Study Finds the Tropics Can Cool

A research team led by scientists at Columbia's Lamont-Doherty Earth Observatory has reported strong new evidence that the tropics cooled along with the rest of the Earth during the last ice age.

The finding, reported in the July 21 issue of Science, contradicts the long-held belief that the tropics remained warm and stable while ice sheets waxed and waned nearer the Earth's poles. If tropical climate has shifted in the past, the potential for future climate change in this widespread, populous region may have to be reassessed, the scientists say.

The new finding is the latest and perhaps strongest clue in a growing body of evidence on tropical climate that is leading scientists to reconsider how Earth's climate system works and how it could change in the future. If tropical climate is changeable, the greenhouse effect could raise average temperatures in the region; an increase of even a few degrees would significantly affect the tropics' vegetation and habitability.

A scientific team led by Lamont-Doherty scientist Martin Stute essentially took the past temperature of an equatorial region in Brazil by tapping an aquifer and analyzing waters that seeped underground tens of thousands of years ago.

They found that the mean annual ground temperature 35,000 to 10,000 years ago, when the last ice age ended, was 5.4 degrees C, or just under 10 degrees F, lower than today.

"On the basis of these results, it appears that the tropical Americas are characterized by a temperature sensitivity comparable to that found in higher latitudes," the scientists said in Science. The research was conducted by Stute, Jordan Clark, Peter Schlosser and Wallace Broecker of Lamont-Doherty, Columbia's earth sciences research institute in Palisades, N.Y.; Markus Forster of Hydroisotop in Schweitenkirchen, Germany; Horst Frischkorn and Alfredo Serejo of the Universidade Federal Do Ceara, in Fortaleza, Brazil, and Georges Bonani of the Institut für Mittelenergiephysik in Zürich, Switzerland.

The tropics' role in global climate change, both in the past and in the future, is a central and controversial issue. In the early 1980s, a major global ocean survey called CLIMAP concluded that tropical oceans showed little or no temperature change during the last ice age--even during intervals of dramatic climate swings in temperate and polar regions. CLIMAP was based on studies of microscopic plankton, whose fossilized remains were buried in seafloor sediments. Species that thrive in warm waters like today's also appeared abundant during the last ice age, leading scientists to conclude that tropical water temperatures had remained warm.

More recent measurements of oxygen isotopes and alkenones found in fossilized planktonic remains--which also are believed to correlate with sea surface temperatures--support the idea of a stable tropics. But other records point to a colder, changeable tropics. Past snowlines and fossilized pollen show that mountain glaciers and vegetation moved more than 3,000 feet down to lower altitudes, indicating that tropical temperatures cooled about 10 degrees F during the last ice age. Still, CLIMAP supporters argue that these do not reflect changes at lower altitudes or in the ocean.

The new evidence reported in Science by Stute and his co-workers comes from an aquifer just south of the equator in northeastern Brazil. The aquifer is "relatively close to the ocean and at a low altitude" and thus provides a link between high-altitude continental and oceanic records, the scientists said. Analyzing groundwater that percolated into the aquifer thousands of years ago during the last ice age, the scientists measured minute concentrations of noble gases, such as neon, argon, krypton and xenon.

The noble gas thermometer technique is based on the straightforward physical principle that the amount of noble gases dissolving in groundwater depends on the temperature of the ground: the colder the air, the colder the ground; the colder the ground, the more noble gases dissolve in the groundwater. When rainwater or meltwater has penetrated some 100 feet into the ground, it has no further contact with the atmosphere and because noble gases are the most stable and non-reactive of elements, their concentrations do not change once they enter an aquifer.

The results indicate that the mean annual temperature during the last ice age was 5.4 degrees C plus or minus 0.6 degrees C (8.6 degrees F to 10.8 degrees F) lower than today, suggesting that equatorial South America did cool significantly at low altitudes, the scientists said.

"Our noble gas record, derived from a site relatively close to the ocean and at low altitude, suggests that the CLIMAP reconstruction has to be questioned, at least in the western equatorial Atlantic."

Studying groundwater in aquifers in Texas, New Mexico and Georgia, the Lamont-Doherty scientists have also found similar ice-age cooling almost 40 degrees north of the equator. Combined with evidence of snowline and vegetation zone shifts in South America, the evidence appears to show that "a broad zone from 40 degrees S to 40 degrees N had cooled more or less uniformly by at least 5 degrees C (9 degrees F)" during the last ice age, the scientists said.

Whether adjacent oceans cooled similarly is still an open question, but the debate heated up last year when Thomas Guilderson and Richard Fairbanks of Lamont-Doherty presented evidence that the tropical ocean was 9 degrees F cooler during the last ice age. They analyzed strontium and calcium isotopes preserved in corals off Barbados, which serve as a thermometer recording sea surface temperatures 20,000 years ago, when ice sheets covered the poles.

"On the basis of the combined evidence, it seems that the glacial tropical oceans bordering the Americas have been significantly cooler than they are today," Stute and colleagues wrote in Science.

The idea that the tropics are sensitive to climate changes is already provoking scientists to reassess how Earth's climate system works.

A changeable tropics would help explain the mystery of how climate change occurred synchronously in both hemispheres in the past.

And it raises the possibility that equatorial regions could undergo dramatic climate fluctuations in the future.

The research was supported by the National Science Foundation, the Department of Energy, the W.M. Keck Foundation, the Lamont-Doherty Earth Observatory Climate Center and the German-Brazilian Project WAVE 5.

Columbia University Record -- September 15, 1995 -- Vol. 21, No. 2