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Arctic sea ice minimum in 2007 "not a one off"

  • 13 Oct 2011, 14:10
  • Verity Payne

Since Arctic sea ice reached an unprecedented low during September 2007, speculation has been rife each summer - will ice retreat be as dramatic again? This summer was no exception as early signs indicated ice levels might reach another record low.

However, it all got a bit confusing as different research groups began announcing their findings. First came the news that the Polar Science Center, University of Washington, had found that Arctic sea ice volume reached a new record minimum in 2010. Then researchers at the University of Bremen announced that Arctic sea ice extent reached an "historic low" in 2011. Finally, the National Snow and Ice Data Center (NSIDC), announced their finding that Arctic sea ice had neared the record low of 2007 but had not surpassed it.

Now, scientists from the University of Bremen have written to the journal Nature, pointing out that debate over whether or not Arctic sea ice reached a record low this year "misses the crucial point."

The scientists say that the important story this summer ought to have been that:

"...This year's [Arctic sea ice] minimum is evidence that the unprecedented seasonal minimum of 2007 was not a one-off."

As we explained in a previous post, there are a number of different measures of sea ice cover (sea ice extent and sea ice volume, for example). Plus, different research groups can use different methods for calculating ice cover. However, the findings of the various research groups were remarkably similar, as the Bremen group points out:

"All of these groups agree that the seasonal 2011 minimum is very close to the 2007 minimum."

The Bremen researchers suggest the recurrence of a retreat as severe as 2007's indicates that such dramatic lows are unlikely to simply be anomalies:

"That [2007] value was about 25% less than the previous low in 2005, and almost 40% less than the climatological mean for the seasonal minimum (1979-2007). In autumn 2007, this value could be considered an outlier, caused by unusually warm weather over large parts of the Arctic Ocean. However, the five seasonal minima since 2007 are the lowest on record. Although it is too early to speak of a trend, other observations, such as the thinning of Arctic sea ice over the past two decades, also suggest that the 2007 and 2011 minima are not single outliers."

The US National Snow and Ice Data Center (NSIDC) has also highlighted the thinning of Arctic sea ice, which results from the loss of the region's oldest, multi-year ice in excess of the predictions in the International Governmental Panel on Climate Change's 2007 Fourth Assessment Report (AR4). They say:

"Continued loss of the oldest, thickest ice has prevented any significant recovery of the summer minimum extent. In essence, what was once a refuge for older ice has become a graveyard."

Arctic sea ice loss has been accelerating over the last few years, at a faster rate than projected in the AR4 report. Recent research suggests that the IPCC models used to project Arctic sea ice cover in the future cannot capture features like thinning of the sea ice particularly well, leading to the suggestion that AR4 projections for ice-free Arctic summer seas by 2100 are likely to be too conservative, and it is certainly conceivable that the next IPCC report might contain quite different predictions about the future of the Arctic sea ice.

 

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