Ice Age In The Tropics

Did the Ice Age cool down the Tropics? According to most scientists, no. But new findings from the 21,463-foot summit of Bolivia's Mount Sajama challenge that Long-held view. In 1997 an international team headed by glaciologist Lonnie Thompson, of the Byrd Polar Research Center at Ohio State University, sampled the mountaintop glacier (see "Thompson's Ice Corps," Natural History, February 1998). Using solar-powered drills, the team extracted two 435-foot-long ice cores, corroborating the tale of past global climates that Thompson had pieced together from other cores drilled in Peru, Antarctica, Greenland, and Tibet.

Sajama's frozen record of past climates--the oldest cores yet recovered from the Tropics--begins with ice formed at least 24,000 years ago, as evidenced by pollen, dust, nitrates, and even intact insects trapped in the lowest layers. During a period of widespread glaciation, the Amazon rainforest was perhaps as much as 50 percent smaller, and tropical temperatures were possibly five to six degrees Fahrenheit cooler than today's--and even tower near the top of the Andes. Thompson's analyses show that beginning about 15,500 years ago, the climate in the Tropics became substantially drier and warmer but then reversed abruptly a thousand years Later. By 9,000 years ago, however, the record shows that global temperatures were similar to those of the present. Another warming trend, beginning only about two hundred years ago, is now thawing the glaciers. Thompson feels there is an urgent need to extract and analyze these records of the earth's climatic history before they become meltwater. ("A 25,000-year tropical climate history from Bolivian ice cores," Science 282, December 4, 1998)