon (OC), and 2030 versus 2000 for sulfate in order to analyze the largest changes for each particle. GISS values for the sulfate burden changes include only the portion of sulfate not absorbed onto dust, as this portion alone is radiatively important.The U.S. Climate Change Science Program Chapter 3

40 sions of ozone-depleting substances (primarily halogens). The forcing from the tropospheric portion of the ozone changes is substantially more important, however (Shindell et al. , 2007). (The NCAR group did not calculate the radiative forcing, but forcing in their model is likely to have been similar, as they found an increase in the tropospheric ozone burden from 2000 to 2050 of 15.0 Dobson Units [DU], very close to the GISS value of 16.2 DU [Table 3.2].) As shown previously, however, the apparent sensitivity of ozone burden to changes in NO x emissions differs substantially between the GISS and GFDL models. Thus the similarity in the radiative forcing may be largely fortuitous, resulting from a cancellation of changes in emissions and of sensitivities of ozone to NO x emissions.
Thus, at 2030, differences in the physical processes in the two models dominate the dif

ferences in radiative forcing between the two models. The large divergence in radiative forc

ing from sulfate stems from both the chemical conversion efficiency of SO 2 to sulfate being more than a factor of two larger in the GFDL model than in the GISS model, and the greater role of sulfate in producing aerosol optical depth in the GFDL model. In addition, the GISS model includes a substantial absorption of sulfate onto dust, a process that is highly uncer

tain. Such a process would reduce the radiative forcing due to sulfate. At 2050, emissions and concentrations of sulfur dioxide have returned to near their 2000 level, so that these differ

ences are not so important at this time. Hence, the 2050 differences between the two models are dominated by differences in black carbon emissions projections and not by differences in physical processes. Differences in the residence times and radiative efficiencies for black carbon are substantial but tend to offset.
On a hemispheric scale, the GISS and GFDL models again differ greatly (Table 3.8). The GFDL model shows a very large positive forcing in the Northern Hemisphere in 2050 due primarily to reductions in emissions of sulfate precursors and increased emissions of black carbon. Increases in sulfate precursor emissions from developing countries lead to a negative forcing in the Southern Hemisphere in the GFDL model. In the GISS model, the sign of the total net forcings are reversed, with negative values in the Northern Hemisphere and positive in the Southern Hemisphere (Table 3.8). The GISS results are primarily due to the reduction in BC in the Northern Hemisphere and the influence of increased ozone and re

duced OC in the Southern Hemisphere (where sea salt dominates the aerosol optical depth, so that anthropogenic particle emissions changes are relatively less important).

3.3.3.2 R e g io na l F o rc i ng P a

tte rn s The differences in hemispheric and global forc

ings can be attributed to strong forcing changes in particular regions, and hence to regional emissions as the radiative forcing is typically localized relatively close to the region of emis

sions. Comparison of the spatial patterns of radiative forcing in the GISS and GFDL models reveals that the starkest discrepancies occur in the developing nations of South and East Asia (Figure 3.4). The emissions scenario used by the GISS model projects strong increases in SO 2 emissions from India, with little change over China. In contrast, the scenario used by the GFDL model has large decreases in sulfate emissions in both regions, especially China.

The scenarios are much more similar for the developed world, with both projecting reductions in sulfate pre

cursor emissions for North America and Europe, for example, leading to a positive radiative forcing in both cases. Differences between the scenarios are even larger for black carbon, which increases throughout most of the Northern Hemisphere in the GFDL model but decreases in the Model 2030 2050 2100

Northern Hemisphere GFDL .15 1.09 1.91
GISS -.15 -.14 NA

Southern Hemisphere GFDL -.09 -.14 .42
GISS .16 .18 NA Values are the net annual average forcings at the tropopause in each hemisphere from particles and ozone. GISS forcing values do not include particle indirect effects that were present in that model. Table 3.8 Hemispheric radiative forcing (W per m 2 )
.

Increases in lowlevel ozone provide much more potential for future warming than the increase in upper-level ozone.41 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

GISS model. Again, however, the divergence is especially large over South and East Asia, where the GISS model has large reductions while the GFDL model has large increases (Figure 3.4). Thus the differences in the global total emissions discussed previously (Figure 3.1; Table 3.1) and in the global radiative forcing (Figure 3.2; Table 3.6) arise primarily from dif

ferences in projected emissions from developing countries in Asia.
The radiative forcing from organic carbon is generally similar in its spatial pattern to black carbon, but of opposite sign and substantially reduced magnitude (25 to 40 percent of the black carbon radiative forcing). Substantial Figure 3.4 Annual average instantaneous radiative forcing (W per m 2 ) near 2050 relative to 2000 for the indicated individual short-lived gases and particles in the GISS (left) and GFDL (right) models. Radia

tive forcing from long-lived gases is largely spatially uniform over the globe. (Note that the instantaneous forcings shown here for the GFDL model differ from the adjusted forcings shown in Table 3.6.)The U.S. Climate Change Science Program Chapter 3

42 differences again occur between the emis

sions scenarios of the two models, this time primarily over African biomass burning regions. As discussed previously, the GFDL model assumed that biomass burning emis

sions would scale with one-half the factor used for purely anthropogenic emissions, while the GISS model instead used regional biomass burning emissions projections (Streets et al. , 2004), with substantial reductions in African biomass burning.
The spatial pattern of radiative forcing from ozone is also very different in the two models (Figure 3.4). However, this forcing is not so closely tied to the region of precursor emissions in the GISS model where much of the forcing is related to an increased flux of ozone into the troposphere owing to the recovery of lower stratospheric ozone. This leads to substantial positive forcing in that model at high latitudes, even without including the effects of climate change on circulation (Section 3.3.3.4). At low latitudes, GISS shows little forcing as the modeled increase in upper stratospheric ozone causes negative radiative forcing, offsetting some of the forcing from tropospheric ozone increases, and alters lower level photochem

istry. Furthermore, the particle indirect effect in that model influences cloud cover and wet deposition, which seems to reduce tropospheric ozone at low latitudes in comparison with simu

lations not including the particle indirect effect. The GFDL model does not show a similar high latitude enhancement, however, but instead shows maximum ozone forcing in the tropics. This may reflect a greater geographic shift in emissions to lower latitudes, a greater efficiency in transporting ozone and its precursors to the upper troposphere, where ozone has the greatest positive forcing efficiency, and differences in the relative importance of change in the overly

ing stratospheric ozone column. The GFDL ra

diative forcing is similar to results from models with tropospheric ozone only and no particle indirect effects (Gauss et al. , 2003).

3.3.3.3 E ff e c ts o f u nc e r t a i n ties i n meth an e c o nc e n t ra tio n s o n ra di a

tive f o rc i ng The SAP 3.2 simulations included methane concentrations prescribed to A1B values from the AIM integrated assessment model, for consistency with the long-lived gas runs. To investigate the potential uncertainty in the methane value derived by that integrated as

sessment model, the GISS model performed an additional 2050 simulation using its internal methane cycle model (Shindell et al. , 2007). The simulation included prescribed anthropogenic emissions increases from the AIM model to al

low comparisons with the AIM results used in the results in this chapter. Natural spatially and seasonally varying emissions and soil adsorp

tion were the standard amounts described in Shindell et al. (2003). Both the methane emis

sions from wetlands and the biogenic isoprene emissions were interactive with the climate in this run (Guenther et al. , 1995; Shindell et al. , 2004), though the distribution of vegetation did not respond to climate change.
Methane’s oxidation rate is calculated by the model’s chemistry scheme in both the tropo

sphere and stratosphere. Thus methane can affect its own lifetime (which is primarily gov

erned by tropospheric oxidation rates), as can other molecules that compete with methane for hydroxyl radicals (the main oxidizing agent), such as isoprene. The simulations included 2050 surface climate (sea surface temperatures and sea ice, taken from an earlier climate model run). Changes in water vapor induced by the altered climate affect methane oxidation in those runs. Methane was initialized with estimated 2050 abundances and the simulations were run for three years. We note that the IMAGE integrated assess

ment model projected a continuous increase in methane emissions; this is rather different from the increase through 2030 and slow decrease thereafter in the AIM integrated as

The differences in the global total emissions arise primarily from differences in projected emissions from developing countries in Asia.43 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

sessment model. At 2050, for example, this led to projected anthropogenic methane emissions of 512 Tg C per year in the IMAGE model, substantially greater than the 452 Tg C per year from the AIM model used here (compared with 323 Tg C per year for 2000).

We find that methane emissions from wetlands increase from 195 to 241 Tg C per year while emissions of isoprene increase from 356 to 555 Tg C per year. Additionally, even in the absence of changes in emissions from natural sources, the projected anthropogenic emissions of ozone precursors (including methane itself) increase the lifetime of methane while climate change reduces it via increased temperature and water vapor (Table 3.9). These responses to anthropogenic emissions and to climate change without interactive emissions are qualitatively consistent with those reported from a range of models (using different emissions projections) in Stevenson et al. (2006). The effect of precur

sor emissions is stronger in our scenario, so that the net effect of anthropogenic emissions and climate changes is to increase methane’s lifetime. When natural emissions are also al

lowed to respond to climate change, increased competition from isoprene and increased meth

ane emissions from wetlands lead to further increases in methane’s lifetime (Table 3.9) and enhanced methane abundance.

The 2050 simulation with the model’s inter

nal methane cycle had a global mean surface methane value of 2.86 ppmv in year three, with sources exceeding sinks by 80 Tg C per year (a growth rate that may reflect an overestimate of the loss rate in the AIM model used in the initial guess). Extrapolating the change in methane out to equilibrium using an exponential fit to the three years of model results yields a 2050 value of 3.21 ppmv.
We have calculated radiative forcings using the standard calculation (Table 6.2 in Ramaswamy et al. , 2001) assuming an increase in N 2 O from 316 to 350 ppb in 2050, following the A1B “marker” scenario (using the AIM integrated assessment model). The 2050 methane forcing using the methane concentration specified in the A1B “marker” scenario would be 0.22 W per

m 2 while using the larger methane concentra

tions of 2.86 or 3.21 ppmv calculated with our model gives 0.36 or 0.46 W per m 2 , respectively. Of course, it is difficult to estimate methane’s abundance at a particular time without perform

ing a full transient methane simulation. How

ever, uncertainty in the forcing from methane appears to be at least 0.1 to 0.2 W per m 2 . Note that use of the 40 percent larger anthropogenic methane emissions increase from the IMAGE integrated assessment model would have led to a substantially larger forcing. Should the results of our modeling of the methane cycle prove to be robust, this would imply that future positive forcing from methane might be substantially larger than current estimates based on inte

grated assessment model projections.
We note that while the A1B projections assume a substantial increase in atmospheric methane in the future, the growth rate of methane has in fact decreased markedly since the early 1990s and leveled off since ~1999 (Dlugokencky et al. , 2003). Hence, the projections may over

estimate future atmospheric concentrations. However, there are indications that the growth rate decrease was primarily due to reduced an

thropogenic emissions, and that these have been increasing again since 1999 (though masked by a coincident decrease in natural methane emissions) (Bousquet et al. , 2006). All of this suggests that atmospheric methane may in fact increase substantially again in the future, as assumed by the integrated assessment models, although other methane studies have argued for an increase in its principal loss path as the explanation, rather than changes in emissions ( e.g. , Fiore et al. , 2006). Other emissions, such as NO x from lightning and from soil and dime

thyl-sulfide from the oceans, are also expected to respond to climate change. Changes in land There is considerable uncertainty in methane-driven climate changes in future scenarios.They

could be larger than currently anticipated.

Run Lifetime (years)
2000 9.01
2030 9.96
2050 10.39 2030 with climate change 9.72 2050 with climate change 10.01 2050 with methane cycle 10.42 Table 3.9 Methane lifetime in GISS simulations. Includes calculated photochemical loss (in tropo
sphere and stratosphere) and prescribed 30 Tg C per year loss to soils.The U.S. Climate Change Science Program Chapter 3

44 cover would also affect both emissions and re

moval of trace gases and particles. Further work is required to gauge the importance of these and other climate-chemistry feedbacks.

3.3.3.4 E ff e c ts o f C lim a te C h ang e o n R a di a tive F o rc i ng The chemical composition simulations (Section 3.2) did not include the effects of climate change on the short-lived gases and particles, only the effects of projected changes in anthropogenic emissions. Separate sets of simulations with the GISS model included climate change via prescribed sea-surface temperatures and sea-ice cover taken from prior runs. Climate change increased the radiative forcing from ozone by increasing stratosphere-troposphere exchange (STE) and hence ozone near the tropopause where it is most important radiatively (Hansen et al. , 1997). This effect outweighed increased reaction of excited atomic oxygen with the enhanced tropospheric water vapor found in a warmer climate, which led to ozone reductions in the tropical lower troposphere. The overall impact was to increase radiative forcing by .07 W per m 2 in 2050. Climate change slightly increased the negative forcing from sulfate (by .01 W per m 2 ), consistent with an increase in tropospheric ozone in these runs (as ozone aids in sulfur dioxide oxidation both directly and via hydroxyl formation).
Dust emissions decreased slightly (about five percent at 2050) in these climate runs, but there was more sulfate on dust, suggesting that this played only a minor role in the sulfate forcing response to climate change. The reduction in dust would itself lead to a slight negative forc

ing (approximately 0.02 W per m 2 ). However, emissions in the model respond only to changes in surface wind speeds, and not to changes in sources due to either CO 2 fertilization or climate-induced vegetation changes which have a very uncertain effect on future dust emissions (Mahowald and Luo, 2003; Woodward et al. , 2005).
Much of the increase in ozone forcing results from an increase in stratosphere-troposphere exchange in the GISS model of 134 Tg per year (about 20 percent of its present-day value) as climate warms. An increase in transport rates between the stratosphere and the troposphere is a robust projection of climate models (Butchart et al. , 2006). Combined with the expected recovery of stratospheric ozone, this should enhance the influx of stratospheric ozone into the lower atmosphere. However, the net effect of climate change on ozone is more difficult to determine as it results from the difference between enhanced stratosphere-troposphere exchange and enhanced chemical loss in the troposphere in a more humid environment, which is not consistent among climate models (Stevenson et al. , 2006).

3.3.4 Climate Model Simulations 2000 to 2050 As discussed in Section 3.3.1, the experimental design consists of two sets of simulations: (1) the effects of changes in short-lived and long-lived gases and particles in the twenty-first century (employing the A1B scenario for the evolution of the long-lived gases and the output from the composition models discussed in Section 3.2 for the short-lived gases and particles); (2) the effects of changes in the long-lived gases only, with the short-lived gases and particles concentrations held at 2000 values. The effects of short-lived gas and particle changes are de

termined by subtracting the climate responses of the runs with changes in long-lived gases only from those with changes in long-lived and short-lived gases and particles. This procedure is justified by studies showing that the climate response to changes in radiatively active spe

cies is generally linear (Ramaswamy and Chen, 1997; Haywood et al. , 1997; Cox et al. , 1995). Simulations where only the short-lived gases and particles change were not included in the experimental design, because such scenarios are neither realistic nor policy relevant.

3.3.4.1 S u rfac e T empe ra tu r e C h ang es The global-mean annual-mean surface tem

perature responses to short-lived gases and particles in the three different models are not as dissimilar as one might have expected, given the different emissions used and the different physical processes included. The CCSM model ran only a single simulation, which showed little or no statistically significant 5 effects of the short-lived gases and particles on global mean surface temperatures. The GFDL and

5 The statistical methods used to assess significance

are discussed in Box 3.3. There may be significant feedbacks of climate change on ozone concentration and dust emissions.45 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

GISS models both ran three-member ensemble simulations, and both show a statistically sig

nificant 5 warming effect from short-lived gases and particles from around 2030 to the end of the runs (Figure 3.5). The GFDL model shows a warming of 0.28°C (ensemble mean 2046 to 2050). This value is commensurate with the adjusted radiative forcing of about 0.48 W per

m 2 computed for 2050. The GISS model shows substantially more warming (approximately 0.13°C near 2050) than would be expected from the direct radiative forcing in that model and its climate sensitivity (λ = approximately 0.6°C [W per m 2 per year]) owing to the presence of the particle indirect effect, which contributes ad

ditional warming as particle loading decreases in the future (Shindell et al. , 2007).
Changing levels of short-lived gases and par

ticles contribute approximately 20 percent of the overall global annual average warming in these two models (17 percent for GISS and 27 percent for GFDL based on 2046 to 2050 vs

. the first five years of the run). It is important to note, however, that these models respond as they do for different reasons.
In the GFDL simulations, reduced sulfate and increased black carbon and ozone all combine to cause warming. In contrast, in the GISS model, the warming results from increased ozone and a reduced particle indirect effect, with a substantial offset (cooling) from reduced black carbon. The lack of a substantial effect from short-lived gases and particles in the CCSM simulations is attributable to the emis

sions used, which produce small increases in sulfate (cooling) and small increases in black carbon (warming) that largely offset one another (thus their radiative forcing changes little from 2000 to 2050).
Hemispheric temperatures show trends largely consistent with the radiative forcings (Table 3.8), namely substantial warming in the North

ern Hemisphere in the GFDL model and in the Southern Hemisphere in the GISS model (Figure 3.6). The Northern Hemisphere warm

ing in the GFDL model is driven primarily by the large decreases projected for sulfate and the large increase projected for black carbon in that model for the industrialized areas of the

Figure 3.5 Global mean annual average temperature change (°C) in the simu

lations with time-varying long-lived (top) and short-lived (bottom) gases and aerosols. Results are three-member ensemble means for GFDL and GISS and single-member simulations for CCSM. Results for the short-lived gases and aerosols are obtained by subtraction of the (long-lived) calculations from the (short + long-lived) calculations. Two of the three models show substantially increased global warming as a result of changes in short-lived gases and particles. BOX 3.3: Statistical Methods

A result is deemed to be statistically significant if it is unlikely to have occurred by chance ( i.e. , the probability that it occurred by chance is less than some specified threshold). A 95 percent confidence level means that the odds are 20:1 against the result having occurred by chance.
Statistical significance in the GFDL climate model results was evaluated using two approaches. For global-mean, or hemispheric-mean results involving a temperature departure from the initial (2000) value, the range (highest to lowest temperature change) of the three ensemble members used to obtain the ensemble-mean result was computed. The ensemble-mean result was deemed significant if that range was entirely different from zero. For regional (latitude-longitude) results comprising the difference of two time series, as in the evolution of temperature change due to short-lived species, the Student’s- t test for significance was applied at each model grid point, with the result deemed significant if the statistical test showed significance at the 95 percent confidence level.The U.S. Climate Change Science Program Chapter 3

46 Northern Hemisphere (Levy et al. , 2008). This causes the aerosol optical depth from sulfate to drop by one-third in the Northern Hemisphere by 2050 while the aerosol optical depth from black carbon increases by 50 percent. The large change in sulfate dominates the overall aerosol optical depth change in that model (Table 3.5). The magnitude of the Northern Hemisphere warming is ~0.5°C by 2050, consistent with the ~1.1 W per m 2 radiative forcing in that model, when one accounts for the fact that the warming has not been fully realized due to the lag-time for oceanic heat adjustment (Stouffer, 2006). There is an overall negative forcing in the Southern Hemisphere in the GFDL model, as sulfate precursor emissions increase in the developing world while black carbon changes little. Some of the negative forcing from par

ticles in the Southern Hemisphere is offset by positive forcing from ozone, which increases rather uniformly over much of the world in that model (Levy et al. , 2008), leading to a small net effect and minimal temperature change from short-lived gases and particles (Figure 3.6).
The change in the forcing due to the particle indirect effect in the GISS model was argued to be on the order of 0.1 W per m 2 in 2050 (Shin

dell et al. , 2007). Combining this with the GISS hemispheric radiative forcings (excluding the indirect effect) in Table 3.8 yields a Northern Hemisphere radiative forcing near zero and a Southern Hemisphere forcing of about 0.3 W per m 2 . These forcings are consistent with the warming of about 0.15°C seen in that model in the Southern Hemisphere and the lack of response in the Northern Hemisphere. Northern Hemisphere aerosol optical depth changes are dominated by a substantial reduction in black and organic carbon (the black carbon aerosol optical depth in the Northern Hemisphere falls by nearly 50 percent), which more than offsets a slight increase in sulfate (particularly as this model is less sensitive to sulfate). These particle changes lead to negative Northern Hemisphere forcing. In the Southern Hemisphere, the GISS model shows only small changes in particles, so that positive forcing from ozone dominates the net radiative forcing. The particle indirect effect further accentuates the positive forcing owing to reductions in black carbon and organic carbon. The signs of the temperature response in the two hemispheres are thus opposite in the GISS model to what they are in the GFDL model.
As for the global case, trends in the CCSM model are not significantly different in the runs with and without short-lived gases and particles. This is the result of only a miniscule change in aerosol optical depth in the Northern Hemi

sphere (-2 percent), as sulfate and carbonaceous particle precursor emissions are both near their present-day values by 2050 in that model. In the Southern Hemisphere, there is an increase in aerosol optical depth from 2000 to 2050, which seems to be primarily due to sulfate, but this is largely offset by increased ozone in the Southern Hemisphere as stratospheric ozone

recovers. Figure 3.6 Hemispheric mean annual average temperature change (°C) in the simulations with time-varying long-lived and short-lived gases and particles. All the results are three-member ensemble means for GFDL and GISS and single member simulations for CCSM. Results for the short-lived gases and particlesare obtained by subtraction of the (long-lived) calculations from the (short+longlived) calculations.47 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

Thus it is clear that at global and especially at hemispheric scales, the three climate models are being driven by substantially different trends in their short-lived gases and particles. These differences in particles are largely related to the differences in the projected emissions of particle precursors, though there is some contri

bution from differences in particle modeling as discussed previously. Additionally, the climate response of each model is different to some ex

tent owing to the inclusion of different physical processes in the models, especially the inclusion of the particle indirect effect in the GISS model. However, the above analysis strongly suggests that the largest contributor to the inter-model variations in projected warming arise from dif

ferent assumptions about emissions trends.
At smaller spatial scales, the annual average patterns of surface temperature changes in

duced by the short-lived gases and particles show even larger divergences (Figure 3.7). Around 2030, the largest responses are seen at Northern middle and high latitudes. These show large regions of both cooling and warming that are characteristic of the response to changes in atmospheric circulation. Most of the response at middle and high latitudes is not statistically significant in the models owing to large natural variability. Surprisingly, all three models show similar patterns of cooling near Alaska and a region of warming over Siberia. Other regions, such as the Labrador Sea/Baffin Island area or Scandinavia, show substantial variations between models, again suggesting these middle and high latitude dynamic responses are not robust.
In the tropics, where dynamic variability is much smaller, the models find much greater areas with statistically significant responses, especially by 2050. The CCSM model finds a small but significant cooling over tropical oceans, while the other models find warming. It is clear that especially at hemispheric scales,

the three climate models are being driven by substantially different trends in

short-lived gases and particles.

Figure 3.7 Annual average surface temperature response (°C) in the climate models to short-lived gases and particles (left and center columns) and long-lived gases (right column) changes for the indicated times. The changes at 2030 are 2020 to 2029 in the CCSM and GFDL models and 2028 to 2033 in the GISS model. At 2050, they are 2040 to 2049 in the CCSM model, 2046 to 2055 in the GFDL model, and 2040 to 2050 in the GISS model. Hatching indicates statistical significance (95 percent) for the response to short-lived gases and particles. All colored values above 0.1°C are statistically significant in the response to long-lived gases. Values in the upper right corners give the global mean.The U.S. Climate Change Science Program Chapter 3

48 As in the global-mean case, this appears to arise from differences in particle burdens and aerosol optical depths.
In the Arctic, the GISS and CCSM models find primarily a cooling effect from projected changes in short-lived gases and particles (especially near 2030 for GISS, and 2050 for CCSM). In contrast, the GFDL model finds a substantial warming there. This may be due in part to the increasing trend in black carbon in that model.
In the Antarctic, the GISS model shows warm

ing related primarily to stratospheric ozone re

covery. The GFDL model shows a similar result by 2050 (after which stratospheric ozone was unchanged in that model). CCSM does not show as clear an Antarctic warming, however, even though this model also included recovery of ozone in the Antarctic lower stratosphere. This is surprising given that the CCSM model ap

peared to show a substantial response to ozone depletion in analyses of the Southern Hemi

sphere circulation in IPCC AR4 simulations (Miller et al. , 2006b). That analysis showed that most climate models found a general strength

ening of the westerly flow in the Southern Hemisphere in response to stratospheric ozone depletion. A stronger flow isolates the polar region from lower latitude air, leading to cool

ing over the Antarctic interior and warming at the peninsula. Conversely, recovery should lead to warming of the interior (enhanced by the direct positive radiative forcing from increased ozone), as in the GISS and GFDL simulations. However, in the GISS model the effect dimin

ishes with time, suggesting that other aspects of the response to short-lived gases and particles become more important in these scenarios over time, presumably as projected particle changes grow ever larger.
Warming over the central United States is pres

ent in the GISS model at all times (but is not statistically significant), in the GFDL model from about the 2040s on, and in the CCSM model around 2030, but not at 2050. The United States and other Northern Hemisphere indus

trialized regions might be especially sensitive to the projected reduction in sulfate precursor emissions in the Northern Hemisphere. This effect is especially large in the GFDL model, where forcings from sulfate decreases and black carbon increases both contribute to warming, though it should be noted that the largest radia

tive forcing is over Asia, not over the United States and Europe (Figure 3.4). In the CCSM model, the warming effect vanishes by 2050 as both sulfate and black carbon decrease, producing temperature responses that cancel. In the GISS model, reductions in sulfate and increases in ozone both contribute to warming; however, these are partially offset by cooling from reduced black carbon.
The surface temperature changes induced by the long-lived gases are clearly much larger than those induced by short-lived gases and particles over most of the Earth by 2050 (Figure 3.7). In some regions, however, the two are of comparable magnitude ( e.g. , the polar regions and parts of the Northern midlatitude conti

nents in the GFDL model, parts of the Southern Ocean in the GISS model) though the statisti

cal significance of the signal for short-lived gases and particles is marginal. Consistency between the models is also clearly greater in their response to long-lived than to short-lived gases and particles.
Overall, it is clear that the regional surface temperature response does not closely follow the regional radiative forcing patterns based on either GISS or GFDL results. Both models show very large forcings over East Asia, for example, yet have minimal response there. This is especially clear when comparing the seasonal radiative forcings and climate response (Figure 3.8). Though some of the spatial mismatches could result from a lag in the climate response The United States and other Northern Hemisphere industrialized regions might be especially sensitive due to the projected reduction in sulfate precursor emissions in the Northern Hemisphere.49 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

to seasonally varying forcings, the di

vergence between the patterns of forcing and response is large even for areas with minimal seasonality in the forcing ( e.g. , Africa, subtropical Asia). See Section 9.2.2.1 of Chapter 9 in the Fourth IPCC assessment (Hegerl et al. , 2007) and ref

erences therein for further discussion of this issue.

3.3.4.2 P r e c ipit a

tio n , S e a

- level , an d othe r v ar i a bles

. Changes in other climate variables (such as precipitation and sea level) due to short-lived gases and particles are typi

cally too small to isolate statistically. In the case of precipitation, this is due to the high variability of the precipitation signal. The relatively short (50 year) length of the integrations discussed here accounts for the lack of a significant sea-level signal. Sea level is expected to rise in a warming climate, chiefly due to thermal expansion of the oceans. In the GISS and GFDL models, the thermal expansion would be enhanced by about 20 to 25 percent due to changes in short-lived gases and particles. Similarly, the enhancement of precipitation along the equator and drying of the subtropics that is a robust feature of climate models in a warming climate (Held and Soden, 2006) would also be ac

centuated in the GFDL and GISS models with their significant tropical warming, though probably not under the CCSM scenario. Such a feature can indeed be seen in the GISS response in the Atlantic and Indian Oceans.
On a regional scale, there are some suggestions of precipitation trends but statistical signifi

cance is marginal for either annual or seasonal changes. The CCSM model shows reductions in winter precipitation due to short-lived gases and particles across most of the United States in the 2040s, and reductions in summer pre

cipitation in the southeastern part of the United States. That model also suggests an increase in summer monsoon rainfall over South Asia. In contrast, the GISS model shows slight increases in winter precipitation over the central United States, and a mixed signal in summer (and spring) with increased precipitation over the Southeast and Southwest United States, but decreases over the Northeast United States. During fall, precipitation decreases over most of the country. As in the CCSM model, there is an increase in summer (and fall) precipitation over South Asia. In the annual average, the GFDL model shows no statistically significant trend over the United States. Given that significant trends are hard to identify in any of the models, and that the models do not agree on the trends themselves, we believe that it is not possible to reliably estimate precipitation trends owing to short-lived gas and particle changes under the A1B storyline. The surface temperature changesinduced by the longlived gases are clearly much larger than those induced by short-lived gases and particles

over most of the Earth by 2050 (Figure 3.7). In some regions, however, the two are of comparable magnitude. Figure 3.8 Seasonal average instantaneous radiative forcing (W per m 2 ) in 2050 from short-lived gases and particles (left column) and surface temperature response in °C (right column) in the GISS and GFDL models. Boreal winter (December through February) is shown in the top two rows, while boreal summer (June through August) is shown in the bottom two rows. The temperature changes at 2050 are 2046 to 2055 in the GFDL model and 2041 to 2050 in the GISS model. Values in the upper right corners give the global mean. (Note that the instantaneous forcings shown here for the GFDL model differ from the adjusted forcings show in Table 3.6.)The U.S. Climate Change Science Program Chapter 3

50 3.3.4.3 D is c ussio n In the transient climate simulations, three cli

mate models examined the response to pro

jected changes in short-lived gases and par

ticles. The results shown in Figure 3.7 differ substantially among the models, particularly by 2050. Comparison has shown that the dif

ferences in the underlying emissions projec

tions are the dominant source of inter-model variations in projected particle trends. These variations result from differences between the various integrated assessment models that provided those projections and to assumptions made about emissions not provided by the in

tegrated assessment models.

Uncertainties in the representation of radiative processes are an additional source of differ

ences in the temperature response of the three models. For example, the GFDL model’s aerosol optical depth is substantially more sensitive to sulfate than the GISS model, with the CCSM model in between. This is partially due to the inclusion of sulfate absorption onto dust being present only in the GISS model. Additionally, the indirect effect of particles is included only in the GISS model. Thus, the inclusion of differ

ent physical processes plays a role in the intermodel differences, and is especially important near 2030, when sulfur dioxide (SO 2 ) emissions are near their peak. With the inclusion of the particle indirect effect, the GFDL model might yield a substantially larger warming, given that sulfate is the largest contributor to particle mass globally and that the GFDL sulfate concentra

tions decrease beyond 2030.
Differences in the models’ representation of the hydrologic cycle, which removes soluble gases and particles, and in oxidation may also produce variations in the temperature response. Inter-model differences between the GFDL and GISS models in the residence times of particles are substantial for sulfate, and dif

ferences in the radiative effect of black carbon are also potentially sizeable. In nearly all cases, however, these are outweighed by emissions differences. Exceptions are sulfate in 2030 and tropospheric ozone; differences in the modeled conversion of SO 2 to sulfate and the sensitiv

ity of ozone to NO x emissions are larger than differences in projected precursor emissions. Hence, in these cases uncertainties in physical processes, including chemistry, dominate over uncertainties in emissions. These differences between GFDL and GISS model results appear to be representative of other intermodel differ

ences, such as those identified for modeling of sulfate and black carbon particles in recent comparisons between a large suite of models (Schulz et al. , 2006).

We also reiterate that uncertainties in the particle indirect effect and in internal mixing between particle types are either not included at all or only partially in these simulations. Sensitivity studies and analysis of the GISS model results indicates that the forcing from reductions in the particle indirect effect is roughly 0.1 to 0.2 W per m 2 , while the inclu

sion of sulfate absorption onto dust reduces the negative forcing from sulfate at 2030 or 2050 by up to 0.2 W per m 2 (Shindell et al. , 2007), These sensitivities suggest that uncertainties in these processes could alter the global mean projected temperature trends by up to 0.1°C at 2030 or 2050, a value comparable to the total temperature trend in that model. Hence without the particle indirect effect, the GISS model would likely have shown minimal warming, while without sulfate absorption onto dust sur

faces, it would likely have shown a substantially greater warming trend (at least at 2030).
The responses of methane (and other hydrocar

bon) emissions and of stratosphere-troposphere exchange to climate change can also potentially have significant impacts on radiative forcing, and these processes were not included in these simulations. As discussed in sections 3.3.3.3 and 3.3.3.4, the resulting changes to radiative forcing could again substantially alter the pro

jected temperature trends. Additionally, given the large influence of uncertainties in emis

sions projections, we stress that the magnitude and even the sign of the effects of short-lived gases and particles on climate might be dif

ferent were alternative emissions projections used in these same models. Thus, the response of short-lived gases and particles and methane to emissions changes and climate changes has been only partially characterized by the present study, and substantial work remains to reduce uncertainties and further clarify their potential role in future climate change.51 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

The results clearly indicate that the spatial distribution of radiative forcing is generally less important than the spatial distribution of climate response in predicting climate impact. Thus, both short-lived and long-lived gases and particles appear to cause enhanced climate re

sponses in the same regions of high sensitivity rather than short-lived gases and particles hav

ing an enhanced effect primarily in or near pol

luted areas. This result is supported by analysis of the response to larger radiative perturbations in these models for the future (Levy et al. , 2008) and the past (Shindell et al. , 2007). It is also consistent with earlier modeling studies exam

ining the response to different inhomogeneous forcings than those investigated here (Mitchell et al. , 1995; Boer and Yu, 2003; Berntsen et al. , 2005; Hansen et al. , 2005). This suggests that the mismatch between model simulations of the regional patterns of twentieth century climate trends and observations is likely not attributable to unrealistic spatially inhomogeneous forcings imposed in those models. Instead, the models may exhibit regional climate sensitivities that do not match the real world, and/or some of the observed regional changes may have been unforced ( i.e. , the result of internal variability [see Knutson et al. , 2006 for a discussion of this issue]).

3.3.5 Climate Simulations Extended to 2100 Following the A1B “marker” scenario into thesecond half of the 21st century for both threemember ensembles, the GFDL simulations (Levy et al. , 2008) find significant climate impacts due to emissions of sulfur dioxide (SO 2 ), the precursor of sulfate particle, (which decrease to ~35 percent of 2000 levels by year 2100) and of black carbon (scaled to carbon monoxide emissions in the GFDL model) which continue to increase. This is confirmed by their respective radiative forcing values for 2100 in Table 3.6. By 2080 to 2100, these projected changes in emissions levels of short-lived gases and particles contribute a significant portion of the total predicted surface temperature warming for the full A1B scenario; 0.2°C in the Southern Hemisphere, 0.4°C globally, and 0.6°C in the Northern Hemisphere as shown in the time series of yearly average surface tem

perature change in Figure 3.9. In Figure 3.10 we examine Northern Hemi

sphere summer surface temperature change between the 2090s (average over 2091 to 2100) and the 2000s (average over 2001 to 2010) due to the changes in emissions of short-lived gases and particles between the first decade and the last decade of the 21st century. Note the large warming in the Northern Hemisphere mid-latitudes with the major hot spots over the continental United States, Southern Europe and the Mediterranean. Eastern Asia, the re

gion with the strongest radiative forcing due to changes in emissions and loading of short-lived gases and particles, is not one of the hot spots. The mid-latitude warming belt is statistically significant to the 95th percentile, with some regions significant to the 99th percentile. Bycontrast, the annual-average and seasonalaverage patterns of change in precipitation due to changes in emissions of short-lived gases and particles (not shown) are, in general, not statistically significant.
We now focus on the large summertime warm

ing over the United States and consider the twenty-first century time series shown in Figure 3.11. By 2100, the change in short-lived gases and particles, primarily the decrease in sulfate and increase in black carbon particles over Asia, contributes ~1.5°C of the total temperature warming of ~4°C predicted for the summertime continental United States when the effects of changes in both short-lived and long-lived gases and particles are included. The geographic distribution of radiative forcing is generally less important than the spatial distribution of climate response in predicting climate impact.

Figure 3.9 Surface temperature change in °C (2000 to 2100) due to short-lived gases and particles in the GFDL model.The U.S. Climate Change Science Program Chapter 3

52 In Figure 3.12, we focus more narrowly on the central United States where the strong sum

mertime warming was predicted for 2100. Monthly-mean area-averaged values of tem

perature, precipitation, and available root-zone soil water are shown for both the full A1B emissions scenario (dashed lines), where both long-lived greenhouse gases and short-lived gases and particles change and the emissions scenario with only long-lived greenhouse gases changing and short-lived gases and particle levels fixed at 2001 values (solid lines). The values are ensemble averages over the last 40 years (2061 to 2100) for each simulation. Here the climate model does predict a statistically significant (at the 95 percent confidence level) decrease in precipitation due to the change in

Figure 3.11 Surface temperature change in °C from 2000 over the twenty first century due to short-lived gases and particles over the continental United States during Northern Hemisphere summer in the GFDL model. short-lived gases and particles (blue curves in Figure 3.12 and green curve in Figure ES.1). We next consider root-zone soil wa

ter, a quantity that integrates and responds to both temperature and precipitation. There is a statisti

cally significant (at the 95 percent confidence level) decrease of up to 50 percent in available root-zone soil water in the Central United States during late summer (July through September), which could have important consequences for United States grain production, and merits future attention. This is the result of a global increase in short-lived gas and particle forc

ing, located primarily over Asia, which in turn results from the large changes projected by the A1B “marker” scenario for Asian emissions of SO 2 and black carbon.
We also find, as already discussed for year 2050 in Section 3.3.4, that the regional patterns of climate change in 2100, due to changes in emissions of short-lived gases and particles, are the result of regional patterns in the climate system’s response rather than regional patterns in radiative forcing. The global patterns of surface temperature change in 2100 are similar for the short-lived gases and particles and the well-mixed greenhouse gases with the stron

gest surface temperature warming occurring over the summer continental United States and Mediterranean and the winter Arctic, while the major change in radiative forcing is over Asia (Levy et al. , 2008). The predicted summertime warming over the United States is greatly enhanced by projected reductions in SO 2 emissions and increased black carbon emissions and the resulting positive radiative forcings over Asia. In the A1B scenario, this is assumed to be the result of Asian decisions addressing their local and regional air quality. The integrated assessment model projections for A1B assume that SO 2 emissions will be reduced in the future in order to improve air quality, but did Figure 3.10 Surface temperature change in °C due to short-lived gases and particles during Northern Hemisphere summer for 2100 to 2091 vs . 2010 to 2001 in the GFDL model.

Model simulations indicate that short-lived gases and particles could account for a significant portion of the warming in 2100.53 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

not explicitly project carbonaceous particle emissions. Scaling future carbonaceous emis

sions according to carbon monoxide emissions projections does not lead to similar reductions in emissions of these particles, so that there is an issue of consistency in projecting the influence of future air quality decisions that deserves further study.



### 3.4 R egional E mission S S ector P ert u rbations and R egional M odels
3.4.1 Introduction to Regional Emissions Sector Studies An additional set of simulations used global models to examine the impact of individual emissions sectors in specific regions on short-lived gases and particles. This study, in which the GISS and NCAR groups partici

pated, was designed to examine the climate effects of short-lived gases and particles in a more policy-relevant way by focusing on the economic activities that could potentially be subject to regulation or reduction in us

age ( e.g. , by improved efficiency). We look at reductions in total emissions from a given sector in particular regions (North America and Asia), and do not consider any changes in technology or the relative contributions within a sector. As such, these are more useful for as

sessing the potential impacts of reductions in total power/fuel usage rather than changes in the mix of power generation/transportation types or in emissions control technologies targeted at specific pollutants.

3.4.2 Global Models The GISS model setup for the regional emis

sions sector perturbation experiments was the same as that used in the transient climate studies (Section 3.2 and 3.3; see Appendix C and D, sections on Goddard Institute for Space Studies). The NCAR regional/sector perturba

tion simulations used the CAM-chem model (Lamarque et al. , 2005b), in which an updated version of the MOZART chemical transport model (Horowitz et al. , 2003) is embedded within the Community Atmosphere Model (CAM3, Collins et al. , 2006).
CAM-chem has a representation of tropospheric chemistry with non-methane hydrocarbons (NMHCs) treated up to isoprene, toluene andmonoterpenes. The particle simulation in CAMchem includes the bulk particle mass of black carbon (BC, hydrophobic and hydrophilic), primary organic carbon (POA, hydrophobic and hydrophilic), secondary organic carbon (SOA), ammonium and ammonium nitrate, and sulfate particles. Further details on the CAM-chem model are found in Appendix C in the section on National Center for Atmospheric Research.
3.4.3 Impact of Emissions Sectors on Short-Lived Gases and Particles This set of experiments consisted of six simula

tions each reducing the present-day emissions by 30 percent in one sector for one region. By using present-day emissions, the results are not tied to any particular scenario. For present-day emissions, the IIASA 2000 inventory, based on the 1995 EDGAR 3.2 inventory extrapolated to 2000 using national and sector economic development data, was used (Dentener et al. , 2005), as in the GISS simulations described above. The exception to this is biomass burning emissions, which are taken from the Global Fire Emission Database (GFED) averaged over 1997 Figure 3.12 Monthly-mean time-series of available root-zone soil water (green lines, scaled by a factor of 1/240 for plotting purposes), precipitation (blue lines), and two-meter air temperature (red lines), averaged over the Central United States (105 to 82.5°W longitude; 32.5 to 45°N latitude). Dashed lines are for the ensemble mean of the A1B experiments, averaged over the years 2061 to 2100; solid lines are for the ensemble mean of the A1B * experiments, also averaged over the years 2061 to 2100. The asterisks represent those A1B monthly aver

age values that are different from their companion A1B * values at the 95 percent confidence level. Temperature in K equals temperature in °C plus 273.15.The U.S. Climate Change Science Program Chapter 3

54 to 2002 (Van der Werf et al. , 2003) with emis

sion factors from Andreae and Merlet (2001) for particles. The regions were defined as North America (60 to 130°W, 25 to 60°N) and Asia (60 to 130°E, 0 to 50°N) and the economic sectors defined according to the IIASA inventory as the domestic sector, the surface transportation sector, and a combined industry and power sec

tor (the CAM-chem model did not perform the transportation sector simulations).
A control run with no perturbations was also performed to allow comparison. The goal of these simulations was to calculate the radia

tive forcing from all the short-lived gases and particles to identify the relative contribution of the given economic sectors in these two regions. This complements prior work examining the response to a subset of the gases and particles included here ( e.g., Koch et al. , 2007; Unger et al. , 2008). As the forcings were expected to be small, we concentrate on simple metrics rather than the climate response. The CAM-chem model did not calculate radiative forcing, so we also use aerosol optical depth, which is a good indicator of the radiative forcing from particles. All simulations were 11-year runs, with analysis performed over the last ten years.
The simulations were not performed using a full methane cycle, but the methane response to the imposed perturbations can be estimated by examining the changes in methane’s oxida

tion rate. In these simulations, methane was prescribed at present-day values. Thus any change in methane oxidation is due solely to changes in the abundance of oxidizing agents. The difference in the steady-state abundance of methane that would occur as a result of this oxidation change is a simple calculation ([CH 4 ]′/ [CH 4 ] = L/L′ for the global mean where L is the methane loss rate and the “prime” notation indicates the adjusted amounts). Use of the model’s oxidation rate in the perturbation runs fully captures spatial and seasonal variations, and thus provides an accurate estimate of the equilibrium response of methane to the emis

sions changes. Finally, the radiative forcing resulting from these indirect methane changes is calculated using the standard formulation (Ramaswamy et al. , 2001).
We first examine the global mean annual average radiative forcing in the GISS model from the regional perturbations and those by economic sectors (Table 3.10). The effect of the

Region Sector Sulfate BC OC Nitrate Ozone Methane (indirect) All

North America Domestic 0 -3 2 1 2 1 4 Surface Transportation -3 -5 0 1 -5 4 -9 Industry/power 14 -2 -1 0 5 2 18

Asia Domestic 0 -42 13 1 -12 -2 -41 Surface Transportation 2 -8 1 2 -5 7 -2 Industry/power 13 -4 0 -1 -1 5 12 Table 3.10 Radiative forcing in milliWatts per square meter (mW per m 2 ), from regional emission sector perturbations in the GISS model.

Perturbations are 30 percent reduction in emissions of all species from the indicated economic sector in the given region. Direct forcings are shown for sulfate, black carbon (BC), organic carbon (OC), nitrate, and ozone. The effect of ozone precursor species on methane is included as methane “indirect”. Note that particle indirect effects are not included.55 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

perturbations is generally larger for Asian than for North American emissions. The only radia

tive forcing from an individual gas or particle to exceed 10 milliWatts per square meter (mW per

m 2 ) from a North American perturbation is the sulfate forcing from a reduction in industrial/ power emissions. In contrast, forcings from sulfate, black carbon, organic carbon and ozone all exceed 10 mW per m 2 in response to per

turbations in developing Asia, with the largest response for reductions in black carbon when domestic emissions are reduced (-42 mW per

m 2 ). The spatial pattern of the radiative forcing is also shown (Figure 3.13). The two largest net forcings are in response to changes in North American industrial/power emissions, whose forcing is positive and is dominated by reductions in forcing from sul

fate, and the Asia domestic sector, whose forc

ing is negative and dominated by reductions in black carbon and ozone. The spatial pattern of the aerosol optical depth changes capture the bulk of the radiative forcing in these two cases (Figure 3.14 vs . Figure 3.13). The sign is oppo

site, however, in the case of industrial/power emissions as these are dominated by reflective sulfate particles, so decreased aerosol optical depth causes positive radiative forcing.

The GISS and CAM-chem models show very similar patterns of aerosol optical depth changes for these two perturbation experi

ments. For emissions reduction in the Asia domestic sector, the global mean aerosol opti

cal depth decreases by 0.15 in the GISS model and 0.13 in the CAM-chem model, while for the North American industrial/power sector the decreases are 0.09 and 0.13, respectively. Hence the particle response appears to be fairly robust across these two models. Results suggest that the calculation of radiative forcing from aerosol optical depth introduces an additional inter-model difference that is less than that from the aerosol optical depth calculation (Schulz et al. , 2006), so that the total inter-model variation in radiative forcing from particles is probably on order of 50 percent. Results for ozone show marked differences, however, with the response of the tropospheric ozone column in the GISS model nearly always a factor of two to three greater than in the CAM-chem model. We be

lieve that these differences primarily reflect the The effect of the perturbations is generally larger for Asian than for North American emissions. Figure 3.13 Short-lived gas and particle annual average radiative forcing (mW per m 2 ) due to 30 percent reduc

tions in emissions from the given region and economic sector in the GISS model.The U.S. Climate Change Science Program Chapter 3

56 inclusion of the stratosphere in the GISS model, which leads to enhanced forcing as ozone near the tropopause has a particularly large radiative impact. Hence, the ozone radiative forcing is not yet robust to inter-model differences. However, particles typically have a larger influence on cli

mate than ozone, so that the net radiative forcing remains a relatively more robust quantity.
We also examine changes in surface pollution levels in these simulations. Changes in surface ozone are typically small, with annual average local reductions of up to about 1 to 1.5 partsper billion by volume (ppbv) in both the CAMchem and GISS models in response to reduction in transportation or industrial/power emissions. These increase to levels of 1 to 3 ppbv during boreal summer. Both these annual and summer increases are statistically significant. Changes in particles are larger. In most cases, substantial reduction in surface particle concentrations result from the regional economic sector emis

sions reductions. This is especially so in the Asia domestic analysis, where summer sulfate concentrations are lowered by 100 to 250 parts per trillion by volume (pptv) locally, and black carbon concentrations drop by 1800 to 2000 pptv for both summer and annual averages. Smaller air quality improvements are also clear in the response to industrial/power and trans

portation emissions reductions in both regions. These reductions in particles are generally quite similar in the two models, with differences of only 5 to 20 percent in most cases.
The analysis shows that reductions in surface transportation emissions have a net negative forcing from short-lived gases and particles in both regions, primarily due to reductions in ozone and black carbon. As these are both pol

lutants at the surface, reducing emissions trans

port offers a way to simultaneously improve human health and mitigate climate warming (though the climate impact is quite small for Asia). The total climate mitigation would of course be larger adding in the effect of reduced emissions of long-lived greenhouse gases. In contrast, industrial/power sector emissions have their largest effect on climate through sulfate, and hence yield a positive forcing. Thus, the net effect of changes in short-lived gases and particles to industrial/power emissions reduc

tions will offset a portion of any climate benefit from reduced emissions of long-lived gases. The domestic sector presents a similar picture to that seen for surface transportation. The effects are substantially larger in Asia, however. Hence, reductions in domestic emissions from Asia In most cases, substantial reduction in surface particle concentrations result from the regional economic sector emissions reductions. This is especially so in the Asia domestic analysis

. Figure 3.14 Annual average aerosol optical depth change due to 30 percent reductions in emissions from the given region and economic sector in the GISS (left column) and CAM-chem (right column) models. Values in the upper right give the global mean.57 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

offer another means to improve human health and mitigate warming. Note that the effects become particularly strong in Northern Hemi

sphere summer (Table 3.11), offering a potential path to mitigate increased summer heat over the Northern Hemisphere continents.
Overall, the Asia domestic emissions offer the strongest leverage on climate via short-lived gases and particles. This is partially a result of their magnitude, and partially their occurrence at lower latitudes than North American (or Euro

pean) emissions. This enhances their impact as photochemistry is faster and incoming radiation is more abundant at lower latitudes. Perturbing the Asia domestic sector in the IIASA 2000 emissions inventory used here yields a much greater effect via black carbon changes than via sulfate changes. This reflects the influence of domestic fuel usage, for which black carbon is the dominant emission, and hence reductions from emissions in this sector in particular seem attractive for warming mitigation. As domestic usage and emissions are extremely difficult to quantify in the developing world, further studies of this sector are especially needed to characterize the uncertainty in these emissions. The GISS and CAM-chem results differ in the magnitude of the aerosol optical depth change resulting from the Asia domestic sector per

turbations by only 13 percent, and this sector/ region has the largest influence in both models for both radiative forcing and surface pollution. The stronger aerosol optical depth response in the CAM-chem model suggests that the radiative forcing in that model might be even larger than the 50 mW per m 2 global mean annual average seen in the GISS results.
Further work is required to more thoroughly characterize the ro

bustness of these conclusions across a larger number of models, to explore the impact of particle indirect effects on clouds, and to examine alternative emissions scenarios considering changes in the mix of sources constituting a given sector and the influence of potential technological changes. The latter could be designed to reduce emissions of particular pollutants, while not affecting others. Our results for the radiative forcing from individual gases and particles give an idea of the potential impact of such technologies. However, we note that these technologies could also have effects on overall fuel consumption by altering the efficiency of a particular process.
Interestingly, both the transient climate projec

tions and the present-day perturbations find that emissions from Asia are the most important controllers of climate trends or mitigation. Given that the radiative forcing reduction from decreases in Asia domestic emissions extends over much of the Northern Hemisphere (Figure 3.13), and the conclusion from the transient Overall, the Asia domestic emissions offer the strongest leverage on climate via short-lived gases and particles. This is partially a result of their magnitude,

and partially their
occurrence at lower latitudes than North American (or European) emissions. Region Sector Total forcing

North America Domestic 6 Surface Transportation -10 Industry/power 34

Asia Domestic -69 Surface Transportation -3 Industry/power 10 Perturbations are 30 percent reduction in emissions of all species from the indicated economic sector in the given region. Direct forcings are shown for sulfate, black carbon (BC), organic carbon (OC), nitrate, and ozone. The effect of ozone precursor species on methane is included as methane “indirect”. Note that particle indirect effects are not included. Table 3.11 Total short-lived gas and particle radiative forcing (in mW per m 2 ) as in Table 3.10 but for summer ( J une through August).The U.S. Climate Change Science Program Chapter 3

58 climate simulations that the climate response to short-lived gas and particle changes is not closely localized near their emissions, it seems plausible that emissions from this region may have as large or larger an effect on other parts of the Northern Hemisphere as changes in local emissions.
3.4.4 Regional Downscaling Climate Simulations The sector-based simulations presented in Section 3.4.3 suggest that reductions in sur

face level ozone or black carbon would have a negative radiative forcing, while reductions in sulfate particles from the utility sector would have a positive radiative forcing. If concentra

tion levels for ozone and small-diameter par

ticles (PM 2.5 ) 6 exceed threshold standards under the United States Clean Air Act, emissions control strategies must be developed. Since

6 PM 2.5 are all small particles with diameters less than

2.5 micrometers these levels vary spatially and are affected by local emissions, regional scale models are needed to develop the emissions control strategies that meet these standards at county and state levels. To achieve reductions in small-diameter particles , emis

sions scenarios often include utility sector emissions reductions to lower sulfate particle levels. As shown in the previous section, lowering sulfate particle concentrations could actually have negative implications for radia

tive forcing and climate temperature increases. Regional downscaling studies shown in this section suggest that future changes in regional cli

mate could reduce the benefits from anticipated emissions reductions on lowering ozone.
Downscaled regional scale cli

mate simulations ( e.g. , Leung and Gustafson, 2005; Liang et al. , 2006; Liang et al. , 2004) rely on a global cli

mate model to provide boundary con

ditions for the regional domain as well as the radiative effect of well-mixed greenhouse gases within the domain for the radiation calculations. Region

ally downscaled climate simulations are needed by a number of applications that must consider local changes in future climate. Since ozone and small particles exceedances of regulatory thresholds are substantially affected by local scale changes in emissions and meteo

rology, several recent studies using regionally downscaled climate scenarios have been used to study the sensitivity of air quality to potential changes in future climate. The primary purpose of these studies was to study how increases in temperature and other future climate changes could affect ozone and small-diameter particles and potentially decrease the effectiveness of anticipated emissions reductions.
Downscaled regional climate model simulations by Gustafson and Leung (2007) and Nolte et al. (2008) have used the EPA/NOAA air pollution model to test the impact of future ( ca. 2050) cli

mate on ozone and particles with current emis

sions scenarios. Biogenic emissions, which are meteorologically dependent, were recalculated59 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

Climate change itself could have a significant impact on low-level ozone concentrations,

leading to yet further changes in climate. under the future climate scenario. Results sug

gest that future climate changes could increase maximum ozone levels by approximately ten percent in some regions (Figure 3.15). With anticipated emissions reductions under United States Clean Air Act requirements, these re

sults suggest that future climate could dampen the effectiveness of these emissions controls. Evaluation of ensemble regional climate model results are essential for this application before quantitative conclusions can be made about the impact of future climate on specific emissions control strategies; however, these results sug

gest that climate change is a factor that needs to be considered in air quality management.
These regional downscal

ing climate studies rely on climate forcing link

ages from global climate simulations with future trends for long-lived gas

es including CO 2 , CH 4 , N 2 O, and halocarbons. The influence of short-lived gases and particles on fu

ture climate has not been included in those studies to date; however, more recent developments are under

way to include direct and indirect radiative effects in the regional chemis

try model. Based on the known positive radiative forcing effect of ozone, the increases in ozone in response to future climate in Figure 3.15 should have a positive radiative forcing that could dampen the net negative radiative forcing anticipated from future emissions reductions for ozone (Section 3.4.3).
While regional downscal

ing climate impacts from short-lived gases and par

ticles cannot be directly re

ported on, future emissions scenarios were considered by several regional downscaling studies for the purpose of air quality impacts under future climate. The impact of climate only and then an emissions change scenario were tested by Nolte et al. (2008) and Hogrefe et al. (2004). As presented in Figure 3.15, results looking only at the climate change without future changes in emissions suggest that future climate changes could increase maximum ozone levels by ap

proximately eight ppbv or ten percent in some regions of North America. Looking at future emissions scenarios for nitrogen oxides and sulfur dioxide reductions demonstrates that the uncertainty in the future emissions scenarios Figure 3.15 From Nolte et al. (2008), the change in ozone at the upper end of the ozone distribution (average of the ≥95th percentile values for each grid) for (2050 to present) years of simulation under A1B regional climate model simulations.The U.S. Climate Change Science Program Chapter 3

60 introduces a much larger variation in the air quality conclusions depending on the scenario (Hogrefe et al. , 2004; Nolte et al. , 2008). Similar to the findings here about short-lived gases’ and particles’ impact on climate, the range of plau

sible air quality impacts from future emissions scenarios suggest very different outcomes, and the future scenarios of emissions for short-lived gases and particles have a great deal of obvious, inherent uncertainty.

# F indings, Issues, Opportunities, and Recommendations

Lead Authors: Hiram Levy II, NOAA/GFDL; Drew T. Shindell, NASA/GISS; Alice Gilliland, NOAA/ARLCHAPTER 

# 4

61 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols



### 4.1 I ntrod u ction This Chapter, which is intended for both tech

nical and non-technical audiences, provides a summary of the key findings, presents a number of new questions that were revealed by our study, and identifies new opportunities for future research.



### 4.2 Key F indings
The key findings of Synthesis and Assessment Product 3.2 are summarized below:
Our results suggest that the projected 1. changes in short-lived 1 radiatively active 2 gases and particles may significantly in

fluence the climate, even out to year 2100. Their projected changes can account for up to 40 percent of the projected warming over the summertime continental United States by 2100.

1 Atmospheric lifetimes for the short-lived radiatively

active gases and particles of interest in the lower at

mosphere are about a day for nitrogen oxides, a day to a week for most particles, and a week to a month for ozone. As a result of their short lifetimes, their concentrations are highly variable in space and time and they are concentrated in the lowest part of the atmosphere, primarily near their sources.
2 “Radiatively active” indicates the ability of a sub

stance to absorb and re-emit radiation, thus changing the temperature of the lower atmosphere. We find that spatial patterns of radiative 2. forcing 3 and climate response are quitedistinct. Thus, both short-lived and longlived 4 gases and particles cause enhanced climate responses in the same regions rather than short-lived gases and par

ticles having an enhanced effect primar

ily in or near polluted areas. This means that regional pollution control strategies will have large-scale climate impacts.
Reductions of short-lived gas and particle 3. emissions from the domestic fuel-burning sector in Asia appear to offer the greatest potential for substantial, simultaneous

3 Radiative forcing is a measure of how the energy

balance of the Earth-atmosphere system is influenced when factors that affect climate, such as atmospheric composition or surface reflectivity, are altered. Whenradiative forcing is positive, the energy of the Earthatmosphere system will ultimately increase, leading to a warming of the system. In contrast, for a negative radiative forcing, the energy will ultimately decrease, leading to a cooling of the system. For technical de

tails, see Box 3.2.
4 Atmospheric lifetimes for the long-lived radiatively

active gases of interest range from ten years for methane to more than 100 years for nitrous oxide. While carbon dioxide’s lifetime is more complex, we can think of it as being more than 100 years in the climate system. As a result of their long atmospheric lifetimes, they are well-mixed and evenly distributed throughout the lower atmosphere. Global atmospheric lifetime is the mass of a gas or a particle in the atmo

sphere divided by the mass that is removed from the atmosphere each year.The U.S. Climate Change Science Program Chapter 4

62 

### K EY FINDINGS improvement in local air quality and reduc

tion of global warming. Reduction in emis

sions from surface transportation would have a similar impact in North America.
The three comprehensive climate models 4. 5 , their associated chemical composition mod

els 6 and the different projections of shortlived emissions all lead to a wide range of projected climate impacts by short-lived gases and particles. Each of the three stud

ies represents a thoughtful, but incomplete, characterization of the driving forces and processes that are believed to be important to the climate or to the global distributions of the short-lived gases and particles. Much work remains to be done to characterize the sources of the differences and their range.
The two most important uncertainties in 5. characterizing the potential climate impact of short-lived gases and particles are found to be the projection of their future emissions and the determination of the indirect effect of particles 7 on climate. See Section 4.3 for a discussion of the fundamental difference between uncertainties in future emissions and uncertainties in processes such as the indirect effect of particles on climate.
Natural particles such as dust and sea salt 6. also play an important role and their emis

sions and interactions differ significantly among the models, with consequences to the role of short-lived pollutants. This

5 Comprehensive climate models are state-of-the-art

numerical representations of the climate based on the physical, chemical, and biological properties of its components, their interactions and feedback pro

cesses that account for many of the climate’s known properties. Coupled Atmosphere-Ocean (-sea ice) General Circulation Models (AOGCMs) provide a comprehensive representation of the physical climate system.
6 Chemical composition models are state-of-the-art

numerical models that use the emission of gases and particles as inputs and simulate their chemical inter

actions, global transport by the winds, and removal by rain, snow, and deposition to the earth’s surface. The resulting outputs are global three-dimensional distributions of the initial gases and particles and their products.
7 Particles may have an indirect effect on the climate

system by modifying the optical properties and life

time of clouds. A detailed technical discussion is given in Box 3.1. inconsistency among models should be addressed in future studies.
The SAP 2.1a emissions scenarios (Clarke 7. et al. , 2007) for long-lived gases produce climate projections that are within the IPCC range, although it should be noted that the lower-bound stabilization emissions scenario 8 , which is equivalent to a carbon dioxide stabilization level of 450 parts per million, results in global surface tempera

tures below those calculated for the IPCC storyline emissions scenario with the most moderate carbon dioxide increases (B1), particularly beyond 2050.



### 4.3 I ss u es R aised
It is important to recognize the difference between uncertainties in processes and un

certainties in future emissions. Uncertainties in chemical and physical processes, which are discussed in Sections 4.3.2 and 4.3.3, represent the state of our current knowledge. The fact that one modeling group chooses to include a process such as indirect forcing of climate by particles, while another group chooses not to, shows that our knowledge about short-lived gases and particles and their interactions with climate is still evolving. Eventually, with further research, uncertainties in chemical and physical processes can be significantly reduced. How

ever, uncertainties in future emissions, which are discussed in Section 4.3.1, will always be with us. What we can do is develop a set of internally consistent emissions scenarios that include all of the important radiative gases and particles and bracket the full range of possible future outcomes.
4.3.1 Emissions Projections The analysis presented in Chapter 3 showed that the main contributors to the divergence among model projections of future particle loading and climate forcing were the differ

ences in the underlying emissions projections.

8 Stabilization emissions scenarios are a representation

of the future emissions of a substance based on a coher

ent and internally consistent set of assumptions about the driving forces (such as population, socio-economic development, technological change) and their key rela

tionships. These emissions are constrained so that the resulting atmospheric concentrations of the substance level off at a predetermined value in the future.63 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols
Those differences arose from two primary factors. First, different integrated assessment models 9 interpret a common socio-economic ‘storyline’ in different ways, as demonstrated in Chapter 2. Second, emissions scenarios were not produced for some short-lived gases and particles and had to be added in later by other emissions modeling groups or by the climate modeling groups themselves. These same is

sues were also encountered in Chapter 2 which focused on the SAP 2.1a stabilization emissions scenarios (Clarke et al. , 2007). While emissions scenarios for short-lived gases and particles were outside of SAP 2.1a’s mandate, climate projections require them. Part of the reason for the different emission inventories used here and in the IPCC studies was that the integrated assessment models did not recognize that they were necessarily important when the scenarios were first constructed.
Just consider two of the integrated assessment models used to generate the sulfur dioxide and nitrogen oxide emissions for the A1B scenario used in Chapter 3. The two models project dif

ferent rates of growth; total energy use is also different in the two models, with 3 percent greater use in one model by 2030, but 9 percent less usage at 2050. That same model is less optimistic about emissions controls.
None of the emissions models predict black and organic carbon particle emissions. The GFDL composition model followed the IPCC sugges

tion to scale future carbon particle emissions by the emissions for carbon monoxide, whichare projected to increase throughout the twentyfirst century. By 2050, the projected carbon monoxide emissions increase ranges from 8 percent to 119 percent across the collection of integrated assessment models used for the last IPCC report. In contrast, the GISS global chem

ical composition model used recent estimates by Streets et al. (2004) that project a substantial decrease in future emissions of carbon particle (Figure 3.1; Table 3.1). Ammonia emissions present a similar problem. They are sometimes scaled by default to follow nitrous oxide, which

9 Integrated assessment models are a framework of

models, currently quite simplified, from the physical, biological, economic, and social sciences that interact among themselves in a consistent manner and can be used to evaluate the status and the consequences of environmental change and the policy responses to it. is projected to increase significantly. Given the number of ammonia sources that are discon

nected from nitrous oxide production, this may be questionable. Moreover, newer projections for ammonia emissions have a much slower rate of increase.
Finally, the global chemical composition models all used their own natural emissions. Though these were held constant, they influence the response to anthropogenic emissions by de

termining the background abundance of shortlived gases and particles. The level of a natural particle, dust, can also directly affect the level of an anthropogenic particle, as it does in the GISS model by removing sulfate.
We face very significant problems in project

ing the future emissions for short-lived gases and particles. Future climates are only weakly dependent on projected emissions for the next 20 years when, due to the inertia in major emitters, we may have credibility in forecast

ing emissions trends for short-lived gases and particles such as the nitrogen oxides and sulfur dioxide. However, we have shown that plausible emissions scenarios have the potential for sig

nificant impacts on climate through the rest of the twenty-first century.
Unlike the long-lived greenhouse gases, shortlived gases and particles do not accumulate, so their full impact at year 2100 depends on end of the century emissions. At this time, there is no credible quantitative skill in forecasting these emissions out to 2100. As Chapter 3 demon

strates, it is not even clear that we can currently predict the sign of the change for black carbon emissions over the next decade. This is a prob

lem that requires not just enhanced scientific knowledge, but also the ability to predict social, economic and technological developments as far as 100 years into the future. One needs only to think back to 1907 to realize how difficult that is and will be.
4.3.2 Particles (Indirect Effect, Di
rect Effect, Mixing, Water U ptake) We find that several aspects of particle modeling have large uncertainties (see Figure SPM.2 in IPCC Fourth Assessment Report [IPCC, 2007] for a qualitative estimate of these uncertainties), of which the particle indirect effect, which isThe U.S. Climate Change Science Program

64 Chapter 4

very poorly known, is probably the most critical. Many aspects of the par

ticle-cloud interaction are not well quantified, and hence the effect was left out entirely in the GFDL and CCSM simulations. The GISS model used a highly parameterized ap

proach that is quite crude. The modeling community as a whole cannot yet pro

duce a credible charac

terization of the climate response to particle/cloud interactions. Moreover, the measurements needed to guide this characteriza

tion do not yet exist. All mainstream climate mod

els (including those participating in this study) are currently either ignoring it, or strongly constraining the model response. Attempts have been made using satellite and ground-based ob

servations to improve the characterization of the indirect effect, but major limitations remain.
As discussed in Section 3.2.4, observations of aerosol optical depth 10 are best able to constrain the total extinction (absorption plus scattering) of sunlight by all particles under clear-sky conditions, but not to identify the effect of individual particles which may scatter (cool) or absorb (warm). Improved measurements of extinction and absorption may allow those two classes of particles to be separated, but will not solve the fundamental problem of determining their relative individual importance. As seen in this and other studies, models exhibit a wide range of relative contributions to total aerosol optical depth from the various natural and anthropogenic particles (Figure 3.2). Thus, the direct radiative effect of changes in a particular particle can be substantially different among models depending upon the relative importance of that particle.

10 Aerosol optical depth is a measure of the fraction of

the sun’s radiation at a given wavelength absorbed or scattered by particles while that radiation passes through the atmosphere. Additionally, particles are not independent of one another. They mix together, a process that is only beginning to be incorporated in compo

sition and climate models. In these studies, for example, the GISS model included the influence of sulfate particles sticking to dust, which can decrease the sulfate radiative forcing, by ~40 percent between 2000 and 2030 (Bauer et al. , 2007), but the sticking rates are quite uncertain. Mixing of other particle types is also highly un

certain, but is known to occur in the atmosphere and would also affect the magnitude of particle radiative forcings.
Another process that influences the effect of particles on climate is their uptake of water vapor, which alters their size and optical properties. This process is now included in all state-of-the-art comprehensive climate models. As the uptake varies exponentially with relative humidity, small differences in treatment of this process have the potential to cause large dis

crepancies. However, our analysis in Chapter 3 ( e.g., Table 3.7) suggests that the differences induced by this process may be small relative to the others we have just discussed.
4.3.3 Climate and Air Quality Policy Interdependence Chapter 3 exposes major uncertainties in the climate impacts of short-lived gases and par

ticles that will have to be addressed in future research. We raise important issues linking air quality control and global warming, but are un

able to provide conclusive answers. We are able, however, to identify key questions that must be addressed by future research.
Most future sources of short-lived gases and particles result from the same combustion processes responsible for the increases in atmo

spheric carbon dioxide. However, while reduc

tions in their emissions are currently driven by local and regional air pollution issues that can be addressed independently of any reductions in carbon dioxide emissions, in the future a unified approach could effectively address both climate and air quality issues. Furthermore, theclimate responses to emissions changes in shortlived pollutants can be felt much more quickly because of shorter atmospheric lifetimes. Thegood news is that there is at least one clear winwin solution for climate (less warming) and65 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols
air quality (less pollution): methane reduction. Decreases in methane emissions lead to reduced levels of lower atmospheric ozone, thereby im

proving air quality; and both the direct methane and indirect ozone decreases lead to reduced global warming (Fiore et al. , 2002; Shindell et al. , 2005; West and Fiore, 2005; West et al. , 2006). Reductions in emissions of carbon mon

oxide or volatile organic compounds (VOCs) have similar effects, namely leading to reduced abundances of both methane and ozone (Bern

tsen et al. , 2005; Shindell et al. , 2005; West et al. , 2006; West et al. , 2007), therefore providing additional win-win strategies for improvement of climate and air quality. Reductions in black carbon particles and nitrogen oxide are poten

tially win-win as well, but the climate impact of reductions in their emissions is uncertain. On the other hand, the reduction of sulfur and organic carbon particles results in a reduction of cooling and increased global warming.
The cases of black carbon (soot) and nitrogen oxide gases are illustrative of the complexi

ties of this issue. A major source of soot is the burning of biofuel, the sources of which are primarily animal and human waste as well as crop residue, all of which are considered carbon dioxide neutral ( i.e., the cycle of production and combustion does not lead to a net increase in atmospheric carbon dioxide). Current suggested replacements result in the release of fossil car

bon dioxide. Therefore this reduction in biofuel burning, while reducing the emission of soot, will increase the net emission of carbon dioxide. The actual net climate response from reduced use of biofuel is not clear. The case of nitrogen oxides appears to be approximately neutral for climate, though clearly a strong win for air qual

ity. Reducing nitrogen oxides reduces ozone, which reduces warming. However, reductions in both lead to reduced hydroxyl radicals and therefore an increased level of methane, which increases warming.
There clearly are win-win, win-uncertain, and win-lose situations regarding climate and actions taken to improve air quality. We are not making any policy recommendations in Synthesis and Assessment Product 3.2, but we do identify the policy relevant scientific issues. At this time we can not provide any quantita

tively definitive scientific answers beyond the well known facts that the decrease of sulfur and organic carbon particles, both of which cool the climate, will increase global warming, while decreased methane, carbon monoxide, and volatile organics will decrease global mean warming. Decreases in the burning of biofuel, as well as decreased emissions of nitrogen ox

ides, are more complex and the net result is not clear at this time.



### 4.4 R esearch O pport u nities and R ecommendations
This last section of the report is a call for fo

cused scientific research in emissions projec

tions, radiative forcing, chemical composition modeling and regional downscaling. Particular emphasis needs to be paid to the future emis

sions scenarios for sulfur dioxide, black carbon particles and nitrogen oxides, to the indirect ra

diative forcing by particles, and to a number of ambiguities in current treatments of transport, deposition, and chemistry.
4.4.1 Emissions Scenario Development Future climate studies must seriously address the very difficult issue of producing realistic and consistent 100-year emissions scenarios for short-lived gases and particles that include a wide range of socio-economic and development pathways and are driven by local and regional air quality actions taken around the globe.
The current best projections used in this re

port and in the Fourth Assessment Report of the IPCC do not even agree on whether black carbon particle and nitrogen oxide emissions trends continue to increase or decrease. While all the current sulfur dioxide emissions projec

tions used in this study assume that emissions in 2100 will be less than at present, how much less is quite uncertain, and all of these projected decreases by 2100 may well be wrong. Part of the reason for the different emission inventories used here and in the IPCC studies was that the integrated assessment models did not recognize that these gases and particles were necessarily important when the scenarios were first con

structed. Clarification of the challenges asso

ciated with emissions projections (not a simple matter of improving quantitative skill, as theseThe U.S. Climate Change Science Program

66 Chapter 4

are a function of difficult-to-anticipate socioeconomic choices) is also necessary.
As the greatest divergences in our study came from the carbon particles that were not projected for the A1B scenario, we strongly recommend that future emissions scenarios pay greater attention to them and provide consistent emis

sions projections for carbon particles and am

monia along with the other gases and particles. We are aware that many integrated assessment models are already capable of providing this information ( e.g., two of the three discussed in Chapter 2 provide carbon particle emissions).

We also recommend that climate models make greater efforts to study the effects of short-lived emissions projections in a manner that isolates their effect from that of the long-lived green

house gases. In particular, we believe there is merit in continuing to use a broad distribution of integrated assessment models to realistically characterize the range of potential futures for a given socio-economic storyline. In order to un

derstand the contribution to uncertainty by the composition and climate models, it would also be worthwhile to perform a controlled experi

ment with identical emissions projections using multiple composition and climate models.
4.4.2 Particle Studies (Direct Effect, Indirect Effect, Mixing, Water U ptake) Calculation of the indirect effect is potentially the single most important deficiency in this study that can be directly improved. None of the models in the IPCC Fourth Assessment Report study or in this Product realistically accounted for the full particle indirect radiative forcing. Given that the inclusion of a crude treatment of the particle indirect effect played a substantial role in one model’s response in this study, it is clear that better characterization of this effect is imperative. The development of a measurement program to characterize particle indirect effects is a critical need.
It is also clear that other potential particle processes need to be examined. An example is interactive dust loading, which can influence the composition of other short-lived gases and particles and can also be influenced by them ( e.g., via changes in solubility due to acid up

take [Fan et al ., 2004; Bauer and Koch, 2005]). Dust emissions will also respond to vegetation changes as the climate warms. It has been speculated that arid regions may contract as a result of fertilization of plants by increased CO 2 , reducing emissions (Mahowald and Luo, 2003), while source regions may expand as a result of global warming and reduced rainfall, and thus increase emissions (Woodward et al. , 2005). Note that the actual trend will depend upon local changes in climate, especially rain

fall, which is among the least robust aspect of current climate projections. Other processes of potential importance that were not included in these transient climate simulations are changes in atmospheric levels of sea-salt particles and changes in darkening of snow and ice surfaces by soot deposition.
Additional observations are clearly needed to better constrain the optical properties of aero

sols. Measurement by devices such as the aero

sol polarimeter on the NASA satellite, Glory, should provide some of the needed information. We recommend emphasis on long-term particle monitoring from ground and space, and on bet

ter characterization of particle interactions with clouds in the laboratory. We also recommend greater use of the distinction between scatter

ing and absorbing particles to characterize their relative importance.
4.4.3 Improvements in Transport, Deposition, and Chemistry The emissions issues become even more prob

lematic when the future distributions employed in the comprehensive climate models are gen

erated by multiple global composition models, all with differing treatments of mixing in the67 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols
lowest layers of the atmosphere, different treat

ment of transport and mixing by turbulence and clouds, different treatments of the removal of gases and particles by rain, snow and contact with the earth’s surface, and different approxi

mate treatments of the very large collection of chemical reactions that we do not yet fully understand. There are research opportunities in the study of all phases of the physical and chemical processes that determine the global distribution of short-lived gases and particles from their emission through their removal from the atmosphere.

4.4.4 Recommendations for Regional Downscaling Regional downscaling, where global climate models introduce climate forcing into regional climate models, is a relatively new develop

ment. Current regional downscaling results have relied on older comprehensive climate model simulations. Data from the newer comprehensive global models are needed and coordination and planning are critical since downscaling require input data every three or six hours. While this will lead to huge amounts of data to be stored and transferred and poses a non-trivial technical problem, it is essentially an infrastructure/practical restriction rather than a source of scientific challenges. A care

fully coordinated set of region climate model predictions using various global and regional scale models and future scenarios is needed to reduce the uncertainty and identify method

ological improvements.
The North American Regional Climate Change Assessment Program (NARCCAP, <http:// www.narccap.ucar.edu>) is an ongoing effort that is actively taking this approach of multiple regional climate model simulations and multiple future scenarios. Four different comprehensive global models and six different regional models are being used to intercompare and evaluate regional climate simulations for North America. This effort and others like it should greatly help to advance regional downscaling approaches and provide important model archives for envi

ronmental quality and resources applications.
It is important to also note that these studies do not currently include short-lived gases and par

ticles in the global climate simulations. Further, many regional models do not include feedbacks between air quality, the radiation budget and cli

mate. These feedbacks may be quite important. For example, The U.S. EPA Clean Air Interstate Rule requires almost a 30 percent reduction in sulfur dioxide emissions in the eastern United States during the next two decades. This could have a significant impact on the climate projec

tions, as was shown in Chapter 3. We also need to separate the impacts on regional climate by the direct and indirect effects of particles in general from the impacts on regional climate by local emissions changes.
More research is clearly needed to determine if downscaled regional climate simulations actu

ally provide more detailed and realistic results. The higher regional resolution is important for a wide range of environmental issues including water and air quality, agricultural productivity, and fresh water supplies. For example, highly resolved regional climate information is needed to accurately predict levels of ozone and small particles, which are strongly influenced by local changes in emissions and climate.
4.4.5 Expanded Analysis and Sensitivity Studies Analyses of surface temperature response to changes in short-lived gases and particles need to be strengthened by additional sensitivity studies that should help to clarify causes and mechanisms. For example, in the GISS model, how much warming did the declining trend in the indirect effect contribute to its climate re

sponse and where? How would the GISS results differ if dust had not been permitted to take up sulfate particles? Determining the relative importance of these and other processes to the climate response would help prioritize the gaps in our knowledge. There are also a wide range of climate-chemistry feedbacks and controls that should be explored. Both the response of the climate system to controls on short-lived gases and particles and the possible feedbacks, and the possible impacts of climate changes on levels of short-lived gases and particles are all fertile areas for future research. While it was not pos

sible, both due to time and resource restraints, for this study to explore these additional analy

ses, we recommend their future study.
The major unfinished analysis question in this study is the relative contribution of a model’sThe U.S. Climate Change Science Program

68 Chapter 4

regional climate response, as opposed to the contribution from the regional pattern of radia

tive forcing, to the observed regional change in seasonal surface temperature. (See Sec

tion 9.2.2.1 of Chapter 9 in the IPCC Fourth Assessment Report [Hegerl et al ., 2007] and references therein for further discussion of this issue.) Specific modeling studies are needed to answer questions such as: Is there a model in

dependent regional climate response? and What are the actual physical mechanisms driving the regional surface temperature patterns that we observe? These questions appear to identify a very important area of study, particularly given the apparently strong climate response in the summertime central United States.



### 4.5 C oncl u sion
After considering all of the issues discussed in this chapter, the net result is that we agree that short-lived species may have significant impact on future climate out to 2100. However, we could not find a consensus in this report on the duration, magnitude or even sign (warming or cooling) of the climate change due to future levels of the short-lived gases and particles. We have presented a plausible case for enhanced climate warming due to air quality policies that focus primarily on sulfate particle reduction and permit the emission of soot to continue to increase as realized in a version of the IPCC’s A1B scenario. Alternative versions of this scenario that follow different pollution control storylines could have less impact. While we do not have definitive answers to the second goal of this report to “assess the sign, magnitude, and duration of future climate changes due to chang

ing levels of short-lived gases and particles that are radiatively active and that may be subject to future mitigation actions to address air quality issues,” we do provide plausible estimates that begin to characterize the range of possibilities and we identify key areas of uncertainty and provide motivation for addressing them.APPENDIX A

69 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols



### A.1 M ean T emperat u re
All Atmosphere-Ocean General Circulation Mod

els (AOGCMs) in Chapter 10 of the Fourth Assess

ment Report (AR4) (Meehl et al. , 2007) project increases in global mean surface air temperature (SAT) throughout the twenty-first century, with the warming proportional to the associated radiative forcing. There is close agreement among globally averaged SAT multi-model mean warming for the early twenty-first century for the three SRES (B1, A1B and A2) scenarios as well as for SAP 2.1a Level 2 through 4 scenarios out to 2050. The warming rate over the next few decades in Chapter 10 (Meehl et al. , 2007) is affected little by different scenario assumptions or different model sensi

tivities, and is similar to that observed for the past few decades. By mid-century (2046 to 2065), the choice of SRES scenario becomes more important and they start to separate, though the range among the collection of AOGCMs is comparable. By the end of the twenty-first century, the SATs gener

ated by MAGICC using the 12 SAP 2.1a scenarios in Chapter 2 as well as the full spread of all of the AOGCMs for the A2, B1 and Committed projec

tions have completely separated, though A1B still has some overlap with A2 and B1.
In general, geographical patterns of projected SAT warming show greatest temperature increases over land (roughly twice the global average temperature increase) and at high northern latitudes, and show less warming over the southern oceans and North Atlantic, consistent with observations during the latter part of the twentieth century. These patterns 

# IPCC Fourth Assessment Report Climate Projections (Supplemental to Chapter 2)

Lead Author: Hiram Levy II, NOAA/GFDL
Contributing Author: Tom Wigley, NCAR

These robust conclusions, which we believe also apply to the climate projections from the SAP 2.1a scenarios (Clarke et al. , 2007), are taken primarily from the Executive Summary of Chapter 10 of the IPCC’s Fourth Assessment Report (Meehl et al. , 2007) as well as some details extracted from the body of Chapter 10, and are summarized below. NCDC/NOA

are similar across the B1, A1B, and A2 scenarios (see Figure 10.8 in Chapter 10 of the AR4; Meehl et al. , 2007) only increasing in magnitude with increasing radiative forcing. Results for the stabilization emission scenarios similar to those studied here should show the same pat

tern similarities at least out to 2100 (Dai et al. , 2001a,b). It should be noted that, in none of the cases considered here, has the climate stabilized by 2100; for the higher stabilization levels this may take centuries. Temperature change patterns may differ as one approaches closer to a stable climate.



### A.2 T emperat u re E xtremes
It is very likely that heat waves will be more intense, more frequent and longer lasting in a future warmer climate. Cold episodes are projected to decrease significantly in a future warmer climate. Almost everywhere, daily minimum temperatures are projected to increase faster than daily maximum temperatures, leading to a decrease in diurnal temperature range. Decreases in frost days are projected to occur almost everywhere in the mid and high latitudes, with a comparable increase in growing season length (Meehl et al. , 2007).



### A.3 M ean P recipitation
Globally averaged mean atmospheric water vapor, evapo

ration and precipitation are projected to increase. By 2100, precipitation generally increases in the areas of regional tropical precipitation maxima (such as the monsoon regimes) and over the tropical Pacific in particular, with general decreases in the subtropics, and increases at high latitudes as a consequence of a general intensification of the global hydrological cycle. The geographical patternsThe U.S. Climate Change Science Program Appendices

70 of precipitation change during the twenty-first century are not as consistent across AOGCMs and across scenarios as they are for surface temperature (Meehl et al. , 2007).



### A.4 P recipitation E xtremes and D ro u ghts
Intensity of precipitation events is projected to increase, particularly in tropical and high latitude areas that experi

ence increases in mean precipitation. There is a tendency for drying of the mid-continental areas during summer, indicat

ing a greater risk of droughts in those regions. Precipitation extremes increase more than the mean in most tropical and mid- and high-latitude areas (Meehl et al. , 2007).



### A.5 S now and I ce

As the climate warms, snow cover and sea ice extent de

crease; glaciers and ice caps lose mass owing to dominance of summer melting over winter precipitation increases. There is a projected reduction of sea ice in the twenty-first century both in the Arctic and Antarctic with a large range of model responses. Widespread increases in thaw depth over much of the permafrost regions are projected to occur in response to warming over the next century (Meehl et al. , 2007).

Note: All of the AR4 predictions for precipitation, snow cover, and sea and land ice are less certain and more vari

able across the suite of AOGCMs than they are for both the global average and the more robust geographic patterns of temperature.



### A.6 C arbon C ycle

Under the SRES illustrative emissions scenarios, for central carbon-cycle model parameters, CO 2 concentrations are pro

jected to increase from its present value of about 380 ppm to 540 to 970 ppm by 2100. The SAP 2.1a Reference scenarios give 2100 concentrations of 740 to 850 ppm (Clarke et al. , 2007). There is unanimous agreement among the simplified climate-carbon cycle models that future climate change would reduce the efficiency of the Earth system (land and ocean) to absorb anthropogenic carbon dioxide. The higher the stabilization emission scenario warming, the larger is the impact on the carbon cycle. Both MAGICC and two of the three integrated assessment models used in SAP 2.1a contain simplified carbon cycle models comparable to those in Chapter 10 of the AR4 (Meehl et al. , 2007).



### A.7 O cean A cidification
Increasing atmospheric CO 2 concentrations lead directly to increasing acidification of the surface ocean. Multi-model projections based on SRES scenarios give reductions in pH of between 0.14 and 0.35 units over the twenty-first century, adding to the present decrease of 0.1 units from preindustrial times.



### A.8 S ea L e v el
“Sea level is projected to rise between the present (1980 to 1999) and the end of this century (2090 to 2099) under the SRES B1 scenario by 0.28 m for the multi-mode average (range 0.19 to 0.37 m), under A1B by 0.35 m (0.23 to 0.47 m), under A2 by 0.37 m (0.25 to 0.50 m) and under A1FI by 0.43 m (0.28 to 0.58 m). These are central estimates with 5 to 95 percent intervals based on AOGCM results, not includ

ing uncertainty in carbon-cycle feedbacks. In all scenarios, the average rate of rise during the twenty-first century very likely exceeds the 1961 to 2003 average rate (1.8 ± 0.5 mm per year). During 2090 to 2099 under A1B, the central es

timate of the rate of rise is 3.8 mm per year. For an average model, the scenario spread in sea-level rise is only 0.02 m by the middle of the century, and by the end of the century it is 0.15 m (Meehl et al. , 2007)”. The projections of sea-level rise for the 12 SAP 2.1a scenarios by MAGICC are within the range reported by AR4 (Wigley et al. , 2007).
“Thermal expansion is the largest component, contributing 60 to 70 percent of the central estimate in these projections for all scenarios. Glaciers, ice caps and the Greenland ice sheet are also projected to contribute positively to sea level. GCMs indicate that the Antarctic ice sheet will receive in

creased snowfall without experiencing substantial surface melting, thus gaining mass and contributing negatively to sea level. Further accelerations in ice flow of the kind recently observed in some Greenland outlet glaciers and West Antarctic ice streams could substantially increase the contribution from the ice sheets. Current understanding of these effects is limited, so quantitative projections cannot be made with confidence (Meehl et al. , 2007)”.



### A.9 O cean C irc u lation

a. There is no consistent change in the ENSO for those AOGCMs with a quasi-realistic base state. b. Among those models with a realistic Atlantic Meridional Overturning Circulation (MOC), while it is very likely that the MOC will slow by 2100, there is little agreement among models for the magnitude of the slow-down. Mod

els agree that the MOC will not shut down completely ( Meehl et al. , 2007 ).



### A.10 M onsoons
Current AOGCMs predict that, in a warmer climate, there will be an increase in precipitation in both the Asian mon

soon (along with an increase in interannual variability) and71 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols
the southern part of the West African monsoon with some decrease in the Sahel in northern summer, as well as an in

crease in the Australian monsoon in southern summer. The monsoonal precipitation in Mexico and Central America is projected to decrease in association with increasing pre

cipitation over the eastern equatorial Pacific. However, the uncertain role of particles complicates the projections of monsoon precipitation, particularly in the Asian monsoon (Meehl et al. , 2007).



### A.11 T ropical C yclones (Hu rricanes and T yphoons )
The Summary for Policymakers finds it likely that in

tense hurricanes and typhoons will increase through the twenty-first century as it warms. Results from embedded high-resolution models and global models, ranging in grid spacing from 1 degree to 9 km, generally project increased peak wind intensities and notably, where analyzed, increased near-storm precipitation in future tropical cyclones (Meehl et al. , 2007). However, these questions of changes in fre

quency and intensity under global warming continue to be the subject of very active research (CCSP, 2008; Emanuel et al ., 2008; Knutson et al ., 2008).



### A.12 M idlatit u de S torms
Model projections show fewer midlatitude storms aver

aged over each hemisphere, associated with the poleward shift of the storm tracks that is particularly notable in the Southern Hemisphere, with lower central pressures for these poleward-shifted storms. The increased wind speeds result in more extreme wave heights in those regions (Meehl et al. , 2007).



### A.13 R adiati v e F orcing
“The radiative forcings by long-lived greenhouse gases computed with the radiative transfer codes in twenty of the AOGCMs used in the AR4 have been compared against re

sults from benchmark line-by-line (LBL) models. The mean AOGCM forcing over the period 1860 to 2000 agrees with the mean LBL value to within 0.1 W per m 2 at the tropopause. However, there is a range of 25 percent in longwave forcing due to doubling CO 2 from its concentration in 1860 across the ensemble of AOGCM codes. There is a 47 percent rela

tive range in longwave forcing at 2100 contributed by all greenhouse gases in the A1B scenario across the ensemble of AOGCM simulations. These results imply that the ranges in climate sensitivity and climate response from models discussed in this chapter may be due in part to differences in the formulation and treatment of radiative processes among the AOGCMs (Meehl et al. , 2007)”. 

### A.14 C limate C hange C ommitment (T emperat u re and S ea L e v el )
“Results from the AOGCM multi-model climate change commitment experiments (concentrations stabilized for 100 years at year 2000 for twentieth century commitment, and at 2100 values for B1 and A1B commitment) indicate that if greenhouse gases were stabilized, then a further warming of 0.5°C would occur (Meehl et al. 2007)”.
“If concentrations were stabilized at A1B levels in 2100, sea-level rise due to thermal expansion in the twenty-second century would be similar to the twenty-first, and would amount to 0.3 to 0.8 m above present by 2300. The ranges of thermal expansion overlap substantially for stabilization at different levels, since model uncertainty is dominant; A1B is given here because most model results are available for that scenario. Thermal expansion would continue over many centuries at a gradually decreasing rate, reaching an eventual level of 0.2 to 0.6 m per degree of global warming relative to present (Meehl et al. , 2007)”.The U.S. Climate Change Science Program Appendices

72APPENDIX B

73 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

MAGICC (Model for the Assessment of Green

house-gas Induced Climate Change) is a coupled gas-cycle/climate model. Various versions of MAGICC have been used in all IPCC assess

ments. The version used here is the one that was used in the IPCC Third Assessment Report (TAR; IPCC, 2001; Cubasch and Meehl, 2001; Wigley and Raper, 2001). A critical assessment focused on its skill in predicting global average sea-level rise is found in Chapter 10 Appendix 1 of the Working Group I contribution to the Fourth Assessment Report (AR4) of the IPCC (Meehl et al., 2007).
The climate component is an energy-balance mod

el with a one-dimensional, upwelling-diffusion ocean (UDEBM). For further details of models of this type, see Hoffert et al. (1980) and Harvey et al. (1997). In MAGICC, the globe is divided into land and ocean “boxes” in both hemispheres in order to account for different thermal inertias and climate sensitivities over land and ocean, and hemispheric and land/ocean differences in forcing for short-lived gases and particles.
In order to allow inputs as emissions, the climate model is coupled interactively to a series of gas-cycle models for CO 2 , CH 4 , N 2 O, a suite of halocarbons and SF 6 . Details of the carbon cycle model are given in Wigley (1991a, 1993, 2000). The carbon cycle model includes both CO 2 fer

tilization and temperature feedbacks, with model parameters tuned to give results consistent with the other two carbon cycle models used in the TAR; viz . ISAM (Kheshgi and Jain, 2003) and the Bern model (Joos et al. , 2001) over a wide range of emissions scenarios. Details are given in Wigley 

# MAGICC Model Description (Supplemental to Chapter 2)

Lead Author: Hiram Levy II, NOAA/GFDL
Contributing Author: Tom Wigley, NCAR

et al. (2007). The other gas cycle models are those used in the TAR (Prather and Ehhalt, 2001; Wigley et al. , 2002). Radiative forcings for the various gases are as used in the TAR. For sulfate particles, both direct and indirect forc

ings are included using forcing/emissions relationships developed in Wigley (1989, 1991b), with central estimates for 1990 forcing values. Sea-level rise estimates use ther

mal expansion values calculated directly from the climate model. Ice melt and other contributions are derived using formulae given in the TAR (Church and Gregory, 2001), except for the glacier and small ice cap contribution which employs an improved formulation that can be applied beyond 2100 (Wigley and Raper, 2005).
The standard inputs to MAGICC are emissions of the various radiatively important gases and various climate model parameters. For the TAR, these parameters were tuned so that MAGICC was able to emulate results from a range of Atmosphere-Ocean General Circulation Model (AOGCMs) (Cubasch and Meehl, 2001; Raper et al. , 2001). For the present calculations, a central set of param

eters has been used. The most important of these is the climate sensitivity, where we have used a value of 2.6°C equilibrium global-mean warming for a CO 2 doubling, the median of values for AOGCMs used in the TAR.The U.S. Climate Change Science Program Appendices

74APPENDIX C

75 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols



### C.1 G eophysical F l u id D ynamics L aboratory
Composition changes for the short-lived gases and particles from 2000 to 2100 in the GFDL experi

ments were calculated using the global chemical transport model MOZART-2 (Model for Ozone And Related chemical Tracers, version 2.4), which has been described in detail previously (Horowitz et al. , 2003; Horowitz, 2006; and references therein). This model was used to generate distributions of ozone, sulfate, black and organic carbon, and dust for the emissions scenarios discussed in Section 3.2.1. Emissions and initial conditions for meth

ane were scaled each decade to match the global average methane abundances specified in the A1B “marker” scenario. The model includes 63 gases, 11 particles and precursor gases to simulate sulfate, nitrate, ammonium, and black and organic carbon, and five size bins for mineral dust (diameter size bins of 0.2 to 2.0 μm, 2.0-3.6 μm, 3.6 to 6.0 μm, 6.0 to 12.0 μm, and 12.0 to 20.0 μm). Hydrophobic black and organic carbon are chemically trans

formed into hydrophilic forms with a lifetime of 1.63 days (Tie et al. , 2005). Different particle types are assumed to be externally mixed and do not interact with one another. Sulfur oxidation in the gas phase and within clouds is fully interactive with the gas-phase oxidant chemistry.
The transport in MOZART-2 is driven with me

teorological inputs provided every three hours by the middle-atmosphere version of the NCAR Community Climate Model (Kiehl et al. , 1998). The meteorology was the same for each decade, thus excluding any feedback from climate change on natural emissions and rates of chemical reactions and removal. Thus, natural emissions, such as those of isoprene, dust, and NO x from lightning, are held constant at present-day levels. Convective mass fluxes are rediagnosed from the large-scale meteorology using the Hack (1994) and Zhang and McFarlane (1995) schemes. Vertical diffu

sion within the boundary layer is diagnosed using the scheme of Holtslag and Boville (1993). Tracer advection is performed using a flux-form semi-Lagrangian scheme (Lin and Rood, 1996).
The horizontal resolution is 2.8° latitude x 2.8° longitude, with 34 hybrid sigma-pressure levels extending up to 4 hPa. Photolysis frequencies for clear-sky are interpolated from a precalculated lookup table, based on a standard ra

diative transfer calculation (TUV version 3.0; Madronich and Flocke, 1998). The values are modified to account for cloudiness (Brasseur et al. , 1998), but do not account for effects of the simulated particles. Heterogeneous hydroly

sis of N 2 O 5 and NO 3 on particle surfaces occurs at a rate based on the simulated sulfate surface area, with a reac

tion probability of 0.04 (Tie et al. , 2005). Stratospheric concentrations of ozone and several other long-lived gases are relaxed to present-day climatological values.
Dry deposition velocities for reactive gases are calculated offline using a resistance-in-series scheme (Wesely, 1989). Deposition velocities for particles are prescribed as by Tie et al. (2005). Wet removal of soluble gases and particles in and below clouds is included as a first-order loss process, based on the large-scale and convective precipitation rates, as described by Horowitz et al. (2003). In-cloud scavenging is based on the parameterization of Giorgi and Chameides (1985), while below-cloud washout 

# Composition Models (Supplemental to Chapter 2)

Lead Authors: Drew T. Shindell, NASA/GISS; M. Daniel Schwarzkopf, NOAA/GFDL; Larry W. Horowitz, NOAA/GFDL
Contributing Authors: Jean-Francois Lamarque, NCAR ; Tom Wigley, NCAR

CDC/NOAThe U.S. Climate Change Science Program Appendices

76 of highly soluble gases and particles follows Brasseur et al. (1998). For gases, the removal rate depends strongly on the temperature-dependent effective Henry’s law constant. Wet deposition of soluble particles (sulfate, hydrophilic BC, hydrophilic OC, ammonium, and nitrate) is calculated by scaling the removal rate to that of highly-soluble HNO 3 , assuming the particles have a first-order loss rate constant equal to 20 percent of that of HNO 3 (Tie et al. , 2005). This scaling introduces a large uncertainty into the calculation of particle burdens. Wet removal of dust is calculated using the formulation of Zender et al. (2003), with below-cloud scavenging efficiencies of 0.02 m 2 per kg for convective and 0.04 m 2 per kg for stratiform precipitation.
The ozone and particle distributions from these simula

tions have been evaluated by Horowitz (2006) and Ginoux et al. (2006), respectively. Simulated ozone concentrations agree well with present-day observations and recent trends (Horowitz, 2006). Overall, the predicted concentrations of particle are within a factor of two of the observed values and have a tendency to be overestimated (Ginoux et al. , 2006). The annual mean surface sulfate concentrations match observed values within a factor of two with values ranging from 0.05 μg per m 3 in the remote marine atmosphere to 13 μg per m 3 in polluted regions. In general, the simulatedconcentrations are over-predicted in summer and underpredicted in winter. Sulfate mass column and zonal mean profiles are comparable to previous studies, although the global mean burden is about 15 percent higher. The annual mean concentration of carbonaceous particles is generally overestimated in polluted regions by up to a factor of two. An exception is West Africa, where other models show significant loadings of carbonaceous particles associated with biomass burning activities during the dry season, while our results do not show any perturbation arising from such activities. The source of this discrepancy seems to be caused in part by the emission inventory in West Africa. The an

nual mean dust concentration at the surface agrees with the observations to within a factor of two, except over Antarctica where it is underestimated by a factor of five.
The three-dimensional monthly mean distributions of ozone, black and organic carbon particles, and sulfate particles from MOZART-2 were archived from simulations for each decade from 2000 to 2100. The results from these simula

tions were then interpolated to intermediate years and used in the transient climate simulations. The distribution of dust from a present-day simulation in MOZART-2 was used in all years of the climate simulations. 

### C.2 G oddard I nstit u te for S pace S t u dies
The configuration of the GISS composition model used here has been described in detail in (Shindell et al. , 2007). The composition model PUCCINI (Physical Understanding of Composition-Climate INteractions and Impacts) includes ozone and oxidant photochemistry in both the troposphere and stratosphere (Shindell et al. , 2006). Photochemistry in

cludes 155 reactions. The model calculates the abundances of 51 chemicals, 26 of which are transported by the model’s ad

vection scheme. It uses “lumped families” for hydrocarbons and PANs. Chemical reactions involving these surrogates are based on the similarity between the molecular bond structures within each family using the reduced chemical mechanism of (Houweling et al. , 1998). This mechanism is based on the Carbon Bond Mechanism-4 (CBM-4) (Gery et al. , 1989), modified to better represent the globally im

portant range of conditions. The CBM-4 scheme has been validated extensively against smog chamber experiments and more detailed chemical schemes. This scheme was modified for use in global models by removing aromatic compounds and adding in reactions important in background conditions, including organic nitrate and organic peroxide reactions, and extending the methane oxidation chemistry. The revised scheme was then readjusted based on the more extensive Regional Atmospheric Chemistry Model (RACM) (Stockwell et al. , 1997), and the modified scheme includes several surrogates designed to compensate for biases relative to the RACM mechanism. The modified scheme was shown to agree well with the detailed RACM reference mechanism over a wide range of chemical conditions including relatively pristine environments (Houweling et al. , 1998).

Rate coefficients are taken from the NASA JPL 2000 hand

book (Sander et al. , 2000). Photolysis rates are calculated using the Fast-J2 scheme (Bian and Prather, 2002), except for the photolysis of water and nitric oxide (NO) in the Schumann-Runge bands, which are parameterized accord

ing to Nicolet (1984; Nicolet and Cieslik, 1980). The particle component simulates sulfate, carbonaceous and sea salt particles (Koch et al. , 2007; Koch et al. , 2006) and nitrate particles (Bauer et al. , 2007). It includes prognostic simu

lations of DMS, MSA, SO 2 and sulfate mass distributions. The mineral dust particle model transports four different sizes classes of dust particles with radii between 0.1 to 1, 1 to 2, 2 to 4, and 4 to 8 microns (Miller et al. , 2006). Most importantly, these components interact with one another, with linkages including oxidants affecting sulfate, gases affecting nitrate, sulfate affecting nitrogen heterogeneous chemistry via reaction of N 2 O 5 to HNO 3 , and sulfate and77 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

nitrate being absorbed onto mineral dust surfaces ( i.e. , the particles are internally mixed as coatings form on dust surfaces (Bauer et al. , 2007). The latter is described by a pseudo first-order rate coefficient which gives the net irreversible removal rate of gases by an particle surface. We use the uptake coefficient of 0.1 recommended from laboratory measurements (Hanisch and Crowley, 2001), though this value is fairly uncertain.
Phase transformation and removal is calculated using a wet deposition scheme in which soluble gases can be re

moved into either moist convective plumes or large-scale clouds as derived from the GCM’s internal cloud scheme (Del Genio and Yao, 1993). During convection, all gases and particles are transported along with the convective plumes, with scavenging of soluble gases and particles within and below cloud updrafts. In large-scale stratiform clouds, soluble gases are removed based on the fraction of the grid box over which precipitation is taking place. Washout of soluble gases and particles is calculated below precipitating clouds. In the case of either evaporation of precipitation before reaching the ground, or detrainment or evaporation from a convective updraft, the dissolved gases and particles are returned to the air. Wet chemistry calculations take place in each grid box at each time step, including the coupling with the convection scheme’s en

training and nonentraining plumes (which are based on the convective instability in the particular grid box at that time), so are entirely consistent with the contemporaneous model physics. The solubility of each gas is determined by an effective Henry’s Law coefficient, assuming a pH of 4.5. Surface dry deposition is calculated using a resistance-in-series model (Wesely and Hicks, 1977) coupled with a global, seasonally varying vegetation data set as given by Chin et al. (1996).
The 2000 simulation uses the 2000 emission inventory of the International Institute for Applied Systems Analysis (IIASA), except for biomass burning which is taken from the Global Fire Emission Database (GFED) averaged over 1997 to 2002 (Van der Werf et al. , 2003) with emission factors from (Andrae and Merlet, 2001) for particles. The IIASA inventory is based on the 1995 EDGAR3.2 inventory (Olivier and Berdowski, 2001), extrapolated to 2000 using national and sector economic development data (Dentener et al. , 2005). Lightning NO x emissions are calculated internally in the GCM (5.6 Tg per year for present-day), and other natural sources are prescribed according to conventional estimates. Dust emissions are constant at 1580 Tg per year, while isoprene emissions are 356 Tg per year. Emissions of DMS are 41 Tg per year. The simulations described here were run with this composi

tion model included within a 23-layer (up to 0.01 hPa), 4° x 5° horizontal resolution version of the ModelE climate model (Schmidt et al. , 2006). This composition model was used for both the transient climate and regional/sector emissions perturbation simulations.
Present-day composition results in the model are generally similar to those in the underlying chemistry and particle models documented previously. The model used here does not include the enhanced convective scavenging of in

soluble gases and particles prescribed in Koch et al. (2007). Therefore our carbonaceous particle burden, especially in the free troposphere, is nearly double that of Koch et al. (2007). Comparison with the limited available observations is comparable between the two simulations (a positive bias replaces a negative bias).



### C.3 N ational C enter for A

tmospheric R esearch

Various methods were used at NCAR to estimate future composition. Present-day tropospheric ozone was taken from calculations performed by Lamarque et al. (2005) using the MOZART-2 model; beyond 2000, tropospheric ozone was calculated by T. Wigley using the MAGICC model <http://www.cru.uea.ac.uk/~mikeh/software/magicc.htm> forced by the time-varying emissions of NO x , methane and VOCs and these average global values were used to scale the present-day distribution. Future carbonaceous particles were scaled from their present-day distribution (Collins et al. , 2001) by a globally uniform factor whose time evolution follows the global evolution of SO 2 emissions. Future levels of sulfate particles were calculated using the MOZART model. Stratospheric ozone changes are prescribed following the study by (Kiehl et al. , 1999).

The Model for Ozone and Related chemical Tracers version 2 (MOZART-2) is described by Horowitz et al. (2003) and references therein. The model provides the distribution of 80 chemical constituents (including nonmethane hydrocar

bons) between the surface and the stratosphere. The model was run at a uniform horizontal resolution of ~2.8° in both latitude and longitude. The vertical discretization of the meteorological data (described below) and hence of the model consists of 18 hybrid levels from the ground to ~4 hPa. The evolution of gases and particles is calculated with a time step of 20 minutes.
The tropospheric photolysis rates use a vertical distribution of ozone based on the simulated ozone in the troposphere andThe U.S. Climate Change Science Program Appendices

78 on the climatology from Kiehl et al. (1999 ) above. For each simulation, this latter distribution is updated to reflect the changes in the lower stratosphere during the twentieth cen

tury, affecting only the photolysis rates and not the amount of ozone transported from the stratosphere.
The NCAR regional/sector perturbation simulations (Sec

tion 3.4) used a version of MOZART chemical transport model (Horowitz et al. , 2003) embedded within the Com

munity Atmosphere Model (CAM3, Collins et al. , 2006). This model, known as CAM-chem, includes an extension of the chemical mechanism presented by Horowitz et al. (2003) to include an updated terpene oxidation scheme and a better treatment of anthropogenic non-methane hy

drocarbons (NMHCs). The MOZART particles have been extended by Tie et al. (2001, 2005) to include a representa

tion of ammonium nitrate that is dependent on the amount of sulfate and ammonia present in the air mass following the parameterization of gas/particle partitioning by Metzger et al. (2002). In brief, CAM-chem simulates the evolution of the bulk particle mass of black carbon (BC, hydrophobic and hydrophilic), primary organic (POA, hydrophobic and hydrophilic), second organic (SOA, linked to the gas-phase chemistry through the oxidation of atmospheric NMHCs as in (Chung and Seinfeld, 2002), ammonium and ammonium nitrate (from NH 3 emissions), and sulfate particles (from SO 2 and DMS emissions). It also considers the uptake of N 2 O 5 , HO 2 , NO 2 , and NO 3 on particles. Results from the CAM-chem model are discussed by Lamarque et al. (2005). A description of sea salt, updated from Tie et al. (2005), is also included. Finally, a monthly-varying climatology of dust is used only for radiative calculations. The CAM-chem model considers only the direct effect of particles and the atmospheric model is coupled with the chemistry solely through the radiative fluxes, taking into account all radia

tively active gases and particles. The horizontal resolution is 2° latitude x 2.5° longitude, with 26 levels ranging from the surface to ~4 hPa.APPENDIX D

79 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols



### D.1 G eophysical F l u id D ynamics L aboratory
Climate simulations at GFDL used the coupled climate model recently developed at NOAA’s Geo

physical Fluid Dynamics Laboratory, which has been previously described in detail (Delworth et al. , 2006). A brief summary is provided here. The model simulates atmospheric and oceanic climate and variability from the diurnal time-scale through multi-century climate change without employing flux adjustment. The control simulation has a stable, realistic climate when integrated over mul

tiple centuries and a realistic ENSO (Wittenberg et al. , 2006). Its equilibrium climate response to a doubling of CO 2 is 3.4°C (Stouffer et al. , 2006). There are no indirect particle effects included in any of the simulations. The resolution of the land and atmospheric components is 2.5° longitude x 2° latitude and the atmospheric model has 24 verti

cal levels. The ocean resolution is 1° latitude x 1° longitude, with meridional resolution equatorward of 30° becoming progressively finer, such that the meridional resolution is 1/3° at the Equator. There are 50 vertical levels in the ocean, with 22 evenly-spaced levels within the top 220 m. The ocean component has poles over North America and Eurasia to avoid polar filtering.
Using a five-member ensemble simulation of the historical climate (1861 to 2003), including the evolution of natural and anthropogenic forcing agents, the GFDL climate model is able to capture the global historical trend in observed surface temperature for the twentieth century as well as many continental-scale features (Knutson et al. , 2006). However, the model shows some tendency for too much twentieth century warming in lower latitudes and too little warming in higher latitudes. Differences in Arctic Oscillation behavior between models and observa

tions contribute substantially to an underprediction of the observed warming over northern Asia. El Niño interactions complicate comparisons of observed and simulated temperature records for the El Chichón and Mt. Pinatubo eruptions during the early 1980s and early 1990s (Knutson et al. , 2006). In Figure 7d of Knutson et al. (2006), where the model ensemble and observa

tions are compared grid box by grid box, ~60 percent of those grid boxes with sufficient observational data have twentieth century surface temperature trends that agree quantitatively with the model ensemble. In general, many observed continental-scale features, including a twentieth century cooling over the North Atlantic, are captured by the model ensemble, as Figures 7a and 7c in Knutson et al. (2006) show. However, the model ensemble does not capture the observed cooling over the southeastern United States, and it produces a twentieth century cooling over the North Pacific that is not observed.



### D.2 G oddard I nstit u te for S pace S t u dies
The GISS climate simulations were performed using GISS ModelE (Schmidt et al. , 2006). We use a 20-layer version of the atmospheric model (up to 0.1 hPa) coupled with a dynamic ocean without flux adjustment, both run at four by five degree horizontal resolution, as in the GISS-ER IPCC AR4 simulations (Hansen et al. , 2007). This model has been extensively evaluated against obser

vations (Schmidt et al. , 2006), and has a climate sensitiv



# Climate Models (Supplemental to Chapter 3)

Lead Authors: Drew T. Shindell, NASA/GISS; M. Daniel Schwarzkopf, NOAA/GFDL; Hiram Levy II, NOAA/GFDL; Larry W. Horowitz, NOAA/GFDL
Contributing Author: Jean-Francois Lamarque, NCAR

CDC/NOAThe U.S. Climate Change Science Program Appendices

80 ity in accord with values inferred from paleoclimate data and similar to that of mainstream GCMs: an equilibrium climate sensitivity of 2.6°C for doubled CO 2 . The modeled radiatively active gases and particles influence the climate in the GCM. Ozone and particles can affect both the short and long wavelength radiation flux. Water uptake on particle surfaces influences the particle effective radius, refractive index and extinction efficiency as a function of wavelength and the local relative humidity (Koch et al. , 2007), which in turn affects the GCM’s radiation field.
The GISS model also includes a simple parameterization for the particle indirect effect (Menon et al. , 2002) (Box 3.1). For the present simulations, we use only cloud cover changes (the second indirect effect), with empirical coefficients selected to give roughly -1 W per m 2 forcing from the preindustrial era to the present, a value chosen to match diurnal tempera

ture and satellite polarization measurements, as described in Hansen et al. (2005). We note, however, that this forcing is roughly twice the value of many other model studies (Penner et al. , 2006). The particle indirect effect in the model takes place only from the surface through ~570 hPa, as we only let particles affect liquid-phase stratus clouds.



### D.3 N ational C enter for A

tmospheric R esearch

The transient climate simulations use the NCAR Com

munity Climate System Model CCSM3 (Collins et al. , 2006). This model had been run previously with evolution of short-lived gases and particles in the future for the IPCC AR4. The model was run at T85 (~1.4° x 1.4° resolution). For this study, a new simulation was performed for 2000 to 2050 in which ozone and particles were kept at their 2000 levels. The equilibrium climate sensitivity of this model to doubled CO 2 is 2.7°C.APPENDIX E

81 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols



# Scenarios (Supplemental to Chapter 4)

Lead Authors: Drew T. Shindell, NASA/GISS; Hiram Levy II, NOAA/GFDL
Contributing Author: Anne Waple, STG Inc.

CDC/NOA



### E.1 T he E mission S S cenarios of the IPCC S pecial R eport on E mission S S cenarios (SRES) (Nakić en ović a n d S wart , 2000)

A1. The A1 storyline and scenario family de

scribes a future world of very rapid economicgrowth, global population that peaks in midcentury and declines thereafter, and the rapid introduction of new and more efficient tech

nologies.
Major underlying themes are convergent among regions, capacity building, and increased cultural and social interactions, with a substantial reduc

tion in regional differences in per capita income. The A1 scenario family develops into three groups that describe alternative directions of technologi

cal change in the energy system. The three A1 groups are distinguished by their technological emphasis: fossil intensive (A1FI), non-fossil en

ergy sources (A1T), or a balance across all sources (A1B) (where balanced is defined as not relying too heavily on one particular energy source, on the assumption that similar improvement rates apply to all energy supply and end-use technologies).

A2. The A2 storyline and scenario family de

scribes a very heterogeneous world. The underlying theme is self reliance and pres

ervation of local identities. Fertility patterns across regions converge very slowly, which results in continuously increasing population. Economic development is primarily region

ally oriented, and per capita economic growth and technological change more fragmented and slower than other storylines.

B1. The B1 storyline and scenario family describes a convergent world with the same global population as in the A1 storyline (one that peaks in mid-century and declines thereafter), but with rapid change in economic structures toward a service and information economy, with reductions in material intensity and the introduc

tion of clean and resource-efficient technologies. The emphasis is on global solutions to economic, social, and environmental sustainability, including improved equity, but without additional climate initiatives.

B2. The B2 storyline and scenario family describes a world in which the emphasis is on local solutions to economic, social, and environmental sustainability. It is a world with continuously increasing global popula

tion, at a rate lower than for the A2 storyline, interme

diate levels of economic development, and less rapid and more diverse technological change than in the B1 and A1 storylines. While the scenario is also oriented toward environmental protection and social equity, it focuses on local and regional levels.

An illustrative scenario was chosen for each of the six scenario groups A1B, A1FI, A1T, A2, B1 and B2. All should be considered equally sound.

The SRES scenarios do not include additional climate initiatives, which means that no scenarios are included that explicitly assume implementation of the United Na

tions Framework Convention on Climate Change or the emissions targets of the Kyoto Protocol.The U.S. Climate Change Science Program Appendices

82 

### E.2 R adiati v e F orcing S tabili z ation L e v els and A pproximate CO 2 

### C oncentrations from the CCSP SAP 2.1 a scenarios ( table 1.2; C lar k e e t al

. , 2007)

The stabilization levels were constructed so that the CO 2 concentrations resulting from stabilization of total radiative forcing in Watts per square meter (W per m 2 ), after account

ing for radiative forcing from the non-CO 2 greenhouse gases (GHGs) included in this research, would be roughly 450 parts per million by volume (ppmv), 550 ppmv, 650 ppmv, and 750 ppmv.

Total Radiative Forcing from GHGs (W per m 2 ) Approximate Contribution to Radiative Forcing from Non-CO 2 GHGs (W per m 2 ) Approximate Contribution to Radiative Forcing from CO 2 (W per m 2 ) Corresponding CO 2 Concentration (ppmv) Level 1 3.4 0.8 2.6 450 Level 2 4.7 1.0 3.7 550 Level 3 5.8 1.3 4.5 650 Level 4 6.7 1.4 5.3 750 Year 1998 2.11 0.65 1.46 365 Preindustrial 0 0 0 275GLOSSARY AND ACRONYMS

83 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols



## G lossary

aerosols very small airborne solid or liquid particles that reside in the atmosphere for at least several hours, with the smallest remaining airborne for days
anthropogenic human-induced

attribution attribution of causes of climate change is the process of establishing the most likely causes for a detected change with some defined level of confidence

black carbon soot particles primarily from fossil fuel burning
climate sensitivity the equilibrium change in global-average surface air temperature following a change in radiative forcing; in current usage, this term generally refers to the warming that would result if atmospheric carbon dioxide concentrations were to double from their pre-industrial levels
cyclone a storm system that rotates around a center of low atmospheric pressure
tropical cyclone a cyclone usually originating in the tropics, with a warm central core
extratropical cyclone a cyclone originating in the mid or high latitudes, with a cold central core. Larger in scale than a tropical cyclone and with less central intensity
diurnal temperature range the difference between maximum and minimum temperature over a period of 24 hours
El Niño-Southern Oscillation (ENSO) the waxing and waning every two to seven years of El Niño and La Niña ocean temperature cycles along with the related atmospheric pressure component of the Southern Oscillation; the primary centers of ENSO variability are in the tropical Pacific, but ENSO effects can be felt across much of the globe forcing a nature or human-induced factor that influences cli

- mate
greenhouse gases gases including water vapor, carbon dioxide, methane, nitrous oxide, and halocarbons that trap infrared heat, warming the air near the surface and in the lower levels of the atmosphere

halocarbons chemical compounds that contain at least one carbon atom and at least one halogen atom, such as chlorine, fluorine, bromine, or iodine
inhomogeneity a break or interruption in an otherwise homogeneous record; for example, moving a weather station from the center of a city to the suburbs will create an inhomogene

- ity in the climate record

monsoon a seasonal change in wind direction (driven by changes in temperature), often accompanied by a seasonal precipita

- tion maximum
organic carbon particles consisting predominantly of organic compounds, mainly carbon, hydrogen, oxygen and lesser amounts of other elements
parameterization a mathematical representation of a process that cannot be explicitly resolved in a climate model
stratosphere the highly stratified region of the atmosphere above the troposphere extending from about 10 kilometers (km) (ranging from 9 km in high latitudes to 16 km in the tropics on average) to about 50 km
troposphere the lowest part of the atmosphere from the surface to about 10 km in altitude in mid-latitudes (ranging from 9 km in high latitudes to 16 km in the tropics on average) where clouds and “weather” phenomena occur, in the tro

- posphere, temperatures generally decrease with heightThe U.S. Climate Change Science Program Glossary and Acronyms

84 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols


## A cronyms A ERONET Aerosol Robotic Network AIM Asian-Pacific Integrated Model AOD Aerosol Optical Depth AOGCM Atmosphere-Ocean (-Sea Ice) General Circulation Model AR4 IPCC Fourth Assessment Report ARL Air Resources Laboratory AVHRR Advanced Very High Resolution Radiometer BC black carbon CaCO 3 calcium carbonate CAM3 Community Atmosphere Model CCSM Community Climate System Model CCSP Climate Change Science Program CH 4 methane CO 2 carbon dioxide DMS dimethylsulfide 

### D U Dobson unit ENSO El Niño-Southern Oscillation EPA Environmental Protection Agency ESM Earth System Model FAR First IPCC Assessment Report GCM General Circulation Model/Global Climate Model GFDL Geophysical Fluid Dynamics Laboratory GHG greenhouse gas GISS Goddard Institute for Space Studies hPa hectopascal HNO 3 nitric acid HO 2 hydroperoxyl radical H 2 O 2 hydrogen Peroxide IAM Integrated Assessment Model IGSM MIT Integrated Global System Model IMAGE Integrated Model to Assess the Greenhouse Effect IPCC Intergovernmental Panel on Climate Change LL long-lived mW per m 2 milliwatts per square meter (after MAGICC) MACCM3 NCAR Middle Atmosphere Community Climate Model, version 3 MAGICC Model for the Assessment of Greenhouse-gas Induced Climate Change MODIS Moderate Resolution Imaging Spectroradiometer MOZART Model for Ozone and Related Chemical Tracers NASA National Aeronautics and Space Administration NCAR National Center for Atmospheric Research NCDC National Climatic Data Center NH Northern Hemisphere NH 3 ammonia NMHC non-methane hydrocarbons N 2 O nitrous oxide N 2 O 5 nitric pentoxide NOAA National Oceanic and Atmospheric Administration NO 2 nitrogen dioxide NO 3 nitrate radical NO x reactive nitrogen oxides NRC National Research Council NSF National Science Foundation O 3 ozone OC organic carbon ppbv parts per billion by volume ppm parts per million ppmv parts per million by volume pptv parts per trillion by volume RF radiative forcing SAP Synthesis and Assessment Product SAR IPCC Second Assessment Report SH Southern Hemisphere SL short-lived SO 2 sulfur dioxide SO 4 sulfate SOA secondary organic aerosol SRES Special Report on Emissions Scenarios SST sea surface temperature TAR Third IPCC Assessment Report Tg teragrams, 1Tg = 1 x 10 12 grams TUV Tropospheric Ultraviolet and Visible Radiation Model VOCs Volatile Organic Compounds W per m 2 watts per square meterREFERENCES

85 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols



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## P hotography credits

Cover/Title Page/Table of Contents Cover, table of contents, page 61, Image for chapter 4, (Smoke plume) Grant Goodge, STG Inc., Asheville, N.C.

Executive Summary Page 5, (Hanging smog) Grant Goodge, STG Inc., Asheville, N.C.

Chapter 2 Page 18, (Aerial blue clouds, haze) Grant Goodge, STG Inc., Asheville, N.C.97 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

Chapter 3 Page 29, (Golden fog in valley) Grant Goodge, STG Inc., Asheville, N.C. Page 35, (Mountains between clouds) Grant Goodge, STG Inc., Asheville, N.C. Page 54, (Aerial view smog over highway) Grant Goodge, STG Inc., Asheville, N.C. Page 57, (Philippines traffic) Deborah Misch, STG Inc., Asheville, N.C. Page 58, (Satellite image) Jeff Schmaltz, NOAA/Modis.The U.S. Climate Change Science Program References

9899 Climate Projections Based on Emissions Scenarios for Long-Lived and Short-Lived Radiatively Active Gases and Aerosols



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