Global warming and the vulnerability of Greenland's ice sheet

News continues to emerge in the scientific literature on the future direction of the world’s largest ice sheets; they are undergoing major changes as the climate continues to warm. Among the latest, a new study by Mathieu Morlighem, Eric Rignot, and others has taken a new approach to improve our understanding of the flowing ice atop Greenland.

This work is important because in the past decade or so, it has been found that Greenland is losing ice, lots of ice, to the world’s oceans. As a consequence, Greenland is one of the significant contributors to sea level rise. The level our oceans will rise to in the next decades and centuries depends strongly on how fast the Greenland ice sheet will melt.

This study is novel because of the way it combined measurements with mass-conservation calculations. The method allowed far better resolution of the thickness of Greenland glaciers and the shape of the valleys beneath the glaciers. The shape and depth of these valleys is important for moderating the speed of ice flow to the oceans. In fact, in the paper, authors state, “the overall state of mass balance of the ice sheet is affected by considerable uncertainties in bed topography and ice thickness.”

While measurements of ice sheet thickness have been made for decades using airborne radar systems, by NASA’s Operation IceBridge in particular, these measurements are not sufficient in number and quality in important ice sheet regions – near the edges and in the South. There, the use of radar is more challenging because of the presence of water in the ice. It is the edges of the ice sheets, particularly the portions that have glaciers that terminate in oceans, which dominate the flow of Greenland’s frozen water into the ocean.

Sukkertoppen glacier, Courtesy Michael Studinger Sukkertoppen glacier. Photograph: Michael Studinger

In order to overcome the paucity of high-quality radar measurements, the authors included ice motion information from satellite imagery. This motion, when input into a mass-conservation model, allowed the researchers to extrapolate “sparse ice thickness data to larger areas with few or no data.”

When they compared their results with other studies, the authors found, “widespread presence of well-eroded, deep-bed troughs along the ice-sheet periphery, generally grounded below sea level, coincident in location and spatial extent with fast-flow features and extending over considerable distance inland.” These features were not previously known.

These findings allow a few conclusions. Aside from the importance of deep troughs to ice motion, the extension inland means that glaciers will have to retreat further than anticipated inland in order to reach a position above sea level. “Some of them will stay in contact with the ocean for centuries, when we thought that in a couple of decades they would stabilize.” said Mathieu Morlighem.

The ice sheet is therefore more vulnerable than predicted, and existing projections of sea level rise contribution from Greenland are too conservative and need to be revised. The research also shows that also means that these troughs are old – it takes 10,000 to 100,000 years for these troughs to be created through erosive action. Also startling is that while only 8% of these regions correspond to ice-grounding below sea level, they are responsible for 88% of the total ice discharge. These are the parts of Greenland that really matter.

As the authors state in the paper, “Our findings imply that the outlet glaciers of Greenland, and the ice sheet as a whole, are probably more vulnerable to ocean thermal forcing and peripheral thinning than inferred previously from existing numerical ice-sheet models.”

Dr. Rignot gave a great summary when I contacted him, "Observations published in a number of recent, independent studies in the scientific literature provide unquestionable and timely evidence that climate change has already significantly shaken up the giant ice sheets in Greenland and Antarctica."

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Posted by John Abraham on Friday, 30 May, 2014

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