Global Warming Could Forestall Ice Age

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, to be published 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,” Dr. Kaufman said.

Several scientists who were not involved with the study concurred that the pace of the temperature reversal far exceeded the natural variability in Arctic temperatures, supporting the idea that the warm-up is human-caused and potentially disruptive.

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.

“The fast rate of recent warming is the scary part,” Dr. Overpeck said. “It means that major impacts on Arctic ecosystems and global sea level might not be that far off unless we act fast to slow global warming.”

In the very long term, the ability to artificially warm the climate, particularly the Arctic, could be seen as a boon as the planet’s shifting orientation to the Sun enters a phase that could initiate the next ice age.

As a result of such periodic shifts, 17 ice ages are thought to have come and gone in two million years. The last ice age ended 11,000 years ago and the next one, according to recent research, could be 20,000 or 30,000 years off discounting any influence by humans. The last ice age buried much of the Northern Hemisphere under a mile or more of ice.

With humans’ clear and growing ability to alter the climate, Dr. Overpeck said, “we could easily skip the next opportunity altogether.”