Arctic climate change is usually viewed as caused by the retreat of sea ice, which reduces high-latitude albedo - a measure of the amount of sunlight reflected off a surface - a change most pronounced in winter.
"Summer warming is more pronounced over land than over sea ice, and atmosphere and sea-ice observations can't explain this," said Terry Chapin, professor of ecology at the University of Alaska Fairbanks' Institute of Arctic Biology and lead author of the paper which appears in the September 22, 2005 advance online publication Science Express.
Using surface temperature records, satellite-based estimates of cloud cover and energy exchange, ground-based measurements of albedo and field observations of changes in snow cover and vegetation, Chapin and co-authors argue that recent changes in the length of the snow-free season have triggered a set of interlinked feedbacks that will amplify future rates of summer warming.
"It's the changes in season length rather than increases in vegetation that explains this observation," Chapin said. Summer warming correlates with a lengthening of the snow-free season that has increased atmospheric heating locally by an amount similar in magnitude to the regional heating expected over multiple decades from a doubling of atmospheric carbon dioxide, say the authors.
"Snowmelt is 2.5 days earlier for each decade we studied," Chapin said.
Two mechanisms explain the pronounced warming over land during the summer. First, the early snow melt increases the length of time the land surface can absorb heat energy. Second, the increase in snow-free ground permits increases in vegetation such shrubs and advances of treelines.
"Continuation of current trends in shrub and tree expansion could further amplify this atmospheric heating 2-7 times," Chapin said.
"This mechanism should be incorporated into climate models," Chapin said. Improved understanding of the controls over rates of shrub expansion would reduce the likelihood of surprises in the magnitude of high-latitude amplification of summer warming.
Researchers were funded by the National Science Foundation, Office of Polar Programs, ARCtic System Science program - the goal ARCSS is to answer the question: What do changes in the arctic system imply for the future?
Journalists may obtain a copy of the paper by contacting Science at: 202-326-6440 or by e-mail: firstname.lastname@example.org
F. Stuart (Terry) Chapin III, Professor of Ecology, Institute of Arctic Biology, University of Alaska Fairbanks. 907-474-7922, email@example.com.
Dr. Matthew Sturm, Research Physical Scientist, U.S. Army, Cold Regions Research and Engineering Laboratory Alaska. 907-353-5183, Matthew.Sturm@erdc.usace.army.mil
Marie Gilbert, Publications and Information Coordinator, Institute of Arctic Biology, University of Alaska Fairbanks. 907-474-7412, firstname.lastname@example.org.