If You Think the Weather Is Bad . . .

By MICHAEL D. LEMONICK

Weather is almost impossible to predict more than a week or two in advance. But when it comes to climate -- the long-term weather averages that make the U.S. temperate and the tropics torrid -- scientists are confident that they understand the overall pattern. Over the past million years or so, the planet has swung between ice ages lasting on the order of 100,000 years and interglacial periods of about 10,000. During each phase, the climate is pretty steady. It's the stability of the current interglacial epoch, which began 10,000 years ago, that made the invention of agriculture, and thus the rise of civilization, possible.

Now it looks as though the concept of long-term stability may be wrong. According to two articles in last week's Nature, deep holes drilled into the ancient ice of Greenland have brought up evidence of sudden, dramatic swings in climate during the last interglacial period, about 120,000 years ago. Several times, average global temperatures dropped as much as 25 degreesF, plunging the planet back into ice-age conditions, and stayed there for tens or hundreds of years before recovering. And the changes happened not over centuries, as scientists would have predicted, but in as little as a decade. The tranquillity of recent centuries may be a climatic fluke.

The discovery, says British scientist David Peel, co-author of one of the reports, is "staggering." Worldwide temperature shifts of a few degrees over half a century -- the kind envisioned in theories of global warming -- would disrupt weather patterns, change sea levels and be difficult for animal and plant life to adjust to. The changes Peel measured, though, are roughly three times as severe and rapid.

One ominous sign: normal temperatures during the last interglacial epoch were about 4 degreesF warmer than they are this time around, and levels of carbon dioxide in the atmosphere were significantly higher. As humans pump more and more CO2 into the air and temperatures rise, the planet will approach the state it was in back then. And if those conditions tend to be inherently unstable -- an idea scientists consider plausible -- people may someday look back on the early 1990s as an idyllic time when the weather was benign.

With reporting by Barry Hillenbrand/London