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 providing 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 Treasury 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 Administration 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 Integration
TABLE 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 ............................................................................51
3.4 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 Chemistry............................................... 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).....................................71

TABLE OF CONTENTS
Appendix 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......................................................................................................................................... 85
The 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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SYNOPSIS

The 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 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 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. 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 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 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 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. Horowitz, (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. 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.
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 Research, Washington, D.C.
The U.S. Climate Change Science Program Preface

VIII
IX

Climate Projections Based on Emissions Scenarios for 

Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

PREFACE

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.

Introduction

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 incorporating 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 Management and Budget. 

A primary objective of the U. S. CCSP is to provide the best possible scientific information to 

support public discussion and government and 

private 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 highest 

priority research, observation, and decisionsupport needs.

The CCSP is conducting 21 such activities, covering topics such as the North American carbon 

budget and implications for the global carbon 

cycle, 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 effects 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 synthesis 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.

Background and Goals

The initial mandate for Synthesis and Assessment Product 

3.2 (SAP 3.2), which is still listed on the official CCSP 

website <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 Program”. 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 scenarios1 were for the 

long-lived2, 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 coherent 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 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 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 explicitly address the direct influence of short-lived3 drivers of 

climate: carbon and sulfate particles and lower atmospheric 

ozone. Therefore, we expanded our mandate to include 

“Short-Lived Radiatively Active4 Gases and Aerosols5”. 

These short-lived gases and aerosols (particles) are largely 

of 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 pollutants in the lower atmosphere: ozone and three types of 

particles (soot/elemental carbon, organic carbon, and 

sulfate), usually identified in scientific terms as aerosols5. 

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 remaining 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 conditions) 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

ReaDER’S GUIDE TO SYNTHESIS AND 

ASSESSMENT PRODUCT 3.2

This Product includes an Executive Summary and four 

Chapters.

The Executive Summary presents the key results and findings, 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 contains 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, MAGICC6, 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 representative 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 composition models7 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 

models8, 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 result7 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, identifies 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 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-lived1

(e.g., carbon dioxide) and short-lived2 (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 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.

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 atmosphere 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

New Results and Findings

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 assessment 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.

1. Our results suggest that changes in short-lived gases and particles (pollutants) may significantly 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 Section 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). 

2. The geographic patterns of factors that drive climate change due to short-lived gases and 

particles and the patterns of the resulting surface temperature responses are quite different. 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 control strategies for short-lived 

pollutants will thus have global 

impacts on climate. The geographic 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.

3. Reductions of short-lived pollutants 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 KEY 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 difference, 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 reduction of global warming. Reduction in 

emissions from surface transportation in 

North America would have a similar impact. 

4. The three comprehensive climate models3, 

their associated chemical composition 

models4 and their differing projections of 

short-lived emissions all lead to a wide range 

of 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 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 are found to be the projection 

of future emissions and the determination of 

the indirect effect5 of particles on clouds. The 

fundamental difference between uncertainties 

in projecting future emissions and uncertainties in processes, such as the indirect effects 

of particles, is discussed in Section 4.3.

5. The range of plausible short-lived emissions projections is very large, even for a 

single well-defined global emission scenario (see Figures and discussion in Section 

3.2 for details). Figure ES.3 clearly demonstrates this situation for the different

emission projections of black carbon particles (soot) used by the three research 

groups. This currently limits our ability 

to provide definitive statements on their 

contribution to future climate change. 

6. Natural particles such as dust and sea 

salt also play an important role in climate 

and their emissions and interactions differ 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 distributions 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 emissions 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) radiative 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, leading 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 studies. This is discussed in Section 3.2.4 and demonstrated in Figure 3.2 and Table 3.5.

7. Climate projections based on new emissions scenarios where atmospheric concentrations 

of the long-lived greenhouse gases level off, or stabilize6, 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 Recommendations for

Future Research

The five most critical areas for future research 

identified in this Report are: 

1. The projection of future human-caused 

emissions for the short-lived gases and 

particles; 

2. 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.

1. Plausible emissions scenarios for the second 

half of the twenty-first century show significant 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 particles is currently extremely difficult to predict. 

Improvements in our ability to predict social, 

economic and technological developments affecting 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 climate modeling community as a whole cannot yet produce a credible characterization 

of the climate response to particle/cloud 

interactions. All models (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 observations 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 different 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 mechanisms 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 Guide FOR Readers

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 descriptions 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 language 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.
Introduction

CHAPTER

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

 

PROLOGUE

Comprehensive climate models1 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-lived2 greenhouse gases and short-lived3 radiatively active4

 gases and particles. A number of standard emissions scenarios5 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 components, 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 treatments 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 Historical Overview

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 concentration of carbon dioxide. Model development continued on a wide range of numerical 

models, especially in the increasing sophistication of the ocean model.

In 1979, Manabe and Stouffer developed a 

global model at NOAA’s Geophysical Fluid 

Dynamics Laboratory (GFDL) useful for estimating 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 

Report6

 (IPCC, 1990).

In the late 1980s, Washington and Meehl (1989) 

at NCAR and Stouffer et al. (1989) at GFDL 

developed the first comprehensive climate models (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 perform 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 Assessment 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.) developed 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 emissions 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 Assessment Report of Working Group I (IPCC, 

2007), 24 comprehensive climate models participated. The component models continued to 

become more sophisticated and included more 

physical processes. The new components allowed 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 Goals and Rationale

As described in the Prospectus outlining the 

purpose of this report, Synthesis and Assessment Product 3.2 has two primary goals:

1. Produce climate projections for research 

and assessment based on the stabilization 

emission scenarios of long-lived greenhouse 

gas emissions developed by Synthesis and 

Assessment Product 2.1a. 

2. Assess the sign, magnitude, and duration 

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 (CO2) emissions

 scenarios? 
The U.S. Climate Change Science Program Chapter 1

10

This Report focuses 

on the direct 

effect 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 intended 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. 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 control. 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 Limitations

The first goal, assessing the climate projections 

for the SAP 2.1a stabilization emissions scenarios for long-lived greenhouse gases, is relatively 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 effect8 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 effects9

of pollutant particles on climate, nor do we address 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 held 

constant 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 radiative forcing by long- and short-lived greenhouse 

gases and particles.

1.4 Methodology

In addressing the questions posed above, we 

rely on several different types of computer models to project the climate changes that would 

result from the scenarios of emissions of greenhouse 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 concentrations 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 discussed 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 addressing 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 listed 

in section 1.2, we focus on the effects of shortlived radiatively active gases and particles, and 

use three different state-of-the-art comprehensive 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 comprehensive 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 Terms and Definitions

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 “stabilization 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 formulation 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, chemical, 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 Atmospere-Ocean General Circulation Models) include interacting components describing atmospheric, oceanic, 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 development paths and rates of technological innovation, population growth, and social-political 

development. Each group used somewhat different 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 exception 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 different, 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 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. 

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: Useful Definitions
The U.S. Climate Change Science Program Chapter 1

14

The regional 

patterns of climate 

change 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 atmospheric lifetimes ranging from a decade to more 

than a century. As a result, they are uniformly 

mixed and their radiative forcing is also relatively 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 concentrated 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 intended primarily for the scientific community, 

though the Questions and Answers and the 

Introduction sections of each chapter are written 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.
Climate Projections from 

Well-Mixed Greenhouse Gas 

Stabilization Emission Scenarios 

CHAPTER

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

 

QUESTIONS 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 scenarios1 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. 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 Intergovernmental 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 forcings2

, 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 model3 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 Introduction

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:

1. Three integrated assessment models4 that 

were used in Synthesis and Assessment 

Product 2.1a to produce stabilization emissions scenarios for long-lived greenhouse 

gases; 

2. A simplified global climate model, Model 

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 active6

gases and particles, global-average surface 

temperature increases and global-average 

sea-level rise (due only to thermal expansion of water, not melting ice caps) for the 

four stabilization emissions scenarios. 

The second section, 2.2, introduces the stabilization 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 (CO2) 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 [SF6]). 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 physical, 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 substance to either absorb or emit sunlight or infrared 

radiation, thus changing the temperature of the atmosphere.

million (ppm) of carbon dioxide, and attainment 

was required within 100 to 200 years. For reference, 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 scenario, as well as four stabilization emissions 

scenarios for long-lived greenhouse gas emissions that required a range of policy choices. 

The scenarios generated by each integrated assessment 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 simplified 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 stabilization emissions scenarios introduced in the 

previous section 2.2. While the three integrated 

assessment models used in Synthesis and Assessment 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 temperature 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 concentrations 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 greenhouse 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 contribution of the short-lived pollutants (ozone, 

elemental and organic carbon particles and 

sulfate particles) to radiative forcing calculations by MAGICC for the stabilization emissions 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 pollutants 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 emissions 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 series: 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 warming 

in the twentyfirst century 

shows 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 patterns 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 Assessment Product 2.1a. 

2.2 Well-Mixed Greenhouse

Gas EmissionS Scenarios

From SAP 2.1a

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 different levels of complexity in their modeling of 

socioeconomic, energy, industry, transport, and 

land-use systems. With respect to emissions, 

EPPA and MiniCAM are similarly comprehensive, and produce output for emissions of 

the following: all the major greenhouse gases 

(carbon dioxide [CO2], methane [CH4], nitrous 

oxide [N2O], and a suite of halocarbons and 

sulfur hexafluoride [SF6]); sulfur dioxide (SO2), 

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 tropospheric ozone change. MERGE produces emissions output for the major greenhouse gases and 

idealized short-lived and long-lived halocarbons 

(characterized by HFC-134a and SF6), 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 CO2 and 

for the other gases that are potentially controlled 

under the Kyoto Protocol (CH4, N2O, halocarbons, and SF6). 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 particles, carbon (both elemental and organic) and 

the precursors of sulfate particles and tropospheric 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 CO2-

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 

m2 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 CO2 stabilization at roughly 450 ppm.

As SAP 2.1a (Clarke et al., 2007) notes, “The 

three models display essentially the same relationship between greenhouse gas concentrations and radiative forcing, so the three reference scenarios also all exhibit higher radiative 

forcing, growing from roughly 2.2 W per m2

above preindustrial in 2000 for the Kyoto Gases 

to between 6.4 W per m2 and 8.6 W per m2 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 represent 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 cumulative emissions than to annual emissions. In 

particular, this is true for CO2, the gas responsible for the largest contribution to radiative 

forcing. This relationship can be seen for CO2

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 CO2 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 CO2 uptake, the aggregate 

response to increased emissions is very similar. 

This basic linear relationship also holds for 

other long-lived gases, such as N2O, SF6, 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, radiative 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 components. 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 Gas7

radiative forcing, and total radiative forcing as7

 “Kyoto Gases” refers to those long-lived greenhouse 

gases covered by the Kyoto Protocol (carbon dioxide, 

methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, 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 Simplified Global

Climate Model (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-balance 

model 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 CO2, CH4, N2O, 

a suite of halocarbons, and SF6. The carbon 

cycle model includes both CO2 fertilization and 

temperature feedbacks, with model parameters 

tuned to give results consistent with the other 

carbon cycle models used in the Third Assessment 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 emissions 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 complex 

global 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 CO2
21

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 Long-Lived Greenhouse

Gas Concentrations and

Radiative Forcings

Figure 2.1 compares the concentrations of the 

primary greenhouse gas, CO2, calculated by 

MAGICC for the 12 SAP 2.1a stabilization 

emission scenarios with earlier calculations of 

CO2 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 CO2 lifetime, 

although the extreme Level 1 (L1) scenarios 

start to separate noticeably by 2030. By year 

2100, CO2 concentrations for the MiniCAM 

and EPPA Level 1 scenarios have converged on 

values close to 450 ppm. For MERGE, the 2100 

value is lower. CO2 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 lowest CO2 for Levels 2 through 4. The CO2 levels 

for the lower bound Level 1 scenario, which 

requires immediate reductions in CO2 emissions 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 Assessment Report (IPCC, 2001) for the B1, A1B, 

and A2 storyline emission scenarios. The evolution of the 12 radiative forcing time series over 

the twenty-first century is very similar to that 

of CO2, in Figure 2.1, which should not be surprising. 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 integrated 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 targets 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 CO2 concentrations (ppm) calculated by MAGICC for the 12 SAP 

2.1a stabilization emission scenarios (Clarke et al., 2007) plotted with calculations 

of CO2 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 m2) 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 

CO2 lifetime.
The U.S. Climate Change Science Program Chapter 2

22

2.5 Short-Lived Gases

and PArticles and Total

Radiative Forcing

While EPPA and MiniCAM produce emissions 

of sulfur dioxide (SO2), elemental or black 

carbon and organic carbon particles and their 

precursors, and the key precursors of tropospheric 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 emissions for the elemental and organic particles, 

it is assumed that the elemental and organic 

particle radiative forcings track the sulfur dioxide 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, tropospheric ozone, halocarbons controlled under 

the Montreal Protocol, and stratospheric ozone 

(Wigley et al., 2008 and supplementary material referenced therein) with the total radiative 

forcing calculated by MAGICC for the B1, A1B, 

and A2 scenarios used in the IPCC Fourth Assessment Report (IPCC, 2007a). Again, just as 

for CO2 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 emissions, which produce significant increases in 

total radiative forcing by 2100 (approximately 

1 W per m2). In the EPPA stabilization emission 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 significantly 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 m2 ) 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 Surface Temperature: 

MAGICC AND IPCC 

Comparisons

Figure 2.4 compares multi-model global-mean 

surface temperature changes reported in Chapter 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 scenarios (Clarke et al., 2007). As we might expect, 

the general behavior is quite similar to that observed 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 SO2

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 temperature 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 CO2,

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 CO2) 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 stabilization emission scenarios are well bounded 

by the comprehensive climate model simulations 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 Climate Projections for

SAP 2.1a Scenarios

The 2.1a stabilization emissions scenarios 

(Clarke et al., 2007) are derived in a fundamentally 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 emissions 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 temperatures 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 surface 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 geographical 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 preserved 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 and 

with increases in the concentration of the longlived greenhouse gases. 

Scenario

CO2

(ppm)

(Figure 2.1)

Kyoto Gases 

Radiative 

Forcing

(W per m2

)

(Figure 2.2)

Total

(W per m2

) 

(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 arrived at in Chapter 10 of the Fourth Assessment 

Report (Meehl et al., 2007) regarding the predicted 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 conclusions 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 uncertainty 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 determination 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 expected 

for 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 simulations and across scenarios as they are for surface temperature.

• Multi-model projections based on SRES scenarios give reductions in ocean 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.

• 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: Uncertainty
CHAPTER

3

27

Climate 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

QUESTIONS 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 Introduction

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 the 

projected 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 chemical 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 evolution 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 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 processes are more important.

The third part of Chapter 3, Section 3.3, discusses the three global comprehensive climate 

models (Geophysical Fluid Dynamics Laboratory [GFDL]; Goddard Institute for Space Studies [GISS]; Community Climate System Model 

[CCSM] developed in part at the National Center for Atmospheric Research [NCAR]) that 

have been used to calculate the impact of the 

short- and long-lived gases and particles1

 on 

the climate, focusing on the changes in surface 

temperature and precipitation. Supplementing 

the climate model results are calculations of 

the changes in radiative forcing2 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 extensive 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, discusses the effects of changes in regional particle 

and ozone and particle precursor emissions, using models that separate emissions by economic 

sector. The results show that regional air quality 

emissions control strategies for short-lived pollutants 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 atmosphere).

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 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 details, 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 EmissionS Scenarios

and Composition Model

Descriptions

3.2.1 Emissions Scenarios

The long-lived (well-mixed) greenhouse gases 

included in this study were carbon dioxide 

(CO2), nitrous oxide (N2O), methane (CH4), and 

the minor gases (chlorofluorocarbons, sulfur 

hexafluoride). Projected global mean values 

were prescribed following the A1B “marker” 

storyline emissions scenario for all three modeling groups. Emissions for anthropogenic 

sources of particles and precursor gases and 

particles for all three composition model calculations were based on an international emissions inventory maintained in the Netherlands 

(Olivier and Berdowski, 2001). 

Though the three groups in this study all prescribed future emissions following a specific 

socioeconomic scenario (A1B) that was highly 

studied in the Fourth Assessment by the Intergovernmental 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 sufficient 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 (Figure 3.1, Table 3.1). For sulfur dioxide (SO2), the 

precursor to sulfate particle, the emissions follow reasonably similar trajectories, with globally 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 compared with GFDL. Thus even two composition 

models using anthropogenic emissions projections from the same integrated assessment 

model show large differences in the evolution of 

their total emissions, presumably owing to differences in the present-day emissions inventories. At 2050, the GFDL model has substantially 

reduced emissions compared with 2000, while 

the other models show enhanced emissions relative 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 emissions decreasing dramatically (~70 percent) by 
The U.S. Climate Change Science Program Chapter 3

30

2050 in one integrated assessment model 

(MINICAM) while 

decreasing only moderately (~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 particle emissions, which 

were not provided by 

any of the integrated assessment models. We 

focus on black carbon (BC) as the more important radiative perturbation. For this particle 

(and for organic carbon [OC]), the GFDL composition model uses the IPCC recommendation 

to scale carbonaceous particle emissions to 

carbon monoxide emissions, leading to substantial 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 SO2. On the other 

hand, the GISS group used emissions projections 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 primary precursor in most regions, NOx (nitrogen 

oxides = NO + NO2), 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). Hydrocarbons and carbon monoxide show analogous differences. 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

NOx (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

SO2 (Tg SO2 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 1012 g) include both natural 

and anthropogenic sources. Values in parentheses are changes relative to 2000.

Figure 3.1 A1B emissions trends used in the three models for SO2 (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 exception 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 produce short-lived gas and particle concentrations 

for the GFDL, GISS, and CCSM climate models 

were driven by the emissions projections discussed 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 concentrations 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 temperature, moisture, and wind). Further details about 

the chemical composition models are provided 

in Appendix C. 

3.2.2.1 Geophysical Fluid Dynamics

Laboratory (GFDL)

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 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 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). Further details on the MOZART model are found 

in Appendix C, in the section on Geophysical 

Fluid Dynamics Laboratory.

3.2.2.2 Goddard Institute for Space

Studies (GISS)

The configuration of the GISS composition 

model used here has been described in detail 

in (Shindell et al., 2007). In brief, 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., 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, composition, and performance are found in Appendix 

C, in the section on Goddard Institute for Space 

Studies. 

3.2.2.3 community climate system

model (CCSM)

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 NOx, methane and volatile 

organic compounds (VOCs) and these average 

global 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 SO2 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 Burden

The 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 use 

identical 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 annual 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 dioxide 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 (SO2) 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 SO2 emissions (Table 3.4). This ratio can be analyzed 

in terms of the total sulfur dioxide burden (in 

Tg [teragrams]) per SO2 emission (in Tg per 

yr); the change in SO2 burden per SO2 emission 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 SO2 to sulfate (versus 

physical removal of SO2 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, accounting for at least some of the 10 to 15 percent 

difference in residence times. Sulfate production can vary even more from model to model, 

as its production from the emitted sulfur diSpecies 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 SO4

=) 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. Values 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), probably 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 SO2 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 SO2 decrease, a 

larger fraction of the sulfate production comes 

from DMS oxidation rather than 

from emitted SO2. 

The conversion 

efficiency from 

SO2 to sulfate 

also varies over 

time in the GISS 

model, decreasing to 2030 and 

increasing thereafter (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 variations 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 carbonaceous 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 SO2 per year emitted as SO2. Ozone ratios are in Tg ozone 

divided by Tg N per year emitted as NOx. 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 1012 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 different 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). Examining the ozone changes relative to the NOx

emissions changes, there are very large differences 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 NOx 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 O3-

NOx chemistry (Stewart et al., 1977), and it 

has been shown that the ozone production efficiency can vary substantially with time (Lamarque 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 NOx 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 difference 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 optical 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 similar 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. Instead, 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 predominantly due to sea salt. The relative contribution 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 contribution 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 optical 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 anthropogenic 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 Climate Studies

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 composition model. For the first ensemble, the GFDL 

simulations used particle and ozone distributions 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 both 

sets 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 Geophysical Fluid Dynamics

Laboratory (GFDL)

Climate simulations at GFDL used the comprehensive 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 evolution of observed surface temperature for the 

twentieth century as well as many continental-scale features (Knutson et al., 2006). Its 

equilibrium climate sensitivity to a doubling 

of CO2 is 3.4°C4

 (Stouffer et al., 2006). The 

model 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 Goddard Institute for Space

Studies (GISS)

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 

CO2 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 Goddard Institute for Space Studies.

3.3.2.3 community climate system

model (CCSM)

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 CO2 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 Global and Hemispheric Average

Values: GFDL and GISS

Radiative forcing calculations were performed 

by GFDL (adjusted forcing) and GISS (instantaneous 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 forcing that is largely balanced by positive forcings 

from increased black carbon particle and ozone. 

In the GISS model, increased sulfate and reduced 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 forcing 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 contribution to the latter from reduced levels of black 

carbon. Both models show a partial cancellation 

of the black carbon forcing by an opposing forcing 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 calculation of radiative properties in the models. This 

can be seen clearly by examining the radiative 

forcing-to-burden ratio, which we term the radiative 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 location of projected black carbon changes as well 

as differing treatments of the radiative properties 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 m2) 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 m2) 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 diagonal 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 location 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 agreement 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 concentrations (Table 3.2) due to increased NOx emissions 

(Table 3.1) and partially from the recovery of 

stratospheric ozone due to reductions in emisTable 3.6 Global mean radiative forcing for short-lived gases and particles 

in Watts per square meter (W per m2

).

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 m2

)/

Tg

(W per m2)/

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 NOx

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 NOx

emissions.

Thus, at 2030, differences in the physical 

processes in the two models dominate the differences in radiative forcing between the two 

models. The large divergence in radiative forcing from sulfate stems from both the chemical 

conversion efficiency of SO2 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 uncertain. 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 differences 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 reduced 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 Regional Forcing Patterns

The differences in hemispheric and global forcings 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 emissions. 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 

SO2 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 precursor 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 m2).

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 differences 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 m2) near 2050 relative to 2000 for 

the indicated individual short-lived gases and particles in the GISS (left) and GFDL (right) models. Radiative 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 emissions scenarios of the two models, this time 

primarily over African biomass burning 

regions. As discussed previously, the GFDL 

model assumed that biomass burning emissions 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 photochemistry. 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 simulations 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 overlying stratospheric ozone column. The GFDL radiative forcing is similar to results from models 

with tropospheric ozone only and no particle 

indirect effects (Gauss et al., 2003). 

3.3.3.3 Effects of uncertainties in

methane concentrations on radiative forcing

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 assessment 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 allow comparisons with the AIM results used in 

the results in this chapter. Natural spatially and 

seasonally varying emissions and soil adsorption were the standard amounts described in 

Shindell et al. (2003). Both the methane emissions 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 troposphere and stratosphere. Thus methane can 

affect its own lifetime (which is primarily governed 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 assessment 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 asThe 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 precursor 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 allowed to respond to climate change, increased 

competition from isoprene and increased methane 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 internal 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 N2O 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 

m2 while using the larger methane concentrations of 2.86 or 3.21 ppmv calculated with our 

model gives 0.36 or 0.46 W per m2

, respectively. 

Of course, it is difficult to estimate methane’s 

abundance at a particular time without performing a full transient methane simulation. However, uncertainty in the forcing from methane 

appears to be at least 0.1 to 0.2 W per m2. 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 integrated 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 overestimate future atmospheric concentrations. 

However, there are indications that the growth 

rate decrease was primarily due to reduced anthropogenic 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 NOx from lightning and from soil and dimethyl-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 troposphere 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 removal of trace gases and particles. Further work 

is required to gauge the importance of these and 

other climate-chemistry feedbacks.

3.3.3.4 Effects of Climate Change on

Radiative Forcing

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 m2

 in 2050. Climate change slightly 

increased the negative forcing from sulfate (by 

.01 W per m2), 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 forcing (approximately 0.02 W per m2

). However, 

emissions in the model respond only to changes 

in surface wind speeds, and not to changes 

in sources due to either CO2 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 determined 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 species 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 Surface Temperature Changes

The global-mean annual-mean surface temperature 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 significant5 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 significant5

 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 

m2 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 m2 per year]) owing to the presence of the 

particle indirect effect, which contributes additional warming as particle loading decreases 

in the future (Shindell et al., 2007).

Changing levels of short-lived gases and particles 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 emissions 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 Northern Hemisphere in the GFDL model and in 

the Southern Hemisphere in the GISS model 

(Figure 3.6). The Northern Hemisphere warming 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 simulations 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 m2 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 particles 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 m2 in 2050 (Shindell 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 m2. 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 Hemisphere (-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 particles 

are 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 contribution from differences in particle modeling as 

discussed previously. Additionally, the climate 

response of each model is different to some extent 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 different assumptions about emissions trends.

At smaller spatial scales, the annual average 

patterns of surface temperature changes induced 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 warming related primarily to stratospheric ozone recovery. 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 appeared to show a substantial response to ozone 

depletion in analyses of the Southern Hemisphere circulation in IPCC AR4 simulations 

(Miller et al., 2006b). That analysis showed that 

most climate models found a general strengthening 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 cooling 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 diminishes 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 present 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 industrialized 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 radiative 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 continents in the GFDL model, parts of the Southern 

Ocean in the GISS model) though the statistical 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 divergence 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 references therein for further discussion of 

this issue.

3.3.4.2 Precipitation, Sea-level, 

and other variables.

Changes in other climate variables (such 

as precipitation and sea level) due to 

short-lived gases and particles are typically 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 accentuated 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 significance 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 precipitation 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 changes 

induced 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 m2) 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.)
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3.3.4.3 Discussion

In the transient climate simulations, three climate models examined the response to projected changes in short-lived gases and particles. The results shown in Figure 3.7 differ 

substantially among the models, particularly 

by 2050. Comparison has shown that the differences in the underlying emissions projections 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 integrated assessment models. 

Uncertainties in the representation of radiative 

processes are an additional source of differences 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 different physical processes plays a role in the intermodel differences, and is especially important 

near 2030, when sulfur dioxide (SO2) 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 concentrations 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 differences 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 SO2 to sulfate and the sensitivity of ozone to NOx 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 differences, 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 m2

, while the inclusion of sulfate absorption onto dust reduces the 

negative forcing from sulfate at 2030 or 2050 

by up to 0.2 W per m2 (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 surfaces, it would likely have shown a substantially 

greater warming trend (at least at 2030). 

The responses of methane (and other hydrocarbon) 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 projected temperature trends. Additionally, given 

the large influence of uncertainties in emissions projections, we stress that the magnitude 

and even the sign of the effects of short-lived 

gases and particles on climate might be different 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 responses in the same regions of high sensitivity 

rather than short-lived gases and particles having an enhanced effect primarily in or near polluted 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 examining 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 the 

second 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 

(SO2), 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 temperature change in Figure 3.9.

In Figure 3.10 we examine Northern Hemisphere 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 region 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. By 

contrast, 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 warming 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 summertime warming was predicted for 2100. 

Monthly-mean area-averaged values of temperature, 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 water, a quantity that integrates and 

responds to both temperature and 

precipitation. There is a statistically 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 forcing, located primarily over Asia, 

which in turn results from the 

large changes projected by the A1B “marker” 

scenario for Asian emissions of SO2 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 strongest 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 SO2 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 SO2 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 emissions 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 Regional EmissionS 

Sector Perturbations and

Regional Models

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 participated, 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 usage (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 assessing 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 emissions 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 perturbation 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 and 

monoterpenes. 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 simulations 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 average 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 emission 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 sector (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 radiative 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 oxidation 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 ([CH4]′/

[CH4] = 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 emissions 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 m2

), 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 radiative forcing from an individual gas or particle to 

exceed 10 milliWatts per square meter (mW per 

m2) 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 m2 in response to perturbations in developing Asia, with the largest 

response for reductions in black carbon when 

domestic emissions are reduced (-42 mW per 

m2). 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 sulfate, and the Asia domestic sector, whose forcing 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 opposite, 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 experiments. For emissions reduction in the Asia 

domestic sector, the global mean aerosol optical 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 believe 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 m2) due to 30 percent reductions 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 climate 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 parts 

per 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 emissions 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 transportation 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 pollutants at the surface, reducing emissions transport 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 reductions 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 Hemisphere 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 European) 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 perturbations 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 m2

 global mean 

annual average seen in the GISS 

results.

Further work is required to more 

thoroughly characterize the robustness 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 projections 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 m2

) as in Table 3.10 but for summer 

(June 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 surface 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 concentration levels for ozone and small-diameter particles (PM2.5)

6

 exceed threshold standards under 

the United States Clean Air Act, emissions 

control strategies must be developed. Since 

6 PM2.5 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, emissions 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 radiative forcing and climate temperature 

increases. Regional downscaling 

studies shown in this section suggest 

that future changes in regional climate could reduce the benefits from 

anticipated emissions reductions on 

lowering ozone.

Downscaled regional scale climate simulations (e.g., Leung and 

Gustafson, 2005; Liang et al., 2006; 

Liang et al., 2004) rely on a global climate model to provide boundary conditions for the regional domain as well 

as the radiative effect of well-mixed 

greenhouse gases within the domain 

for the radiation calculations. Regionally 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 meteorology, 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) climate on ozone and particles with current emissions scenarios. Biogenic emissions, which are 

meteorologically dependent, were recalculated 
59

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 suggest 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 results 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 suggest that climate change is a factor that needs to 

be considered in air quality 

management.

These regional downscaling climate studies rely 

on climate forcing linkages from global climate 

simulations with future 

trends for long-lived gases including CO2, CH4, 

N2O, and halocarbons. The 

influence of short-lived 

gases and particles on future climate has not been 

included in those studies to 

date; however, more recent 

developments are underway to include direct and 

indirect radiative effects 

in the regional chemistry 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 downscaling climate impacts from 

short-lived gases and particles cannot be directly reported 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 approximately 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 plausible 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.
Findings, Issues, Opportunities, and 

Recommendations 

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

 NASA/GISS; Alice Gilliland, NOAA/ARL

CHAPTER

4

61

Climate Projections Based on Emissions Scenarios for 

Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

4.1 Introduction

This Chapter, which is intended for both technical 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 Findings

The key findings of Synthesis and Assessment 

Product 3.2 are summarized below:

1. Our results suggest that the projected 

changes in short-lived1 radiatively active2

gases and particles may significantly influence 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 atmosphere 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 substance to absorb and re-emit radiation, thus changing 

the temperature of the lower atmosphere.

2. We find that spatial patterns of radiative 

forcing3 and climate response are quite 

distinct. Thus, both short-lived and longlived4

 gases and particles 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. This means 

that regional pollution control strategies 

will have large-scale climate impacts.

3. Reductions of short-lived gas and particle 

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. When 

radiative 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 details, 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 atmosphere divided by the mass that is removed from the 

atmosphere each year.
The U.S. Climate Change Science Program Chapter 4

62

KEY FINDINGS

improvement in local air quality and reduction of global warming. Reduction in emissions from surface transportation would 

have a similar impact in North America. 

4. The three comprehensive climate models5, 

their associated chemical composition models6

 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 studies 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.

5. The two most important uncertainties in 

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 particles7

 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.

6. Natural particles such as dust and sea salt 

also play an important role and their emissions 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 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.

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 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.

7 Particles may have an indirect effect on the climate 

system by modifying the optical properties and lifetime of clouds. A detailed technical discussion is given 

in Box 3.1.

inconsistency among models should be 

addressed in future studies.

7. The SAP 2.1a emissions scenarios (Clarke 

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 

scenario8, which is equivalent to a carbon 

dioxide stabilization level of 450 parts per 

million, results in global surface temperatures below those calculated for the IPCC 

storyline emissions scenario with the most 

moderate carbon dioxide increases (B1), 

particularly beyond 2050.

4.3 Issues Raised

It is important to recognize the difference 

between uncertainties in processes and uncertainties 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. However, 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 differences in the underlying emissions projections. 

8 Stabilization emissions scenarios are a representation 

of the future emissions of a substance based on a coherent and internally consistent set of assumptions about 

the driving forces (such as population, socio-economic 

development, technological change) and their key relationships. These emissions are constrained so that the 

resulting atmospheric concentrations of the substance 

level off at a predetermined value in the future.
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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 

models9 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 issues 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 different 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 suggestion to scale future carbon particle emissions 

by the emissions for carbon monoxide, which 

are 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 chemical 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 disconnected 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 determining 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 projecting 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 forecasting 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 significant 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 demonstrates, 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 problem 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, Direct Effect, Mixing, Water Uptake)

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 is 
The U.S. Climate Change Science Program

64

Chapter 4

very poorly known, is 

probably the most critical. 

Many aspects of the particle-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 approach that is quite crude. 

The modeling community 

as a whole cannot yet produce a credible characterization of the climate 

response to particle/cloud 

interactions. Moreover, 

the measurements needed 

to guide this characterization do not yet exist. All 

mainstream climate models (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 observations to improve the characterization of the 

indirect effect, but major limitations remain.

As discussed in Section 3.2.4, observations of 

aerosol optical depth10 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 composition 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 uncertain, 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 discrepancies. 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 particles that will have to be addressed in future 

research. We raise important issues linking air 

quality control and global warming, but are unable 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 atmospheric carbon dioxide. However, while reductions 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, the 

climate responses to emissions changes in shortlived pollutants can be felt much more quickly 

because of shorter atmospheric lifetimes. The 

good news is that there is at least one clear winwin solution for climate (less warming) and 
65

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 improving 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 monoxide or volatile organic compounds (VOCs) 

have similar effects, namely leading to reduced 

abundances of both methane and ozone (Berntsen 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 potentially 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 complexities 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 carbon 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 quality. 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 quantitatively 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 oxides, are more complex and the net result is not 

clear at this time.

4.4 Research Opportunities

and Recommendations

This last section of the report is a call for focused scientific research in emissions projections, radiative forcing, chemical composition 

modeling and regional downscaling. Particular 

emphasis needs to be paid to the future emissions scenarios for sulfur dioxide, black carbon 

particles and nitrogen oxides, to the indirect radiative 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 report 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 projections 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 constructed. Clarification of the challenges associated with emissions projections (not a simple 

matter of improving quantitative skill, as these 
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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 emissions projections for carbon particles and ammonia 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 greenhouse 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 understand the contribution to uncertainty by the 

composition and climate models, it would also 

be worthwhile to perform a controlled experiment with identical emissions projections using 

multiple composition and climate models.

4.4.2 Particle Studies (Direct Effect, 

Indirect Effect, Mixing, Water Uptake)

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 uptake [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 

CO2, 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 rainfall, 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 aerosols. Measurement by devices such as the aerosol 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 better characterization of particle interactions with 

clouds in the laboratory. We also recommend 

greater use of the distinction between scattering and absorbing particles to characterize their 

relative importance.

4.4.3 Improvements in Transport, 

Deposition, and Chemistry 

The emissions issues become even more problematic when the future distributions employed 

in the comprehensive climate models are generated by multiple global composition models, 

all with differing treatments of mixing in the 
67

Climate Projections Based on Emissions Scenarios for 

Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

lowest layers of the atmosphere, different treatment 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 approximate 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 development. 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 carefully 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 methodological 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 environmental quality and resources applications.

It is important to also note that these studies do 

not currently include short-lived gases and particles in the global climate simulations. Further, 

many regional models do not include feedbacks 

between air quality, the radiation budget and climate. 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 projections, 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 actually 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 response 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 possible, both due to time and resource restraints, 

for this study to explore these additional analyses, we recommend their future study.

The major unfinished analysis question in this 

study is the relative contribution of a model’s 
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Chapter 4

regional climate response, as opposed to the 

contribution from the regional pattern of radiative forcing, to the observed regional change 

in seasonal surface temperature. (See Section 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 independent 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 Conclusion

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 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,” 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 Mean Temperature

All Atmosphere-Ocean General Circulation Models (AOGCMs) in Chapter 10 of the Fourth Assessment 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 sensitivities, 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 generated 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 projections 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 pattern 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 Temperature Extremes

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 Mean Precipitation

Globally averaged mean atmospheric water vapor, evaporation 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 patterns 
The 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 Precipitation Extremes and

Droughts

Intensity of precipitation events is projected to increase, 

particularly in tropical and high latitude areas that experience increases in mean precipitation. There is a tendency for 

drying of the mid-continental areas during summer, indicating 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 Snow and Ice

As the climate warms, snow cover and sea ice extent decrease; 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 variable across the suite of AOGCMs than they are for both the 

global average and the more robust geographic patterns of 

temperature.

A.6 Carbon Cycle

Under the SRES illustrative emissions scenarios, for central 

carbon-cycle model parameters, CO2 concentrations are projected 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 Ocean Acidification

Increasing atmospheric CO2 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 Sea Level

“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 including 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 estimate 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 increased 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 Ocean Circulation

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. Models agree that the MOC will not shut down completely 

(Meehl et al., 2007).

A.10 Monsoons

Current AOGCMs predict that, in a warmer climate, there 

will be an increase in precipitation in both the Asian monsoon (along with an increase in interannual variability) and 
71

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 increase in the Australian monsoon in southern summer. The 

monsoonal precipitation in Mexico and Central America 

is projected to decrease in association with increasing precipitation 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 Tropical Cyclones

(Hurricanes and Typhoons) 

The Summary for Policymakers finds it likely that intense 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 frequency 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 Midlatitude Storms

Model projections show fewer midlatitude storms averaged 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 Radiative Forcing

“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 results 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 m2 at the tropopause. 

However, there is a range of 25 percent in longwave forcing 

due to doubling CO2 from its concentration in 1860 across 

the ensemble of AOGCM codes. There is a 47 percent relative 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 Climate Change Commitment

(Temperature and Sea Level)

“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

72
APPENDIX 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 Greenhouse-gas Induced Climate Change) is a coupled 

gas-cycle/climate model. Various versions of 

MAGICC have been used in all IPCC assessments. 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 model 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 CO2, CH4, N2O, a suite of 

halocarbons and SF6. Details of the carbon cycle 

model are given in Wigley (1991a, 1993, 2000). 

The carbon cycle model includes both CO2 fertilization 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 forcings are included using forcing/emissions relationships 

developed in Wigley (1989, 1991b), with central estimates 

for 1990 forcing values. Sea-level rise estimates use thermal 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 parameters 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 CO2 doubling, the 

median of values for AOGCMs used in the TAR.
The U.S. Climate Change Science Program Appendices

74
APPENDIX C

75

Climate Projections Based on Emissions Scenarios for 

Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

C.1 Geophysical Fluid

Dynamics Laboratory

Composition changes for the short-lived gases and 

particles from 2000 to 2100 in the GFDL experiments 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 methane 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 transformed 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 meteorological 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 NOx 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 diffusion 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 radiative 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 hydrolysis of N2O5 and NO3 on particle surfaces occurs at a rate 

based on the simulated sulfate surface area, with a reaction 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/NOA
The 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 HNO3, 

assuming the particles have a first-order loss rate constant 

equal to 20 percent of that of HNO3 (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 m2 per kg for convective and 

0.04 m2 per kg for stratiform precipitation.

The ozone and particle distributions from these simulations 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 m3 in the remote marine atmosphere to 

13 μg per m3 in polluted regions. In general, the simulated 

concentrations 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 annual 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 simulations 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 Goddard Institute for Space

Studies

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 includes 155 reactions. The model calculates the abundances of 

51 chemicals, 26 of which are transported by the model’s advection 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 important 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 handbook (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 according 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 simulations of DMS, MSA, SO2 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 N2O5 to HNO3, and sulfate and 
77

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 removed 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 entraining 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 NOx 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 composition 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 insoluble 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 National Center for

Atmospheric Research

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 NOx, 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 SO2 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 hydrocarbons) 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 and 
The 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 century, affecting only the photolysis rates and not the amount 

of ozone transported from the stratosphere.

The NCAR regional/sector perturbation simulations (Section 3.4) used a version of MOZART chemical transport 

model (Horowitz et al., 2003) embedded within the Community 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 hydrocarbons (NMHCs). The MOZART particles have been 

extended by Tie et al. (2001, 2005) to include a representation 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 NH3 emissions), and sulfate particles (from 

SO2 and DMS emissions). It also considers the uptake of 

N2O5, HO2, NO2, and NO3 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 radiatively 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 Geophysical Fluid

Dynamics Laboratory

Climate simulations at GFDL used the coupled 

climate model recently developed at NOAA’s Geophysical 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 multiple centuries and a realistic ENSO (Wittenberg

et al., 2006). Its equilibrium climate response to 

a doubling of CO2 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 vertical 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 observations 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 observations 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 Goddard Institute for Space

Studies

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 observations (Schmidt et al., 2006), and has a climate sensitivClimate 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/NOA
The 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 CO2.

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 m2 forcing from the preindustrial 

era to the present, a value chosen to match diurnal temperature 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 National Center for

Atmospheric Research

The transient climate simulations use the NCAR Community 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 CO2 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 The EmissionS Scenarios

of the IPCC Special Report

on EmissionS Scenarios

(SRES) (Nakićenović and 

Swart, 2000)

A1. The A1 storyline and scenario family describes a future world of very rapid economic 

growth, global population that peaks in midcentury and declines thereafter, and the rapid 

introduction of new and more efficient technologies.

Major underlying themes are convergent among 

regions, capacity building, and increased cultural 

and social interactions, with a substantial reduction in regional differences in per capita income. 

The A1 scenario family develops into three groups 

that describe alternative directions of technological change in the energy system. The three A1 

groups are distinguished by their technological 

emphasis: fossil intensive (A1FI), non-fossil energy 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 describes a very heterogeneous world. The 

underlying theme is self reliance and preservation of local identities. Fertility patterns 

across regions converge very slowly, which 

results in continuously increasing population. 

Economic development is primarily regionally 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 introduction 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 population, at a rate lower than for the A2 storyline, intermediate 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 Nations Framework Convention on Climate Change or the 

emissions targets of the Kyoto Protocol. 
The U.S. Climate Change Science Program Appendices

82

E.2 Radiative Forcing Stabilization

Levels and Approximate CO2 

Concentrations from the CCSP 

SAP 2.1a scenarios (table 1.2; Clarke

et al., 2007)

The stabilization levels were constructed so that the CO2

concentrations resulting from stabilization of total radiative 

forcing in Watts per square meter (W per m2), after accounting for radiative forcing from the non-CO2 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 m2

)

Approximate

Contribution to

Radiative Forcing from

Non-CO2 GHGs 

(W per m2)

Approximate

Contribution to 

Radiative Forcing 

from CO2 

(W per m2)

Corresponding 

CO2

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 0 275
GLOSSARY AND ACRONYMS

83

Climate Projections Based on Emissions Scenarios for 

Long-Lived and Short-Lived Radiatively Active Gases and Aerosols

Glossary

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 height
The U.S. Climate Change Science Program Glossary and Acronyms

84

Acronyms

AERONET 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

CaCO3 calcium carbonate

CAM3 Community Atmosphere Model

CCSM Community Climate System 

 Model

CCSP Climate Change Science Program

CH4 methane

CO2 carbon dioxide

DMS dimethylsulfide

DU 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

HNO3 nitric acid

HO2 hydroperoxyl radical

H2O2 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 m2 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

NH3 ammonia

NMHC non-methane hydrocarbons

N2O nitrous oxide

N2O5 nitric pentoxide

NOAA National Oceanic and 

 Atmospheric Administration

NO2 nitrogen dioxide

NO3 nitrate radical

NOx reactive nitrogen oxides

NRC National Research Council

NSF National Science Foundation

O3 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

SO2 sulfur dioxide

SO4 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 1012 grams

TUV Tropospheric Ultraviolet and 

 Visible Radiation Model

VOCs Volatile Organic Compounds

W per m2 watts per square meter
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Photography 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.
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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.
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