Researchers have revealed that the movement of glaciers in Greenland is more complicated than previously believed, with deformation in sections of warmer ice holding tiny quantities of water that account for motion that had typically been supposed to be produced by sliding where the ice contacts the bedrock underneath. The multinational team of researchers lead by the University of Cambridge utilised computer modelling methods based on prior fiber-optic readings from the Greenland Ice Sheet to develop a more precise picture of the behaviour of the world’s second-largest ice sheet.
Their observations, detailed in the journal Science Advances, might be used to build more precise projections of how the Greenland Ice Sheet will continue to move in response to climate change. Mass loss from the Greenland Ice Sheet has grown sixfold since the 1980s and is currently the single biggest contributor to global sea-level rise. Around half of this mass loss is via surface meltwater runoff, while the other half is caused by discharge of ice directly into the ocean by fast-flowing glaciers that reach the sea.
The RESPONDER project is examining the dynamics of the Greenland Ice Sheet using a mix of physical data and computer models.
The present study expands on prior discoveries revealed by the RESPONDER team in 2021 utilising fiber-optic connections. In that experiment, the researchers observed that the temperature of ice sheets does not fluctuate as a continuous gradient, but is significantly more heterogeneous, with regions of highly localised deformation warming the ice more.
The borehole tests also indicated that the ice at the base contains modest amounts—up to around two percent—of water. In certain areas of the ice sheet, this mixed ice-water layer, termed temperate ice, was roughly eight metres deep, while in other places it was up to 70 metres thick.
“The addition of even tiny amounts of water softens the ice considerably, transforming it into a unique material with substantially altered mechanical characteristics,” said first author Dr. Robert Law, who completed the work while based at Cambridge’s Scott Polar Research Institute and is now based at the University of Bergen. “We wanted to discover why the thickness of this layer changed so considerably, since if we don’t completely grasp it, our models of ice sheet activity won’t adequately replicate the physical processes happening in nature.”
“The textbook perspective of glacier motion is that it happens with a tidy separation of basal sliding and interior deformation, and that both are well known,” stated co-author and RESPONDER project head Professor Poul Christoffersen, who is based at SPRI. “But that’s not what we saw when we examined attentively in boreholes using new technology. Using fewer comprehensive observations in the past, it was difficult to gain a truly accurate image of how the ice sheet moves and much more complex to duplicate it with computer models.”
Law, Christoffersen and his colleagues from the U.K., U.S., Switzerland and France built a model based on their previous borehole data that can account for all of the new findings.
Importantly, they accounted for natural differences in the topography at the foot of the ice, which, in Greenland, is full of steep hills, basins and deep fjords. The researchers observed that when a glacier travels over a significant obstruction or slope, there is a deformation and heating effect which sometimes reaches several hundred metres from the ice sheet base. Previously, this influence was disregarded in models.
“The load on the ice base is strongest at the summits of these hills, which leads to increased basal sliding,” said Law. “But so far most models have not accounted for all of these differences in the terrain.”
By including these differences, the model built by the researchers revealed that a changeable layer of temperate ice accumulates as the glacier travels across the terrain, whether the glacier itself is fast- or slow-moving. The thickness of this temperate ice layer corresponds with the prior drilling observations, but diverges dramatically from traditional modelling approaches used to anticipate sea level rise from ice sheets.
“Because of this mountainous topography, the ice may shift from sliding over its base nearly totally to rarely sliding at all, across short distances of only a few kilometres,” stated Law. “This directly effects the thermal structure—if you’ve got less basal sliding then you’ve got more internal deformation and heating, which may lead to the layer of temperate ice being thicker, affecting the mechanical properties of the ice across a large region. This moderate bottom ice layer may really operate as a deformation bridge between hills, allowing the quick motion of the much colder ice just above it.”
The researchers intend to utilise this better knowledge to construct more accurate descriptions of ice motion for the ice sheet models used in projecting future sea level rise.
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