Though it seems counter-intuitive, Greenland’s thick ice sheet actually insulates the bedrock below from the cold air temperatures at the surface. As a result, the bottom of the ice—slowly warmed by heat coming from Earth’s depths—can be tens of degrees warmer than at the top. Knowing whether Greenland’s ice lies on wet, slippery ground or is anchored to dry, frozen bedrock is essential for understanding how the ice sheet will react to a warming climate and flow in the future.

But the thermal conditions below 3 kilometers (2 miles) of ice are hard to determine. Scientists have very few direct observations beneath the ice sheet, and those were obtained through fewer than two dozen boreholes that have reached the bedrock. Now, a new study synthesizes NASA airborne and satellite data from the past two decades to infer the Greenland Ice Sheet’s basal thermal state—whether the bottom of the ice is melted or not.

The results of that synthesis are represented on the map above. Red areas show where the ice is likely thawed on the bottom (43 percent of the ice sheet), and blue areas show where it is likely frozen (24 percent). Gray areas are where the thermal state is uncertain.

“We’re ultimately interested in understanding how the ice sheet flows and how it will behave in the future,” said Joe MacGregor, lead author of the study and a glaciologist at NASA’s Goddard Space Flight Center. “If the ice at its bottom is at the melting point temperature, or thawed, then there could be enough liquid water there for the ice to flow faster and affect how quickly it responds to climate change.”

For this study, published in July 2016 in the Journal of Geophysical Research–Earth Surface , MacGregor’s team combined four different approaches to investigate the basal thermal state. First, they examined results from eight computer models of the ice sheet that predict bottom temperatures. Second, they studied the layers that compose the ice sheet itself, detected by radars onboard aircraft flown during NASA’s Operation IceBridge mission. This information suggests where the bottom of the ice is melting rapidly. Third, they looked at where the ice surface speed (as measured by satellites) exceeds its “speed limit”—the maximum velocity at which the ice could flow and still be frozen to the rock beneath it. Fourth, they studied imagery from the Moderate Resolution Imaging Spectroradiometer on NASA’s Terra and Aqua satellites, looking for rugged surface terrain that is usually indicative of ice sliding over a thawed bed.

“Each of these methods has strengths and weaknesses. Considering just one isn’t enough,” MacGregor said. “By combining them, we produced the first large-scale assessment of Greenland’s basal thermal state.”

For each method, MacGregor’s team looked for areas where the technique confidently inferred that the bed of Greenland’s ice sheet was thawed or frozen. They then looked at the places where these methods agreed and classified these areas as likely thawed or likely frozen. The zones where there was insufficient data or where the methods disagreed were classified as uncertain.

MacGregor said the team’s map is just one step in fully assessing the thermal state of the bottom of Greenland’s ice sheet. “I call this the piñata, because it’s a first assessment that is bound to get beat up by other [scientific] groups as techniques improve or new data are introduced,” MacGregor said. “But that still makes our effort essential, because prior to our study, we had little to pick on.”

NASA Earth Observatory image by Jesse Allen, based on data provided by Joe MacGregor (NASA/GSFC). Caption by Maria-José Viñas, adapted for Earth Observatory by Kathryn Hansen.