Polar Bear Population in Canada's Western Hudson Bay Unlikely to Survive Climate Disruption

Polar bear with two cubs near Churchill, Canada, along the western shore of Hudson Bay in November 2010. Source: Geoff York, WWF.

Declining sea ice in Canada's Western Hudson Bay already has been linked to a decline in the area's polar bear population. Today, researchers report that as the ice-free period in the region becomes longer, fewer females will successfully reproduce and those that do will have smaller litters. The authors say their findings "serve as another indicator that the western Hudson Bay population will probably not remain viable under predicted climatic conditions."

Predicting climate change impacts on polar bear litter size, by Péter K. Molnár, Andrew E. Derocher, Tin Klanjscek and Mark A. Lewis, was published today (8 February 2011) in Nature Communications. They introduce their paper as follows:

"Climatic warming affects ecosystems worldwide, and is a major conservation threat to Arctic species. Sea ice-obligate species, such as polar bears (Ursus maritimus), are particularly vulnerable because their habitat is disappearing. Polar bear body condition, reproduction, survival and abundance are already declining in some populations, and further declines are expected with continued warming."

They go on to summarize the connections between warming and polar bear litters as follows:

"The mechanisms linking sea ice to litter size are well understood. For about 8 months of the year, Hudson Bay is frozen and bears hunt for seals on the sea ice. Each summer, the sea ice melts and the population is forced ashore. With little to no terrestrial food available, bears rely on their energy stores for survival and reproduction while on land. Pregnant females enter terrestrial maternity dens in early October, where they give birth and nurse 1–3 altricial cubs until den emergence in February or March. Food continues to be unavailable to females while denning, and energetic expenses of survival, gestation and lactation are met from fat and protein stores accumulated during the previous hunting season. Storage energy thus limits the number of cubs that can be raised to den emergence, and the amount of storage energy available to denning females depends on the length of the previous sea ice season. Historically, polar bears came ashore in early August, but because of rising temperatures sea ice break-up has been occurring about 7–8 days earlier per decade in recent years. Polar bear on-shore arrival has shifted accordingly, resulting in shortened on-ice feeding and prolonged on-shore fasting. The trends towards a progressively earlier sea ice break-up and progressively earlier polar bear on-shore arrival are expected to continue with continued warming, and consequent food stress is expected to lead to reduced energy stores at den entry. Declines in litter size are likely because less energy is available for gestation and lactation."

They add that the length of the previous sea-ice season not only is reduced by earlier sea-ice breakups but also by later freeze-ups in the fall (as happened in the Fall of 2010, when the bears were unable to return to the ice until early December). The consequences for the litters are dramatic:

"In western Hudson Bay, we predict climate warming-induced litter size declines that jeopardize population viability: ~28% of pregnant females failed to reproduce for energetic reasons during the early 1990s, but 40–73% could fail if spring sea ice break-up occurs 1 month earlier than during the 1990s, and 55–100% if break-up occurs 2 months earlier. Simultaneously, mean litter size would decrease by 22–67% and 44–100%, respectively."

The study has implications well beyond western Hudson Bay:

"Outside western Hudson Bay, over one-third of the global polar bear population follows similar patterns of seasonal on-ice feeding and on-shore fasting. The physiological arguments put forth in our study also suggest future litter size declines in these populations, although the rates of decline may differ from the western Hudson Bay population and must be evaluated separately to account for population-specific sea ice changes."

The research was funded in part by WWF Canada, and WWF has funded polar bear research in the region for decades.

“This new analysis clearly strengthens the concerns from both scientists and Northern people on the changes unabated climate warming will bring to key species across the Arctic," says Geoff York, Species Lead for WWF’s Global Arctic Programme. "Climate change is fundamentally altering long held assumptions for wildlife management and will require a new approach as we work together to conserve strong populations into a very uncertain future. The circumstance for polar bears is both dynamic and complex and will require solutions that understand regional differences and embrace adaptation. There are 19 sub populations of polar bears across the Arctic and this paper presents one of what will be many different stories unfolding as conditions change.”

Another Year of Very Poor Ice Conditions for Hudson Bay Polar Bears.

The study comes after another year of very poor ice conditions for the polar bears. Temperatures in 2010 were the warmest on record for the region -- and for Canada as a whole (see Climate Trends and Variations Bulletin - Annual 2010 ). The ice breakup came early in 2010 and most bears were off the ice by mid-July- up to 2.5 weeks earlier than in past decades. According to the Canadian Ice Service, Hudson Bay – and the entire Canadian Arctic – saw the lowest seasonal average ice cover on record (since 1971) during the summer 2010, largely because of wind patterns and above normal air temperatures (see Seasonal Summary for the Canadian Arctic. Summer 2010. Canadian Ice Service, 15 December 2010).

What’s more, temperatures remained well above normal into the fall, delaying the formation of sea ice. Ice covered less than 2 per cent of Hudson Bay in mid-November 2010, where normally 20% would be covered.

The sea ice begins this year on a troubling note. So far this year, extraordinary warmth has dominated the region. In Arctic Oscillation brings record low January extent, unusual mid-latitude weather, a press release (2 Feb 2011) from the National Snow and Ice Data Center (NSIDC), the center report that sea ice extent was at a record low in January 2011. In particular, the NSIDC said sea ice extent "remained unusually low in Hudson Bay, Hudson Strait (between southern Baffin Island and Labrador), and Davis Strait (between Baffin Island and Greenland). Normally, these areas freeze over by late November, but this year Hudson Bay did not completely freeze over until mid-January. The Labrador Sea remains largely ice-free." See also Record Low Arctic Sea Ice Extent for January (8 Feb 2011) from NASA's Earth Observatory.

Monthly January sea ice extent, 1979 to 2011. The extent has declined at a rate of 3.3% per decade. Source: National Snow and Ice Data Center.

Polar Bears International reports that this delayed the hunting season for the western Hudson Bay polar bears (Weird Weather: January Arctic Sea Ice Hits Record Low, press release, 3 Feb 2011). The impacts of the late formation of sea ice will be compounded if the bears are forced off the ice by an early break-up of the ice in Summer -- and another extended fasting period onshore punctuated by delayed ice formation in the Fall.

Other Recent Developments Related to the Arctic and Polar bears

Today's research results follow publication in the scientific literature of several other alarming articles related to the Arctic and polar bears:

In 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 (by George M. Durner, John P. Whiteman, Henry J. Harlow, Steven C. Amstrup, Eric V. Regehr, Merav Ben-David), in Polar Biology (20 December 2010), the authors summarize the implications for one polar bear of sea ice decline: “Between an initial capture in late August and a recapture in late October 2008, a radio-collared adult female polar bear in the Beaufort Sea made a continuous swim of 687 km over 9 days and then intermittently swam and walked on the sea ice surface an additional 1,800 km ... Between captures, this polar bear lost 22% of her body mass and her yearling cub. The extraordinary long distance swimming ability of polar bears, which we confirm here, may help them cope with reduced Arctic sea ice. Our observation, however, indicates that long distance swimming in Arctic waters, and travel over deep water pack ice, may result in high energetic costs and compromise reproductive fitness.”

In "Enhanced Modern Heat Transfer to the Arctic by Warm Atlantic Water" (Science, 28 January 2011), Robert F. Spielhagen et al report that "early–21st-century temperatures of Atlantic Water entering the Arctic Ocean are unprecedented over the past 2000 years and are presumably linked to the Arctic amplification of global warming." See Warming North Atlantic water tied to heating Arctic, according to new study. Press release (27 Jan 2011). from University of Colorado Boulder.

U.S. Marine Mammal Commission: "Unfortunately, our political, social and economic systems have not yet responded to the already strong evidence of climate disruption."

Meanwhile, the U.S. Fish and Wildlife Service (FWS) is about to announce its decision on whether to list the Pacific Walrus as threatened or endangered under the Endangered Species Act (ESA). The walruses are found elsewhere in the Arctic, mainly in the Bering and Chukchi seas. In a 3 Jan 2011 letter to the FWS [PDF], the U.S. Marine Mammal Commission (MMC) recommended that walruses be listed as threatened under ESA. "Without question, the warming of the Arctic is destroying, modifying, and curtailing walrus habitat and will continue to do so in the foreseeable future," wrote MMC Executive Director, Timothy Ragen. "The loss of sea ice habitat is perhaps the most significant threat to the walrus population," he said.

He pointedly noted that the Intergovernmental Panel on Climate Change provides "a compelling case that sea ice will continue to decline for the foreseeable future unless human societies are willing to take meaningful action to address the factors disrupting the climate." He continues:

"Those studies also indicate that, even if meaningful actions were taken soon, their effects would not be clearly evident until the latter half of this century because of the lagged effects of greenhouse gases that already have been emitted and that will persist in the atmosphere for decades. Unfortunately, our political, social and economic systems have not yet responded to the already strong evidence of climate disruption. Given that inertia, plus the fact that the underlying causes may worsen over time, the Marine Mammal Commission sees no basis for confidence that climate disruption and its effects on walrus habitat are being or soon will be brought under control."

Online Resources:

Predicting climate change impacts on polar bear litter size . By Péter K. Molnár, Andrew E. Derocher, Tin Klanjscek & Mark A. Lewis in Nature Communications (8 February 2011).

WWF