Clues. Boulders dropped by a retreating glacier in this New Zealand valley reveal an uninterrupted warming at the end of the ice age in the Southern Hemisphere.

George Denton

'Deep Freeze' Didn't Affect Southern Hemisphere

Approximately 13,000 years ago, as the last ice age was winding down, Earth's Northern Hemisphere reverted to a near-glacial period called the Younger Dryas. Temperatures dropped by 15˚C, and giant ice sheets again advanced south from the Arctic. But things were much different in the Southern Hemisphere. New data reveal that the globe's bottom half continued to warm its way out of the ice age, even as the north temporarily plunged back into a another deep freeze.

Scientists blame the Younger Dryas on a disruption of ocean currents. As the ice age ended, melting glaciers poured huge volumes of cold freshwater—enough to fill all of the Great Lakes several times over—onto the surface of the North Atlantic Ocean. The freshwater killed off the Gulf Stream, which brings warm surface water up from the tropics to North America and Western Europe. Lacking that warmth, temperatures in the northern latitudes plunged.

The Gulf Stream represents just one segment of a giant ocean current called the conveyor belt, which loops around the globe and affects the temperatures of all the continents. When the glacial meltwater blocked the Gulf Stream, scientists think it set off a climate cascade all over the planet. But until now, scientists haven't had enough data to figure out what exactly happened on the southern half of the world.

To get more answers, an international team led by paleoclimatologist Michael Kaplan of the Lamont-Doherty Earth Observatory in Palisades, New York, concentrated on a well-studied part of New Zealand called Irishman Stream, in the central part of the South Island. The Irishman valley is strewn with soil deposits and large boulders that had been pushed downstream by a glacier during the ice age. Kaplan says he and colleagues chose the valley because the team's previous research had shown that the location remains essentially undisturbed from when it emerged from the ice—the boulders and soil have not budged since.

Based on their chemical analyses, the researchers concluded that the boulders came to rest as the glacier retreated back up the valley sometime during the Younger Dryas—an indication that this part of the world was warming as the north was cooling. To determine the timetable more precisely, the team employed a technique that analyzes the isotope beryllium-10 in quartz crystals in the rocks to compute exactly when the rocks were first exposed to open air (i.e., when they emerged from under the glacier) and thus bombarded by cosmic-ray particles.

Reporting in the 9 September issue of Nature, Kaplan and colleagues found that all but one of the rocks' exposure to cosmic rays began between 13,700 and 11,100 years ago—well within the Younger Dryas. In other words, he says, the glaciers in New Zealand were retreating at the same time as the glaciers covering the northern half of the globe were on the move again.

Kaplan says the phenomenon probably occurred because the disrupted conveyor belt was not bringing enough deep, cold water south. That triggered other climate mechanisms, such as changes in winds, which allowed the Southern Hemisphere to continue to warm. So while temperatures took a nosedive in the north, they must have been warm enough to melt the ice age glaciers in New Zealand.

It's "an exquisite piece of work, in terms of detailed observations of the glacial deposits, the precise and accurate dating of the deposits, and the shear number of precise dates," says paleoclimatologist R. Lawrence Edwards of the University of Minnesota, Twin Cities. And geomorphologist Martin Kirkbride of the University of Dundee in the United Kingdom says the study pinpoints "the timing of glacier advances as precisely as is technologically possible." That's very important, he says, for understanding how ocean circulation transmitted climate change across Earth's surface at the end of the ice age.

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