Abrupt summer sea ice decline has not affected polar bear numbers as predicted

Yes, Arctic sea ice has declined since satellite records began in 1979 but polar bears have adjusted well to this change, especially to the abrupt decline to low summer sea ice levels that have been the norm since 2007.

Global pb population size sea ice 2017 July PolarBearScience

Some polar bear subpopulations have indeed spent more time on land in summer than in previous decades but this had little negative impact on health or survival and while polar bear attacks on humans appear to have increased in recent years (Wilder et al. 2017), the reasons for this are not clear: reduced summer sea ice is almost certainly not the causal factor (see previous post here).

Ultimately, there is little reason to accept as plausible the computer models (e.g. Atwood et al. 2016; Regehr et al. 2016) that suggest polar bear numbers will decline by 30% or more within a few decades: even the IUCN Red List assessment (Wiig et al. 2015) determined the probability of that happening was only 70%.

Arctic sea ice has never been a stable living platform (Crockford 2015): it shifts from season to season, year to year, and millennia to millennia. Without the ability to adapt to changing conditions, Arctic species like polar bears and their prey species (seals, walrus, beluga, narwhal) would not have survived the unimaginably extreme changes in ice extent and thickness that have occurred over the last 30,000 years, let alone the extremes of sea ice they endured in the last 200,000 years or so.

Some biologists continue to hawk doomsday scenarios for polar bears due to summer sea ice loss but the truth is that their previous predictions based on sea ice declines failed so miserably (e.g. Amstrup et al. 2007) that it’s impossible to take the new ones seriously — especially since the basic assumptions that caused the first predictions to fail have not been corrected, as I’ve stated in print (Crockford 2017:27):

In summary, recent research has shown that most bears are capable of surviving a summer fast of five months or so as long as they have fed sufficiently from late winter through spring, which appears to have taken place since 2007 despite marked declines in summer sea ice extent.

The assumption that summer sea ice is critical feeding habitat for polar bears is not supported.

Recent research shows that changes in summer ice extent generally matter much less than assumed in predictive polar bear survival models of the early 2000s as well as in recent models devised to replace them (Amstrup et al. 2010; Atwood et al. 2016a; Regehr et al. 2015; Regeher et al. 2016; Wiig et al. 2015), while variations in spring ice conditions matter more.

As a consequence, the evidence to date suggests that even if an ‘ice-free’ summer occurs sometime in the future ­ defined as sea ice extent of 1 million km2 or less (Jahn et al. 2016) ­ it is unlikely to have a devastating impact on polar bears or their prey. [my bold]

The abrupt drop in summer sea ice that occurred in 2007 was not predicted by experts to occur until mid-century yet the predicted decimation of polar bears worldwide expected under those conditions (a loss of 2/3 of the global total, to only about 6660-8325 bears) not only did not happen, it did not come even close to happening (Crockford 2017; see also my recent books, Polar Bear Facts & Myths, and Polar Bears: Outstanding Survivors of Climate Change, sidebar).

Instead, the global population grew from about 22,550 bears in 2005 to about 28,500 bears in 2015. And while this might not be a statistically significant increase (due to the very wide margins of error for polar bear estimates), it is absolutely not a decline.

The present reality is that low summer sea ice cover since 2007 has not caused polar bear numbers to decline and therefore, polar bears are not a species in trouble. This suggests that even if the Arctic should become briefly ice-free in summer in the future, polar bears are likely to be only minimally affected and not become threatened with extinction. Polar bears are outstanding survivors of climate change: recent research and their evolutionary history confirm this to be true.


Sea ice at LGM_vs today_PolarBearScience 14 July 2017

As I pointed out a few years ago (see previous post here which contains all pertinent references, including those used to construct the LGM sea ice map used above), according to the genetic data (Lindqvist et al. 2010, see Figure 2 below), polar bear numbers dropped most significantly during the LGM – due to much more expanded sea ice and colder conditions — not during the warmer-than-present Eemian:

“While the polar bear is an Ice Age species, genetic and fossil evidence suggests it barely survived the profound sea ice changes associated with the Last Glacial Maximum, one of the most severe glacial periods of the Pleistocene.

The genetic evidence suggests that sea ice conditions during the LGM had a huge impact on polar bear numbers, larger than previous glacial periods (even-numbered MIS) and the warm conditions of interglacials (odd-numbered MIS), including the last Interglacial (the Eemian or MIS 5e, ca. 130-110 thousand years ago).” [bold in original]

PNAS_polar_bears_fig2 WCR

Figure 2 (top): Estimates of the effective population size over the past 1 million years of the different bear species studied (as in Figure 1). The larger gray-shaded area on the right refers to the Early Pleistocene, and the other gray areas (from right to left) refer to the interglacial Marine Isotope Stages (MIS) 15, 13, and 11, and the Eemian, respectively. The arrows point to major events in bear population history discussed by the authors. H, Holocene epoch. (source: Miller et al., 2012). Figure 2 (bottom): General climate history of the earth for the past 1 million years as derived from a collection of ocean core sediments. The numbers are the various MIS (source: Lisiecki and Raymo, 2005). From World Climate Report critique, see original post for references.

Compared against the profound changes that occurred during the Last Glacial Maximum, sea ice variation since 1979 has been minor. It is therefore not surprising to see polar bears doing well despite the sea ice changes, especially since most of the change has been confined to summer. Thinning of multiyear ice is a boon to polar bears, since both they and their prey species do best with first year ice that’s about 2 m thick or less.

And as I’ve also pointed out before (see previous post here, which is fully referenced and has more pertinent images), warmer conditions than today — with less summer and winter sea ice — occurred in the Eemian Interglacial (about 130,000-110,000 years ago) but appears to have had minimal impact on polar bears as a whole:

“What you need to know is that the future polar bear survival models generated by Amstrup and his colleagues (e.g., Amstrup et al. 2008, 2011; Durner et al. 2009; Obbard et al. 2010) predicted that a 1 degree increase in global temperature would have a much less profound effect on sea ice habitat than the Eemian actually experienced.

Something approaching the Eemian sea ice situation (approximated in Fig. 1), with no ice in the Bering Sea in winter, occurs in only a few of the model runs (Fig. 2). Those ice-free Bering Sea projections do not appear until the end of the 21st century and assume that global temperatures would have increased by almost 3 degrees Celsius by that time (Durner et al. 2009:Fig.8c, using IPCC AR4 (2007) scenario A1B, “business as usual” 1.7-4.4 degree increase, average 2.8).

What we have from the Eemian is irrefutable evidence that polar bears are capable of surviving a significant contraction of winter sea ice habitat without getting anywhere close to extinction. In fact, the only evidence we have (from genetics) suggests the population did not drop as low as it did during historic times.”

References

Amstrup, S.C., Marcot, B.G. & Douglas, D.C. 2007. Forecasting the rangewide status of polar bears at selected times in the 21st century. US Geological Survey. Reston, Virginia. pdf here.

Atwood, T.C., Marcot, B.G., Douglas, D.C., Amstrup, S.C., Rode, K.D., Durner, G.M. et al. 2016. Forecasting the relative influence of environmental and anthropogenic stressors on polar bears. Ecosphere 7(6): e01370. http://onlinelibrary.wiley.com/doi/10.1002/ecs2.1370/full

Crockford, S.J. 2015. The Arctic Fallacy: sea ice stability and the polar bear. GWPF Briefing 16. The Global Warming Policy Foundation, London. Pdf here.

Crockford, S.J. 2017 V3. Testing the hypothesis that routine sea ice coverage of 3-5 mkm2 results in a greater than 30% decline in population size of polar bears (Ursus maritimus). PeerJ Preprints 2 March 2017. Doi: 10.7287/peerj.preprints.2737v3 Open access. https://doi.org/10.7287/peerj.preprints.2737v3 pdf here.

Lindqvist, C., Schuster, S.C., Sun, Y., Talbot, S.L., Qi, J., Ratan, A., Tomsho, L., Kasson, L., Zeyl, E., Aars, J., Miller, W., Ingólfsson, Ó., Bachmann, L. and Wiig, Ø. 2010. Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear. Proceedings of the National Academy of Sciences USA 107:5053-5057. Open access http://www.pnas.org/content/107/11/5053.abstract

Regehr, E.V., Laidre, K.L, Akçakaya, H.R., Amstrup, S.C., Atwood, T.C., Lunn, N.J., Obbard, M., Stern, H., Thiemann, G.W., & Wiig, Ø. 2016. Conservation status of polar bears (Ursus maritimus) in relation to projected sea-ice declines. Biology Letters 12: 20160556. http://rsbl.royalsocietypublishing.org/content/12/12/20160556

Wiig, Ø., Amstrup, S., Atwood, T., Laidre, K., Lunn, N., Obbard, M., et al. 2015. Ursus maritimus. The IUCN Red List of Threatened Species 2015: e.T22823A14871490. Available from http://www.iucnredlist.org/details/22823/0 [accessed Nov. 28, 2015]

Wilder, J.M., Vongraven, D., Atwood, T., Hansen, B., Jessen, A., Kochnev, A., York, G., Vallender, R., Hedman, D. and Gibbons, M. 2017. Polar bear attacks on humans: implications of a changing climate. Wildlife Society Bulletin, in press. DOI: 10.1002/wsb.783 http://onlinelibrary.wiley.com/doi/10.1002/wsb.783/full

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