WASHINGTON -- Warming in the Arctic is stimulating the growth ofvegetation and could affect the delicate energy balance there, causingan additional climate warming of several degrees over the next fewdecades. A new study indicates that as the number of dark-coloredshrubs in the otherwise stark Arctic tundra rises, the amount of solarenergy absorbed could increase winter heating by up to 70 percent. Theresearch will be published 7 September in the first issue of theJournal of Geophysical Research-Biogeosciences, published by theAmerican Geophysical Union.
The study in western Alaska during the winters in 2000-2002 shows howthe increasing abundance of high-latitude vegetation, particularlyshrubs, interacts with the snow and affects Earth's albedo, or thereflection of the Sun's rays from the surface. The paper, which alsoanalyzes the ramifications of continued plant growth in the tundraregions, written by researchers at the U.S. Army Cold Regions Researchand Engineering Laboratory and at Colorado State University. Itpresents the first evidence that shrub growth could alter the winterenergy balance of the Arctic and subarctic tundra in a substantial way.
The authors measured five adjacent sites in subarctic Alaska. Theyincluded areas covered by continuous forest canopy, others dotted withshrubs, and some of barren tundra. They found that mid-winter albedowas greatly reduced where shrubs were exposed and that melting beganseveral weeks earlier in the spring at these locations, as compared tosnow-covered terrain. The researchers note, however, that the shrubs'branches produced shade that slowed the rate of melting, so that thesnowmelt finished at approximately the same time for all the sites theyexamined.
Matthew Sturm, lead author of the study, notes that warming in theregion seems to have stimulated shrub growth, which further warms thearea and creates a feedback effect that can promote higher temperaturesand even more growth. This feedback could, in turn, accelerateincreases in the shrubs' range and size over the four million squarekilometer [1.5 million square mile] tundra and effect significantchanges over the region.
"Basically, if tundra is converted to shrubland, more solar energy willbe absorbed in the winter than before," Sturm says. And while previousresearch has shown that warmer temperatures during the Arctic summerenhance shrub growth, "our study is important because it suggests thatthe winter processes could also contribute to and amplify the rate ofthe [growth]."
Sturm cites satellite and photographic evidence showing increasingplant growth across the Alaskan, Canadian, and Euro-Asian Arctic andnotes that continued warming will likely produce thicker stands ofbrush that protrude above the snow. The new, brushy landscape wouldreplace the smooth, white environment that currently dominates theArctic during its 8-10 month winter.
In addition, the increasing shrub cover would impact more thanjust the energy balance in the Arctic. With nearly 40 percent of theworld's soil carbon is stored in Arctic soils, any change in vegetationand energy is likely to trigger a response in the Arctic carbon budget.Scientists are still trying to understand the nature of this response,but Sturm and his coauthors conclude that the feedback effects theydescribe would undoubtedly accelerate its rate. They conclude thatcombined effects of increasing shrubs on both energy and carbon couldchange the Arctic in a way that affects the rest of the world.
The research was supported by the National Science Foundation.
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