Barents Sea polar bear condition varies with AMO and spring sea ice conditions

Figure 1. NSIDC sea ice extent at March 8, 2014 (a “MASIE” product), with labels added. Click to enlarge.

In its end of February report, the US National Snow and Ice Data Center (NSIDC) noted that Barents Sea ice was below average for this time of year (see Fig. 1 above, and Fig. 5 below) but suggested this was primarily due to natural variation driven by the Atlantic Multidecadal Oscillation (AMO):

“The Barents Sea has experienced consistently low extents, particularly in winter, and this year has been no different. While the Barents and Kara seas normally have close to 2 million square kilometers (772,000 square miles) of ice in February, recent years have seen 500,000 square kilometers (193,000 square miles) of ice extent or lower. This year, the Kara Sea is near average, but the Barents Sea remains low (Figure 4a). Unlike other regions in the Arctic, longer records of Barents Sea ice extent exist from records of fishing, whaling, and other activities. A recent paper (Miles et al., 2013 [2014, now in print]) examined these records, along with paleoproxy data, to examine extent over the past four hundred years. They found a 60- to 90-year cycle in Barents and Greenland seas ice extent related to the Atlantic Multidecadal Oscillation (AMO); the AMO is a basin-wide cycle of sea surface temperature variability similar to the El Niño and La Niña cycles in the Pacific, but varying over much longer periods. This research shows that in addition to the warming trend in the Arctic, some sea ice regions are likely also responding to natural climate variability.” [my bold]

The paper they cite (Miles et al. 2014, discussed elsewhere in December 2013 here) described the AMO this way:

“The AMO is a coherent pattern of basin-wide sea surface temperature (SST) variations with a period of roughly 60–90 years. ..Paleoenvironmental studies suggest that the AMO has persisted through previous centuries [Gray et al., 2004] and even millennia [Knudsen et al., 2011].”

Note that Miles and colleagues were looking at ice records on or around the sea ice maximum in winter/spring.

The Polar Bear Twist: Norwegian biologists Jon Aars and Magnus Andersen, who I’ve discussed before, have pointed out that the condition of polar bear males and females around Svalbard (Fig. 2) they examined over the last 20 years varied with the AMO and sea ice levels in spring and early summer. [research results posted at the website for Environmental Monitoring of Svalbard and Jan Mayen (MOSJ), Norwegian Polar Institute].

Figure 2. The Barents Sea polar bear subpopulation boundaries, courtesy the IUCN Polar Bear Specialist Group. Svalbard is the largest archipelago, closest to the East Greenland Sea.

That makes a lot of sense to me, given that spring/early summer is the most critical feeding season for polar bears because it’s when fat young seals are most easily available.

It also makes sense to me that you may need a record hundreds of years long to understand the natural variability of Arctic Sea ice in its various regions. Recall that natural variation, not global warming, is now being used to explain the large variation in annual sea ice cover in the Bering Sea (home to Chukchi Sea polar bears).

Figure 3. From Aars and Andersen’s 2013 online data: “Proportion of females with cubs of the year – COYs (upper panel) and yearlings (lower panel), based on data from the annual capture-recapture program 1993-2013.”

Aars and Anderson say this about Fig. 3 above:

“The figure shows the number of cubs of the year (COYs) pr adult female based on data from the annual capture-recapture program 1993-2013. The dotted line shows a significant decreasing trend over time (p = 0.049). A major part of the interannual variation is explained by variations in the Arctic Oscillation (AO) in spring (Apr-Jun) the preceding year. Higher values of AO correlate with lower cub production the year after (p < 0.01).” [my bold]

They also used variation in the AMO and resulting variation in spring/early summer conditions to explain most of the variation in the condition of adult males caught in spring.

It’s clear that the changes in sea ice conditions affecting polar bears year to year are short term fluctuations in the AMO (these can be seen in panel (a) of Fig. 4, below, from the Miles et al. paper).

However, understanding that there is a long term pattern of variability is also important for understanding changes to polar bear habitat over extended periods of time.

Figure 4. These are panels (a) and (e) from Miles et al. 2014:Fig. 2, showing the short-term instrument portion of the AMO record (a) and the extended record going back 400 years (e).

From the “Results: Multidecadal Sea-ice Signal” section of the Miles et al. paper cited:

“The salient feature is the presence of pervasive multidecadal variability, upon which interannual-to-decadal fluctuations are superposed (Figure 1). Four aspects should be noted: (1) less sea ice is generally seen in most recent decades—despite the fact that these series do not extend into the 2000s—however, a common characteristic amongst several of the records (Figures 1a–1e) is sharply reduced sea ice at the onset of the ETCW, heralding the termination of the Little Ice Age in the region; (2) multidecadal variability is apparent in all of the records, except for the Baltic Sea. The wavelet-filtered signals (bold lines in Figure 1) have predominately 60–90 year timescales; e.g., the 400 year Iceland and “Western Nordic Seas” series have five successive peaks and troughs; the multidecadal fluctuations amongst the records are broadly consistent in their periods and in phase (though offset several years for the Newfoundland and the Barents Sea); (3) the multidecadal signal is persistent in all records where it is found—in no cases does the signal disappear through time; and (4) multidecadal signals are strongest in the Greenland Sea region (Figures 2b–2e) and weaker on either side, i.e., Newfoundland and the “Eastern Nordic Seas” (i.e., Barents Sea). Further, this is consistent with findings from twentieth century records from the Russian Arctic seas [Polyakov et al., 2003], which indicate that a multidecadal fluctuation dissipates eastward from the Kara Sea, supporting the notion that the North Atlantic is the source of this variability.” [my bold]

Later, Miles and colleagues add this:

“Here for the first time, we explicitly compare sea-ice variability and the AMO over several centuries, thereby linking the two. We focus further on three independent sea-ice records from the Greenland Sea region, together with two AMO indices. The sea-ice records are the 180 year series Fram Strait ice export reconstruction, the 400 year historical series from Iceland, and the 800 year paleoproxy reconstruction of the Western Nordic Seas.
..
“…the lowest sea-ice values before the twentieth century occurred in the middle to late 1600s (W5 in Figure 2), as shown here in the Icelandic historical record and the Western Nordic Seas proxy (Figures 2d and 2e) and in a pan-Arctic summer sea-ice reconstruction [Kinnard et al., 2011]. This period coincided with a period of high AMO-proxy index values (Figure 2b). Moreover, the three highest sea-ice values (e.g., early 1600s, late 1700s to early 1800s, and late 1800s–1920) occurred during the three lowest AMO index values on record, thus demonstrating persistent covariability.”

They concluded:

“The pervasive multidecadal variability in observed sea ice is here not considered to represent truly oscillatory cycles but rather irregular, broadly multidecadal fluctuations between warmer (colder) periods with less (more) ice that are related to AMV [Atlantic Multidecadal Variability].”

So, we have marked natural variability in winter/spring sea ice conditions going back at least 400 years — variability that has been shown to affect polar bear condition over the short term.

I suggest this means that before we take anyone seriously who suggests that declining summer ice or delayed winter ice formation due to global warming is to blame for changing conditions of polar bears in East Greenland or the Barents Sea (e.g. Derocher et al. 2011; Obbard et al. 2010) — or predicting such effects into the future — we need to see that they have taken into account the influence of the variable AMO on spring ice conditions.

Figure 5. Sea ice extent for the month of February, 2014, published in the monthly sea ice update by NSIDC.

References
Derocher, A.E., Andersen, M., Wigg, Ø., Aars, J., Hansen, E., and Biuw, M. 2011. Sea ice and polar bear den ecology at Hopen Island, Svalbard. Marine Ecology Progress Series 441:273-279. http://www.int-res.com/abstracts/meps/v441/p273-279/

Durner, G.M., Douglas, D.C., Nielson, R.M., Amstrup, S.C., McDonald, T.L. and 12 others. 2009. Predicting 21st-century polar bear habitat distribution from global climate models. Ecological Monographs 79:25-58. http://www.esajournals.org/doi/abs/10.1890/07-2089.1

Miles, M. W., D. V. Divine, T. Furevik, E. Jansen, M. Moros, and A. E. J. Ogilvie. 2014. A signal of persistent Atlantic multidecadal variability in Arctic sea ice.Geophysical Research Letters 41, doi:10.1002/2013GL058084. http://onlinelibrary.wiley.com/doi/10.1002/2013GL058084/abstract

Obbard, M.E., Theimann, G.W., Peacock, E. and DeBryn, T.D. (eds.) 2010. Polar Bears: Proceedings of the 15th meeting of the Polar Bear Specialists Group IUCN/SSC, 29 June-3 July, 2009, Copenhagen, Denmark. Gland, Switzerland and Cambridge UK, IUCN.