Sulfate Aerosol and Global Warming

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Were it not for the greenhouse effect at work in the earth's atmosphere, our world would be a chilly 60 degrees Fahrenheit colder than it really is. In a loose analogy to the process by which sunlight warms the air inside a greenhouse or a car, visible light from the sun passes through the window of our atmosphere, is absorbed by the ground, reradiated at longer wavelengths, and absorbed by molecules of carbon dioxide, water vapor, and other "greenhouse gases" such as methane in the air, thereby warming the atmosphere.

Human activities--notably the burning of gasoline and other fossil fuels--are adding more greenhouse gases to the atmosphere, and appear to be adding them faster than they can be removed by natural processes. The net result: global warming, a gradual increase in the average global temperature. A lively debate continues among scientists about how significant the warming will be and how fast it will occur. Scientists' models of atmospheric processes are imperfect, and they can't predict with certainty what will happen.

Meanwhile, UW professor Robert J. Charlson and colleagues have obtained data to show that the composition of our atmosphere has changed over the years in other important ways. The resultant shifts in the heat balance of the planet, called "forcings," are not globally uniform and may bring about dramatic changes in regional weather patterns. Charlson's pioneering work at the UW over the past 30 years has established the role of sulfate aerosols in global atmospheric processes.footnote 1 dash footnote 3

An aerosol is a suspension of very fine particles of a solid, or of droplets of a liquid, in a gaseous medium. Fog, smoke, and volcanic dust are naturally occurring examples of aerosols. Sulfates are salts that contain a charged group of sulfur and oxygen atoms: SO42minus, the basic constituent of sulfuric acid. The sulfate particles in the atmosphere are about 0.1 to 1.0 micrometer (a millionth of a meter) in diameter and are particularly concentrated over industrial areas. They contribute to acid rain, cause lung irritation, and have been a main culprit in causing the haze that obscures a clear view of the Grand Canyon.

Much of the sulfate aerosol in the atmosphere derives from the oxidation of sulfur dioxide produced in the combustion of fossil fuels. Industrial activities are not the only source: Natural aerosols--mostly dust, sea salt, and other compounds of marine origin--do exist, but they have remained in nearly constant concentrations in the atmosphere for a long time. However, the man-made version has increased dramatically since about 1950.

Aerosols can cool the climate in two basic ways: either directly, under clear sky conditions, by reflecting away some of the incoming solar radiation, or indirectly, by increasing the reflectivity of clouds.

During the late 1960s, in the process of trying to measure and understand the optical clarity of the atmosphere, Charlson realized that existing instrumentation for measuring aerosols was inadequate. The old measurements were done by eye and were only approximate. So he designed and built a new device, patented by the University, to analyze the light-scattering power of atmospheric aerosols. The technology currently forms the basis of the Model 3550/3560 Integrating Nephelometer marketed by a Minnesota-based company called TSI Incorporated. The nephelometer made it possible for the first time to quantitatively assess the amount of sunlight reflected back into space by sulfate aerosols.

Charlson's original work on the role of aerosols in the global heat balance, published in 1969, did not attract much attention, nor did a subsequent paper on sulfate in 1976. It wasn't until the global warming debate heated up, so to speak, in the late 1980s and early 90s that the importance of his work began to be fully appreciated.

Charlson and colleagues have shown that the cooling effect of sulfate aerosols does not neatly cancel out the effects of greenhouse warming, but rather, makes the situation more complex. "Aerosol cooling and the greenhouse effect have characteristics that prevent them from neatly offsetting each other," note Charlson and colleague Tom Wigley, who heads the Office for Interdisciplinary Earth Studies at the University Corporation for Atmospheric Research in Boulder, Colorado.footnote 1

First, the cooling and warming occur mostly over different parts of the world: the aerosol effect is focused over industrial areas in the Northern Hemisphere, whereas warming effects may be greatest over subtropical oceans and deserts. There are also temporal variations. Aerosol effects are most pronounced during daylight hours during the summer season; the activity of greenhouse gases differs very little over the course of a day, or over a year.

The work of Charlson and colleagues suggests that forcing by sulfate aerosol is not evenly distributed over the globe--it can vary by roughly a factor of five from region to region. As a result, the world might expect to see dramatic changes in regional weather patterns in the future, not just an increase in average global temperature.

Furthermore, Charlson's work demonstrates how to incorporate particular chemical and physical measurements made on the local scale into models of atmospheric dynamics on a global scale. For that achievement, Charlson was awarded an honorary doctoral degree from Stockholm University in 1993. Sulfate aerosol was named by the journal Science as one of nine runners-up for Molecule of the Year in 1995.footnote 4

  1. "Sulfate Aerosol and Climatic Change," Robert J. Charlson and Tom M. L. Wigley, Scientific American, 270 (2), 48-57, February 1994.
  2. "Climate Forcing by Anthropogenic Aerosols," R. J. Charlson, S. E. Schwartz, J. M. Hales, R. D. Cess, J. A. Coakley, Jr., J.E. Hansen, D. J. Hofmann, Science, 255, 423 (1992).
  3. "Radiative Forcing of Climate Change," The 1994 Report of the Scientific Assessment Working Group of IPCC, Intergovernmental Panel on Climate Change, World Meterological Organization and United Nations Environment Programme.
  4. "Molecule of the Year: A New Form of Matter Unveiled," Science, 270, 1902 (1995).

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