An already relentless melting of the Arctic greatly accelerated this summer, a warning sign that some scientists worry could mean global warming has passed an ominous tipping point. One even speculated that summer sea ice would be gone in five years. Greenland's ice sheet melted nearly 19 billion tons more than the previous high mark, and the volume of Arctic sea ice at summer's end was half what it was just four years earlier, according to new NASA satellite data obtained by The Associated Press. "The Arctic is screaming," said Mark Serreze, senior scientist at the government's snow and ice data center in Boulder. Just last year, two top scientists surprised their colleagues by projecting that the Arctic sea ice was melting so rapidly that it could disappear entirely by the summer of 2040. This week, after reviewing his own new data, NASA climate scientist Jay Zwally said: "At this rate, the Arctic Ocean could be nearly ice-free at the end of summer by 2012, much faster than previous predictions." The Arctic is often cited as the canary in the coal mine for climate warming," said Zwally. "Now, as a sign of climate warming, the canary has died. It is time to start getting out of the coal mines." What happens in the Arctic has implications for the rest of the world. Faster melting there means eventual sea-level rise and more immediate changes in winter weather because of less sea ice. In the United States, a weakened Arctic blast moving south to collide with moist air from the Gulf of Mexico can mean less rain and snow in some areas, including the drought-stricken Southeast, said Michael MacCracken, a former federal climate scientist who now heads the nonprofit Climate Institute. Some regions, including Colorado, would likely get extra rain or snow.

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