Ice age sent shivers through the tropics.

Among climate scientists, the tropics have the reputation of unwavering stability. When the rest of the globe turned frosty during the last ice age, some 115,000 to 10,000 years ago, Earth's midsection seemed to weather the glacial epoch with little or no cooling.

But gathering evidence is shaking this appraisal by showing that parts of the tropics chilled substantially during the ice age. If duplicated at other sites, the new findings will remove one of the major problems plaguing people trying to forecast Earth's climate.

Martin Stute of Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y., and his colleagues took the temperature of the tropics during the ice age by analyzing groundwater from aquifers in northeastern Brazil at a latitude of 7oS. The researchers measured the concentrations of four noble gases--neon, argon, krypton, and xenon--in water samples pulled from enclosed aquifers hundreds of meters below the surface.

Because gas is more soluble in cooler water, the scientists could gauge the temperature of the water at the time it percolated down into the aquifers.

Using the carbon-14 dating technique, Stute and his colleagues determined an age for each water sample. Samples less than 10,000 years old had an average temperature of 29.6oC, while older samples, from the glacial epoch, averaged 24.2oC, the scientists report in the July 21 Science.

Evidence of a marked drop in temperature so close to the equator conflicts with data collected by oceanographers in the 1970s, which indicate that the low-latitude oceans cooled by less than 2oC.

Another recent study from South America also challenges the idea of unvarying tropical warmth during the ice age. In the July 7 Science, Lonnie G. Thompson of Ohio State University in Columbus and his coworkers describe new climate data collected during an expedition to the Peruvian Andes. The scientists drilled into a glacier at an altitude of 6,048 meters and pulled up two long ice cores containing evidence about past conditions going back into the ice age.

The cores represent the first ice this old found at a tropical site, and they will enable researchers to compare the low-latitude ice record with those of the Arctic and Antarctic. Using the ratio of two oxygen isotopes as a thermometer, the Ohio State team calculated that the Andes were roughly 8oC cooler during the ice age.

The work by Stute and Thompson supports other reports of low-latitude cooling. Last year, scientists studying coral from Barbados found evidence that the tropical western Atlantic Ocean chilled by 5oC during the ice age (SN: 2/19/94, p.124).

"All these diverse types of information appear to give a very consistent story--that temperatures were probably on the order of 5oC colder at the lower elevations and probably 8oC colder at higher elevations," says Ohio State's Ellen Mosley-Thompson.

Lloyd D. Keigwin of the Woods Hole (Mass.) Oceanographic Institution calls the new results intriguing. He notes that oceanographers who study oxygen isotopes in sediments are beginning to find hints of substantial low-latitude cooling during the ice age. But other evidence, based on the species of plankton living in the ocean, continues to support the story of unchanging ice age temperatures--a discrepancy that oceanographers have yet to resolve.

Climate modelers eagerly await an answer. They have always had trouble simulating ice age conditions, because it has proved difficult to cool off the poles and midlatitudes while keeping the low latitudes at today's temperatures. This chronic problem has caused some researchers to wonder whether the models lack some critical element. But the new evidence from Brazil and Peru may vindicate the models after all.