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Warming and Melting Mass loss from the ice sheets of Greenland and Antarctica account for a large fraction of global sea-level rise. Part of this loss is because of the effects of warmer air temperatures, and another because of the rising ocean temperatures to which they are being exposed. Joughin et al. (p. 1172) review how ocean-ice interactions are impacting ice sheets and discuss the possible ways that exposure of floating ice shelves and grounded ice margins are subject to the influences of warming ocean currents. Estimates of the mass balance of the ice sheets of Greenland and Antarctica have differed greatly-in some cases, not even agreeing about whether there is a net loss or a net gain-making it more difficult to project accurately future sea-level change. Shepherd et al. (p. 1183) combined data sets produced by satellite altimetry, interferometry, and gravimetry to construct a more robust ice-sheet mass balance for the period between 1992 and 2011. All major regions of the two ice sheets appear to be losing mass, except for East Antarctica. All told, mass loss from the polar ice sheets is contributing about 0.6 millimeters per year (roughly 20% of the total) to the current rate of global sea-level rise.

Ice Sheets and Sea Level Rise By: Claire L. Parkinson, Oceans and Ice Branch, Code 971, NASA Goddard Space Flight Center, Greenbelt MD 20771, e-mail: clairep@neptune.gsfc.nasa.gov Exercise: Determine the amount that sea level would rise, averaged around the globe, in response to the complete melting of (a) the Greenland ice sheet, (b) the Antarctic ice sheet, and (c) both the Greenland and Antarctic ice sheets.

Experts Agree Global Warming Is Melting the World Rapidly Richard A. Kerr Forty-seven glaciologists have arrived at a community consensus over all the data on what the past century's warming has done to the great ice sheets: a current annual loss of 344 billion tons of glacial ice, accounting for 20% of current sea level rise. Greenland's share-about 263 billion tons-is roughly what most researchers expected, but Antarctica's represents the first agreement on a rate that had ranged from a far larger loss to an actual gain. The new analysis, published on page 1183 of this week's issue of Science, also makes it clear that losses from Greenland and West Antarctica have been accelerating, showing that some ice sheets are disconcertingly sensitive to warming.

"An Accelerating Problem A glacier speeds up unexpectedly and scientists pay attention to it. "

Warming to Cause Catastrophic Rise in Sea Level? Stefan Lovgren for National Geographic News Updated April 26, 2004

"Is Warming Causing Alaska Meltdown? Hillary Mayell for National Geographic News December 18, 2001"

"Greenland's Melting Ice One of the world's frigid landscapes is losing a lot of its ice. "