Ice Cores May Hold Clues to Weather 200,000 Years Ago : Environment: Scientists hope sample from Tibet will yield secrets through 4 ice ages about ‘global warming.’

American and Chinese scientists working on a 21,000-foot Tibetan plateau have recovered ice cores that they believe will yield information about the Earth’s atmosphere as it existed at least 200,000 years ago, Ohio State University’s Byrd Polar Research Center disclosed Tuesday.

Analysis of a 1,013-foot deep core from the Guliya icecap, one of three taken by a Chinese-American expedition last summer, is still in its early stages. But expedition leader Lonnie G. Thompson said it could provide extraordinary insight into climatic patterns through the last four ice ages.

The ability to study climatic patterns has been critical to the debate over the phenomenon called “global warming.”

Some scientists believe--and some ice core studies seem to indicate--that humanity’s production of carbon dioxide is leading to a potentially dangerous overheating of the planet. But skeptics contend there is no evidence the warming exceeds the climate’s natural variations.

In ice cores, where there has been no melting through the centuries, air bubbles, dust, cosmic particles and gases such as carbon dioxide are trapped to form what amounts to fossils of the atmosphere.

“These long-term archives,” Thompson said, “will let us look at the natural variability of the climate over long periods of time and consequently determine if and how the 20th Century is different. There are so many aspects of the climate system recorded in this ice. We should be able to really look at the environment and start to seek the linkages between mechanisms and forces which drive the climate system.”

One study of a core drilled at the former Soviet Union’s Vostok research station in Antarctica has shown that the temperature and carbon dioxide content of the atmosphere have risen and fallen together for the last 170,000 years.

But scientists have yet to resolve the ultimate question raised by that stunning discovery: Do the changes in carbon dioxide levels cause the changes in temperature?

Both American and European scientists also are studying deep cores taken at the summit of the Greenland ice sheet to expand on the findings of the Antarctica project.

European scientists reached bedrock last summer, and the American team expects to complete its retrieval of a 10,000-foot core next summer. Project scientists plan to report some of their early findings next week at a San Francisco meeting of the American Geophysical Union.

Because Greenland experiences a heavier snowfall than Antarctica, it is expected that the new cores there will provide clearer long-term climate data. But there is no guarantee that the results from Greenland or Antarctica are representative of the global climate, making the new core recovered in Tibet all the more important.

While the biggest ice-coring projects have been carried out in polar regions, Thompson has concentrated on lower latitudes and the pursuit of the climate record of more recent geologic history. The Guliya icecap lies at 35 degrees latitude--the same distance from the Equator as Oklahoma City.

“That’s within the range of latitudes where most human beings live,” Thompson said, “which makes this a very significant climate record.”

An earlier expedition to Tibet produced evidence that the climate of Central Asia has been warmer over the last 50 years than at any time since the last ice age.

The 1992 expedition, like the previous project in the Far East, was a joint venture of Ohio State’s Byrd Center, the Polar Ice Coring Office of the University of Alaska at Fairbanks and the Lanzhou Institute of Glaciology and Geocryology in China. It was funded by the National Science Foundation and the National Geographic Society and included Russian and Peruvian scientists, as well as Americans and Chinese.

Depending on how well the bottom of the core is preserved, Thompson said, it is conceivable that it could contain ice that formed from the first snows that fell on the Tibetan Plateau a million years ago.

Retrieving detailed data from the lower portion of the core is extremely difficult, however, because the ice is more highly compressed at greater depths. But while the party was still on the icecap last summer, scientists were able to count annual cycles going back 8,000 years by noting layers of dust blown off the Gobi desert.

The dust layers, providing markers as reliable as the rings that reveal tree ages, were especially distinct during the last ice age, when the climate is believed to have been drier than it has been over the last 10,000 years since the great polar ice sheets receded.

American scientists working on the Greenland ice sheet project have been able to discern the annual cycles going back 40,000 years, so far.