Update: 18 DEC 2001

Mark Twickler cuts a freshly extracted ice core into sections. Photos © Michael Morrison, University of New Hampshire. National Ice Core Laboratory, Summit Greenland.

on the rocks.
If you don't like the climate, just wait a while...
Climate: It's a calming word, giving a picture of stability and gradual change. But recent reports have shown that climate can change radically, at least in certain places at certain times. Evidence from deep ice cores shows that in Greenland, at least, the temperature changed rapidly toward the end of the last ice age.

Around 15,000 years ago, temperatures were gradually rising as Earth rebounded from the ice ages. Then,about 13,000 years ago, during the so-called "Younger Dryas" period, a cold snap temporarily halted the warming. slab o'iceThe period was named for an Arctic plant that first gave paleoclimatologists a signal of the cooling. When the warming resumed, temperatures rose 15 degrees C in about 40 to 50 years.

That's fast.


James White, an associate professor of geological science at the University of Colorado and an expert on ice core studies, says that the record includes a rapid burst of warming: 5 to 6 degrees C within just five years.

Did these jumps occur outside Greenland? Nobody's sure, and ice core records are rare. But there is some evidence that a rapid cooling like the Younger Dryas occurred in Antarctica at least.

This kind of erratic behavior could cause havoc on a highly industrialized planet where greenhouse gases are rapidly accumulating in the atmosphere. Yet they also indicate that rapid warming took place long before the invention of the internal combustion engine.

Enormous changes at the last minute
If such rapid temperature changes did occur, at least in Greenland, what drove them? About 10 years ago, Wallace Broecker of Columbia University pinned the rapid cooling of the Younger Dryas on changes in ocean circulation. He suggested that the warming before the Younger Dryas had melted vast amounts of glacial ice, feeding fresh water to the seas around Labrador and Greenland and preventing the downturn of cold, salty ocean water to the sea bottom. This, in turn, altered the global ocean conveyor interrupting the flow of heat to Greenland and Northern Europe.

ocean conveyor belt

Whether that change -- or whatever else caused the cooling -- could also affect global temperatures was one of many disputes triggered by the surprising finding. But if global temperatures have jumped around as much as they did in Greenland, that could undermine forecasts for slow, steady warming.

Sherwood Rowland, an atmospheric chemist at the University of California at Irvine, who helped raise the alarm about stratospheric ozone depletion about 20 years ago, cautions that unknown climate mechanisms could produce unknown results. "I refer back to the Antarctic ozone hole, which was revealed to the world in 1985. It was occurring in a way that was not part of [computer] models, because it involved a chemical reaction that was not known, on a surface that was not known to exist."

Rowland, who shared the 1995 Nobel Prize in chemistry for predicting that chlorofluorocarbons would eventually deplete ozone, threatening the planet with a cancerous bath of ultraviolet light, says, "It was not possible to predict the ozone hole because the ingredients were not known."

Applying that experience to the even more complicated picture of global warming, Rowland says, "If there are physical or biological things that you don't know, you can't expect to predict very accurately. I'd not want to indicate that all surprises are over and everything is known."

Rapid changes could bring even bigger problems, says White, the ice-core expert. "There's an underlying assumption in the global warming debate that this is something we can adapt to. I have the utmost faith that individuals can adapt to almost anything short of an asteroid. But the larger the change, and the shorter the time, the more challenging the adaptation and economic impact will be."

Could agriculture play a role in rapid warming?


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