Staggering through the Ice Ages: What Made the Planet Careen between Climate Extremes?

Article excerpt

Climate experts once viewed Earth as a well-mannered dancer moving to the stately and predictable beat of the ice ages. The world cooled and warmed with a slow, steady rhythm governed by the well-understood wiggles of its orbit. Vast ice sheets spread across the northern lands and then gradually retreated. Sea levels dipped and rose. Forests moved south and later returned. The changes kept a pace as measured as that of Earth's voyage around the sun.

But discoveries about the most recent ice age are transforming the image of a climatic waltz to something resembling a drunken lurch. During the last glacial epoch, Earth repeatedly swayed from extremely frigid conditions to warmth and back again with startling speed. As part of these shifts, the great North American ice sheet vomited huge numbers of icebergs that filled the North Atlantic.

The new revelations have left scientists reeling, because the steady orbital cycles thought to control the ice ages cannot easily account for the evidence of quick climatic jitters.

"It's clear that the climate theory is not complete. The quest is now to develop the rest of the theory to explain the shorter-term events that--when we look at them in the context of our lifespan--are much more dramatic," says Gerard Bond, a marine geologist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y.

Without knowing what made the ice-age climate so intemperate, scientists cannot tell whether today's interglacial period is immune to the sudden swings so common when ice covered large swaths of the globe.

Much of the news about climate instability has emerged from two drilling projects in the middle of Greenland, where hardy crews bored through the 3-kilometer-thick glacial cap. Like counting tree rings, scientists can look back through the annual ice layers to trace how temperature, gas concentrations, and other factors varied during the last glacial epoch, which persisted from 115,000 until 10,000 years ago.

Although the term "ice age" conjures up a picture of unrelenting cold, temperatures during this period actually rose close to their interglacial values several times, only to plummet back toward full glacial conditions after a respite of a few hundred to a few thousand years. Willi Dansgaard of the University of Copenhagen and Hans A. Oeschger of the University of Bern in Switzerland first uncovered hints of these swings in Greenland ice cores drilled in the late 1960s and early 1980s. But the questionable quality of the oldest ice undermined confidence in their findings.

The two new Greenland cores, which match each other almost perfectly, provide dual confirmation that so-called Dansgaard-Oeschger events did indeed send temperatures bouncing up and down. In fact, the jumps from one extreme to the other occurred over decades, and in some cases over a few years--much faster than previously deemed possible.

But what does the weather over Greenland have to do with conditions in the rest of the world? As long as the evidence of climate blips remained confined to the top of Greenland, researchers could only speculate whether the fast shifts they detected there had affected other parts of the globe.

Unknown to the ice-core workers, however, a German oceanographer had already reported evidence that would draw the oceans into the picture. In 1988, Hartmut Heinrich, working in the eastern North Atlantic, described finding six unusual layers of seafloor sediments containing abnormally high percentages of rock grains.

To explain these bands, Heinrich, who works at the Federal Maritime and Hydrographic Agency in Hamburg, surmised that huge armadas of icebergs sailed across the Atlantic a half dozen times during the last ice age, dropping pulverized rock as they melted. The bergs apparently calved off of the Laurentide ice sheet, which covered Canada and parts of the United States.

Heinrich's discovery made few waves until 1992, when Lamont-Doherty's Wallace S. …