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Glacial retreat. Image 1 of 2. Muir Glacier, Glacier Bay National Park, Alaska, photographed in 1941. Glaciers are large, slow-moving expanses of ice, formed from compacted layers of snow. They are sensitive to climate change and in recent years most have retreated dramatically. See image E235/391 for the same glacier seen in 2004. The National Snow and Ice Data Center (NSIDC) is a US body that supports research into snow, ice, glaciers, frozen ground and climate interactions that make up the Earth's cryosphere. The World Data Center for Glaciology (WDC) in Boulder, USA, is responsible for archiving all available glaciological information. (KEYSTONE/SCIENCE PHOTO LIBRARY/WILLIAM O. FIELD, NSIDC, WDC)