More than two trillion tons of land ice in Greenland, Antarctica and Alaska have melted since 2003, according to new NASA satellite data which scientists say is the latest sign that global warming is accelerating.
More than half of the loss of landlocked ice in the past five years has occurred in Greenland, based on measurements of ice weight by NASA's GRACE satellite.
NASA geophysicist Scott Luthcke said the Greenland melt seems to be accelerating.
NASA scientists will present their findings on Thursday at the American Geophysical Union conference in San Francisco. Mr Luthcke said Greenland figures for the summer of 2008 aren't complete yet, but this year's ice loss, while still significant, won't be as severe as 2007.
The news was better for Alaska. After a precipitous drop in 2005, land ice increased slightly in 2008 because of large winter snowfalls, Luthcke said. Since 2003, when the NASA satellite started taking measurements, Alaska has lost 400 billion tons of land ice.
In assessing climate change, scientists generally look at several years to determine the overall trend.
Melting of land ice, unlike sea ice, increases sea levels very slightly. In the 1990s, Greenland didn't add to world sea level rise; now that island is adding about half a millimeter of sea level rise a year, NASA ice scientist Jay Zwally said in a telephone interview from the conference.
Between Greenland, Antarctica and Alaska, melting land ice has raised global sea levels about one-fifth of an inch in the past five years, Luthcke said. Sea levels also rise from water expanding as it warms.
Other research, being presented this week at the geophysical meeting point to more melting concerns from global warming, especially with sea ice.
"It's not getting better; it's continuing to show strong signs of warming and amplification," Zwally said. "There's no reversal taking place."
Scientists studying sea ice will announce that parts of the Arctic north of Alaska were 9 to 10 degrees warmer this past autumn, a strong early indication of what researchers call the Arctic amplification effect. That's when the Arctic warms faster than predicted, and warming there is accelerating faster than elsewhere on the globe.
As sea ice melts, the Arctic waters absorb more heat in the summer, having lost the reflective powers of vast packs of white ice. That absorbed heat is released into the air in the fall. That has led to autumn temperatures in the last several years that are six to 10 degrees warmer than they were in the 1980s, said research scientist Julienne Stroeve at the US National Snow and Ice Data Centre in Boulder, Colorado.
That's a strong and early impact of global warming, she said.
"The pace of change is starting to outstrip our ability to keep up with it, in terms of our understanding of it," said Mark Serreze, senior scientist at the National Snow and Ice Data Center in Boulder, Colo., a co-author of the Arctic amplification study.
Two other studies coming out at the conference assess how Arctic thawing is releasing methane - the second most potent greenhouse gas. One study shows that the loss of sea ice warms the water, which warms the permafrost on nearby land in Alaska, thus producing methane, Stroeve says.
A second study suggests even larger amounts of frozen methane are trapped in lakebeds and sea bottoms around Siberia and they are starting to bubble to the surface in some spots in alarming amounts, said Igor Semiletov, a professor at the University of Alaska in Fairbanks.
In late summer, Professor Semiletov found methane bubbling up from parts of the East Siberian Sea and Laptev Sea at levels that were 10 times higher than they were in the mid-1990s, he said based on a study this summer.
The amounts of methane in the region could dramatically increase global warming if they get released, he said.
That, Professor Semiletov said, "should alarm people."
AP