Polar Bears and Climate Change:
A Summary of the Robust Body of Science
Showing Polar Bears are Imperiled by the Arctic Meltdown
Recently a scientist at the Department of the Interior, Dr. Charles Monnett, was placed on administrative leave pending the completion of an Interior Inspector General investigation relating to Dr. Monnett’s observations of drowned polar bears in 2004, the publication of those observations in a peer-reviewed scientific journal, and an agency contract for polar bear research. U.S. Senator James Inhofe and others have asserted that this investigation could undermine the scientific basis for the 2008 decision to list polar bears under the Endangered Species Act.
The actions taken by the Interior Department against Dr. Monnett to date are highly irregular and disturbing. But regardless of the outcome of the investigation, this episode in no way undermines the scientific evidence demonstrating that global warming imperils the polar bear. To date no valid criticism of the paper (Monnett and Gleason 2006) has been raised, but even if one were raised it could not possibly refute the large, robust body of scientific literature linking observations of starvation, cannibalism, drowning, increasing mortality of both adults and cubs, and shrinking populations to the rapid decline of sea ice occurring in the Arctic.
Key studies and observations documenting climate change impacts to polar bears are summarized below.
Polar bear populations are declining:
- The Polar Bear Specialist Group of the IUCN determined that 8 of 19 of the world’s polar bear populations are declining, 3 are stable, 1 is increasing, and the status of 7 is unknown (Obbard et al. 2010).
- The Western Hudson Bay population declined by 22% between 1987 to 2004, which was attributed to earlier sea-ice breakup in spring, shortening the time that bears can hunt on the ice (Regehr et al. 2007).
- The Southern Beaufort Sea population appears to have declined from an estimated 1,800 bears in 1986 to 1,526 bears in 2006, which has been attributed to loss of sea ice (Obbard et al. 2010).
Polar bear survival and reproductive success are declining as sea ice disappears:
- Female survival, breeding rates, and cub litter survival declined as the ice-free period increased during 2001 to 2006 in the Southern Beaufort Sea (Regehr et al. 2010).
- The survival of juvenile, subadult and older bears declined from 1984 to 2004 in the Western Hudson Bay, which was linked to earlier sea-ice breakup (Regehr et al. 2007).
- The survival of polar bears of all age classes in the Northern Beaufort Sea decreased with declines in the sea-ice concentration over shelf waters in the Northern Beaufort Sea (Stirling et al. 2011).
Declines in polar bear body size linked to nutritional stress:
- Skull size and body length of polar bears three years and older declined over time between 1982 and 2006 in the Southern Beaufort Sea, which was attributed to increased nutritional stress (Rode et al. 2010).
- Body condition and cub production of bears in Western Hudson Bay declined between 1981 and 1998, linked to earlier sea-ice breakup and nutritional stress (Stirling et al. 1999).
Degradation of denning habitat due to sea-ice loss and increasing coastal erosion:
- The proportion of polar bear maternal dens on pack ice decreased between 1985 and 2005 in the Southern Beaufort Sea as fall ice freeze-up was delayed and stable ice and snow cover declined (Fischbach et al. 2007).
- Denning habitat along the Alaska coast is being threatened by increasing coastal erosion due to climate change (Durner et al. 2006).
Starvation and fasting:
- Three (and likely four) polar bears that had starved to death were found in the Southern Beaufort Sea during the spring of 2006 (Regehr et al. 2006).
- Researchers found that two to three times as many polar bears were in a fasting state in 2005 and 2006 compared with 1985 and 1986, indicating increased nutritional stress (Cherry et al. 2009).
Increased long-distance swimming and drowning linked to sea-ice loss:
- Surveys during September 2004 in the Southern Beaufort Sea reported 14 of 55 polar bears (25%) in open water, of which 4 bears were drowned. Prior surveys during September 1987-2003 had observed only 4% of bears swimming in open water and none drowned (Monnett and Gleason 2006).
- An adult female was documented making a 687-km continuous swim over 9 days to reach the distant sea-ice edge, followed by an 1800-km walk and swim, in fall 2008 in the Beaufort Sea during which time she lost 22% of her body mass and her yearling cub (Durner et al. 2011).
- A survey in the Chukchi Sea in August 2008 recorded 10 polar bears swimming in open water, with one bear more than 60 miles from shore (Clarke et al. 2011).
- Bears in the Southern Beaufort and Chukchi Seas are being forced to swim increasingly longer distances to find stable ice or reach land, increasing mortality of their cubs (Pagano et al. 2011).
- Three instances of cannibalism were documented in 2004 and one in 2006 in the Southern Beaufort Sea region, including an unprecedented incident in which a male polar bear stalked, killed, and ate a mother polar bear in her den (Amstrup et al. 2006, Stirling et al. 2008).
- Four incidents of adult male polar bears killing cubs for food were reported in Western Hudson Bay in 2009, which was higher than prior reports of one to two incidents per year (PBI 2009).
Desperate hunting behaviors linked to nutritional stress:
- Polar bears were documented using abnormal and inefficient hunting behaviors in spring of 2004 to 2006 in which they clawed holes through solid ice to try to catch seals, suggesting high nutritional stress (Stirling et al. 2008).
Bears are being forced onto land due to sea-ice loss and must wait longer to begin hunting on the ice:
- Polar bears have shifted from offshore pack ice to the coast in fall as sea ice has retreated increasingly far from shore in the Southern Beaufort Sea (Schliebe et al. 2008, Gleason and Rode 2009).
- Increasing numbers of polar bears have been observed on the Chukchi Sea coast in November and December in the past 10-15 years (Kochnev 2006), and bears in this region have been delayed from returning to sea-ice in fall by two to three weeks compared to the 1980s (S.E. Belikov cited in Durner et al. 2009).
More bears are entering human settlements due to nutritional stress and are being shot:
- Human and polar bear conflicts in Western Hudson Bay are increasing as the longer ice-free season forces bears onto land for longer periods and intensifies their nutritional stress (Towns et al. 2009).
- During the summer of 2011, three bears have already been shot and killed in the town of Churchill.
Amstrup, S.C. et al. 2006. Recent observations of intraspecific predation and cannibalism among polar bears in the southern Beaufort Sea. Polar Biology 29:997-1002.
Cherry, S.G. et al. 2009. Fasting physiology of polar bears in relation to environmental change and breeding behavior in the Beaufort Sea. Polar Biology 32:383-391.
Clarke, J.T. et al. 2011. Chukchi Offshore Monitoring in Drilling Area (COMIDA) Distribution and Relative Abundance of Marine Mammals: Aerial Surveys. Final Report, OCS Study BOEMRE 2011-06.
Durner, G.M. et al. 2006. Polar bear maternal den habitat in the Arctic National Wildlife Refuge, Alaska. Arctic 59:31-36.
Durner, G.M. et al. 2009. Predicting 21st-century polar bear habitat distribution from global climate models. Ecological Monographs 79:25-58.
Durner, G.M. et al. 2011. Consequences of long-distance swimming and travel over deep-water pack ice for a female polar bear during a year of extreme sea ice retreat. Polar Biology 34:975-984.
Fischbach, A.S . et al. 2007. Landward and eastward shift of Alaskan polar bear denning associated with recent sea ice changes. Polar Biology 30:1395-1405.
Kochnev, A. A. 2006. Research on polar bear autumn aggregations on Chukotka, 1989-2004. Pages 157-165 in J. Aars, N.J. Lunn, and A.E. Derocher, eds. Polar Bears: Proceedings of the 14th Working Meeting of the IUCN/SSC Polar Bear Specialist Group, June 2005.
Monnett, C., and J.S. Gleason. 2006. Observations of mortality associated with extended open-water swimming by polar bears in the Alaskan Beaufort Sea. Polar Biology 29:681-687.
Obbard, M.E. et al. 2010. Polar Bears: Proceedings of the 15th Working Meeting of the IUCN/SSC Polar Bear Specialist Group, Copenhagen, Denmark, 29 June–3 July 2009. Gland, Switzerland and Cambridge, UK: IUCN. vii + 235 pp.
Pagano, A.M. et al. 2011. Long-distance swimming events by adult female polar bears in the southern Beaufort and Chukchi seas. Talk presented at the July 2011 International Bear Association (IBA) Conference held in Ottawa, Canada.
PBI. 2009. Increased number of cannibalisms observed in Western Hudson Bay polar bears by tourists and wildlife officials, press release by Polar Bears International, available at www.polarbearsinternational.org.
Regehr, E.V. et al. 2006. Polar Bear Population Status in the Southern Beaufort Sea. U.S. Geological Survey Open-File Report 2006-1337, 20 pp.
Regehr, E.V. et al. 2010. Survival and breeding of polar bears in the southern Beaufort Sea in relation to sea ice. Journal of Animal Ecology 79:117-127.
Regehr, E.V. et al. 2007. Effects of earlier sea ice breakup on survival and population size of polar bears in Western Hudson Bay. Journal of Wildlife Management 71:2673-2683.
Rode, K.D. et al. 2010. Reduced body size and cub recruitment in polar bears associated with sea ice decline. Ecological Applications 20:768-782.
Stirling, I. et al. 1999. Long-term trends in the population ecology of polar bears in western Hudson Bay in relation to climate change. Arctic 52:294-306.
Stirling, I. et al. 2011. Polar bear population status in the northern Beaufort Sea, Canada, 1971–2006. Ecological Applications 21:859-876.
Stirling, I. et al. 2008. Unusual predation attempts of polar bears on ringed seals in the Southern Beaufort Sea: possible significance of changing spring ice conditions. Arctic 61:14-22.
Towns, L. et al. 2009. Spatial and temporal patterns of problem polar bears in Churchill, Manitoba. Polar Biology 321:1529-1537.