posted 06-02-2002 07:34 PM            

A study of past climate changes in the South American tropics has challenged traditional understanding of the mechanisms that triggered the advance and retreat of glaciers during the last ice age. The National Science Foundation (NSF)-funded study was published in this week's issue of Science.

A research team found that glaciers in the tropical Andes Mountains retreated several thousand years earlier than North American glaciers during a period of wet climate conditions, and during a time when the sun's warming radiation (solar insolation) was at a minimum.

The finding contradicts traditional thinking that climate conditions in the northern latitudes generate the advance and retreat of global glaciation, and that glaciers retreated during dry climate conditions and when solar insolation was at its maximum, said lead researcher Geoffrey Seltzer of Syracuse University. "If the tropics warmed earlier than the northern latitudes, as our study demonstrates, that means there is something else influencing climate change that we don't yet understand."

David Verardo, director of the NSF's paleoclimate program, which funded the project, said that Seltzer and his colleagues "offer remarkable insights into the intricate movements of a climate tango of sorts between the low and high latitudes as one hemisphere alternately leads or follows in influencing long-term weather. Getting the moves of this tango down is important for understanding climate."

Added Seltzer, "We know from our experiences with El Nino, that the tropics are the focus of energy and water vapor that drives the global climate system. Our research provides a hint that something important happened in the tropics that could be an important trigger for the last retreat of the global ice age."

The researchers compared climate data derived from sediment cores they collected from Lake Titicaca, located on the border of Peru and Bolivia, and Lake Junin, located in Peru, with published data from ice cores collected in Greenland and Antarctica. A key component of the research was an analysis of inorganic sediment accumulation in the lakes. The researchers found that during periods of maximum glaciation, the tropical lakes overflowed and the sediment that flowed into the lakes from the surrounding region contained a high concentration of fine-grained magnetic minerals. During periods of glacial retreat, the sediment was trapped behind moraines (mounds of rock and debris left by glaciers), resulting in a lower concentration of magnetic minerals flowing into Lakes Titicaca and Junin.

"From the analysis, it is clear that the tropical Andes deglaciated several thousand years earlier than higher latitude warming," the authors wrote. "In contrast, maximum glaciation in the U.S. Sierra Nevada persisted several thousand years after deglaciation had commenced in the tropical Andes." If early warming occurred throughout the tropics, this climate change could have been transmitted both atmospherically and by ocean circulation processes to produce deglaciation of alpine and continental ice sheets in the Northern Hemisphere, the scientists believe.