Editorial: 鈥�Arctic melt, smash and grab ahead鈥�
IT IS smaller, patchier and thinner than ever 鈥� and rotten in parts. The extent of the Arctic ice cap has hit a record low, and the consequences of what is arguably the greatest environmental change in human history will extend far beyond the North Pole.
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For at least 3 million years, and most likely 13 million, says Louis Fortier of the University of Laval in Quebec City, Canada, the Arctic Ocean has been covered by a thick, floating ice cap, the breadth of which fluctuates with the seasons and currents. Each summer, the cap shrinks to an annual minimum in mid-September before growing out again, fuelled by plummeting winter temperatures and long nights.
Climate change has had more of an impact here than anywhere else on Earth. Air temperatures are rising twice as fast as the global average, and models predict that the region could see temperatures rise by between 6聽掳C and 14聽掳C by 2100, depending on how fast the sea ice disappears. Nobody believes the world鈥檚 northern ice cap could survive such scorching summers.
Constant, real-time monitoring made it evident last week that this would be a landmark year, even worse than 2007, when the summer thaw opened the North-West Passage for the first time in living memory. On 27 August, the daily update from the (see diagram). That is 70,000 km2 smaller than the 2007 low, and there are several weeks to go before the ice starts growing back as winter approaches. (To see an interactive diagram showing the summer ice extent for every year from 1979 to 2011, see our interactive map.)
Less ice in the Arctic means more light enters the ocean beneath, fuelling more life (see 鈥�What ice-free summers will mean for Arctic life鈥�), but it also means warmer surface waters and more energy released into the atmosphere. That energy drives cyclones, which generate mammoth waves capable of ripping into the ice pack, degrading it further. The consequences are not good for this highly specialised environment, and the knock-on effects for the rest of us could be severe.
Even the NSIDC鈥檚 figures for ice coverage, from satellite data, could be overestimations, as satellite images cannot distinguish between pack ice and slushy, 鈥渞otten鈥� ice (Geophysical Research Letters, ).
Measurements of ice thickness are also worrying. From 1979 to 2000, the average volume of Arctic ice in September was 12,000 cubic kilometres. This year, it is less than 3000 cubic kilometres. 鈥淚n plain words,鈥� says Fortier, 鈥渨e are three-quarters of the way to a summertime ice-free Arctic Ocean, with all the climatic, geopolitical, environmental and economic consequences.鈥�
鈥淲e are three-quarters of the way to an Arctic Ocean free of ice in the summer鈥�
Several factors are to blame. Warm spring temperatures didn鈥檛 help, then on 5 August a major cyclone crossed into the Arctic Circle. Such events used to be rare, but are now more frequent, stronger and last longer than before, says at the University of Alaska Fairbanks (Journal of Climate, ).
These storms carry heat from the lower latitudes, and stir up the highly stratified Arctic Ocean waters, bringing warm bottom-waters up to the surface, where they help further melting of the floating ice. More dramatic is the storms鈥� direct physical influence. With the Arctic water no longer protected by thick multiyear ice, near-hurricane-force winds shove it around, forming powerful waves capable of shredding any ice floes they encounter.
and at the University of Manitoba in Winnipeg, Canada, witnessed this in September 2009 aboard an ice-breaker called Amundsen. They were near an ice floe 10 kilometres across and up to 10 metres thick in places. 鈥淲e were about to put a team down on the ice when all of a sudden I saw a wave enter the ice pack,鈥� Barber recalls. 鈥淭he entire floe rose up on top of the wave trains, and as it came down on the other side, it split. As I watched, it broke into small pieces 100 metres across.鈥�
Smaller pieces melt faster, so the researchers believe cyclones are an important positive feedback mechanism, further reducing the pack ice (Journal of Geophysical Research 鈥� Oceans, ).
Zhang points out that cyclones also carry clouds, which boost downward radiation. He says the August cyclone probably contributed to this year鈥檚 low. Bill Chapman at the University of Illinois at Urbana-Champaign agrees. 鈥淪ea-ice area decreased rapidly during the period of the storm,鈥� he says, 鈥渂ut it was decreasing rapidly already during the height of this melt season.鈥�
The upheaval will not be confined to the Arctic Circle. The atmospheric-temperature gradient between the hot tropics and the cold poles powers much of our planet鈥檚 weather and water cycles. It drives the jet stream, for instance, which pushes global weather systems, it fuels the winds that power ocean currents, and it distributes moisture around the planet.
What鈥檚 more, the accelerating melt of Arctic ice is injecting a layer of freshwater into the surface of the North Atlantic and Baffin Bay, off the coast of Greenland, says Fortier. This may be slowing the circulation in the Atlantic, which pushes water all through the world鈥檚 oceans.
The long-term trends are damning. The latest climate models predict that by around 2050 the Arctic will be completely ice-free during the summers (Geophysical Research Letters, ). When that happens, the Arctic routine, stable for millions of years, will be flipped on its head. 鈥淭he changes under way in the Arctic,鈥� concludes Barber, 鈥渁re the most significant, in scale and rate, that we have seen in the history of human civilisation.鈥�
Read more: 鈥�What ice-free summers will mean for Arctic life鈥�
When this article was first posted, Xiangdong Zhang鈥檚 name was misspelt
