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First map of thawing under Greenland ice

Greenland’s ice sheet insulates its own bottom layer, which is warmed from below by Earth’s heat. Which bottom layers are now thawed, and which are still frozen?

This first-of-a-kind map, showing which parts of the bottom of the Greenland Ice Sheet are likely thawed (red), frozen (blue) or still uncertain (gray), will help scientists better predict how the ice will flow in a warming climate. Image via NASA Earth Observatory/Jesse Allen

Likely thawed (red), frozen (blue), still uncertain (gray). Image via NASA Earth Observatory/Jesse Allen.

NASA said on August 3, 2016 that its researchers have helped produce a first-of-its-kind map showing where the Greenland ice sheet is likely thawed at its base. The thawing comes in part from Earth’s heat and affects how fast the ice flows to the sea. The Journal of Geophysical Research – Earth Surface published the new study last month. It comes on the heels of an unrelated study showing ice melt in Greenland could release Cold War-era toxic waste.

How can we know what’s happening under Greenland’s ice? The sparse direct observations available come from through fewer than two dozen boreholes that have reached the bottom. The new study synthesizes several methods to infer whether the bottom of the ice is melted or not. For the study, the researchers:

– Examined results from eight recent computer models of the ice sheet, which predict bottom temperatures.
– Studied the layers that compose the ice sheet itself, which are detected by radars onboard NASA’s Operation IceBridge aircraft.
– Looked at where the ice surface speed measured by satellites exceeds its “speed limit,” that is, the maximum velocity at which the ice could flow and still be frozen to the rock beneath it.
– Studied imagery from the Moderate Resolution Imaging Spectroradiometers on the NASA Terra and Aqua satellites looking for rugged surface terrain that is usually indicative of ice sliding over a thawed bed.

Joe MacGregor, lead author of the study and a glaciologist at NASA’s Goddard Space Flight Center, explained:

Each of these methods has strengths and weaknesses. Considering just one isn’t enough. By combining them, we produced the first large-scale assessment of Greenland’s basal thermal state [that is, what’s frozen or solid at the base of the ice sheet].

I call this the piñata, because it’s a first assessment that is bound to get beat up by other groups as techniques improve or new data are introduced. But that still makes our effort essential, because prior to our study, we had little to pick on.

We’re ultimately interested in understanding how the ice sheet flows and how it will behave in the future.

NASA pointed out that Greenland’s ice sheet insulates the bedrock below from the cold temperatures at the surface. That means the bottom of the ice is often tens of degrees warmer than at the top, because the ice bottom is slowly warmed by heat coming from the Earth’s depths. NASA added:

Knowing whether Greenland’s ice lies on wet, slippery ground or is anchored to dry, frozen bedrock is essential for predicting how this ice will flow in the future.

Read more about this map from NASA

Ice melt could release Cold War-era waste

Bottom line: Researchers used a combination of methods to create the first-ever map of thawing at the bottom of Greenland’s ice sheet. The map shows that, predictably, there’s more solid ice toward the center of the ice sheet.

Deborah Byrd

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