WASHINGTON - 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 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, Colo.
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 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."
So scientists in recent days have been asking themselves these questions: Was the record melt seen all over the Arctic in 2007 a blip amid relentless and steady warming? Or has everything sped up to a new climate cycle that goes beyond the worst-case scenarios presented by computer models?
It is the burning of coal, oil, and other fossil fuels that produces carbon dioxide and other greenhouse gases, responsible for man-made global warming, scientists say.
For the past several days, diplomats have been debating in Bali, Indonesia, the outlines of a new climate treaty calling for tougher limits on these gases.
Yesterday, Senator John F. Kerry, Democrat of Massachusetts, said that a delegation from China, one of the biggest producers of the gases linked to global warming, told him at the international climate-change conference that it was ready to cut greenhouse gas emissions if the United States first set mandatory reductions of its own - a plan the Bush administration has consistently rejected.
The message from China "was very, very significant," Kerry told reporters in a teleconference yesterday. "Every country there [at the Bali conference] emphasized the importance of the US to lead. Up to now, our absence has been an excuse for countries to postpone action."
John Donnelly of the Globe staff contributed to this report.