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.