Never mind that the sea ice maximum this year came almost a month later than last year (and close to latest since 1979) – and was lower by only .02 – the US National Sea Ice Data Center (NSIDC) today trumpeted a new record low. What this means to polar bears, if anything, remains to be seen.
2015: maximum set February 25 (day 56), at 14.54 mkm2
2016: maximum set March 24 (day 84), at 14.52 mkm2
[The difference in area? Smaller than the Islands of the Bahamas]
Latest maximum extent (since 1979) occurred in 2010 on April 2 (Day 92).
The average date for maximum extent is March 12.
I note, however, that given the lateness of the winter sea ice surge meant that the amount of ice present at 24 March 2016 (see NSIDC Interactive) was more than was present on the same date in 2006, 2007 and 2015.
Clearly, there was plenty enough sea ice in the spring of those years for most polar bears to hunt seals successfully and put on the weight they needed to survive the summer fast ahead. I see no reason to expect 2016 to be different.
Overall, the situation is better than many recent years – lower than average but higher than even 2012 and 2007, when all the polar bears didn’t die despite record-low extents for September. In several areas of the Eastern Arctic, sea ice levels continue to run higher than average. There is plenty of ice in the Arctic Basin for polar bears spending the summer under the midnight sun.
According to NSIDC MASIE (see map above):
2007 at 25 August – 5.2 mkm2
2012 at 25 August – 4.9 mkm2
2015 at 25 August – 5.3 mkm2
Sea ice looks low for this time of year but how does it compare to 2007, when summer ice habitat for polar bears hit a record-breaking low? What can the impact of 2007 ice levels on polar bears tell us about what to expect this year?
By this date in 2007 (8 August, Day 220, NSIDC Masie map below), there was almost 1 million km2 less ice than there is this year (map above). However, look which polar bear subpopulations not only survived, but thrived, through the 2007 low ice summer: Chukchi Sea, Southern Beaufort, Barents Sea, Davis Strait, Foxe Basin, Western Hudson Bay, and Southern Hudson Bay. That’s all of the subpopulations for which we have recent data.
There is more than a month left in the melt season, of course. However, while 2012 finished with a lower minimum ice extent due to a massive mid-August storm that broke up a lot of ice (Simmonds and Rudeva 2012), by the end of the first week of August (i.e, the 8th), there was more ice in 2012 than in 2007 and a bit less than this year (2012, 6.3 mkm2; 2007, 5.6 mkm2; 2015, 6.5 mkm2).
This means if less summer ice for a longer period of time impacts polar bear health and survival, conditions in 2007 should have had a noticeable impact on polar bears around the world. They didn’t. That suggests even if this September sea ice minimum is as low as 2007, it won’t have any negative impact on polar bear health or survival. The most profoundly negative documented impacts have come from thick sea ice in spring or suboptimal spring snow levels (Crockford 2015) and the evidence shows that variation in the extent of summer ice is simply irrelevant to polar bears.
Posted in Life History, Sea ice habitat
Tagged Arctic, extent, habitat loss, hunting, melt, polar bear, population, record-breaking, sea ice, summer, survival, threatened