According to the National Snow and Ice Data Center (NSIDC, Sept. 20 report), the annual sea ice minimum extent was reached on Sept. 13, 2013.
At 5.10 million square kilometers, this year’s low was a whopping 1.69 million square kilometers above the minimum extent for 2012 (which was the lowest since 1979) and well within two standard deviations of the 1979-2010 average. (Two standard deviations: “Measurements that fall far outside of the two standard deviation range or consistently fall outside that range suggest that something unusual is occurring that can’t be explained by normal processes”).
The minimum extent for 2013 is virtually indistinguishable from the minimum for 2009, which was 5.13 million square kilometers. The ice was distributed a bit differently in 2009 – more in the east and less in the west — than it was this year (see Fig. 1 below).
You’ll know from previous discussions here that the annual minimum reached in late summer has little impact on polar bear health and survival (see excellent summary of the evidence for that here). What matters most to polar bears is the presence of ample ice in spring and early summer (March-June), which is their critical feeding period.
But after the fast that many polar bears endure over the height of the summer, they are eager to get back onto the ice and resume hunting. When in the fall does that become possible?
I wondered what the similarity in extent for 2013 and 2009 might tell us about polar bear habitat development over the next month or so.
In other words, what might polar bears this year expect in the way of sea ice development by say, the end of October? When might they be able to start hunting?
So I used the JAXA (Japan Aerospace Exploration Agency) “Sea ice monitor” feature, which I used to generate Fig. 1 (which maps sea ice concentration for any date you choose back to 2002, see here), to plot the date the 2013 minimum was reached (September 13) with an overlay for October 31, 2009. See Fig. 2 below.
As Fig. 2 shows, by 31 October 2009, virtually all regions of the Arctic where polar bears reside had ice that could be used as a hunting platform, with a few notable exceptions: western and southern Hudson Bay, Davis Strait and the Chukchi Sea.
Bears in southern and western Hudson Bay had to wait another 5 weeks before they could resume hunting in 2009 (see Fig. 2 in previous post here). However, the late Hudson Bay freeze-up that fall was offset by the late breakup of ice that summer. The late breakup meant that polar bears got to hunt well into August and most arrived on shore in very good condition to start their summer fast (TundraBuggy News; pdf backup here).
Davis Strait bears summering on Baffin Island, and Chukchi Sea bears (see here, here, and here) summering in western Alaska and eastern Russia, had to wait another 2-3 weeks for substantial amounts of ice.
Time will tell if conditions similar to 2009 develop over the coming weeks or if a totally different pattern emerges. I’ll post updates as they transpire.