Rhythm of the ice age: north versus south.

Linked together like Siamese twins, Earth's northern and southern hemispheres should dance in tandem to the beat of climatic cycles. Indeed, every 100,000 years for the last million years, both halves of the globe have jointly entered into prolonged ice ages.

A close look at events within the last ice age, however, reveals that the two hemispheres often fall out of step with each other, sometimes even moving in opposite climatic directions, according to a team of European scientists. Their analysis of ancient ice from Greenland and Antarctica raises questions about what drove the ice-age temperature changes and whether similar factors are operating today.

"If we want to understand the climate today, we have to understand the climate of the past," says Thomas Blunier of the University of Bern in Switzerland.

Blunier and his colleagues studied ice cores pulled from a deep drill hole in central Greenland and from two drill holes in Antarctica. The ice in these places has piled up, layer by layer, over hundreds of thousands of years, preserving chemical clues to the climate of ancient times. By pulling up cores of this hoary ice, scientists can trace how conditions have changed.

During the last ice age, the climate in both hemispheres see-sawed from extremely cold to mild about every 3,000 years--a relatively quick swing by geological standards. Researchers have long wondered whether the warmings, called interstadials, happened simultaneously in the north and the south. They have had trouble, however, comparing the timing of the events in the records from the two hemispheres.

In the August 20 Nature, Blunier and his coworkers describe a technique for matching up evidence of interstadials in the different ice cores. They focus on methane gas trapped in tiny bubbles of air from tens of thousands of years ago. Because worldwide concentrations of methane rose and fell markedly throughout the ice age, the researchers could use spikes in the ice's methane as benchmarks for lining up the three ice cores.

The new analyses of the matched cores showed that the short-term warmings during the ice age occurred at different times in Antarctica and Greenland. Temperatures in the south often rose a thousand years or more ahead of those in the north.

The work won praise from researchers studying other ice cores from Greenland and Antarctica. "They've taken timing to a new level," says James W.C. White, an ice-core researcher at the University of Colorado in Boulder.

Climate scientists have previously discovered hints that the northern and southern hemispheres did not always dance together as the last ice age ended. Wallace S. Broecker of Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y., suggests that deep ocean currents in the far north and south may alternate in strength, cooling off one hemisphere while bringing warmth to the other--a pattern that agrees with the new discovery, he says.

One complicating factor has emerged from West Antarctica, however. Studies of a new ice core drilled there show climate fluctuations at the same time as those in Greenland, and therefore out of synch with the other Antarctic drill sites, says White. That leaves researchers with a cool conundrum to ponder.