21 Jun 2012 by Jim Fickett.
The latest issue of the Economist has a useful overview of the rapidly shrinking ice cover in the Arctic, and the likely consequences. Go there for thoughts on changes to shipping, mining, Inuit culture, etc. Here are a few extracts on the melt itself:
The melt has become quite extreme:
Monitoring sea ice is a fairly recent activity. It began seriously in the 1950s, from aboard nuclear submarines. Satellite monitoring started in 1979. Since then the summer sea ice has shrunk by 12% a decade. … Using Viking epics, whaling and pollen records, log books, the debris shed by melted ice rafts, diatoms (silicon-armoured algae found in marine sediments), ice cores and tree rings, scientists have constructed a record of the Arctic past which suggests that the summer sea ice is at its lowest level for at least 2,000 years.
The consensus prediction is that sea level will rise a couple of feet over the next few decades. But there is an outside chance that positive feedback loops will accelerate global warming and cause a much higher rise. So it is interesting to know, just for an upper bound, what the effect of melting the full ice caps would be:
Greenland’s [ice sheet] is up to 3km (1.9 miles) deep and contains enough water to raise the sea level by 7.5 metres; the Antarctic ice sheets are much bigger and could potentially cause a 57-metre rise. In recent years they have also been monitored by satellite, with the first data obtained in 1992. Until about 2000 the ice sheets seemed to be more or less stable, with increased snowfall on their tops compensating for increased melting at the margins.
But in Greenland something has changed. The ice sheet’s recent rate of loss—around 200 gigatonnes a year—represents a fourfold increase on a decade ago. Half this melting is thought to be due to the warming atmosphere. The other half is due to warmer seawater, caused by global warming or a shift in Atlantic currents, or both. As a result, the sea is eating away the edge of the ice sheet at a faster rate. Between 2002 and 2007 the Jakobshavn Isbrae, a big glacier in western Greenland, retreated by 3km a year, shedding a total of over 36 billion tonnes of ice.
If global warming does get into a really dangerous positive feedback loop, it will probably involve methane released from melting permafrost:
Roughly a quarter of the northern hemisphere, including most of the Arctic land, is covered by [Arctic permafrost]. Formed over millennia, it varies in depth from a few centimetres to up to 1,500 metres in Siberia. Much of the Arctic’s shallow continental shelf is also covered by permafrost. According to an estimate made in 2009, terrestrial permafrost holds about 1.7 trillion tonnes of carbon, roughly twice as much as the atmosphere. By another estimate subsea permafrost stores an additional 0.5 trillion tonnes. And underlying it there may be another 0.8 trillion tonnes in the form of methane hydrates …
Methane is a short-lived greenhouse gas—it stays in the atmosphere for 6-10 years before being oxidised—but it is 25 times more efficient than carbon dioxide at trapping heat. And no one is sure how stable the hydrates are.
Given the scale of these risks, it is extraordinary how little research has been done on permafrost. … an explosive methane release is very unlikely. Ice cores going back 800,000 years show no trace of such an event. Nonetheless, the release of permafrost or subsea carbon could be gradual and still cause a lot of warming, and that does seem likely. …
A 2011 study for America’s National Oceanic and Atmospheric Administration found evidence of thawing permafrost in most parts of the Arctic. … Last year an American-Russian research team observed plumes of methane, some more than a kilometre across, bubbling up from the Siberian Sea, suggesting that its permafrost layer was no longer sealing the methane hydrates below. …
One of the best available guides to this risk is a survey of 41 permafrost scientists published in Nature last year. They predicted that at the current rate of global warming between 48% and 63% of terrestrial permafrost would be thawed to a depth of 3 metres by 2100. In the process, they expected between 7% and 11% of its stored organic matter to be released into the atmosphere. Only a little over 2% of that would be in the form of methane, but this would be responsible for 30-50% of the resultant warming. It would be impossible to prevent these emissions: they would probably continue for centuries.