Now that Bering Sea ice cover is high again, variability is normal

Sea ice development in the Bering Sea is trending higher than average again this year, even though it is early in the season. The NSIDC sea ice extent image for Dec 6 2012 below (Fig. 1) shows extensive ice development in the Bering Sea. [courtesy WUWT sea ice reference page]

“Seven of the last 10 years have produced above-average freezing in the waters west of Alaska” says Kathleen Cole of the Alaska National Weather Service (Dec. 6, 2012) – see “Featured Quote” #20 in the right column (also filed in the “Quote Archive“).

Figure 1. NSIDC sea ice extent at Dec. 6 2012.
The orange line is the median extent for this date for 1979-2000. Click to enlarge.

The NSIDC, in their report on the annual freeze-up of the Arctic (November 2012), have this to say about the ice in the Bering Sea:

“…ice extent in the Bering Sea by the end of the month was greater than average, continuing a pattern seen in recent years. Extent in the Bering Sea was at record high levels last winter.”

Why is this significant for polar bears? Polar bears of the Bering Sea are considered part of the “Chukchi Sea” subpopulation, of which very little is known. In a previous post, I noted a statement made by US Fish & Wildlife polar bear biologist Eric Regehr:

“In 2009, when the PBSG [Polar Bear Specialist Group] issued its population status reports, it listed the Chukchi Sea population, which Alaska shares with Russia, as being of unknown size, but one thought to be in decline because of anecdotal reports about possible over-harvesting in Russia. But now, newer research yet to be published has scientists reconsidering the status designations of the Chukchi population, Regehr said. It appears the bears in this area are reproducing well and maintaining good body condition.” [Oct 9, 2012 Alaska Dispatch “How many polar bears live in the Arctic?” Jill Burke]

It is unclear from Regehr’s statement whether this “newer research yet to be published” includes the Bering Sea. However, it appears that sea ice habitat for polar bears in the Bering Sea is not in fact disappearing and putting polar bears at risk, but expanding. And now that the ice cover trend is going up instead of down, sea ice researchers now contend that sea ice extent that varies extensively – high some years, low in others – is normal for the Bering Sea.

In a study published last year (2011), Zachary Brown and colleagues Gert van Dijken and Kevin Arrigo report that while the Bering Sea was thought to be “rapidly warming and losing sea ice,” the results of their study indicate that “mean annual sea ice extent in the Bering Sea has exhibited no significant change over the satellite record (1979-2009)” and that “the Bering Sea as a whole has undergone no significant change in the timing of ice retreat or advance, or in the length of the ice-free season.”

Brown et al. also say that sea ice cover in the Bering Sea ice is “most aptly characterized by its intense interannual variability.”  Sea ice extent in the Bering Sea varied over their period of study (1979-2009) from a low of 130,000 km2 (in 1979) to a high of 256,000 km2 (in 1999).

However, 2009 is now three years ago. Bering Sea ice is not only still above average but the NSIDC says that last year’s maximum extent for the Bering Sea exceeded the highest amount recorded in the satellite record period. The May 2012 extent (presumably the annual maximum) was 350,000 km2, considerably more than the previous annual high of 256,000 km2 and almost 3x more than the lowest extent reported by Brown et al. (see the June 2, 2012 NSIDC report).

In summary, maximum ice cover in the Bering Sea is now acknowledged to have varied naturally between 130,000 km2 (1979) and 350,000 km2 (2012), a huge amount of habitat change for polar bears. Too bad we have no data on Bering Sea bears, we might have learned a lot about how they coped with this change.

References
Brown, Z.W., van Dijken, G.L. and Arrigo, K.R. 2011. A reassessment of primary production and environmental change in the Bering Sea. Journal of Geophysical Research 116:C08014. doi:10.1029/2010JC006766.