Credit: NASA Earth Observatory
This is a re-post from Carbon Brief
Credit: NASA Earth Observatory
Over the past week, news that sea ice extent has fallen to a new low according to satellite measurements has prompted speculation about how long it will be before the Arctic Ocean is completely free of ice in summer.
The fate of Arctic sea ice has always captured attention. The ice grows in winter and retreats in summer. Records since 1979 show Arctic sea ice losing around 3% of its area per decade. The amount of ice at the summer minimum is shrinking faster, at around 12% per decade, and the sea ice is also becoming thinner.
This long-term decline is clear. But it's not possible to confidently predict how much ice there will be in a particular year. In 2007 Arctic sea ice coverage fell to an unusual low - unprecedented in the satellite record, and well below what was expected.
This prompted widespread coverage, a suggestion from US National Snow & Ice Data Center (NSIDC) director Mark Serreze that the sea ice had entered a " death spiral", and speculation from others that the Arctic ocean might be clear of sea ice in summer sooner than expected.
But 2008 through 2011 didn't see new records broken, although sea ice levels remained consistent with the long-term downward trend:
Subsequent studies suggest that unusual weather conditions contributed to 2007's record low, prompting more conservative predictions for when we might sea ice-free summers.
But this year has seen another dramatic sea ice low, with Arctic sea ice extent already below 2007 levels and a couple of weeks of the melt season left:
Commentators are once again questioning whether this year's fall in ice cover marks a fundamental change in the pattern of Arctic sea ice melt, or whether it's the result of natural variability. Some scientists point out that the unusual weather conditions that contributed to 2007's record low haven't happened this year. Serreze explains:
"The previous record, set in 2007, occurred because of near perfect summer weather for melting ice. Apart from one big storm in early August, weather patterns this year were unremarkable. The ice is so thin and weak now, it doesn't matter how the winds blow."
Predictions are once again being made about when the Arctic Ocean might be ice-free in summer. There's quite a range of opinions. For example, Professor Peter Wadhams, ocean physicist at the University of Cambridge, tells the Scotsman:
"The entire ice cover is now on the point of collapse [...] It is truly the case that it will be all gone by 2015."
And Mark Drinkwater, mission scientist for the European Space Agency's CryoSat satellite tells Yale Environment 360:
"If this rate of melting [in 2012] is sustained in 2013, we are staring down the barrel and looking at a summer Arctic which is potentially free of sea ice within this decade."
Other scientists offer estimates in terms of decades. Ted Scambos, NSIDC senior research scientist, suggests that Arctic sea ice-free summers aren't likely to be reached until 2030, "plus or minus a decade."
And in 2011, NSIDC director Mark Serreze wrote:
"[W]e may [...] be looking at ice-free summers only a few decades from now."
Peer reviewed research in this area has developed over the last few years. In 2007, before the dramatic melt that year, the most considered scientific opinion came from the Intergovernmental Panel on Climate Change (IPCC). The worst case scenario, it said, was ice-free summer seas by the end of the century.
But a 2009 review of newer scientific literature by a group of scientists concludes:
"The observed summer-time melting of Arctic sea-ice has far exceeded the worst-case projections from climate models of IPCC AR4 … The warming commitment associated with existing atmospheric greenhouse gas levels means it is very likely that in the coming decades the summer Arctic Ocean will become ice-free, although the precise timing of this remains uncertain."
Indeed, subsequent research confirms the climate models used for IPCC's AR4 underestimate future Arctic sea ice loss.
Projections made since 2007 suggest the Arctic ocean could be seasonally free of ice sooner. A 2007 paper concludes "decreasing ice coverage might lead to an ice-free Arctic in summer sometime within the upcoming decades", whilst a 2009 paper, which uses 2007/2008 September sea ice measurments as a starting point for six IPCC climate models predicts "a nearly sea ice free Arctic in September by the year 2037."
A more recent assessment of sea ice projections using the latest generation of IPCC climate models finds that the updated models more realistically represent the present state of the sea ice cover, and projects
"[A] seasonally ice-free Arctic sooner than [the older IPCC models], leading to the conclusion that a seasonally ice-free Arctic Ocean within the next few decades is a distinct possibility."
But the paper's authors point out that the updated models have not reduced the range in model projections of when there might be a seasonally ice-free Arctic Ocean.
So there are recent climate models that project seasonal ice-free Arctic seas within the next few decades, but they cannot yet pinpoint a precise year, or even decade. Lars-Otto Reiersen, head of the Arctic Monitoring and Assessment Programme, tells Reuters most models indicate ice-free conditions in 30 to 40 years, although "there are models that indicate 2015 as an extreme."
Overall, this Reuters headline probably offers the best summary of the state of scientific opinion:
"Arctic summer sea ice might thaw by 2015 - or linger for decades"
Posted by Verity on Sunday, 9 September, 2012
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