The ice sheet covering Greenland is growing darker in response to global warming, new satellite data show, an effect that reaches into the interior and has altered virtually the entire surface of the island.

The darkening also feeds on itself: The less reflective the ice sheet becomes, the more warmth it absorbs, and the more melting accelerates.

The ice sheet now reflects as much as 20 percent less sunlight during summer than it did from 2000 to 2006.

“The signal is not just localized on the coast,” said Eric Rignot, a research scientist at UC Irvine and NASA’s Jet Propulsion Laboratory who specializes in polar ice but was not involved in the study.

Dirty Ice

Dirty ice along a stream of meltwater in Greenland.
The darker ice absorbs more heat, causing even faster melting.
Photo by Henrik Egede Lassen/Alpha Film.

“You can see the impact of warming over the whole ice sheet,” Rignot said. “That was a surprising result to me.”

The finding appears as part of the 2011 Arctic Report Card from the National Oceanic and Atmospheric Administration.

The study relies on data from NASA satellites and was led by researcher Jason Box of Ohio State University.

Along the edges of Greenland, bare ground is exposed and pools of water form. Meanwhile, snow melts, exposing the less reflective ice beneath, which can be further darkened by windblown dust.

But the darkening has another cause in Greenland’s interior. There, the ice sheet rises in a dome nearly two miles above sea level, and melting is not a factor.

Ice Crystals

Fresh snow crystal, left, has sharp edges that reflect sunlight.
Warming causes them to become rounded, right, and to clump together, reducing reflectiveness.
Courtesy Electron and Confocal Microscopy Laboratory, USDA Agricultural Research Service.

Instead, ice crystals change shape as temperatures increase. They lose their sharp, reflective edges and begin to clump; both changes result in lower reflectiveness.

“So the snow absorbs more energy from the sun, and gets warmer,” said Rignot, whose own research has shown that ice-sheet melting appears to be accelerating at both poles.


Wednesday, January 11, 2012