Climate Change Threatens Polar Bear Populations

October 2010 CLIMATE CHANGE AND POLAR BEARS2895than the observed frequency of poor years from 1979 to2005, the available record from satellite imagery. Theobserved frequency of poor ice years, q ¼ 0.2, from 1979to 2005 (the available satellite record) exceeds thisthreshold, and would lead to a stochastic growth rate oflog k s ¼ 0.012 (parametric model) or 0.006 (nonparametricmodel).Arctic sea ice extent at the end of the summer of 2005was the lowest ever observed. This record was broken in2007, and approached in 2008. If thinning multiyear ice,more extensive open water and albedo effects createpositive feedback loops leading to further reductions(Maslanik et al. 2007), the fate of polar bears in otherlocations, even where populations have been stable (e.g.,the northern Beaufort Sea) will also be in doubt. Recentshipboard surveys in the southern Beaufort Sea (Barberet al. 2009) found summer sea ice there to be eventhinner and more broken than shown in satellite images.Comparisons of these growth rates estimated herewith trajectories of population size are hampered by thelack of reliable population size estimates. The claim,commonly repeated in the press, that worldwide polarbear populations have increased fourfold since the 1960shas no basis in the scientific literature (Dykstra 2008).Some increases in polar bear populations occurred in the1970s as harvesting was reduced, but there is no evidencethat these increases continued, and such recoveries,where they occurred, are irrelevant to the effects ofrecent changes in the availability of sea ice. In thesouthern Beaufort Sea, Amstrup et al. (1986) reported acrude estimate of population size of N ’ 1800 bears inthe mid-1980s. Regehr et al. (2006) estimated N ’ 1500bears in 2006. Amstrup et al. (2001) suggested thepopulation may have remained constant or evenincreased between the 1980s and the late 1990s.However, if the difference between 1800 bears and1500 bears represents a real decline, and if that declineoccurred over 15 years, it would represent an averageannual growth rate of 0.012. Over 20 years, such adecline would imply an average annual growth rate of0.009. This further suggests that our estimates ofstochastic growth rate are reasonable. This comparisonwith our calculated growth rate is very rough, because ofthe different sources of historical data and the highdegree of uncertainty around these population estimates.Nevertheless, the comparison supports the modelresults.The mechanisms linking sea ice to survival andreproduction of polar bears are not known in detail.Reduced prey availability no doubt plays a role (Stirlingand Smith 2004), because polar bears are dependentupon sea ice for capturing prey (Amstrup 2003) andtheir preferred foraging area is the water over thecontinental shelf. In addition, reduced sea ice extent mayforce polar bears to swim longer distances, increasingtheir risk of drowning or starvation (Monnett andGleason 2006). The thinner ice that reforms after largesummer ice retreats also may be less suitable for polarbear foraging (Stirling et al. 2008). Reduced sea ice willundoubtedly also affect other ice-dependent species (see,e.g., Gaston et al. 2005) and many food web links maybe involved in these effects (Huntington and Moore2008, Moline et al. 2008).We have shown that global warming is likely to haveprofoundly negative effects on future growth rates ofpolar bear populations. A warmer world will have lesssea ice and hence less polar bear habitat. Geographicalvariation in the nature of sea ice and in predictions ofthe retreat of sea ice habitat (Durner et al. 2009) suggeststhat polar bears in different parts of their current rangewill be affected at different rates. Nonetheless, ouranalyses, which incorporated sea ice and other uncertainties,projected that by mid century, the effects ofglobal warming on polar bears will be severe. If currentGCM outcomes are correct, there is a high probabilitythat the Beaufort Sea population of polar bears willdisappear by the end of the century. Because all polarbears are dependent on sea ice for securing their prey, itis reasonable to expect that the effects of global warmingon polar bears of the southern Beaufort Sea willultimately extend to polar bears throughout their range.This and other related findings provided the primarymotivation for listing polar bears, in May of 2008, as athreatened species under the U.S. Endangered SpeciesAct (U.S. Fish and Wildlife Service 2008).ACKNOWLEDGMENTSWe acknowledge primary funding for model developmentand analysis from the U.S. Geological Survey and additionalfunding from the National Science Foundation (DEB-0343820and DEB-0816514), NOAA, the Ocean Life Institute and theArctic Research Initiative at WHOI, and the Institute of ArcticBiology at the University of Alaska–Fairbanks. Funding for thecapture–recapture effort in 2001–2006 was provided by the U.S.Geological Survey, the Canadian Wildlife Service, the Departmentof Environment and Natural Resources of the Governmentof the Northwest Territories, and the Polar ContinentalShelf Project, Ottawa, Canada. We thank G. S. York, K. S.Simac, M. Lockhart, C. Kirk, K. Knott, E. Richardson, A. E.Derocher, and D. Andriashek for assistance in the field. D.Douglas and G. Durner assisted with analyses of radiotelemetryand remote sensing sea ice data. We thank J. Nichols, M.Udevitz, E. Cooch, and J.-D. Lebreton for helpful commentsand discussions.LITERATURE CITEDAars, J., N. J. Lunn, and A. E. Derocher. 2006. Polar bears:proceedings of the 14th working meeting of the IUCN/SSCpolar bear specialists group. International Union forConservation of Nature and Natural Resources, Gland,Switzerland and Cambridge, UK.Amstrup, S. C. 2003. Polar bear, Ursus maritimus. Pages 587–610 in G. A. Feldhamer, B. C. Thompson, and J. A.Chapman, editors. Mammals of North America: biology,management, and conservation. Johns Hopkins UniversityPress, Baltimore, Maryland, USA.Amstrup, S. C., G. Durner, I. Stirling, N. J. Lunn, and F.Messier. 2000. Movements and distribution of polar bears inthe Beaufort Sea. Canadian Journal of Zoology 78:948–966.Amstrup, S. C., B. G. Marcot, and D. C. Douglas. 2008. ABayesian network modeling approach to forecasting the 21stcentury worldwide status of polar bears. Pages 213–268 inE. T. DeWeaver, C. M. Bitz, and L.-B. Tremblay, editors.