The amount of ice that has melted away from Greenland's ice sheet this year could cover an area twice the size of the United States, with more melting occurring in 2007 than the average going back to 1988, a new study finds.

Using satellite data, NASA scientists compared the average snow that melted from Greenland annually between 1988 and 2006 with the snow that melted away this summer and found an overall rising trend in the amount of melt, especially at high altitudes where melt was 150 percent higher this year than the average.

The melting index—an indicator of where melting is occurring and its duration—for Greenland's high altitude areas (those over 1.2 miles above sea level) was significantly higher than usual so far this year. Melting in these areas lasted for 25 to 30 days longer this year than for the average over the last 19 years.

"When snow melts at those high altitudes and then refreezes, it can absorb up to four times more energy than fresh, unthawed snow," said study leader Marco Tedesco of NASA's Goddard Space Flight Center in Greenbelt, Md. "This can affect Earth's energy budget by changing how much radiation from the sun is absorbed by the Earth versus that reflected back into the atmosphere. Refrozen snow can also alter the snow density, thickness and snow-water content."

As this refrozen snow absorbs more energy, it can cause even more snow to melt in subsequent seasons.

The 2007 melting index for lower altitudes was also high, ranking in fifth place behind 2005, 2002, 1998 and 2004, in that order.

"Increases in the overall melting trend over Greenland have an impact that stretches beyond its icy shores," Tedesco said. "Aside from contributing to direct sea level rise, melting especially along the coast can speed up glaciers since the meltwater acts like a lubricant between the frozen surface and the bedrock deep below. The faster glaciers flow, the more water enters the ocean and potentially impacts sea level rise."

Tedesco's findings are detailed in the Sept. 25 issue of the American Geophysical Union's Eos newspaper.