Indeed, the Arctic had been cooling for nearly two millennia before reversing course in the last century and starting to warm as human activities added greenhouse gases to the atmosphere.
"If it hadn't been for the increase in human-produced greenhouse gases, summer temperatures in the Arctic should have cooled gradually over the last century," said Bette Otto-Bliesner, a National Center for Atmospheric Research scientist and co-author of a study of Arctic temperatures published in Friday's edition of the journal Science. The most recent 10-year interval, 1999-2008, was the warmest of the last 2,000 years in the Arctic, according to the researchers led by Darrell S. Kaufman, a professor of geology and environmental science at Northern Arizona University.
Summer temperatures in the Arctic averaged 2.5 degrees Fahrenheit (1.4 degrees Celsius) warmer than would have been expected if the cooling had continued, the researchers said.
The finding adds fuel to the debate over a House-passed climate bill now pending in the Senate. The administration-backed measure would impose the first limits on greenhouse gases and eventually would lead to an 80 percent reduction by putting a price on each ton of climate-altering pollution.
It is the latest in a drumbeat of reports on warming conditions in the Arctic, including:
- A marine scientist reports that Alaskan waters are turning acidic from absorbing greenhouse gases faster than tropical waters, potentially endangering the state's $4.6 billion fishing industry.
- NASA satellite measurements show that sea ice in the Arctic is more than just shrinking in area, it is dramatically thinning. The volume of older crucial sea ice in the Arctic has shrunk by 57 percent from the winter of 2004 to 2008.
- Global warming effects in Alaska also include shrinking glaciers, coastal erosion and the march north of destructive forest beetles formerly held in check by cold winters.
And with the melting of land-based ice, such as the massive Greenland ice cap, sea levels could rise across the world, threatening millions who live in coastal cities.
The new report is based on a decade-by-decade reconstruction of temperatures over the past 2,000 years developed using information from ancient lake sediments, ice cores, tree rings and other samples. The findings were then compared with complex computer climate model simulations created at the National Center for Atmospheric Research in Boulder, Colo.
"This study provides us with a long-term record that reveals how greenhouse gases from human activities are overwhelming the Arctic's natural climate system," commented NCAR scientist David Schneider, a co-author on the study.
Added Jonathan T. Overpeck, a University of Arizona professor of geosciences: "The Arctic should be very sensitive to human-caused climate change, and our results suggest that indeed it is."
In addition, he pointed out, as the Arctic warms there is less snow and ice to reflect solar energy back into space and the newly exposed dark soil and dark ocean surfaces absorb solar energy and warm further, accelerating the warming process.
The Arctic cooling had been the result of a 21,000-year cycle in the Earth's movement that caused the far north to get progressively less summertime energy from the sun for the last 8,000 years. That process won't reverse for another several thousand years.
The research was funded by the National Science Foundation.