Every year since 2002, Greenland has lost 264 gigatons of ice, causing sea levels to rise by 0.8mm annually. This may not initially seem like a huge rise, but when you consider that 10% of the world’s population live within 5km of the coast at elevations near sea level, it soon becomes an alarming statistic.
The loss of Greenland’s ice is largely driven by warming air and sea temperatures linked to anthropogenic global warming. However, a new study led by the University of Leeds has just highlighted a relatively unexplored feature that is amplifying this loss - the meltwater lakes forming at the end of Greenland’s retreating glaciers.
As a glacier melts and retreats up the valley it formed in, it exposes deep, bowl-shaped hollows in the surrounding landscape that quickly fill with meltwater. These lakes are known as ice-marginal lakes (or IMLs) and they can grow as large as 117km2, which is roughly the same area occupied by the urban subdivision of Leeds in England.
The recent study, published last week in the journal Communications Earth and Environment, demonstrates IMLs aren’t just the results of retreating glaciers, but rather active agents in their demise, destabilising them, triggering movement, and increasing thinning, all of which contribute to ice loss.
When a glacier flows into an IML, its front is partly lifted, exposing its underside to increased melting. This reduces the friction that typically slows the glacier’s flow and increases the likelihood of large slabs breaking away via a process known as ‘calving’.
Using satellite data, the researchers found that the speed of glaciers ending in IMLs was three times faster at their fronts than those terminating on land. These alarming levels of acceleration weren’t just confined to the glaciers’ edges either; the researchers detected this effect up to 3.5km inland.
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“When glaciers flow faster, they deliver more ice to lower elevations, where it can melt, or to their fronts, where it can break away,” said the study’s lead author Connie Harpur. “By showing that lakes at the ice margin can substantially speed up glaciers, we identify an important process that needs to be included in predictions of future ice loss.”
While this isn’t the first time such an effect has been documented - earlier observations from the Himalayas have shown glaciers ending in IMLs can move twice as fast as their land-based counterparts - it is the first time it has been properly examined and measured in Greenland.
So far, ice dynamic models of the Greenland ice sheet have seldom taken into account the role IMLs play. The authors involved in this latest study argue such models urgently need to factor in IMLs if they’re to accurately project the short and long term health of this important ice sheet.
“If we do not account for lake effects, we may underestimate how dynamically parts of the ice sheet respond to future warming, and in turn how much Greenland will contribute to future sea level rise,” said co-author Mark Smith. “Understanding ice-marginal lakes’ influence on glacier flow is crucial for accurate projections.”
As well as having disastrous implications for our own species, rising sea levels threaten other species too, particularly those living on the icy frontiers of the Arctic. The waters surrounding Greenland were once covered in floating sea ice, home to a variety of different cold-adapted animals, including polar bears, walruses, narwhals and seals.
However, in recent decades sea levels have been on the decline – in Southeast Greenland summer sea ice has essentially vanished since 2003. This has had profound effects on the already fragile Arctic ecosystem, reducing the size of available habitat, altering migration routes, and leading many species towards potentially irreversible decline.
Top image: Not all of Greenland’s glaciers flow into the ocean, around 10% of the landmass’ ice edge is currently bordered by freshwater lakes known as ice-marginal lakes. Credit: Connie Harpur
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