Time history of global emissions of sulfur dioxide and estimates of the global and Northern Hemisphere natural fluxes. Anthropogenic sulfur is emitted mainly (-90%) in the Northern Hemisphere and emissions greatly exceed the natural emissions. Width of shading represents the uncertainty (from IPCC, 1994 and Charlson et.al., 1992).

What are the sources of aerosols and what are their concentrations in the atmosphere? How much of a climate cooling influence do aerosols presently exert relative to the warming influence exerted by greenhouse gases? What influence will aerosols likely exert in the future? How well do climate models account for the influence of aerosols? What are the uncertainties and how large are those uncertainties? Will cooling due to aerosols counter warming due to greenhouse gases?

INTRODUCTION:

Dr. John H. Seinfeld, Louis E. Nohl Professor and Chair, Div. of Engineering and Applied Science, Calif. Institute of Technology, Pasadena, CA

SPEAKERS:

Dr. Robert J. Charlson, Department of Atmospheric Sciences, Environmental Studies and Geophysics Program, University of Washington, Seattle, WA

Dr. Joyce E. Penner, Atmospheric Science Division, Lawrence Livermore National Laboratory, Livermore, CA

Aerosol is a term used to describe the many types of small particles in the atmosphere. Aerosol particles vary greatly in size, source, chemical composition, amount and distribution in space and time, and how long they survive in the atmosphere. Only over the past few years has enough become known about aerosol effects on solar radiation that scientists can start to estimate their large-scale influence on climate. While the results are still relatively new and uncertain, what has been learned is providing important new insights about how humans are affecting the climate.

Measurements now show that a substantial fraction of the aerosols in the lower atmosphere are a by-product of human activities. The highly visible haze that persists in all of the industrialized regions of the world consists mainly of sulfate and organic compounds from emissions of sulfur dioxide, organic gases, and smoke from the burning of fossil fuels and vegetation. Emissions of sulfur dioxide, mainly from coal combustion, have risen dramatically in the Northern Hemisphere over the last century, and, while now decreasing in the United States, are continuing to rise in the industrializing nations of the Northern Hemisphere.

Small aerosol particles affect the natural energy balance of the Earth mainly by reflecting (and in some cases absorbing) solar radiation and by, some evidence suggests, influencing the reflective and absorbing properties of clouds. Larger particles can also affect the fluxes of long-wave terrestrial radiation, but this is generally a smaller effect. Aerosol particles can also affect atmospheric chemistry by providing sites on which chemical reactions can take place  It is this role of aerosols, in this case mainly aerosols created by sulfur dioxide injections from volcanic eruptions, that is contributing to depletion of stratospheric ozone and affecting the radiative effects of ozone in the atmosphere.

The current mean global warming influence from anthropogenic increases in greenhouse gas concentrations and from associated changes in atmospheric chemistry since the 18th century is estimated to be about +2.5 watts per square meter. By comparison, the continuing emissions of sulfur dioxide from coal combustion and other sources are estimated, with considerable uncertainty, to be causing an average cooling influence of about -1.3 watts per square meter, thus reducing the current warming influence of greenhouse gases by about half.

Recent projections of increases in global mean surface temperature suggest a best estimate of 2°C temperature rise by the year 2100, with a range from about 0.8 to 3.5 °C depending on emissions projections for greenhouse gases and aerosols and climate sensitivity. These projections by the Intergovernmental Panel on Climate Change (IPCC) are roughly one degree lower than similar projections in 1990 because account is being taken of past and future cooling influences from changes in the aerosol concentration.

While the radiative effects of aerosols work in the opposite direction of greenhouse gases, their climatic effects are not simply opposing because of the very different spatial and temporal distributions of their influence. Greenhouse gases exert their influence night and day, all year long. By contrast, forcing by anthropogenic aerosols occurs mostly by day, mostly in the summer, and mostly near and downwind of aerosol sources. While the cooling influence of aerosols is thus seasonal regional, the climate response can extend hemispherically, and even globally, as the atmospheric circulation patterns adjust to the differential patterns of warming and cooling.

The long lifetime of greenhouse gases in the atmosphere (typically decades to centuries) and the short lifetime of sulfate aerosols (typically days to weeks) means that the greenhouse-gas-induced warming will more and more strongly dominate the aerosol cooling influence in the future. Also, because of the health effects of fine aerosol particles, allowing a greater build-up of aerosols cannot be used to continue to offset the greenhouse gas effect without leading to deleterious health and ecological effects.

Because of their many influences, however, improving understanding of aerosols effects is essential. Their effects determine how rapidly or slowly warming will occur, how soon human and natural influences on the climate can be distinguished, and what the regional patterns of climate change will be. This seminar will provide an overview of scientific understanding and of the importance of gaining better estimates of the influence of aerosols on climate.

Dr. Robert J. Charlson began his career as an instrument engineer at the Boeing Company while simultaneously studying meteorology. Upon completion of his Ph.D. in meteorology at Stanford University, Dr. Charlson joined the faculty of the University of Washington. After 21 years in the College of Environmental Engineering, he joined the Atmospheric Sciences and Chemistry faculties. His research interests focus on connecting chemical and physical properties and processes of atmospheric aerosols, focusing especially on the role of sulfur and organic aerosols in climate forcing. He is a member of the National Academy of Sciences Panel on Aerosol Forcing of Climate, serves on the Science Advisory Committee of the Atmosphere/Ocean Experiment, and was co-lead author of the aerosol sections of the 1994 and 1995 IPCC reports. Dr. Charlson is a former Fulbright Scholar and NATO Lecturer in Meteorology. He is also the recipient of the Gerbier-Mumm Award of the World Meteorological Association, and holds an honorary doctorate degree from Stockholm University. He received his BS and MS degrees in Chemistry from Stanford University, and his Ph.D. in Atmospheric Sciences from the University of Washington.

Dr. Joyce E. Penner is presently Group Leader of the Atmospheric Science Division of the Lawrence Livermore National Laboratory. She is responsible for developing, conducting and supervising research programs that are aimed at evaluating the role of fossil fuel emissions in altering chemical cycles and climate. She has over 90 peer-reviewed publications in the scientific literature, including studies of stratospheric chemistry and ozone change, regional and urban air pollution, nuclear war effects on global climate, and chemical and aerosol effects on radiation and climate. She is a leading expert on the interactions of chemistry, aerosols, and their effects on the climate system. She has played an active role in recognition of the effects of aerosols on climate and organized several meetings including on the subject, most recently, the 5th International Conference on Carbonaceous Particles in the Atmosphere. She has served on several scientific advisory committees, including the National Academy of Sciences Atmospheric Chemistry Committee and the National Academy of Sciences Panel on Aerosol Forcing and Climate Change. Dr. Penner received a BA degree from the University of California-Santa Barbara, and her MS and Ph.D. degrees from Harvard University.