Arctic sea ice extent total for March does not equal global polar bear habitat

Only half or less of the estimated 2.6% loss per decade of March sea ice extent since 1979 (Fig. 1, below) represents a decline in polar habitat. That’s because several regions with sea ice that are not home to polar bears, like the Sea of Okhotsk, are included in Arctic sea ice totals.

Figure 1. Average monthly Arctic sea ice extent for March 1979-2015 (which includes ice in the Sea of Okhotsk, the Sea of Japan, and the Baltic, where polar bears do not live), shows a decline of 2.6% per decade. NSIDC, March summary 2015.

Both the Sea of Okhotsk and northern Sea of Japan (Fig. 2) have sea ice in winter (which is included in total Arctic sea ice records) but they are not truly “Arctic” – neither is connected to the Arctic by continuous ice, even when the ice is at its maximum extent (nor is the Baltic Sea — in contrast to Hudson Bay and the east coast of North America, which are connected to the Arctic by continuous ice).

Figure 2. Location of the Sea of Okhotsk and Sea of Japan. Insert ice map for March 1979 from NSIDC shows it’s position relative to the Arctic proper.

That lack of connection to Arctic pack ice is probably the main reason that polar bears never colonized the Sea of Okhotsk, even though western Arctic seal species (ringed, bearded, spotted, and ribbon) and Arctic whales  (bowhead and beluga) live there. Polar bears don’t currently live in the Sea of Okhotsk and all evidence suggests they never have.

Sea ice maps show that about half of the total ice extent difference between March 1979 and March 2015 was due to a relatively large decline in sea ice cover for Sea of Okhotsk and northern Sea of Japan — regions without polar bears. Surely no reputable scientist or journalist would suggest that the “record low” maximum ice extent for 2015 has any relevance for polar bear health and survival? [or for Northwest Passage travel, for that matter] Sadly, they would.

The Sea of Okhotsk covers 1.58 million kilometres squared (mkm2) – it’s a huge basin that was virtually filled with ice in March 1979 but only about 1/3 filled in 2015.

Detailed figures of ice cover, provided by NSIDC’s MASIE analysis since 2006, shows the ice extent for the Sea of Okhotsk in March 2006 was only 0.59 mkm2¹ — a large decline from its maximum extent in 1979.

However, Fig. 3 (below) shows that there was even less ice in the Sea of Okhotsk in 2015 than there was in 2006. I estimate that in 2015, there was approximately 0.5 mkm2 of sea ice in the Sea of Okhotsk and northern Sea of Japan combined (precise MASIE data is not available for 2015, but looks most like MASIE maps for 2009¹).

Figure 3. Arctic sea ice extent and concentration for 2015 and 2006 compared, courtesy NSIDC. Note that there was less ice in the Sea of Okhotsk in 2015 than in 2006, where MASIE archives show ice covered 0.59 mkm2. Click to enlarge.

Here’s the math for the estimated contribution of Sea of Okhotsk ice to total Arctic ice extent for March:
total extent 1979 (period high), 16.5 mkm2
total extent 2015 (period low),  14.4 mkm2.

The difference between them is 2.1 mkm2, of which approximately 1.1 mkm2 represents the change in ice cover in the Sea of Okhotsk and Sea of Japan between 1979 and 2015 (Fig. 4).

Figure 4. Comparing sea ice extent over the Sea of Okhotsk and northern Sea of Japan for 1979 and 2015 (extents approximate).

Since polar bears do not live in the Sea of Okhotsk, it would therefore be totally disingenuous for a polar bear researcher, journalist, or conservation organization to state or imply that the “record low” maximum ice extent this year has any relevance for polar bear health or survival. But activist organization Polar Bears International recently did just that and so did polar bear biologist Andrew Derocher.

Did they just not think — or were they deliberately misleading the public?

Arctic sea ice extent total for March > (≠) global polar bear habitat.

In fact, I suggest that if ice extent figures for the Sea of Okhotsk, Sea of Japan and the Baltic Sea were removed from “Arctic” ice extent totals for the satellite record, we would find that total polar bear habitat at the end of March since 1979 has been about 15.0 mkm2 with relatively little variation over time.

And as I explained in my last post, polar bear habitat this spring is abundant.

Footnote 1: Sea ice extent totals at the end of March for Sea of Okhotsk (from NSIDC MASIE analysis archives) since 2006, the first available date, are given below. It is not clear if the extent totals given include ice in the northern Sea of Japan but they might.

2006 – 0.59 mkm2 (here, see below)

2007 – 0.8 mkm2 (here)
2008 – 0.68 mkm2 (here)
2009 – 0.51 (here and here, averaged)
2010 – 1.0 mkm2 (here)
2011 – 0.98 mkm2 (here)
2012 – 1.2 mkm2 (here, see below)

2013 – 1.0 mkm2 (here)
2014 – 0.83 mkm2 (here)
2015 ~0.51 mkm2 (MASIE not available, but NSIDC map below suggest extent was most like 2009)