Sulfate Aerosol and Climatic Change; February 1994; Scientific American Magazine; by Charlson, Wigley; 8 Page(s)

The greenhouse effect is a geophysical fact of life. Atmospheric gases such as carbon dioxide and methane trap and hold heat, enabling the earth's biota to survive. Such gases warm the surface of this planet by about 33 degrees Celsius, from below freezing to a current average of about 17 degrees C. Models and analyses of global warming generally agree that most of the long-lived gases that human economic activity adds to the atmosphere make the earth warmer than it would otherwise be. Yet discrepancies between theory and observation persist. The predicted warming based on recent increases in concentrations of greenhouse gases is slightly more than the observed warming of the atmosphere. In addition, the warming trend in North America does not appear to follow the global pattern. What might account for these and other deviations of fact from theory?

The answer is ironic. In all probability, aerosols primarily composed of sulfates, themselves the result of commercial activity, enhance the ability of the atmosphere to reflect sunlight back into space before it can reach the planet's surface and participate in the warming process. The sulfate particles, about 0.1 to one micron in diameter, are particularly concentrated over the industrial areas of the Northern Hemisphere. Their roles as contributors to acid rain, as irritants and as obscurers of such splendid vistas as the Grand Canyon have been known for years. But their capacity to cool by scattering sunlight has become a recognized force in climatic change only recently. Clearly, both the cooling effects of aerosols and the warming caused by greenhouse gases must be taken into account if we are to attain accurate climate models and effective industrial policies.