Were we on our way to becoming a planet of snow and ice before we began producing tons of greenhouse gases every year since the Industrial Revolution?

An international team of scientists thinks so. They looked at the temperatures “recorded” in Arctic ice, tree rings and lake sediments and found a sudden reversal in millenniums of naturally occurring cooling.

Darrell S. Kaufman, climate specialist at Northern Arizona University, led the recently released study (Science, September 4) that shows Arctic temperatures warmed 2.2 degrees Fahrenheit since 1900. For 2,000 years before that, the Arctic had been cooling about half a degree Fahrenheit per millennium as the planet continued slowly sliding toward an ice age. Kaufman notes: “The slow cooling trend is trivial compared to the warming that’s been happening and that’s in the pipeline.”

The long cooling trend resulted naturally from a cyclical wobble in Earth’s axis of rotation, increasing the distance between Earth and Sun and reducing the intensity of summer sunlight in the Arctic. The wobble, still in its 21,000-year cycle, continued into the 20^th century and should be causing Arctic temperatures to drop in the 21^st century. Instead, Arctic temperatures rose to their warmest of the last 2,000 years in just the last decade, 1998 – 2008.

According to a press release from the National Science Foundation, a sponsor of the research, the 2,000-year reconstruction of Arctic temperatures incorporated field-based data that captured the response of different components of the Arctic’s climate system to changes in temperature. The data included sediments from Arctic lakes and records from glacial ice and tree rings. This produced the longest view of past climate yet available. Earlier views covered only 400 years.

A computer simulation that incorporated the rotation wobble and the reduced seasonal sunlight in the Arctic was found to match the field-based data. Together the computer simulation and data from the field give the scientists confidence in their reconstruction. However, their finding that human-induced greenhouse gases would quickly cancel out the long-term cooling was troubling.

Other studies have shown 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.

“Because we know that the processes responsible for past Arctic amplification are still operating, we can anticipate that it will continue into the next century,” says Gifford Miller of the University of Colorado at Boulder, a member of the study team. “Consequently, Arctic warming will continue to exceed temperature increases in the rest of the Northern Hemisphere, resulting in accelerated loss of land ice and an increased rate of sea level rise, with global consequences.”

The NSF press release contains video clips of an interview with Kaufman, photos of data gathering, and a graphic of the wobble.