Haze Clouds the Greenhouse

Abstract

H igh above our heads, two kinds of pollution are waging a tug-ofwar over Earth's climate. Greenhouse gases, with their muchpublicized warming powers, hold decided advantage in this environmental struggle. But another form of pollution is showing unanticipated strength as it pulls against greenhouse forces, reducing rate of warming. Atmospheric experts have long suspected that sulfur haze same kind that obscures skylines in much of industrialized world could exert a cooling effect by reflecting sunlight. Yet they are only now recognizing sulfur's potential power. The Intergovernmental Panel on Climate Change (IPCC) recently recognized importance of sulfur issue in its 1992 review, released in mid-January While international panel stood by its 1990 conclusion that greenhouse gas emissions are likely to raise Earth's temperature significantly it scaled back estimated warming rate, largely because of influence of sulfur pollution. The report concludes that the cooling effect of sulfur emissions may have offset a significant part of greenhouse warming in northern hemisphere during past several decades. One of U.S. participants in IPCC assessment, climate modeler Michael C. MacCracken of Lawrence Livermore (Calif.) National Laboratory, says panel included this point because newer studies have found sulfur emissions more significant than previously thought. It's a topic whose time has come, says Robert J. Charlson, an atmospheric scientist at University of Washington in Seattle who has long studied effects of sulfur pollution. F ormed during combustion of fossil fuels, sulfur pollution pours from same smokestacks and tailpipes that belch out 6 billion tons of carbon dioxide each year. The sulfur dioxide gas wafts into atmosphere, where it eventually turns into tiny sulfuric acid which can be either droplets or particles. Sulfur aerosols tug on climate in two ways one direct, other more subtle. Aerosols exert their most obvious effect by reflecting incoming solar radiation back toward space. They wield indirect climatic power by serving as nuclei around which water vapor can condense to form sunlight-reflecting cloud particles. In both cases, aerosol pollution acts as a giant shade, reducing amount of light reaching Earth's surface. Over last few years, Charlson and other atmospheric experts have gradually realized that aerosol effect warranted consideration. They wondered: Could sulfur shade actually block enough of sun's energy to slow greenhouse warming? To answer that question, researchers would need to estimate power of aerosols and compare it with that of greenhouse gases. Climate modelers have a reasonably good handle on greenhouse part of equation. Since beginning of Industrial Revolution, emissions of carbon dioxide, methane and other gases have added about 2 to 2.5 watts of energy for each square meter of Earth's surface equivalent of hanging two Christmastree lights over every square meter of planet. The aerosol factor isn't nearly so clear. Charlson and six colleagues recently joined together to present a consensus statement-at least for United Stateson aerosols. In Jan. 24 SCIENCE, they estimate that direct influence of sulfur aerosols, averaged over globe, amounts to roughly 1 W/m2. The real value, they say could range from 0.5 to 2.0 W/m . The global average may have little meaning, though, because sulfur aerosols assert themselves most forcefully in northern hemisphere, which contributes 90 percent of world's sulfur pollution. Aerosols remain in atmosphere only a few days, so they haven't time to spread around world, unlike long-lasting greenhouse gases. Charlson and his colleagues estimate that sulfur effect over northern hemisphere may be double global average. For all uncertainty about sulfur's direct effect, climate researchers face an even more daunting task in gauging its indirect influence. Atmospheric scientists do not know to what extent aerosols increase number of particles within a cloud, one of critical determinants of how much sunlight clouds reflect. Charlson's group offers only a very rough estimate, proposing that indirect