New understanding of Greenland’s glaciers show more of them are below sea level, and may melt faster
Pic: Ville Miettinen/Flickr
By Gerard Wynn
Greenland glaciers are lodged in valleys which extend deeper and further below sea level than previously thought, found a study published on Sunday, implying a greater risk of ice loss and sea level rise.
Where they are in contact with the sea, glaciers are more liable to melt given water temperatures far higher than the polar air above.
Their vulnerability is even greater at depths of 300-400 metres, where they meet “ocean heat” transported from the sub-tropical Atlantic Ocean.
Previously, some studies had concluded that the Greenland ice sheet may be less vulnerable to warming oceans, under climate change, because the glaciers may lose ocean contact as they melt, retreating above sea level inland.
The new study concluded that many would in fact remain below sea level, and continue to drain ice from the Greenland interior, because of the depth of the valleys.
“Our results show that the submarine bed channels are more widespread, deeper and extend significantly farther inland than previously thought,” found the paper, ‘Deeply incised submarine glacial valleys beneath the Greenland ice sheet’, published in the journal Nature Geoscience.
“We anticipate that these results will have a profound and transforming impact on model simulations of ice-sheet evolution in Greenland and reveal a more pervasive influence of ocean thermal forcing on these glaciers.”
They found that submarine valleys extended inland some 50% further than in the most recent similar study, published last year, especially in western Greenland, following an improved understanding of their topography.
The authors gave the example of one glacier in West Greenland whose three branches extended below sea level for some 80 kilometres inland, with one submarine arm extending more than 140km. Another glacier, further north, was travelling along a 15km-wide valley below sea level for some 120km.
At their tips, many glaciers extended several hundred metres below sea level.
“This contrasts with existing bed maps that indicate ice fronts grounded at sea level, not in contact with the ocean,” found the authors, who were based at the University of California and the California Institute of Technology.
Thickness
Ice sheet thickness in central Greenland is measured using airborne radar, which bounces off the bed rock.
But this approach is not so effective at the margins of the ice sheet where glaciers drain into the sea, because of a more complicated topography, and the confusing impact of crevasses and melt water on radar soundings.
The latest study combined satellite data of glacier flow with the sparser airborne ice thickness readings to calculate the bed rock topography.
“Our bed topography provides the first comprehensive view of the entire periphery of Greenland,” they concluded.
“Although our compilation is not different from earlier compilations in the interior, for example, the total ice volume is the same, our results differ significantly along the coast,” they said.
University of California researchers, including several of the authors of Sunday’s study, published a paper last week concluding, for the first time, that West Antarctic glaciers were unstable, and that their collapse had “passed the point of no return”.
The basis for that conclusion was also largely due to the underlying topography, and the fact that they were passing along valleys which were not only below sea level, but sloping inland making the ice piled above them even less stable.