The pollutants are sulfate aerosols, tiny particles that help form acid rain and are also unhealthy to breathe. They come from smelting metal and burning fossil fuels such as coal and oil. They reflect sunshine and seed clouds, which in turn bounce sunlight back to space. In both cases the result is a colder planet. That much had been known, in a general way, for years. But according to the latest research, this cooling just about equals the heating effect from “greenhouse gases” such as carbon dioxide (CO,) and so “has likely offset global greenhouse warming to a substantial degree.” In other words, as a result of sulfates, there has been less warming than simple greenhouse models predicted.
Last week’s report follows another discovery: that the same sulfates may be the planet’s best defense against the vanishing ozone layer. In December, Shaw Liu of the National Oceanic and Atmospheric Administration suggested why the thinning ozone layerthe envelope of gas that screens out cancer-causing ultraviolet rays from the sun-hasn’t given us all melanoma yet. Writing in Geophysical Research Letters, he noted that the tiny sulfate particles scatter ultraviolet radiation. That’s why, despite a 5 percent loss of wintertime ozone in the Northern Hemisphere in the last decade, there is not much more UV reaching the ground, he concludes. Sulfates act like little lead umbrellas.
If all these interactions among pollutants and the atmosphere sound complicated, that’s because they are. It was just last autumn that the first complexity got pinned down: a diminishing ozone layer indirectly cools the Earth, and so partially counters greenhouse warming (NEWSWEEK, Nov. 4). While we might get cataracts and skin cancer from the extra UV streaming in, we won’t have to dike Manhattan quite yet.
What does all this mean for environmental policy? Officials of all 166 members of the United Nations have been invited to meet in Rio de Janeiro this June at the first Earth Summit. A chief goal will be an agreement on greenhouse gases, similar to the 1990 pact phasing out gases that destroy the ozone layer. The United States is expected to argue that scientists don’t know enough about global warming to justify expensive CO, controls; as evidence, negotiators can point to the twofold discrepancy between how much warming greenhouse gases should have produced by now and how much they actually have. But what if that gap is due to sulfates having damped down the warming.? The new Clean Air Act will sharply reduce sulfates; some of the offset against the greenhouse effect will thus disappear, and the need to control CO, will be greater than ever. By throwing all sorts of trash into the air, humans have begun the biggest experiment in atmospheric chemistry since physicists wondered whether the first atomicbomb test would ignite the planet’s atmosphere. The physicists at Alamogordo were smart and lucky. This experiment might not turn out so well.
