The hulking southern glaciers of Greenland are melting rapidly -- at a rate quicker than previously thought.
In only five years, the amount of freshwater the melting glaciers have dumped into the Atlantic has nearly doubled, which has caused many scientists to conclude that current projections of how fast sea levels will rise have been too low.
The melting glaciers, moving quickly across the ocean, may account for about 17 percent of the estimated one-10th of an inch annual rise in sea levels, according to a study released today by Eric Rignot, of NASA's Jet Propulsion Laboratory in Pasadena, Calif., and his co-author, Pannir Kanagaratnam, of the University of Kansas.
An increase in surface air temperatures appears to be causing the glaciers to flow faster -- at up to 8 to 9 miles a year at their fastest clip. They have also been dumping increased volumes of ice into the Atlantic.
"The behavior of the glaciers that dump ice into the sea is the most important aspect of understanding how an ice sheet will evolve in a changing climate," Rignot told The Associated Press. "It takes a long time to build and melt an ice sheet, but glaciers can react quickly to temperature changes."
That stepped-up flow accounted for about two-thirds of the net 54 cubic miles of ice Greenland lost in 2005. That compares with 22 cubic miles lost in 1996, Rignot said.
Details of the study were presented Thursday at the annual meeting of the American Association for the Advancement of Science. The study appears Friday in the journal Science.
The increased rise in sea level could have possible runaway effects that could produce a dramatic rise in sea levels, potentially making storm damage far worse, by the time today's toddlers reach middle age, some scientists predict.
Scientists also worry about the effect all this fresh-melt water will have on the Atlantic's Gulf Stream "conveyor belt" currents. These currents have long kept the northeastern United States, Britain and northwestern Europe relatively warm for their northern latitudes by transporting heat up from the tropics. Too much freshwater slows these currents, said scientists.
A few weeks ago, scientists announced a surprising discovery -- currents have slowed by 30 percent in recent years.
A dramatic sign of Greenland's melting is the speeding up of the glacier "tongues" of the ice cap as they push down to the sea, and the disintegration of their front walls, which means they spill out even faster. This acceleration has been recorded in several tongues on Greenland's southern coast and is beginning to happen farther north as well, the scientists said in their report.
This is the latest confirmation that global warming is now accelerating and involving interconnected "positive feedback" effects in which the warming in different Earth systems reinforces overall warming, and it is all now happening faster than scientists recently thought possible, according to the report.
The only way to stem the loss of ice would be for Greenland to receive increased amounts of snowfall, said Julian Dowdeswell at the University of Cambridge, in an accompanying study.
"Human activities are releasing greenhouse gases more than 30 times faster than the natural rate of such emissions that produced a similar period of extreme warming in ancient times," according to his study, which is based on evidence from studies of sediment cores.
Ancient natural warming events started slowly and then accelerated -- worrisome evidence that there are natural "positive feedback loops," such as the release of the greenhouse gases carbon and methane as the frozen ground and seabeds that have trapped them thaw, that become "runaway effects" at some point, his study found.
ABC News' Bill Blakemore contributed to this report.