Air Pollution as a Climate Forcing: A Workshop

Workshop Background

James E. Hansen

Global Warming and Climate Forcing Agents

It is now widely recognized that global warming is occurring. The Earth's average temperature has increased about 0.75°C (1.35°F) in the period of good instrumental measurements, that is since about 1880 (1, 2, 3). This warming is not globally uniform, and where it is largest, for example in Alaska, it is beginning to have noticeable effects (4).

A significant portion of observed global warming may be a result of natural climate variability. Indeed, there is evidence that the amount of solar energy received at the Earth increased slightly over the past few hundred years (5), and this may account, at least in part, for recovery from the "little ice age", i.e., the cold period that was experienced, especially in Europe, during the period approximately 1650-1850 (6).

Many of the forcing agents that drive climate change have been measured during recent years, including natural forcings, such as changes of solar irradiance, and human-made forcings, such as increasing greenhouse gases (1, 7). It is clear that human-made forcings have begun to exceed the magnitude of natural climate forcing agents during the past few decades. Global warming in the past 50 years matches well the expectations based on human-made changes of atmospheric composition (7).

Discussions about potential actions to reduce global warming have focused on carbon dioxide (CO2). This is appropriate, because it is the largest single human-made climate forcing now and it is expected to become the dominant forcing over the next century. However, the sum of other human-made forcings that cause warming exceeds the forcing by CO2 (1, 8). Indeed, it has been suggested that climate forcings associated with air pollution, specifically tropospheric ozone (O3), black carbon (BC) aerosols, and methane (CH4) together represent a greater climate forcing than the forcing by CO2 (8). A strong case can be made that the non-CO2 climate forcings deserve greater attention than they have received.

Policy Relevance

The global warming issue can be expected to continue and probably intensify in the future. The basis for this expectation is evidence that the Earth is presently out of energy balance by 0.5-1 W/m2, i.e., the amount of solar energy now being absorbed by the planet exceeds the heat energy being radiated to space. This energy imbalance was initially inferred from climate models (7), but it has been confirmed by observations of the rate of heat storage in the ocean (9). One consequence of this energy imbalance is that, even if steps are taken to slow greenhouse gas emissions, the Earth will continue to warm during the next few decades and effects of global warming can be expected to become more and more noticeable.

Policy makers, if they wish to influence the course of 21st century climate substantially, must consider ways to reduce the growth of both CO2 and non-CO2 climate forcings. Much of the non-CO2 climate forcing is associated with true air pollution, which has large effects on human health and the environment. Thus policy makers will need to consider the global warming problem in a larger context with multiple issues.

Policy makers can address these issues effectively only if they have quantitative composition-specific information about both the climate forcings and the health and environmental impacts of different atmospheric constituents. At present the magnitude and efficacies of many of these air pollution climate forcings are not well quantified. Similarly, although it is estimated that there are of the order of one million deaths per year from air pollution (10, 11), there is little understanding of which constituents are most responsible.

Most air pollutants have short atmospheric lifetimes. One consequence is that their spatial distributions are inhomogeneous, usually most concentrated near their sources. Thus climate effects are expected to be regional as well as global. Better understanding of the nature and distribution of regional effects is needed, as well as information on the transport of pollutants from one country to another.

Workshop Purpose

Impetus for the workshop derived from recognition that actions to address air pollution and climate forcing agents are being made now at all levels of government, in industry, and by consumers. In addition, we recognize the success that was achieved in avoiding increased greenhouse climate forcing from chlorofluorocarbon substitutes, as a result of open communication among scientists, industry, the public, and governments.

Thus our goal is to communicate current knowledge and uncertainties, including linkages between the air pollution and climate problems. In addition, we aim to define the scientific issues more clearly, in the hopes of speeding progress in research and understanding. Primary workshop objectives are to illuminate the role of air pollutants as climate forcing agents, to make the discussion of health and climate impacts more composition specific, and to identify practical actions that can improve air quality and slow the growth of climate forcings. A Statement of Purpose and Specific Objectives were defined with the help of an Organizing Committee.

Statement of Purpose

This workshop aims to illuminate the role of air pollution as a global climate forcing and to place climate-related impacts of air pollution in the context of its other human health and environmental impacts. The scientific scope of the workshop is limited by a focus on the two significant air pollution climate forcings: (1) aerosols, and (2) the greenhouse gases controlled by air pollution, i.e., tropospheric ozone (O3) and methane (CH4).

A theme of the workshop is composition specificity, of aerosols and of the pollutant emissions affecting O3 and CH4. Knowledge of the effects of specific emissio s is needed to determine the most effective strategies to reduce ir pollution effects on climate, health and the environment. We ask what would be required to halt and reverse the growth of these air pollutants, what impact this would have on climate forcing and climate change, what contributions technologies can make to reducing these air pollutants, and what the air quality benefits would be for human health and the environment.

Recognizing that actions to address air pollution and their impact on climate forcings are being made now at all levels of government, in industry, and by consumers, we attempt to communicate current knowledge and uncertainties, along with the important linkages between these two global environmental problems. Recognizing the success that was achieved in avoiding increased greenhouse climate forcing from chlorofluorocarbon substitutes, as a result of open communication, we hope that communication of information from this workshop can help achieve climate and health benefits.

Specific Objectives

  1. Improve and broaden understanding of climate forcing caused by changing amounts of aerosols, tropospheric ozone and methane, by sharing information across disciplines.
  2. Describe human health and environmental impacts of composition specific aerosols and tropospheric ozone.
  3. Identify practical actions and research activities that could improve air quality, slow the growth of climate forcings, and improve our understanding of the linkages between air quality and climate change.

Organizing Committee

Dan AlbrittonCongbin FuA.P. Mitra
John BachmannDaniel GreenbaumJoyce Penner
John BealeJim HansenMichael Prather
Tami BondDaniel JacobV. Ramanathan
Greg CarmichaelMark JacobsonAnne-Marie Schmoltner
Bill ChameidesTodd JohnsonKirk Smith
Armond CohenJack KayeDavid Streets
Paul CrutzenDina KrugerGeorge Thurston
Jay FeinAlan LloydSusan Tierney

Workshop Organization

The workshop was organized into sessions on Gases (Day 1), Aerosols (Day 2), Technology (Day 3), Health and Agricultural Impacts (Day 4), and Summaries of Days 1-4, including discussion of investments with multiple payoffs (Day 5).

Speakers were asked to lay the groundwork for panel discussion of a specific question at the end of each session. For example, the questions for the two sessions on Day 1 were: (1) Is it possible to achieve a global warming success story with methane, i.e., can we achieve a real world scenario with decreasing CH4?, and (2) Is it feasible to halt the growth of global tropospheric O3 or even achieve a reduction?

It was recognized that the questions posed for Days 1-4 cannot in general be answered given our current knowledge. Thus a primary objective on Day 5 was to discuss what research and observations are needed to address these questions, as summarized in Section 2.

Panel Discussions

Day 1: Gases
A. Is it possible to achieve a global warming success story with methane, i.e., can we achieve a real world scenario with decreasing CH4?
B. Is it feasible to halt the growth of global tropospheric O3 or even achieve a reduction?
Day 2: Aerosols
A. Can we decrease BC aerosols enough to alleviate the global warming "bounce" that will accompany "clear sky" initiatives that are expected to reduce reflecting aerosols in places such as the United States and China?
B. Are aerosols responsible for important regional climate anomalies, for example, the increasing tendency for droughts in northern China, Afghanistan and other places?
Day 3: Technology
A. Are technologies available and practicable for reduction of relevant global air pollution (especially BOCs, O3, O3 precursors including CH4, CO, NOX, VOCs) and what are co-benefit implications for CO2? [BOC = black and organic carbon; VOC = volatile organic compound]
B. What are real-world expectations based on case studies for different cities, states and countries? Are there discrepancies with 3A?
Day 4: Human Health and Agricultural Effects
A. Can we differentiate the effects of specific aerosols and gases on human health?
B. Can we compare quantitatively the impacts of specific air pollutants (e.g., with an economic measure) ideally including other (agricultural and ecosystem) effects, i.e., can we make a "bar chart" comparison?
Day 5: Summaries
A. Summary of Days 1-4 (see Section 2). What research and observations are needed to address the questions defined above for Days 1-4.
B. Discussion of potential investments with multiple payoffs.


"Scientists have cast great darkness on the matter. If they continue their investigations, we shall soon know nothing at all on the topic." (12). During the workshop it was reported on CNN television that 70 percent of Americans do not understand "the scientific method". The practice of challenging every assertion and conclusion is not always recognized as a vehicle to improved understanding. And even as answers are developed to some questions, these only raise more questions.

In view of the workshop objectives, described above, it is important that we do a better job of communicating the science. Decisions are being made now that affect air pollution and climate forcings. Despite our imprecise knowledge of climate forcings and other effects of air pollutants, such decisions should be made with awareness of current scientific understanding.

This workshop report, including the session summaries (Section 2), the presentations (Section 3), most of which are available on-line at, and the extended abstracts (Section 4), is intended as a contribution to this communication. Papers corresponding to some of the workshop presentations are being submitted to appropriate journals. A list of such papers, which will be kept updated on the world wide web version of this workshop report, is given below.


Thanks to all participants for their contributions, and my apologies for ending the chance of further improvements by sending the report to the printer. All summaries were vetted with all attendees, yet as, Michael Prather notes, the summaries "may not necessarily represent a 'consensus'... but are based on the final day discussion". Tami Bond rightly decries the absence of more effort to "merge" the summaries "as much of the 'oomph' of the message is in the cross-benefits". I apologize for all shortcomings, which I hope can be corrected at our next workshop. I look forward to it. Thanks again.

Associated Publications

The following journal publications are associated with workshop presentations:

  • Chung, S. H., and J. H. Seinfeld 2002 Global distribution and climate forcing of carbonaceous aerosols, J. Geophys. Res., in press.
  • Denier van der Gon, H.A.C., M.J. Kropff, N. van Breemen, R. Wassmann, R.S. Lantin, E. Aduna, T.M. Corton, and H.H. van Laar 2002. Optimizing grain yields reduces CH4 emissions from rice paddy fields. Proc. Natl. Acad. Sci. 99, 12021-12024.
  • Edwards, R.D., K.R. Smith, J. Zhang, and Y. Ma 2002. Models to predict emissions of health-damaging pollutants and global warming contributions of residential fuel/stove combinations in China. Chemosphere: Global Change Sci., in press.
  • Edwards, R.D., K.R. Smith, J. Zhang, and Y. Ma 2002. Implications of changes in household stoves and fuel use in China. Energy Policy, submitted.
  • Menon, S., J. Hansen, L. Nazarenko, and Y. Luo 2002. Climate effects of black carbon aerosols in China and India, Science 297, 2250-2253.
  • Novakov, T., V. Ramanathan, J.E. Hansen, T.W. Kirchstetter, M. Sato, J.E. Sinton and J.A. Sathaye 2003. Large historical changes of fossil-fuel black carbon aerosols, Geophys. Res. Lett., 30, no. 6, 1324.
  • Rotstayn, L.D., and U. Lohmann 2002. Tropical rainfall trends and the indirect aerosol effect, J. Climate, in press.
  • Siegl, W.O., T.J. Wallington, M.T. Guenther, T. Henney, D. Pawlak and M. Duffy 2002. R-134a emissions from vehicles. Environ. Sci. Tech. 36, 561.


  • 1. Houghton, J.T., et al., Climate Change 2001: The Scientific Basis (Cambridge Univ. Press, Cambridge/New York, 2001).
  • 2. Jones, P.D., et al., Rev. Geophys. 37, 173-199, 1999.
  • 3. Hansen, J., et al., J. Geophys. Res. 106, 23947-23963, 2001.
  • 4. IPCC Working Group II, Chapter 5 (sec. 5.9) and Chapter 6 (sec. 6.4.6), 2001.
  • 5. Lean, J.L., et al., J. Geophys. Res. 102, 29939-29956, 1997.
  • 6. Shindell, et al., Science 292, 2149-2152, 2001.
  • 7. Hansen, J., et al., J. Geophys. Res. 102, 25679-25720, 1997; 107, in press, 2002.
  • 8. Hansen, J., et al., Proc. Natl. Acad. Sci. 97, 9875-9880, 2000; 98, 14778-14783, 2001; Clim. Change 52, 435-440, 2002.
  • 9. Levitus, S., et al., Science 287, 2225-2229, 2000.
  • 10. Smith, K.R., et al., Thorax 55, 518-532, 2000.
  • 11. Holgate, S.T., et al., Air Pollution and Health, Academic Press, New York, 1065 pp., 1999.
  • 12. Purported to be a quotation of Mark Twain, but we have not found it in his works. Robert Schmunk, based on electronic search, finds that the most common variant of the quotation is "The researches of many commentators have already thrown much darkness on the subject, and it is probable that if they continue we shall soon know nothing at all about it."

Workshop Homepage * Background
Summaries: Overview, Gases, Aerosols, Tech., Health, Agri./Eco.
Abstracts: Day 1, Day 2, Day 3, Day 4, Day 5 * Participants