Arctic Greening Linked To Retreating Sea Ice

Walker will present results of his Greening of the Arctic study at the American Geophysical Union fall meeting in San Francisco on Dec. 16, 2008.

If the Arctic continues to warm as predicted, large changes in vegetation will have important consequences for the status of permafrost, depth of the thaw layer, snow patterns, hydrological cycles, wildlife and human uses of arctic landscapes. There will also be significant feedbacks to climate through changes in the carbon flux and in the amount of light and heat reflected by the land.

Walker’s group combined information from Earth-orbiting satellites with ground-based studies and climate analyses to examine the trends of sea ice, land temperatures and vegetation using a simple numerical indicator of greenness called the Normalized Difference Vegetation Index. Vegetation reflects near-infrared light and absorbs visible light, Walker said: the greater the difference between the infrared and visible channels, the greater the density of green vegetation. The index also accounts for things like shadow and slope-aspect.

After examining 28 years of sea ice, land temperature and NDVI data, Walker’s group found that between 1982 and 2007, summer sea-ice cover declined by 27 percent in a 50-kilometer band along Arctic Ocean coastlines. Corresponding changes in the greening index varied, ranging from a relatively large 24-percent increase along the Beaufort Sea in northern Alaska and Canada to a 12-percent decline along the Laptev Sea in Russia.

Walker’s group is also measuring vegetation changes along an 1800-kilometer line, or transect, running from Alaska’s North Slope to Ellef Ringnes Island in Canada and a 700-kilometer transect in Russia’s Yamal Peninsula. Each crosses five different vegetative and climatic regions, called bioclimate subzones.

“If you have a natural climate gradient and you study it from cold to warm you have a good idea of what will happen when climate changes,” Walker said. “We’re using the transects as an analog for climate change.”

Modelers have predicted that the Arctic Ocean could be ice-free in late summer by as early as the mid-21st century. “If this happens one of our major concerns is that entire ecosystems in the far north where there is currently perennial sea ice will vanish,” said Walker.

The Greening of the Arctic project received funding from NASA and the National Science Foundation. It is part of UAF’s work during the International Polar Year, an international event that is focusing research efforts and public attention on the Earth’s polar regions.

Greening of the Arctic collaborators University of Alaska Fairbanks: Donald “Skip” Walker, Uma Bhatt, Vladimir Romanovsky, Martha Raynolds, Gary Kofinas, Patrick Kuss. University of Virginia: Howie Epstein, Qin Yu, Ben Cook, Domingo Alcaraz; NASA Goddard: Joey Comiso; Earth Cryosphere Institute, Moscow, Russia: Marina Liebman, Nataliya Moskalenko, Pavel Orekov, George Matyshak, Anatoly Gubarkov, Artem Khomutov. Arctic Centre, Rovaniemi, Finland: Bruce Forbes, Florian Stammler, Timo Kumpula, Elina Karlejaärvi. REC-TEA, Chinese Academy of Science: Gensu Jia. Swiss Federal Institute for Forest, Snow, and Landscape Research, Lausanne: Jed Kaplan, Hieke Lieschke.