USGCRP Seminar on The Role of Aerosols and Climate Change, April 25th

Anthony Socci tsocci at
Thu Apr 18 13:29:43 EDT 1996

       U.S. Global Change Research Program Second Monday Seminar Series 

                                        The Role of Aerosols in Climate Change 

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? 

                                                                Public Invited 

                                       Thursday, April 25, 1996, 3:15-4:45 PM 
                                      Rayburn House Office Bldg., Room B369 
                                                         Reception Following 


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


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 is 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 warming influence of greenhouse 
gases by about half. 

Recent projections of increases in global mean surface temperature suggest 
a best estimate of 2 degrees C temperature rise by the year 2100, with a range 
from about 0.8 to 3.5 degrees 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 

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 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, 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 


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 section 
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 and has played an active role in recognition of the effect 
of aerosols on climate, organizing several meetings including, 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 

                                   NEXT SEMINAR: May  20th and 21st, 1996 

 A Closer Look at Global Satellite and Surface Temperature Records and Trends 
                                                         (Parts I and II) 

For more information please contact: 

Dr. Anthony D. Socci, U.S. Global Change Research Program Office 
300 D St., SW, Suite 840, Washington, DC 20024 
Telephone: (202) 651-8244; Fax: (202) 554-6715 

Additional information on the U.S. Global Change Research Program (USGCRP) 
and this Seminar Series is available on the USGCRP Home Page at:  Normally these seminars are held on the second 
Monday of each month. 

More information about the Coral-list-old mailing list