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.