Articles

New York—Global environmental change is one of the most profound potential effects of humans on our planet. Yet, despite a preponderous of evidence that emissions from industry, automobiles, and other human activities have contributed to a change in the composition of the global atmosphere, the huge number of factors involved—and the complexity of how these factors interrelate—have kept scientists and engineers from establishing an unambiguous, cause-and-effect link between these emissions and current or potential climate
changes.

In the "Perspectives" column of the February 2000 issue of AIChE Journal, Dr. John H. Seinfeld, a professor of chemical engineering at the California Institute of Technology and a member of the National Academy of Engineering, suggests one reason for this inconclusiveness—the complexity of the relationship between particles in the air and the clouds that form on those particles. "The relationship among cloud lifetime, precipitation, and aerosols is one of the most complex in all of atmospheric science," he states, "and is not likely to be represented fundamentally in global climate models anytime soon."

His article lays out, in some detail, the reasons for that complexity, observing that the type of cloud, its size and shape, the size and type of droplets that compose it, its liquid water content, and many other factors influence whether a cloud will heat or cool the surface. Even its geographic positioning can play a role. "The clouds that exhibit the greatest sensitivity to changes in the cloud condensation nuclei (which controls the size of the droplets in the cloud and, in turn, the cloud's optical properties) are those in clean maritime air masses. Climatologically, these marine clouds are the most important on Earth, as they cover vast areas of the ocean."

Most of what is known and understood about climatic responses to greenhouse gases is based on models that simulate fundamental geophysical processes, Seinfeld explains, and "most current model simulations of Earth's climate indicate that the increase in greenhouse gases will lead to an increase in the average surface air temperatures; a doubling of current atmospheric CO² is predicted to produce an equilibrium temperature increase of 2.0 ± 0.6° C." But, he continues, "models that have been used to study climate change are necessarily simplified representations of the climate system. The relatively coarse resolution of the models limits their ability to accurately represent processes that occur on a smaller scale, and a major process in this regard is clouds and precipitation. Whether average global cloudiness would increase or decrease in a future greenhouse-enhanced world is not yet established."

So what does this mean when attempting to make realistic predictions on the effects of increases in greenhouse gases on future climate changes? Seinfeld writes "the climatic impact of the increase in greenhouse gases is indisputably warming, but the magnitude of warming is uncertain." In fact, he adds, "the climatic impact of increase in global aerosols is, on the whole, cooling, which is largely confined to the Northern Hemisphere. Rather than serving to cancel greenhouse warming, aerosol cooling establishes gradients in global heating and cooling that may actually exacerbate climatic effects." He concludes, "while cloud responses to a warmer, smoggier atmosphere are uncertain, and could be the gremlin lurking in the climate greenhouse, these responses are unlikely to change the signs of either greenhouse heating or aerosol cooling."

Edited by Julio M. Ottino, associate editor of the Journal, the "Perspectives" column offers easy-to-read, interesting, and thought-provoking overviews of specific topics, written by experts but for a general audience. The column is designed to help R&D professionals keep up with new developments across the spectrum of chemical engineering.

AIChE Journal, published by the American Institute of Chemical Engineers, is the premier research monthly in chemical engineering and related fields. This peer-reviewed and broad-based journal reports on the most important and latest technological advances in core areas of chemical engineering as well as in other relevant engineering disciplines. An online edition of the Journal is also available through Science Direct. For more information on the AIChE Journal, or to subscribe, call 1-800-242-4363.

American Institute of Chemical Engineers (AIChE), 3 Park Avenue, New York City, NY 10016-5901. Tel: 212-591-8100; Fax: 212-591-8894.

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