CO2 jumps before ice sheets slump....

[CO.sub.2] jumps before ice sheets slump ...

To fully understand the threat of rising carbon dioxide levels, scientists are trying to decipher what role this greenhouse gas played during previous shifts in climate, namely the last several ice ages. New work on ice cores from Antarctica suggests that during one episode, about 130,000 years ago, carbon dioxide took center stage by helping to melt the massive ice sheets that covered large sections of the globe.

The new evidence comes from tiny samples of ancient air, locked in bubbles buried deep within the Antarctic ice cap. Soviet researchers have spent nearly two decades drilling a 2.5-kilometer-deep hole at their Vostok base to collect samples of ice that formed as far back as 160,000 ago. When French researchers analyzed the air bubbles, they discovered that atmospheric levels of carbon dioxide dropped substantially during the ice ages and rose again during warm periods. But they could not tell whether the carbon dioxide fluctuations were a cause or an effecct of climate change, partly because they could not directly compare the ice bubble data with information from deep-sea sediments. While the ice bubbles record atmospheric changes in carbon dioxide, the sediments contain information about sea level variations caused by the advance and retreat of continental ice sheets.

Todd Sowers from the University of Rhode Island at Narragansett proposes a way to skirt the problem. Working with U.S., French and Soviet colleagues, he suggests the ice bubbles themselves contain information about the growth of ice sheets in Europe and North America. Oceanographers traditionally study ice sheet history by measuring the ratio of different oxygen isotopes in sea sediments, which reflects the isotopic composition of ancient sea water. But Sowers and his co-workers think the isotopic ratio of the air trapped in ice can serve as a proxy for the ratio in the ancient sea water. They conclude that carbon dioxide levels rose about 3,000 years before the ice sheets melted at the end of the second of the last ice age. Since carbon dioxide helps heat the atmosphere, a rise in its concentration would have spurred the melting. They suggest the gas changes amplify slight variations in Earth's orbit, which serve as the pacemaker for the glacial cycle.

The proposed isotopic link between sea water and the atmosphere is complex, and the researchers must convince others the links is justified. If so, their method could help resolve what drives the changes in carbon dioxide levels.