FAIRBANKS, AK--In a paper that shows dramatic summer warming in arcticAlaska, scientists synthesized a decade of field data from Alaskashowing summer warming is occurring primarily on land, where a longersnow-free season has contributed more strongly to atmospheric heatingthan have changes in vegetation.
Arctic climate change is usually viewed as caused by the retreat of seaice, which reduces high-latitude albedo -- a measure of the amount ofsunlight reflected off a surface - a change most pronounced in winter.
"Summer warming is more pronounced over land than over sea ice, andatmosphere and sea-ice observations can't explain this," said TerryChapin, professor of ecology at the University of Alaska Fairbanks'Institute of Arctic Biology and lead author of the paper which appearsin the September 22, 2005 advance online publication Science Express.
Using surface temperature records, satellite-based estimates of cloudcover and energy exchange, ground-based measurements of albedo andfield observations of changes in snow cover and vegetation, Chapin andco-authors argue that recent changes in the length of the snow-freeseason have triggered a set of interlinked feedbacks that will amplifyfuture rates of summer warming.
"It's the changes in season length rather than increases in vegetationthat explains this observation," Chapin said. Summer warming correlateswith a lengthening of the snow-free season that has increasedatmospheric heating locally by an amount similar in magnitude to theregional heating expected over multiple decades from a doubling ofatmospheric 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 thesummer. First, the early snow melt increases the length of time theland surface can absorb heat energy. Second, the increase in snow-freeground permits increases in vegetation such shrubs and advances oftreelines.
"Continuation of current trends in shrub and tree expansion couldfurther amplify this atmospheric heating 2-7 times," Chapin said.
"This mechanism should be incorporated into climate models," Chapinsaid. Improved understanding of the controls over rates of shrubexpansion would reduce the likelihood of surprises in the magnitude ofhigh-latitude amplification of summer warming.
Researchers were funded by the National Science Foundation, Office ofPolar Programs, ARCtic System Science program -- the goal ARCSS is toanswer the question: What do changes in the arctic system imply for thefuture?
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