We’ve hit the seasonal Arctic sea ice minimum for this year, called this morning by US NSIDC for 19th and 23rd of Septmeber: 4.59 mkm2, the same extent as 2008 and 2010. This is not a “ho-hum” year for polar bears: it means that since 2007, they have triumphed through 10 or 11 years1 with summer ice coverage below 5.0 mkm2 — levels that in 2007 were expected to cause catastrophic declines in numbers.
Summer sea ice below 5.0 mkm2 were not expected to occur until about 2050, according to 2005/2006 sea ice models and polar bear specialists at the US Geological Survey (USGS). Polar bear survival models predicted 2/3 of the world’s polar bears would disappear when ice levels reached this threshold for 8 out of 10 years (Amstrup et al. 2007, 2008; Hunter 2007) but polar bears have been more resilient than expected (Crockford 2017, 2018; Crockford and Geist 2018). In fact, in many areas (like the Chukchi Sea, Barents Sea and Foxe Basin) polar bears are thriving despite dramatic declines in summer sea ice coverage (Aars et al. 2017; ACSWG 2018; Peacock et al. 2013; Regehr et al. 2016; Stapleton et al. 2016).
The sea ice models used to support the addition of polar bears to the US Endangered Species List as ‘threatened’ with extinction suggested sea ice levels from 3-5 mkm2 would not occur unti mid-century, yet they dropped before the ink was dry on the 2007 USGS Reports (ACIA 2005; Hassol 2004; Holland et al. 2006; Solomon et al. 2007; Zhang and Walsh 2006).
The ice extent charts from the University of Bremen (below) show ice that’s 50% concentration or greater at the date of the seasonal minimum (19th September): what polar bear specialists define as preferred habitat (Amstrup et al. 2007).
Compare the minimum shown above to the coverage predicted for 2050 and to coverage at the minimum in 2012 (the NSIDC image is here):
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- Of these 12 years, two were only marginally above 5.0 mkm2 and one (2014) was within the margin of error for the threshold: 2013 (5.05) and 2014 (5.03). Only 2009, at 5.12 mkm2, was above that threshold. NSIDC states (Table 1) that values within 40,000 km2 are considered a tie, so the margin of error for these estimates can be assumed to be about 40,000 km2. That means the true value for the minimum at 2014 could be below 5.0 mkm2. Values used by the USGS were for 50% sea ice concentration, which would in any case be slightly below the 15% concentration value used by NSIDC: that fact means that 2013 was probably below 5.0 mkm2 as well.
Aars, J., Marques,T.A, Lone, K., Anderson, M., Wiig, Ø., Fløystad, I.M.B., Hagen, S.B. and Buckland, S.T. 2017. The number and distribution of polar bears in the western Barents Sea. Polar Research 36:1. 1374125. doi:10.1080/17518369.2017.1374125
AC SWG 2018. Chukchi-Alaska polar bear population demographic parameter estimation. Eric Regehr, Scientific Working Group (SWG. Report of the Proceedings of the 10th meeting of the Russian-American Commission on Polar Bears, 27-28 July 2018), pg. 5. Published 30 July 2018. US Fish and Wildlife Service. https://www.fws.gov/alaska/fisheries/mmm/polarbear/bilateral.htm pdf here.
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, VA. Pdf here
Amstrup, S.C.,Marcot, B.G. and Douglas, D.C. 2008. A Bayesian network modeling approach to forecasting the 21st century worldwide status of polar bears. Pp. 213–268 in Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications, E.T. DeWeaver, C.M. Bitz and L.B. Tremblay (eds.). Geophysical Monograph 180. American Geophysical Union, Washington, D.C.
Crockford, S.J. 2017. 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
Crockford, S.J. 2018. State of the Polar Bear Report 2017. Global Warming Policy Foundation Report #29. London. pdf here.
Crockford, S.J. and Geist, V. 2018. Conservation Fiasco. Range Magazine, Winter 2017/2018, pg. 26-27. Pdf here.
Holland, M.M., C.M. Bitz, and B. Tremblay. 2006. Future reductions in the summer Arctic sea ice. Geophysical Research Letters. 33:L23503. here.Open access, pdf
Hassol, S.J. 2004. Impacts of a Warming Arctic: Arctic Climate Impact Assessment Synthesis Report. Cambridge University Press, Cambridge UK. [summary for policy makers published ahead of the report containing the science background] Pdf here.
Hunter, C.M., Caswell, H., Runge, M.C., Regehr, E.V., Amstrup, S.C. and Stirling, I. 2007. Polar bears in the southern Beaufort Sea II: demography and population growth in relation to sea ice conditions, 2001-2006. Administrative Report US Geological Survey, Reston, VA. Pdf here.
Peacock, E., Taylor, M.K., Laake, J. & Stirling, I. 2013. Population ecology of polar bears in Davis Strait, Canada and Greenland. Journal of Wildlife Management 77:463–476.
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
Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor, and H.L. Miller (Editors). 2007. Summary for policymakers. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the 4th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Cambridge University Press, United Kingdom. Available at https://www.ipcc.ch/publications_and_data/ar4/wg1/en/spm.html Pdf here.
Stapleton, S., Peacock, E. & Garshelis, D. 2016. Aerial surveys suggest long-term stability in the seasonally ice-free Foxe Basin (Nunavut) polar bear population. Marine Mammal Science 32:181-201.
Zhang, X. & Walsh, J.E. 2006. Toward a Seasonally Ice-Covered Arctic Ocean: Scenarios from 991 the IPCC AR4 Model Simulations. Journal of Climate 19: 1730-1747. https://doi.org/10.1175/JCLI3767.1 Open access, pdf here.