Arctic sea ice is melting three times faster than many scientists project, U.S. researchers reported Monday, just days ahead of the next major international report on climate change.
Scientists at the National Center for Atmospheric Research and the University of Colorado in Boulder concluded, using actual measurements, that Arctic sea ice has declined at an average rate of about 7.8 percent per decade between 1953 and 2006.
By contrast, 18 computer models used by the Intergovernmental Panel on Climate Change, a U.N.-sponsored climate research group, estimated an average rate of decline of 2.5 percent per decade over the same period, the researchers said.
International delegates are meeting in Bangkok, Thailand, this week to hammer out the final wording of the third IPCC report.
Both the observations cited in the new study and projections from the IPCC computer models are for September, when Arctic sea ice is typically at its low point for the year.
For March, when the ice is typically at its most extensive, the new study found the rate of decline was 1.8 percent per decade, about three times larger than the mean from the computer models.
The researchers said their observations indicate the retreat of summertime Arctic sea ice is about 30 years ahead of the pace projected by climate models.
"While the ice is disappearing faster than the computer models indicate, both observations and the models point in the same direction: the Arctic is losing ice at an increasingly rapid pace and the impact of greenhouse gases is growing," said NCAR scientist Marika Holland, one of the study's co-authors.
Gavin Schmidt, a climate researcher at the Goddard Institute for Space Studies who wasn't involved in the study, said the study is "a good reminder that uncertainty in model projections cuts both ways."
Critics of some global warming scenarios say the models exaggerate the potential problems.
"My feeling (along with the authors) is that it is likely that the models are insufficiently sensitive," Schmidt said in an e-mail to The Associated Press.
He said the reasons for the lack of sensitivity are unclear.
"Overall, the models have a track record of getting large scale changes right, particularly in temperatures, but at the regional scale (like in the Arctic), there is more variability," he wrote.
The Boulder-based researchers used a combination of early reports from aircraft and ships and more recent satellite measurements to come up with their observations of the ice melt.
They said the discrepancy between their observations and computer projections indicate computer models may have failed to portray the entire impact of increasing levels of greenhouse gases such as carbon dioxide in the atmosphere.
The computer models indicated that increased greenhouse gases and natural climate variations were about equally responsible for ice loss between 1979 and 2006, the researchers said. They said their own study indicates greenhouse gases may have a "significantly greater" role than the models suggested.
A number of factors may lead the computer models to underestimate the rate of decline in sea ice, the researchers said.
Several models overestimated the thickness of the ice, and the models may have failed to fully account for changes in currents in the atmosphere and oceans that transfer heat to polar regions, they said.
The study, "Arctic Sea Ice Decline: Faster Than Forecast," will appear in the online edition of Geophysical Research Letters on Tuesday, three days before the IPCC issues its report.