Climate Switch 118,000 Years Ago May Hold Clues To Earth's Future

November 12, 1997

A new Massachusetts Institute of Technology study that uncovered evidence for profound climate change at the end of the last interglacial period 118,000 years ago is a tantalizing piece of research for global-change scientists, according to a University of Colorado at Boulder researcher.

Scott Lehman of CU's Institute for Arctic and Alpine Research said the MIT study published in the Nov. 13 issue of Nature indicates a rapid reduction in global, deep-water ocean circulation roughly 118,000 years ago may have triggered Earth's most recent ice age. Lehman authored a News and Views article in the Nov. 13 issue of the weekly British science magazine which assessed the significance of the MIT study led by Jess Adkins.

The MIT team used geochemical dating methods on a core of seabed sediments more than 150 feet long taken from the Sargasso Sea in the North Atlantic to arrive at their conclusions. The core, which contains sediments and fossils dating back 160,000 years, showed significant chemical changes in the deep sea during the final 400 years of the last interglacial period, which lasted from about 127,000 to 118,000 years ago.

The MIT study indicates that during most of the last interglacial period, the "conveyor belt" circulation which today carries ocean heat north from the tropics and warms much of Europe, remained strong and steady. "Most interestingly, they also find evidence that the onset of ice growth marking the end of the interglacial was accompanied by a sudden reduction in the conveyor circulation, which took only 400 years and from which the climate system apparently never recovered," Lehman wrote in Nature.

"These are important findings," said Lehman, also a research professor in CU-Boulder's geological sciences department. "They show an unexpected blip in the ocean circulation at the end of the last interglacial that appears to have thrown the climate back into an ice age."

Scientists still do not know why or how this may have happened, Lehman said. "Some climate-modeling scenarios suggest warming temperatures on Earth 'choke down' ocean circulation by ice-sheet melting and increasing precipitation." Climate models that factor in the rapidly increasing carbon dioxide levels in Earth's atmosphere indicate the greenhouse gas likely is speeding the warming process.

Air temperatures have been relatively high and steady through most of the current interglacial period that began about 9,000 years ago. A well-known exception is the Little Ice Age -- a period marked by a cooling of the North Atlantic region that began about 1500 and lasted several hundred years.

At the time, semi-permanent snow fields blanketed Labrador and Baffin Island, increasing the reflection of sunlight from Earth back into the atmosphere. Moreover, solar radiation levels striking Earth -- which change in step with a periodic wobble in Earth's axis -- had been on the wane for some time and continue to wane today, said Lehman.

"In this context, a slight weakening of the ocean circulation can be likened to a sledgehammer blow -- one which could conceivably bring the present interglacial to its knees," wrote Lehman. "Such a possibility only adds to concerns about the potential impact of increased greenhouse forcing on the oceans."

One problem, says Lehman, is that scientists don't know how much of current climate change is natural and how much is human-caused. "But it could be double trouble if we confirm both natural and anthropogenic warming in the coming decades or centuries," he said, noting that only additional research will answer that question.

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University of Colorado at Boulder

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