Magazine article Geographical

A Clear Signal

Magazine article Geographical

A Clear Signal

Article excerpt

With the summer ice melting and temperatures rising at unprecedented rates, scientists are all but certain that changes taking place in the Arctic can be attributed to anthropogenic climate change.

In September 2012, the world's attention turned to the Arctic as warmer than average summer temperatures melted the sea ice around the North Pole. By the time temperatures began to cool down again, the average extent of the Arctic sea ice had shrunk to just 3.61 million square kilometres. This was 690,000 square kilometres less than the previous record low in 2007. Between March 2012, when the ice reached its maximum extent for that year, and 16 September, the Arctic Ocean lost a total of 11.83 million square kilometres of ice (which isn't far off the total land area of Antarctica).

But this record loss is merely part of a long-term trend that scientists are all but certain is directly attributable to anthropogenic climate change, which has seen temperatures rise across the Arctic region at a staggering rate. The most pronounced warming has taken place in the East Siberian region, where surface air temperature increased by 5[degrees]C between 2000 and 2005.

RECORD DECLINE

Passive microwave satellites have been circling the North Pole several times a day since 1978, able to look through cloud and the polar night to see where the sea ice is--and where it isn't. Over that time, September Arctic sea ice extent has declined by 13.7 per cent per decade, according to the University of Colorado's National Snow and Ice Data Center (NSIDC). 'It has dropped from around eight million square kilometres over that time to around four million square kilometres,' says Dr Jeremy Wilkinson, lead investigator at the British Antarctic Survey (BAS). 'We can be 99.9 per cent sure that that is due to a warming planet.'

But the retreat of the sea ice--which is mainly occurring around the large continental shelves of Canada, Alaska and Siberia--is just one part of the story. For decades, US and UK nuclear submarines have travelled under the Arctic ice, where they deploy an upward-looking sonar system (partly for safety, allowing them to find thinner ice through which to surface) to measure its thickness.

Analysis of these data has shown how the ice has thinned from an average of 3.5 metres in the 1970s to about 2.5 metres today. Wilkinson can point to another profound development. 'About five years ago, the ice shifted from predominantly multi-year ice [more than one year old] to predominantly first-year ice. These changes influence the thickness and the strength of ice, and as a result, the ice sheet can become much more dynamic.' On top of that, Wilkinson points out, an anomalously warm summer may not significantly affect the thicker multi-year ice, but can have a detrimental impact on the first-year ice.

Another consequence of these developments is the albedo feedback effect. Loss of the sea ice produces larger areas of open water. These darker areas absorb more solar radiation than the more reflective white ice, which in turn warms up the surface waters--which increases the melt-rate of sea ice, as well as delaying when water starts to freeze again. 'The sea ice has an albedo of 0.8 which means it reflects 80 per cent of solar radiation back into space,' says Wilkinson. 'But water has an albedo of 0.2, so it absorbs 80 per cent of the radiation.'

GREENLAND MELT

Away from the sea ice, ice caps and glaciers across the Arctic are also retreating. Indeed, a study published in 2011 found that between 2004 and 2009, Canada's Arctic glaciers lost the equivalent of about three quarters the volume of Lake Erie, while the situation is Greenland has been even more alarming. 'Last summer [2012] was the first time when the entire surface of the Greenland ice sheet was in melt for a few days,' says Professor Peter Wadhams, head of the Polar Ocean Physics Group at the University of Cambridge. …

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