Signs of unsteady climate appear in ice.

Geoscientists have discovered evidence of a widespread cooling 8,200 years ago that raises questions about the stability of the modern climate.

Researchers have usually considered the current geologic time period, the Holocene epoch, immune to the wild temperature shifts of the last ice age, which ended 10,000 years ago. In theory, the giant glacial sheets on North America and Europe during the ice age destabilized the climate; once the ice melted, the climate calmed down.

New findings culled from a borehole in central Greenland now challenge that idea. At a meeting of the American Geophysical Union in Baltimore this week, Richard B. Alley of Pennsylvania State University in University Park and his colleagues reported that the climate of the northern latitudes, and possibly much of the globe, cooled markedly 8,200 years ago, after the ice sheets had retreated. "What this means is we have an event here that we cannot blame on the ice sheets," says Alley.

The researchers discovered signs of the cooling preserved within Greenland's thick ice sheet, a remnant of the giant glacial blankets that existed during the ice age. A U.S. team known as the Greenland Ice Sheet Project 2 (GISP 2) drilled through the 3-kilometer-thick covering during the early 1990s. The ice cores from that hole have provided a climate record of the last 100,000 years.

Measurements of oxygen isotopes in the GISP 2 ice indicate that the temperature at the summit of Greenland dropped 4oC for a century--a relatively short, but intense, climate shift, says Alley. The cooling exceeds all other Holocene temperature changes recorded in GISP 2.

Such findings build on the results of earlier drilling projects in Greenland, which also turned up signs of a temperature drop 8,200 years ago. From the previous studies, however, scientists could not tell whether the cooling was limited to Greenland. The records from GISP 2 and from a nearby European hole suggest that the cool snap affected other regions as well.

By sampling the contents of bubbles trapped in the ice, scientists have determined that concentrations of methane in the atmosphere dropped 20 percent during the cold span. Because methane comes predominantly from wetlands, fluctuations in its concentration reflect changes going on outside ice-covered Greenland. Wetlands in the high latitudes or perhaps in the tropics may have played some role in the climate change 8,200 years ago, says Alley.

The ice record also shows a jump in the concentration of tiny soot particles at that time. The soot peak could have come from increased forest fire activity in eastern Canada during the cooling, suggests Ken C. Taylor of the Desert Research Institute in Reno, Nevada.

For now, scientists remain unsure how much the climate changed during the early Holocene cooling. In his studies of soluble chemicals within the GISP 2 core, Paul A. Mayewski of the University of New Hampshire in Durham finds evidence of several other, more dramatic coolings during the Holocene.