“Not only are we directly altering the climate, but that process in itself is kicking off these things that are gonna actually accelerate the damaging impacts over the rest of the century.” Thomas Crowther professor at ETH Zürich on the worrisome unknowns of feedback loops.
In the Arctic, rising temperatures cause the creation of large surface lakes, called supra-glacial lakes or meltwater lakes. The lakes form in the summer atop the Greenland ice sheet. These lakes can last for months on the surface, but can drain in a matter of hours or days. When they drain, freshwater and heat is carried to the bottom of the ice sheet via a moulin, a “roughly circular, vertical to nearly vertical well-like shaft within a glacier or ice sheet which water enters from the surface”. The water pours into the moulin and travels through the thick ice sheet all the way to the bottom where it becomes a lubricating fluid at the “base of the glacier, enhancing glacial motion and speeding disintegration of the ice sheet. The melting water encourages further ice loss and accelerates the glacier’s flow to the sea, where large chunks break off to form icebergs”.
The surface melt on Greenland has begun to make a measurable difference to sea level rise. And the lakes are appearing further inland and at higher altitudes than what has been recorded in the past according to the study.
Scientists studied large fractures in Greenland's ice sheet produced by the chain-reaction melting events.University of Cambridge reports on the findings.
Previously, it had been thought that these ‘drainage events’ were isolated incidents, but the new research, led by the University of Cambridge, shows that the lakes form a massive network and become increasingly interconnected as the weather warms. When one lake drains, the water quickly spreads under the ice sheet, which responds by flowing faster. The faster flow opens new fractures on the surface and these fractures act as conduits for the drainage of other lakes. This starts a chain reaction that can drain many other lakes, some as far as 80 kilometres away.
These cascading events – including one case where 124 lakes drained in just five days – can temporarily accelerate ice flow by as much as 400%, which makes the ice sheet less stable, and increases the rate of associated sea level rise. The results are reported in the journal Nature Communications.
The study demonstrates how forces within the ice sheet can change abruptly from one day to the next, causing solid ice to fracture suddenly. The model developed by the international team shows that lakes forming in stable areas of the ice sheet drain when fractures open in response to a high tensile shock force acting along drainage paths of water flowing beneath the ice sheet when other lakes drain far away.
“This growing network of melt lakes, which currently extends more than 100 kilometres inland and reaches elevations as high a 2,000 metres above sea level, poses a threat for the long-term stability of the Greenland ice sheet,” said lead author Dr Poul Christoffersen, from Cambridge’s Scott Polar Research Institute. “This ice sheet, which covers 1.7 million square kilometres, was relatively stable 25 years ago, but now loses one billion tonnes of ice every day. This causes one millimetre of global sea level rise per year, a rate which is much faster than what was predicted only a few years ago.”
xMelt, fill, collapse, repeat: the life cycle of a Greenland meltwater lake at 79N as seen by 162 #sentinel2 and #landsat8 images. Check how the lake debris gradually moves while the lakes stays. pic.twitter.com/DrHGX40kGD
— Stef Lhermitte (@StefLhermitte) January 18, 2018Dr Marion Bougamont, Scott Polar Research Institute is quoted “The transformation of forces within the ice sheet when lakes drain is sudden and dramatic, Lakes that drain in one area produce fractures that cause more lakes to drain somewhere elsewhere. It all adds up when you look at the pathways of water underneath the ice.”
A large fracture is visible in a lake bed on the Greenland Ice Sheet after it drained the lake's entire liquid contents.The researchers found “clear evidence of these crevasses at 1,800 metres above sea level and as far 135 kilometres inland from the ice margin. This is much farther inland than previously considered possible.”
A model of the lake-melting chain-reaction process.The scientists found that in one case “all but one of 59 observed lakes drained in a single cascading event. Most of the melt lakes drain in this dynamic way.”
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