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Permafrost — the permanently frozen ground that underlies much of the Arctic land surface — is thawing in many parts of the Arctic. [1] As permafrost thaws, it releases the powerful greenhouse gas methane into the atmosphere, which contributes to further warming in a reinforcing feedback loop. Permafrost plays an essential role in the Arctic ecosystem by making the ground watertight and maintaining the vast network of wetlands and lakes across the Arctic tundra that provide habitat for animals and plants. The thawing of permafrost threatens to drain the wetland breeding sites for numerous species of waterfowl and shorebirds.

Snow cover is also changing in many parts of the Arctic. [2] As temperatures rise, more precipitation is falling as rain instead of snow, which has reduced snow cover in many areas. Snow cover has decreased in spring and summer by 10 percent on average since the early 1970s over the northern hemisphere. [3] Rain-on-snow events, in which rain falls on top of snowfall, have increased significantly across much of the Arctic, with increases of 50 percent recorded over the past 50 years in Arctic Russia. [4] These events change the structure of the snow and create hard ice crusts that can hamper grazing by Arctic herbivores like caribou. Scientists predict that the frequency and extent of rain-on-snow events will increase over the next 50 years in many parts of the Arctic. [5] In the northernmost latitudes of the Arctic, snow cover is becoming deeper in many areas. [6] In winter, warmer air masses that hold more moisture are depositing deeper snow in northern regions, making it difficult for grazers like caribou and muskoxen to reach forage under the snow.

1. Richter-Menge, J., and J. E. Overland, Eds. 2009. 2009: Arctic Report Card 2009, http://www.arctic.noaa.gov/reportcard.
2. McCabe, G.J. and D.M. Wollock. Long-term variability in Northern Hemisphere snow cover and associations with warmer winters. Climatic Change 99: 141-153.
3. ACIA. 2005. Arctic Climate Impact Assessment. Cambridge University Press, Cambridge, UK.
4. Lemke, P., J. Ren, R. B. Alley, I. Allison, J. Carrasco, G. Flato, Y. Fujii, G. Kaser, P. W. Mote, R. H. Thomas, and T. Zhang. 2007. 2007: Observations: Changes in Snow, Ice, and Frozen Ground. in S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor, and H. L. Miller, editors. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK and New York, NY, USA.
5. Rennert, K. J., G. Roe, J. Putkonen, and C. M. Bitz. 2008. Soil thermal and ecological impacts of rain on snow events in the circumpolar Arctic. Journal of Climate 22:2302-2315.
6. McCabe, G.J. and D.M. Wollock. Long-term variability in Northern Hemisphere snow cover and associations with warmer winters. Climatic Change 99: 141-153.

Polar bear photo © Jenny E. Ross/ www.jennyross.com