Polar bear habitat in the Arctic Basin this year appears to have reached its apex days earlier than average. As of 12 September, freeze-up of Arctic sea ice had begun. Unless something dramatic happens over the next few days, this will make 2015 the earliest September minimum since at least 2007, using NSIDC data.1
The two lowest September ice extents (2007 and 2012) were also both later than average; this year’s minimum is the fourth lowest (see chart below).
Of course, all this fuss about how low the September minimum gets is irrelevant to polar bears: they are either on land or in the Arctic Basin, and virtually all are living off stored fat no matter where they are (see Arctic Basin bear here). What matters is when the refrozen ice reaches pregnant females that have preferred denning spots onshore (like in Svalbard) or for bears onshore waiting to return to the ice to hunt (like Davis Strait, and Western and Southern Hudson Bay bears). We won’t know that until October (for Svalbard) or November (for E. Canada).
Again, no sea ice death spiral or polar bears in peril because of it.
UPDATE 15 September 2015, 11:00 am PDT: Just published at the NSIDC website, 2015 minimum has been (tentatively) called at 4.41 mkm2, confirming my figure taken from their interactive graph (see below). However, despite the fact that their own data show that sea ice extent stayed at that value for three days, NSIDC has chosen the last day of that 3-day period rather than the first to represent the 2015 minimum. Go figure. That makes 2015 tied with 2011 for the earliest date for their official records, which seems more than a little self-serving and means I’m not changing the title of my post. NSIDC have also modified slightly some of the official extent figures for past minimums (added below) but it doesn’t really change anything.