Hot Times Under Greenland’s Ice

Over the past two decades, we have been documenting the vast changes in the Earth’s cryosphere, as ice caps, sea ice, and permafrost warm, melt, and shrink. NASA and other agencies have provided critical assets for remote sensing these continental and global phenomena.

Intense interest is focused on Greenland, which has been covered with a permanent ice cap for millennia, and appears to be rapidly melting in recent decades. Setting aside worries about the causes and consequences of this sudden melt off (which cannot be good news for land based animals, such as H. sapiens), can we even measure what is really going on?

One of the more difficult things to measure is the bottom of the ice cap. Where ice rests on bedrock, it may be frozen solid (there the rock itself is cold) or melting (where the rock is a little warm). The latter case may lead to faster melting in the summer, and also enables the ice to slide easily, e.g., downhill to the ocean coast. It isn’t easy to directly measure conditions down there, and without this data it is difficult to model what is happening with the ice overall.

Over the past decades, there have been dozens of borehole studies, but these are a sparse (and unsystematic) sample of the whole continent. These studies indicate that in some areas the bed is frozen solid, and in others it is thawed. But it these measurements could not provide a complete picture.

This month NASA published new research that created the first attempt at a comprehensive map of conditions under the ice for all of Greenland.  (As a first try, one of the authors called it a “piñata”—certain to be beat up by subsequent analysis and criticism.)

This study is another example of the value of remote sensing from space—which gives continent wide coverage—airborne, and surface studies, combined through careful computational modeling. The multiple data sources are essential, because none of them alone can really give enough information.

The satellite data used includes measures of the movement of the ice over time and the roughness of the ice surface (which is an indirect indication of ice sliding over thawed bed). This is combined with radar studies from aircraft, and computer models of the physics based on theory and other data.

The resulting map “… identifies distinct regions, where the bed is likely frozen (24% by area) or thawed (43%) and where this basal thermal state remains most uncertain (34%).” [1], p. 1347

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. Credits: NASA Earth Observatory/Jesse Allen

Assuming these estimates are generally correct, the significant areas of thawed bed are certainly melting, and liable to melt much more rapidly in the coming years. This finding is consistent with other observations that suggest that many parts of Greenland’s ice is melting rapidly, and may disappear in a few decades.

  1. J. A. MacGregor, M. A. Fahnestock, G. A. Catania, A. Aschwanden, G. D. Clow, W. T. Colgan, P. S. Gogineni, M. Morlighem, J. D. Paden, S. F. Price, and H. L. Seroussi, A synthesis of the thermal state of the bed of the Greenland Ice Sheet. American Geophysical Union Fall Meeting, 2015 2015.


Space Saturday

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