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:
â?¢ Arctic sea ice is melting at its fastest pace in almost 40 years (The Guardian)
â?¢ Arctic sea ice melts at fastest rate for 40 years (Telegraph)
â?¢ 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 global warming.
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 been 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
Data from 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 September:
“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 ice together.”
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 year?
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.