On the basis of projected losses of their essential sea-ice habitats, a United States Geological Survey research team concluded in 2007 that two-thirds of the world’s polar bears (Ursus maritimus) could disappear by mid-century if business-as-usual greenhouse gas emissions continue1, 2, 3. That projection, however, did not consider the possible benefits of greenhouse gas mitigation. A key question is whether temperature increases lead to proportional losses of sea-ice habitat, or whether sea-ice cover crosses a tipping point and irreversibly collapses when temperature reaches a critical threshold4, 5, 6. Such a tipping point would mean future greenhouse gas mitigation would confer no conservation benefits to polar bears. Here we show, using a general circulation model7, that substantially more sea-ice habitat would be retained if greenhouse gas rise is mitigated. We also show, with Bayesian network model outcomes, that increased habitat retention under greenhouse gas mitigation means that polar bears could persist throughout the century in greater numbers and more areas than in the business-as-usual case3. Our general circulation model outcomes did not reveal thresholds leading to irreversible loss of ice6; instead, a linear relationship between global mean surface air temperature and sea-ice habitat substantiated the hypothesis that sea-ice thermodynamics can overcome albedo feedbacks proposed to cause sea-ice tipping points5, 6, 8. Our outcomes indicate that rapid summer ice losses in models9 and observations6, 10 represent increased volatility of a thinning sea-ice cover, rather than tipping-point behaviour. Mitigation-driven Bayesian network outcomes show that previously predicted declines in polar bear distribution and numbers3 are not unavoidable. Because polar bears are sentinels of the Arctic marine ecosystem11 and trends in their sea-ice habitats foreshadow future global changes, mitigating greenhouse gas emissions to improve polar bear status would have conservation benefits throughout and beyond the Arctic12.
At a glance
- Predicting 21st century polar bear habitat distribution from global climate models. Ecol. Monogr. 79, 25–58 (2009) et al.
- Survival and breeding of polar bears in the southern Beaufort Sea in relation to sea ice. J. Anim. Ecol. 79, 117–127 (2010) , , , &
- 213–268 (American Geophysical Union, 2008) , & in Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications (eds DeWeaver, E. T., Bitz, C. M. & Tremblay, L.-B.)
- Tipping elements in the Earth’s climate system. Proc. Natl Acad. Sci. USA 105, 1786–1793 (2008) et al.
- Is battered Arctic sea ice down for the count? Science 318, 33–34 (2007)
- The future of ice sheets and sea ice: between reversible retreat and unstoppable loss. Proc. Natl Acad. Sci. USA 106, 20590–20595 (2009)
- The Community Climate System Model version 3. J. Clim. 19, 2122–2143 (2006) et al.
- The thinning of Arctic sea ice, 1988–2003: have we passed a tipping point? J. Clim. 18, 4879–4894 (2005) &
- Future abrupt reductions in the summer Arctic sea ice. Geophys. Res. Lett. 33, L23503 (2006) , &
- National Snow and Ice Data Center. Arctic sea ice extent remains low. NSIDC Notes 69, http://nsidc.org/pubs/notes/69/Notes_69_web.pdf (2009)
- Sentinel species and animal models of human health. Oceanography (Wash. D.C.) 19, 127–133 (2006) &
- How much climate change can be avoided by mitigation? Geophys. Res. Lett. 36, L08703 (2009) et al.
- Rebuttal of “Polar bear population forecasts: a public-policy audit”. Interfaces 39, 353–369 (2009) et al.
- Polar bear Ursis maritimus conservation in Canada: an ecological basis for identifying designatable units. Oryx 42, 504–515 (2008) , &
- Long-term trends in the population ecology of polar bears in western Hudson Bay in relation to climatic change. Arctic 52, 294–306 (1999) , &
- Reduced body size and cub recruitment in polar bears associated with sea ice decline. Ecol. Appl. 20, 768–782 (2010) , &
- Climate change threatens polar bear populations: a stochastic demographic analysis. Ecology 91, 2883–2897 (2010) et al.
- 2000) et al. Emissions Scenarios. A Special Report of Working Group III of the Intergovermental Panel on Climate Change (Cambridge Univ. Press,
- Probabilistic forecast for 21st century climate based on uncertainties in emissions (without policy) and climate parameters. J. Clim. 22, 5175–5204 (2009) et al.
- On avoiding dangerous anthropogenic interference with the climate system: formidable challenges ahead. Proc. Natl. Acad. Sci 105, 14245–14250 (2008) &
- Alaska polar bears called doomed. Anchorage Daily News A1 (September 8 2007)
- The climate change commitment. Science 307, 1766–1769 (2005)
- U.S. Climate Change Science Program Scenarios of Greenhouse Gas Emissions and Atmospheric Concentrations. Sub-report 2.1a of Synthesis and Assessment Product 2.1 (Department of Energy, Office of Biological & Environmental Research, Washington DC, 2007)
- Global warming in the twenty-first century: an alternative scenario. Proc. Natl Acad. Sci. USA 97, 9875–9880 (2000) , , , &
- 111–132 (American Geophysical Union, 2008) in Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications (eds DeWeaver, E. T., Bitz, C. M. & Tremblay, L.-B.)
- Nonlinear threshold behavior during the loss of Arctic sea ice. Proc. Natl Acad. Sci. USA 106, 28–32 (2009) &
- Accelerated Arctic land warming and permafrost degradation during rapid sea ice loss. Geophys. Res. Lett. 35, L11506 (2008) , , , &
- 133–150 (American Geophysical Union, 2008) , , & in Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications (eds DeWeaver, E. T., Bitz, C. M. & Tremblay, L.-B.)
- 7–26 (American Geophysical Union, 2008) & in Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications (eds DeWeaver, E. T., Bitz, C. M. & Tremblay, L.-B.)
- Arctic sea ice decline: faster than forecast. Geophys. Res. Lett. 34, L09501 (2007) , , , &
- Climate change projections for the 21st century and climate change commitment in the CCSM3. J. Clim. 19, 2597–2616 (2006) et al.
- Combinations of natural and anthropogenic forcings in twentieth-century climate. J. Clim. 17, 3721–3727 (2004) et al.
- International Union for Conservation of Nature. Polar Bears: Proceedings of the Fourteenth Working Meeting of the IUCN/SSC Polar Bear Specialists Group (eds Aars, J., Lunn, N. J. & Derocher, A. E.) occasional paper no. 32, 189 (IUCN, 2006)
- 2002) , , , & Resource Selection by Animals: Statistical Design and Analysis for Field Studies (Kluwer Academic,
- 2006) , , & Sea ice concentrations from Nimbus-7 SMMR and DMSP SSM/I passive microwave data (Digital media, National Snow and Ice Data Center, Boulder, Colorado, 1996, updated,
- Predicting survival, reproduction and abundance of polar bears under climate change. Biol. Conserv. 143, 1612–1622 (2010) , , &
- Guidelines for developing and updating Bayesian belief networks applied to ecological modeling and conservation. Can. J. For. Res. 36, 3063–3074 (2006) , , &
- Supplementary Information (2.5M)
The file contains Supplementary Methods, a Supplementary Discussion, Supplementary Figures 1-8 with legends, Supplementary Table 1 and additional references.