There has been a good deal of attention paid to the recent
decline in Arctic sea-ice in the British press over the past week,
with headlines including the following:
Arctic sea ice is melting at its fastest pace in almost 40
years (The Guardian)
Arctic sea ice melts at fastest rate for 40
The Great Thaw: Arctic sea ice levels shrink to a record
low (Daily Mail)
What measure are we using?
It is worth noting of this coverage, first of all that there are
different ways of taking meaningful measures of Arctic ice. The
most frequently used measure is sea ice "extent", defined as the
surface area of the Arctic that is greater than 15% ice. Extent can
be important in terms of ice albedo effects, and their impacts on
regional and global warming, as well as further feedback effects on
Arctic ice and elsewhere - and of course for all manner of other
repercussions as the ice recedes.
Another important measure, however, is the volume of sea-ice,
which can be modelled more roughly. Volume matters in part because
younger, thinner ice can be more vulnerable to adverse weather
conditions, which impact on the ice in the context of long-term
Thus age, extent and volume all matter in weighing up the future
of the Arctic ice, and in particular what point this century we can
expect to see ice-free Arctic summers.
2010 witnessed a record-breaking low for Arctic sea-ice
volume; 2011 looks like going the same way
In the context of the above, it's worth noting that the
Guardian's coverage referenced aforthcoming
paper in Geophysical Research Letters by researchers at
the Polar Science Center of the University of Washington, Seattle,
which notes that:
"the 2010 September ice volume anomaly
did in fact exceed the previous 2007 minimum by a large enough
margin to establish a statistically significant new record."
The Polar Science Center
use the Pan-Arctic Ice Ocean Modeling and Assimilation
System (PIOMAS for short) to assess Arctic ice volume. It has
noted by other blogs that the PIOMAS graphs appear to
indicate that in 2011 Arctic sea ice volume has already surpassed
the minimum set last year - although PIOMAS themselves haven't made
an announcement to that effect.
Extent has reached a record low in one data set this
year, but not yet in others
physicists at the University of Bremen has provided the main basis
for the stories on the Arctic ice. While this is certainly a
legitimate measure, it is worth noting that it is only one data
set, and that others - such as that compiled by the
well-known National Snow and Ice
Data Center (NSIDC) - don't yet confirm this result. Live
Science's slightly more circumspect headline "Arctic
Sea Ice Hits Record Low According to One Measure" - thus gives a
rather more rounded impression on this score.
As the NSIDC noted on 6
"Arctic sea ice extent will likely reach
its minimum extent for the year sometime in the next two weeks.
NSIDC will make a preliminary announcement when ice extent has
stopped declining and has increased for several days in a row."
They add that:
"with the ice cover now thinner than in
years past, there is a greater potential for late-season ice loss,
caused by warm water melting ice from below or winds that push the
Today the NSIDC website showed a slight uptick in ice extent.
This could mean the melt is over - this would make 2011 the second
lowest extent on record after 2007. But we probably need to wait a
week or so to be sure.
How important is the mere fact of a record-breaking
It's worth bearing in mind that the evidence of Arctic decline
this year is worrying enough in and of itself as evidence of the
long-term trend in Arctic sea-ice decline. As Ted Scambos, senior
research scientist at NSIDC, points
out, the most concerning point is that the Arctic seems to have
needed no "extra help" this year to replicate 2007's drastic
minimum. Says Scambos:
"The main message is not so much whether
or not we set a record, but this year, without any noticeably
unusual pattern of weather, we nearly broke a record, which only
four years ago took a very unusual weather pattern plus a warming
Arctic to achieve".
And that in itself should be quite worrying enough.