The human-driven buildup of heat-trapping greenhouse gases in the atmosphere appears to have ended a slide, many millenniums in the making, toward cooler summer temperatures in the Arctic, the authors of a new study report.

Scientists familiar with the work, published on Friday in the journal Science, said it provided fresh evidence that human activity is not only warming the globe, particularly the Arctic, but could also even fend off what had been presumed to be an inevitable descent into a new ice age over the next few dozen millenniums.

The reversal of the slow cooling trend in the Arctic, recorded in samples of layered lakebed mud, glacial ice and tree rings from Alaska to Siberia, has been swift and pronounced, the team writes.

Earlier studies have also shown that the Arctic, more than the planet as a whole, has seen unusual warming in recent decades. But the new analysis provides decade-by-decade detail on temperature trends going back 2,000 years — five times further than previous work at that detailed a scale.

Several climate scientists said the new study was most significant for showing just how powerfully the Arctic climate appears to be responding to a greenhouse-gas buildup that is having more complex and subtle mix of effects elsewhere around the globe.

Darrell S. Kaufman, the lead author and a climate specialist at Northern Arizona University, said the biggest surprise was the strength of the shift from cooling to warming, which started in 1900 and intensified after 1950.

“The slow cooling trend is trivial compared to the warming that’s been happening and that’s in the pipeline,” Kaufman said.

According to the study, after a slow cooling of less than half a degree Fahrenheit per millennium, driven by a cyclical change in the orientation of the North Pole and the Sun, the region warmed 2.2 degrees just since 1900, with the decade from 1998 to 2008 the warmest in 2,000 years.

In theory, summer temperatures in the Arctic region would be expected to cool for at least 4,000 more years, given the growing distance between the Sun and the North Pole during the summer in the Northern Hemisphere, the study says.

But Jonathan T. Overpeck, a study author and climate specialist at the University of Arizona, said the rising concentration of long-lived greenhouse gases guaranteed warming at a pace that could stress ecosystems and cause rapid melting of Greenland’s great ice sheet. — © 2009 The New York Times News Service

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