The world’s climate can change in just a few decades without any human intervention. Jeffrey P. Severinghaus of the Scripps Institution of Oceanography, using a new method of analyzing gases trapped in Greenland ice, showed air temperatures warming rapidly at the end of the last ice age about 15,000 years ago.
The study is based on a new technique that analyzes the isotopic chemistry of argon and nitrogen in the ice cores and allows the detection of temperature change with unprecedented precision.
“There was a 16 degree F abrupt warming at the end of the last ice age,” said Severinghaus, lead author of a study in the journal Science [Science 1999 October 29; 286: 930-934]. “It happened within just a couple of decades. The old idea was that the temperature would change over a thousand years.” The rapid temperature increase may have been touched off by a surge in warm currents in the Atlantic Ocean that brought a melting trend to the vast ice sheet covering the Northern Hemisphere. It still took hundreds of years for the ice to recede, but the start of the great thaw was much more sudden than scientists had once thought.
Pieter P. Tans, a scientist at the National Oceanographic and Atmospheric Administration, said that “both the rapidity and the magnitude of the temperature change is surprising.” But scientific caution demands “another piece of evidence to support it.” Climate models, such as used by the UN-IPCC had predicted that an increase in greenhouse gases, such as carbon dioxide, would cause a gradual global warming, with temperatures rising slowly over many decades.
A quite different study supports the story, however. Scott Lehman and Julian Sachs report (in Science, Oct. 22, 1999) that temperatures in the Sargasso Sea (between the West Indies and the Azores) fluctuated repeatedly by up to 9 degrees Fahrenheit from 60,000 to 30,000 years ago. (The last ice age occurred between 11,000 and 10,000 years ago.)
After analyzing sediment deposited during the last ice age, they discovered that extreme temperature fluctuations occurred during and at the end of the period. Even during an ice age, warm oceans can heat up. “What is new here is clear evidence that the warm Atlantic, like the polar Atlantic, was undergoing very large and very rapid temperature changes during the last glacial period,” said Scott Lehman, a research associate at the University of Colorado Institute of Arctic and Alpine Research. The researchers next hope to determine if similar changes occurred in the much larger Pacific Ocean.
Lehman and Sachs reached their conclusions after studying 50 meters of sediment cores hauled up from several miles deep in the Sargasso Sea near Bermuda by French scientists as part of an international project. They analyzed the saturation state of organic molecules from planktonic algae over the past 100,000 years, revealing sea-surface temperatures during that period.
“The warming at the end of the last ice age, about 10,000 years ago, was supported by the disappearance of enormous ice sheets, a one-third increase in atmospheric carbon dioxide levels and changes in the seasonal distribution of the sun’s energy,” said Lehman. “But the abrupt changes we documented during the last ice age seem to be almost entirely ocean driven.”
