September sea ice ballyhoo and why it doesn’t matter to polar bears
The end of September sea ice summary from the US National Snow and Ice Data Center (NSIDC) finally became available late last week (October 3, “A better year for the cryosphere”).
The summary figure NSIDC provides are the average ice extent for the month (not the maximum achieved at the end of the month), which are compared to previous years.
[There has been considerable ice growth since the end of September (updated daily here].
Here is why the September extent doesn’t matter to polar bears: it is the extent in June that is important to polar bear survival. June is the end of the critical spring feeding period for polar bears (see previous post here) – healthy bears eat more seals over a shorter period of time from March to June than any other time of year. After the end of June, most bears have enough fat to survive a fast of 4 months or more.
In contrast to September – when many bears are taking a time-out on shore – ice extent for June over the last 30 years or so provided an extensive hunting platform for polar bears throughout the Arctic. To show you how extensive, I’ve constructed a composite of ice maps from selected years (Fig.1, below).
Figure 1. Sea ice extent averages for June: 1979 (top left), 1996 (bottom left), 2010 (top right) and 2013 (bottom right). Note that even during the lowest extent on record for June (2010, 10.8 m km2), there was still a considerable amount of ice remaining in all regions where polar bears reside, including Hudson Bay (HB), Davis Strait (DS), East Greenland (EG), Baffin Bay (BB), Barents Sea (BS), Chukchi Sea (CS) and Southern Beaufort Sea (SB), noted on the 1979 map (top left). Maps from NSIDC archive. Click to enlarge.
A bit of variation over time occurred in June but not much: down from 12.6 million kilometers squared in 1979 to 10.8 in 2010, and back up to 11.6 in 2013.
Most importantly, even at the lowest point (2010, 10.8 m km2), there was still considerable amounts of ice remaining in all regions where polar bears reside, including Hudson Bay, Davis Strait, East Greenland, Baffin Bay, Barents Sea, Chukchi Sea and Southern Beaufort Sea.
Here are the numbers (in millions of square kilometers):
June (range 10.8-12.7, a difference of 1.9)
1979 12.6
1982 12.7 [highest]
1996 12.1
2009 11.5
2010 10.8 [lowest]
2012 10.9
2013 11.6
[NSIDC graph of this data here]
I’ve included 1996 in the June comparison because surprisingly, as Fig. 2 below shows (original here), the highest average September extent recorded over the last 34 years was not 1979, but 1996. I wanted to see how June 1996 compared to other years.
Figure 2. The latest update to NSIDC’s average monthly Arctic sea ice extent (not the annual minimum), in millions of kilometers squared. Note that the highest extent for 1979-2013 occurred in the middle of the record, in 1996 (7.9), the lowest in 2012 (3.6). The value for 2013 was 5.3.
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