HEADING FOR APOCALYPSE?

By MICHAEL D. LEMONICK

Like street-corner prophets proclaiming that the end is near, scientists who study the earth's atmosphere have been issuing predictions of impending doom for the past few years without offering any concrete proof. The atmospheric scientists' version of the apocalypse is global warming, a gradual rise in worldwide temperatures caused by man-made gases trapping too much heat from the sun. If the theory is correct, the world could be in for dramatic changes in climate, accompanied by major disruptions to modern society. So far, though, even the experts have had to admit that while the earth has warmed an average of up to 1.1 degree F over the past 100 years, no solid evidence has emerged that this is anything but a natural phenomenon. And the uncertainty has given skeptics--especially Gingrichian politicians--plenty of ammunition to argue against taking the difficult, expensive steps required to stave off a largely hypothetical calamity.

Until now. A draft report currently circulating on the Internet asserts that the global-temperature rise can now be blamed, at least in part, on human activity. Statements like this have been made before by individual researchers--who have been criticized for going too far beyond the scientific consensus. But this report comes from the International Panel on Climate Change (IPCC), a respected U.N.-sponsored body made up of more than 1,500 leading climate experts from 60 nations.

Unless the world takes immediate and drastic steps to reduce the emissions of heat-trapping gases, says the panel, the so-called greenhouse effect could drive global temperatures up as much as 6 degrees F by the year 2100--an increase in heat comparable to the warming that ended the last Ice Age and with perhaps equally profound effects on climate. Huge swaths of densely populated land could be inundated by rising seas. Entire ecosystems could vanish as rainfall and temperature patterns shift. Droughts, floods and storms could become more severe. Says Michael Oppenheimer, a senior scientist with the Environmental Defense Fund: "I think this is a watershed moment in the public debate on global warming."

This shift in scientific consensus is based not so much on new data as on improvements in the complex computer models that climatologists use to test their theories. Unlike chemists or molecular biologists, climate experts have no way to do lab experiments on their specialty. So they simulate them on supercomputers and look at what happens when human-generated gases--carbon dioxide from industry and auto exhaust, methane from agriculture, chlorofluorocarbons from leaky refrigerators and spray cans--are pumped into the models' virtual atmospheres.

Until recently, the computer models weren't working very well. When the scientists tried to simulate what they believe has been happening over the past century or so, the results didn't mesh with reality; the models said the world should now be warmer than it actually is. The reason is that the computer models had been overlooking an important factor affecting global temperatures: aerosols, the tiny droplets of chemicals like sulfur dioxide that are produced along with CO2 when fossil fuels are burned in cars and power plants. Aerosols actually cool the planet by blocking sunlight and mask the effects of global warming. Says Tom Wigley, a climatologist at the National Center for Atmospheric Research and a member of the international panel: "We were looking for the needle in the wrong haystack."

Once the scientists factored in aerosols, their models began looking more like the real world. The improved performance of the simulations was demonstrated in 1991, when they successfully predicted temperature changes in the aftermath of the massive Mount Pinatubo eruption in the Philippines. A number of studies since have added to the scientists' confidence that they finally know what they're talking about--and can predict what may happen if greenhouse gases continue to be released into the atmosphere unchecked. Just last week, a report appeared in Nature that firmly ties an increase in the severity of U.S. rainstorms to global warming.

In general, the news is not good. Over the next century, says the IPCC report:

Sea levels could rise up to 3 ft., mostly because of melting glaciers and the expansion of water as it warms up. That could submerge vast areas of low-lying coastal land, including major river deltas, most of the beaches on the U.S. Atlantic Coast, parts of China and the island nations of the Maldives, the Seychelles and the Cook and Marshallislands. More than 100 million people would be displaced.

Winters could get warmer--which wouldn't bother most people--and warm-weather hot spells like the one that killed 500 in Chicago this past summer could become more frequent and more severe.

Rainfall could increase overall--but the increase wouldn't be uniform across the globe. Thus areas that are already prone to flooding might flood more often and more severely, and since water evaporates more easily in a warmer world, drought-prone regions and deserts could become even dryer. Hurricanes, which draw their energy from warm oceans, could become even stronger as those oceans heat up.

Temperature and rainfall patterns would shift in unpredictable ways. That might not pose a problem for agriculture, since farmers could change their crops and irrigate. Natural ecosystems that have to adapt on their own, however, could be devastated. Observes Oppenheimer dryly: "They cannot sprout legs and move to another climate." Perhaps a third of the world's forests, he says, might find themselves living in the wrong places.

These are all worst-case scenarios, and the report's authors acknowledge that plenty of uncertainties remain in their analysis. For example, as the world warms up, it should get cloudier; depending on what sort of clouds predominate, their shadows could offset the warming effect. And nobody knows how the deep ocean currents--which play a major but still murky role in world climate, channeling heat from one part of the globe to another--would respond to global warming.

Some researchers argue that even with these caveats the report overstates the case. Says Richard Lindzen, an atmospheric scientist at M.I.T.: "The margin of error in these models is a factor of 10 or more larger than the effect you're looking for."

Even if Lindzen is wrong and the IPCC report is right, there might not be much anyone could do. Slashing emissions of greenhouse gases to stave off global warming would be straightforward enough, but that doesn't mean it would be easy. Among the strategies recommended in the new report: switching from coal and oil to natural gas, turning to nuclear and solar energy, slowing deforestation, altering land-use and traffic patterns, curbing automobile use, changing life-styles and employment patterns.

In other words, people in the developed world would have to completely transform their society, and rich countries like the U.S. would have to subsidize poor but fast-developing nations like China. And that's just to roll CO2 emissions back to 1990 levels, the goal most environmentalists endorse. To stave off global warming completely, Lindzen maintains, "you would have to reduce emissions to where they were in 1920." Despite noble proclamations issuing from meetings like the 1992 Earth Summit in Rio, that is virtually inconceivable. As economist Henry Jacoby of M.I.T.'s Sloan School of Management puts it, "If you said, 'Let's design a problem that human institutions can't deal with,' you couldn't find one better than global warming."

Even a Democrat-controlled, Al Gore-inspired Congress would shrink from passing draconian emissions-control measures. And the current Republican House and Senate are unlikely to consider such regulations no matter how many scientists are convinced that global warming is real. Other industrial nations probably won't do much better, and poor countries can't afford to try. A more realistic strategy, some scientists argue, is to spend what research money there is figuring out how best to deal with global warming when it comes. It's already too late, they say, to do much else.

--Reported by David Bjerklie/New York and Michael Riley/Washington

With reporting by DAVID BJERKLIE/NEW YORK AND MICHAEL RILEY/WASHINGTON