Arctic trees will make things warmer

Vegetation feedback effect amplifies climatic changes.

Warming of the Arctic could speed up if more trees and shrubs grow there. Credit: J. Holmgren/ National Science Foundation

Chalk up another piece of dire news for the Arctic in a globally warmed future. Researchers have identified a previously unknown climate feedback effect suggesting that, as vegetation creeps northward, it will accelerate warming trends already in place.

Higher surface temperatures in recent decades are already making it easier for trees to grow farther north. But "when the vegetation moves in, there will be an amplification of the warming", says Inez Fung, an atmospheric physicist at the University of California, Berkeley. She presented the results of her team's study on 14 December at a meeting of the American Geophysical Union in San Francisco, California.

Fung's team ran a computer model that simulated trees moving into a swathe of bare land — a region more than twice the size of Alaska — at latitudes north of 60 degrees. Because vegetation is darker than bare land, it has lower albedo or reflectiveness so absorbs more of the Sun's energy and contributes to warming at the surface. It's a similar effect, says Fung, to that seen when sea ice melts and exposes darker surface water, thus accelerating the melting effect there.

Bring me a shrubbery

Average temperatures in the Arctic are already rising roughly twice as fast as those in the rest of the world. The team's calculations show that, as a result of new tree and shrub growth, the landscape warmed by at least 1 ºC.

In another twist, records of ancient Arctic pollen suggest that deciduous trees colonize the landscape before evergreen trees. And transpiration in deciduous trees is greater than that in evergreens, which means that they release more moisture as water vapour, a heat-trapping greenhouse gas that also accelerates the warming effect, says Fung.

The work is in press in the Proceedings of the National Academy of Sciences.

Robert Anderson, a geologist at the University of Colorado in Boulder, says that Fung's study is part of a larger picture showing that the Arctic is changing far faster and in far more extreme ways than scientists had thought. "The landscape has already experienced much of the predicted twenty-first-century warming," he says. "And here we are in only the first decade of it."

At the conference, Anderson presented a dramatic video of frozen coastlines in northern Alaska eroding rapidly into the Beaufort Sea. In places, the bluffs are being eroded by 9 to 14 metres per year — rates that are "three to four orders of magnitude higher than common", Anderson says. That could be partly because the sea ice that holds fast to the coast for much of the year is breaking up earlier, and also that the water lapping the shore is warmer than it was.

Asked what her team's findings might mean for the international climate negotiations going on in Copenhagen, Fung says she is impatient to see progress. But she also sounds somewhat fatalistic. "Whatever they agree to," she says, "is not fast enough to stop the changes that we are seeing."

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Inez Fung's website

Arctic report card from NOAA

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Witze, A. Arctic trees will make things warmer. Nature (2009). https://doi.org/10.1038/news.2009.1142

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