On Aug. 27, 2012 sea ice extent dipped below the yearly minimum extent reached in 2007.

The sea ice at this point, a five-day average, was 4.1 million km squared and the lowest point reached since satellite records began in 1979. The National Snow and Ice Data Center (NSIDC) thought it was important enough to issue a press release.



There are sure to be more than a few media-hungry folks who will be moaning about the fate of the poor polar bears because of this report. Before people get too hysterical, a bit of rational perspective might be helpful.

Keep in mind that 4.1 m sq. km (Fig. 5) is about the size of Greenland (2.166 m km2) plus Mexico (1.972 m km2), or about the size of India (3.287 m km2) plus Pakistan (0.809 m km2), figures from Wikipedia.

It may be a ‘record’ low, but it’s still a lot of ice: 4.1 m km2 is not anywhere close to an ‘ice-free’ Arctic!



Arctic sea ice melts – or gets pushed out of the Arctic and then melts – every year, leaving various amounts behind. The ice lingering at mid-September adds to next year’s ‘multiyear’ ice.

The maximum extent is usually reached mid-to-late March. Have a look at how this ‘low extent’ developed from the March maximum this year. Even if you’ve looked at some of these maps, you may not have looked at them one after the other. I’ll compare these to the fall maps for 2007, after the Sept. minimum and discuss these in relation to polar bear habitat.

Figure 1 shows the extent of ice this year at the end of May, which was still 13.13 million square km (m km2). This was down from the maximum of 15.24 m km2 in March.

Here’s where polar bears live, in case you haven’t seen this map before:

Now that we know what the Aug. 26, 2012 extent looks like compared to the minimum extent of 2007, let’s look at what the ice was like at its low point in 1979 – with respect to polar bear habitat (Fig. 6).

Sea ice in the fall of 2007, after the September minimum

Now, look at what happened in the fall of 2007, as the ice began to reform, again with respect to polar bear habitat (Fig. 7-8).

Note that Hudson Bay bears routinely go 4 months or more without eating, while bears in other areas fast for 2-3 months, depending on local conditions. (Messier et al. 1992; Ramsay and Stirling 1988:614; Stirling and Oritsland 1995). This summer fast is normal for polar bears and they are well adapted to handle it.

Polar bears need ice from March to June/July when they feed on baby seals. By mid- to late July – well before the minimum extent of sea ice – they are well fed and ready to go 2-4 months or more without eating. See Featured quote #7 in the quote archive for an example. See Quote archive

You might well ask, however…

In regards to sea ice, how ‘thin’ is ‘too thin’ for polar bears?

Newly-formed ice that is present in the fall starts out thin but quickly thickens due to the frigid temperatures. But how ‘thin’ is too thin for polar bears?

We hear Arctic scientists talking about ‘thin’ ice – especially, the perils of thin ice – but what does ‘thin’ ice really mean?

According to NSIDC:

new ice is < 10cm thick

young ice is 10-30cm thick (30cm is about 1 foot)

thin first year ice is 30-120cm (0.3-1.2m) thick

thick first year ice is 120-200cm (1.2-2.0m) thick or more.

So, technically speaking, ‘thin’ ice can be 1.2m (about 4 feet!) thick. Try to remember that the next time you hear or read the media or research spokespersons ranting about the dire state of the Arctic because of ‘thin ice.’

[See Canadian Ice Service definitions of ice thickness here]

Work on polar bears in the Canadian Arctic around Baffin Bay (Ferugson et al. 2000) has shown that the bears routinely hunt on ‘young ice’ (0.1-0.3m) and are known to use thick first year ice (1.2-2.0m) for over-wintering activities, including denning – and are as successful doing so as bears that have access to multiyear ice.

In the Barents Sea (around Svalbard, north of Norway), polar bears are known to den on first year ice (Mauritzen et al. 2001). In other areas that are dominated by 1st year ice (e.g. Southern Beaufort Sea, Chukchi Sea, Laptev Sea, and East Greenland), bears that den on stable annual ice or pack ice that is 1-2 years are common (Amstrup and Gardner 1994; Schliebe et al. 2008:1000).

In fact, it appears that while some polar bears make use of multiyear ice for denning where it is available in winter (Durner and Amstrup 1995:339; Ferguson et al. 2000:769), they rarely use it in summer in the areas that have been studied extensively (e.g. Durner and Amstrup 1995:340).

That said, a Russian researcher recently did a straight-line survey by ship across the Arctic Basin in late summer (2005 and 2007) – the very first survey of any kind to be done in the central Arctic – and found a number of bears in good condition (including a pair of bears feeding on a ringed seal) on multiyear ice over deep water (Ovsyanikov 2010:175-177).

In summary, sea ice only needs to be 0.3m or so for polar bear hunting/travelling and 1.2-2.0m for denning. They do not require multiyear ice for denning.

The take home message is this: “An ice-free September in the Arctic really means nothing in the life cycle of polar bears.” [h/t commenter “Kels”, from last post update]



References

Amstrup, S.C. and Gardner, C. 1994. Polar bear maternity denning in the Beaufort Sea. The Journal of Wildlife Management 58:1-10. http://www.jstor.org/discover/10.2307/3809542?uid=3739400&uid=2&uid=3737720&uid=4&sid=21101008172123

Durner, G.M. and Amstrup,S.C. 1995. Movements of a polar bear from northern Alaska to northern Greenland. Arctic 48:338-341. http://www.jstor.org/discover/10.2307/40511936?uid=3739400&uid=2&uid=3737720&uid=4&sid=21101008172123

Ferguson, S. H., Taylor, M. K., and F. Messier 2000. Influence of sea ice dynamics on habitat selection by polar bears. Ecology 81:761-772. http://www.esajournals.org/doi/abs/10.1890/0012-9658%282000%29081%5B0761%3AIOSIDO%5D2.0.CO%3B2

Mauritzen, M., Derocher, A.E. and Wiig, Ø. 2001. Space-use strategies of female polar bears in a dynamic sea ice habitat. Canadian Journal of Zoology 79:1704-1713. http://www.nrcresearchpress.com/doi/abs/10.1139/z01-126

Messier, F., Taylor, M. K., and Ramsay, M. A. 1992. Seasonal activity patterns of female polar bears (Ursus maritimus) in the Canadian Arctic as revealed by satellite telemetry. Journal of Zoology London 226:219-229. http://onlinelibrary.wiley.com/doi/10.1111/j.1469-7998.1992.tb03835.x/abstract

Ovsyanikov, N. 2010. Polar bear research on Wrangel Island and in the central Arctic Basin. In, Proceedings of the 15th meeting of the Polar Bear Specialists Group IUCN/SSC, 29 June-3 July, 2009., Obbard, M.E., Theimann, G.W., Peacock, E. and DeBryn, T.D. (eds.), pp. 171-178. Gland, Switzerland and Cambridge UK, IUCN. PBSG



Ramsay, M. A. and Stirling, I. 1988. Reproductive biology and ecology of female polar bears (Ursus maritimus). Journal of Zoology London 214:601-634. http://onlinelibrary.wiley.com/doi/10.1111/j.1469-7998.1988.tb03762.x/abstract



Stirling, I., and N. A. Øritsland 1995. Relationships between estimates of ringed seal (Phoca hispida) and polar bear (Ursus maritimus) populations in the Canadian Arctic. Canadian Journal of Fisheries and Aquatic Sciences 52:2594-2612. http://www.nrcresearchpress.com/doi/abs/10.1139/f95-849