Petermann Glacier, Greenland
On Greenland, tens of thousands of years of snowfall have settled and solidified into a massive sheet of ice. Each summer, snow retreats briefly at low elevations, and a narrow strip of rocky coastline emerges. As it thins and fractures, sea ice that packed into the mouths of fjords over the winter takes on the look of a mosaic made from blue and white stained glass. Glaciers flowing down from the highland interior get a little slushy, at least on top; turquoise blue pools of melt water dot the surface. These seasonal changes are apparent in this image of the Petermann Glacier in far northwestern Greenland. The image was captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite on July 5, 2003.
While some seasonal thawing is typical on Greenland, more dramatic changes are probably in store for the Greenland Ice Sheet in coming decades and centuries. Few places on Earth are warming faster than the Arctic, and scientists have observed widespread thinning around the edges of the Greenland ice sheet since at least the late 1990s. Many glaciers are accelerating. One of the most fundamental questions about Arctic climate change is whether the mass of the Greenland Ice Sheet will remain in balance: can snow accumulation keep pace with melting?
New types of satellite data, including ice elevation and mass measurements, are revealing the answer: the emerging estimate is that the ice sheet is shrinking by 150 to 180 gigatons a year on average (a gigaton is one billion metric tons). Despite Greenland's remoteness, large-scale changes to its ice sheet will have global influence. Most significantly, melting on Greenland will raise sea level. (If the whole sheet melted, sea level would rise more than 20 feet.) Because difference in salinity between different areas of the ocean influence currents, increasing flows of freshwater from melting ice could also change ocean circulation patterns, such as the Gulf Stream.