A thin spot in the Earth's crust is enabling underground magma to melt Greenland's ice, scientists at the Ohio State University feel.
According to them, the "hotspot" is located in the northeast corner of Greenland -- just below a site where an ice stream was recently discovered.
The researchers don't yet know how warm the hotspot is, but if it is warm enough to melt the ice above it even a little, it could enable the ice to slide more rapidly out to sea.
To measure actual temperatures beneath the ice, scientists will have to drill boreholes down to the base of the ice sheet-- a mile or more below the ice surface. The effort and expense make such measurements few and far between, especially in remote areas of northeast Greenland.
"The behaviour of the great ice sheets is an important barometer of global climate change," said Ralph von Frese, leader of the project and a professor of earth sciences at Ohio State University.
"However, to effectively separate and quantify human impacts on climate change, we must understand the natural impacts, too," he added.
Von Frese's team combined gravity measurements of the area taken by a Naval Research Laboratory aircraft with airborne radar measurements taken by research partners at the University of Kansas.
The combined map revealed changes in mass beneath the Earth's crust, and the topography of the crust where it meets the ice sheet.
According to the researchers, below the earth's crust is the mantle, the partially molten rocky layer that surrounds its core.
The mantle is so hot that temperatures just a few miles deep in the crust reach hundreds of degrees Fahrenheit, von Frese explained.
"It could be that there's a volcano down there. But we think it's probably just the way the heat is being distributed by the rock topography at the base of the ice," he said.
The ice thickness, the temperature at the base of the ice, and ground topography all contribute to the forming of an ice stream -- a river of ice that flows within a larger ice sheet.
In recent years, Greenland ice streams have been carrying ice out to sea faster, and ice cover on the island has been diminishing.
Once the ice reaches the sea, it melts, and global sea levels rise.
"Where the crust is thicker, things are cooler, and where it's thinner, things are warmer. And under a big place like Greenland or Antarctica, natural variations in the crust will make some parts of the ice sheet warmer than others," von Frese added.
The ice sheet in northeast Greenland is especially worrisome to scientists. It had no known ice streams until 1991, when satellites spied one for the first time.
Dubbed the Northeastern Greenland Ice Stream, it carries ice nearly 400 miles, from the deepest interior of the island out to the Greenland Sea.
The newly discovered hotspot is just below the ice stream, and could have caused it to form, the researchers concluded. But what caused the hotspot to form?
"It could be that there's a volcano down there. But we think it's probably just the way the heat is being distributed by the rock topography at the base of the ice," said von Frese.
Collaborator Kees van der Veen, a visiting associate professor of geological sciences and research scientist at Byrd Polar Research Center at Ohio State, said: "Our map is the first attempt at quantifying spatial variations in geo-heat under Greenland -- and it explains why the Northeast Greenland Ice Stream is where it is."
For now, the researchers are combining theories of how heat flows through the mantle and crust with the gravity and radar data, to understand how the hotspot is influencing the ice.
Once they finish searching the rest of Greenland for other hotspots, they hope to turn their attention to Antarctica.
Timothy Leftwich, von Frese's former student and now a post-doctoral engineer at the Centre for Remote Sensing of Ice Sheets at the University of Kansas, presented the study's early results on Thursday at the American Geophysical Union meeting in San Francisco.