Arctic air temperatures in the 1990s were the warmest in the last 2,000 years and were a result of rising greenhouse gas levels, a new study concludes.

The findings, detailed in the Sept. 4 issue of the journal Science, also suggest that if it weren’t for these manmade pollutants, temperatures around the North Pole would actually be cooling as a result of natural climate patterns.

"This result is particularly important because the Arctic, perhaps more than any other region on Earth, is facing dramatic impacts from climate change," said study team member David Schneider of the National Center for Atmospheric Research (NCAR). "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."

Natural archives

The researchers uncovered this masked cooling trend by reconstructing Arctic temperatures over the past two millennia with data from Arctic lake sediments, glacial ice and tree rings, all of which provide records of the changes in temperatures up there.

These natural archives indicated a pervasive cooling across the Arctic on a decade-by-decade basis that is related to an approximately 21,000-year cyclical wobble in Earth's tilt relative to the sun.

Over the last 7,000 years, the timing of Earth's closest pass by the sun has shifted from September to January. This has gradually reduced the intensity of sunlight reaching the Arctic in the Northern Hemisphere's summertime, when Earth is farther from the sun (the main driver of summer temperatures is the fact that the hemisphere is tilted toward the sun during these months, while it is tilted away from the sun during winter).

The team's temperature analysis shows that summer temperatures in the Arctic, in step with the reduced energy from the sun, cooled at an average rate of about .35 degrees Fahrenheit (0.2 degrees Celsius) per thousand years. The temperatures eventually bottomed out during the "Little Ice Age," a period of widespread cooling that lasted roughly from the 16th to the mid-19th centuries.

The study is useful because it isolates the temperature changes of the Arctic region from the larger signal of the Northern Hemisphere — the orbital changes are known to have more of an effect on the high latitudes than the lower ones, and this is born out in the study's findings, said Michael Mann of Penn State University, who did not work on the new study.

Trend overwhelmed

Even though the orbital cycle that produced the cooling continued, it was overwhelmed in the 20th century by human-induced warming caused by the buildup of greenhouse gases, such as carbon dioxide, in 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 team member Bette Otto-Bliesner, also of NCAR.

The study found that the 10 years from 1999 to 2008 was the warmest in the Arctic in two millennia. Arctic temperatures are now 2.2 degrees Fahrenheit (1.2 C) warmer than in 1900.

"The amount of energy we're getting from the sun in the 20th century continued to go down, but the temperature went up higher than anything we've seen in the last 2,000 years," said team member Nicholas P. McKay of The University of Arizona in Tucson.

The scientists compared the temperatures inferred from the field-based data with computer model simulations. The model's estimate of the reduction of seasonal sunlight in the Arctic and the resulting cooling was consistent with the analysis of the lake sediments and other natural archives. These results give scientists more confidence in computer projections of future Arctic temperatures.

"This study provides a clear example of how increased greenhouse gases are now changing our climate, ending at least 2,000 years of Arctic cooling," says NCAR scientist and team member Caspar Ammann.

The new study follows previous work showing that temperatures over the last century warmed almost three times faster in the Arctic than elsewhere in the Northern Hemisphere. This phenomenon, called Arctic amplification, occurs as highly reflective Arctic ice and snow melt away, allowing dark land and exposed ocean to absorb more sunlight. This amplification could lead to potentially catastrophic melting of Arctic sea ice and land-based glaciers, which could impact Arctic wildlife, indigenous peoples and global sea levels.

"The warming of the past century, which has been shown to very likely be due in substantial part to human-caused increases in greenhouse gas concentrations, is once again seen to be without precedent in a very long-term (at least 2000 year in this case) context," Mann told LiveScience. "Yet one other line of evidence that the changes that are taking place today are indeed without precedent."