Arctic Melt
Earth's North and South Poles are famous for being cold and icy．Last year，however，the amount of ice in the Arctic Ocean(北冰洋)fell to a record low．
Normally，ice builds in Arctic waters around the North Pole each winter and shrinks(缩小)during the summer．But for many years，the amount of ice left by the end of summer has been declining．
Since 1979，each decade has seen an 11.4 percent drop in end-of-summer ice cover．Between 1981 and 2000．ice in the Arctic lost 22 percent of its thickness-becoming 1.13 meters thinner．
Last summer，Arctic sea ice reached its thinnest levels yet．By the end of summer 2007，the ice had shrunk to cover just 4.2 million square kilometers．That's 38 percent less area than the average cover at that time of year．And it's a very large 23 percent below the previous record low，which was set just 2 years ago．This continuing trend has made scientists concerned．
There may be several reasons for the ice melt，says Jinlun Zhang，an oceanographer(海洋学家)at the University of Washington in Seattle．Unusually strong winds blew through the Arctic last summer． The winds pushed much of the ice out of the central Arctic．1eaving a large area of thin ice and open wa ter．
Scientists also suspect that fewer clouds cover the Arctic now than in the past．Clearer skies allow more sunlight to reach the ocean．The extra heat warms both the water and the atmosphere．In parts of the Arctic Ocean last year，suce temperatures were 3.5? Celsius warmer than average and 1.5℃ warmer than the previous record high．
With both air and water getting warmer，the ice is melting from both above and below．In some parts of the Beaufort Sea，north of Alaska and western Canada，ice that measured 3.3 meters thick at the beginning of the summer measured just 50 cm by season's end．
The new measurements suggest that melting is far more severe than scientists have seen by just looking at ice cover from above．Some scientists fear that the Arctic is stuck in a warming trend from which it may never recover．
The word“builds”in paragraph 2 could be best replaced by______．