Melting Sea Ice is Forcing Polar Bears to Swim Longer, Increasing Cub Mortality
According to a new study, sea ice loss from climate change is causing polar bears to swim longer distances to find stable ice or reach land, resulting in greater risk to their cubs. The study, Long-distance swimming events by adult female polar bears in the southern Beaufort and Chukchi seas, was presented on July 19 at the International Bear Association (IBA) Conference held in Ottawa, Canada.
“Climate change is pulling the sea ice out from under polar bears’ feet, forcing some to swim longer distances to find food and habitat,” said Geoff York, a WWF Polar Bear expert who is a co-author of the study.
According to Geoff York, Head of Arctic Species Conservation in WWF's Global Arctic Programme, this is the first study to identify a multi-year trend of polar bears swimming longer distances. Previous research, only reported single incidents.
The study, conducted between 2004 and 2009, used global positioning system (GPS) collars on female bears along with satellite imagery of sea ice to detect incidences of bears swimming more than 30 miles at a time. Swimming distance for the bears ranged up to 426 miles and lasted up to 12.7 days.
The cubs, accompanied by their mothers and forced to swim long distances, suffered a 45% mortality rate, compared to cubs (18% mortality rate) that did not have to swim the extended distance. To learn more about the specifics of the study, see WWF's press release.
Why Long Swims put Polar Bears at Risk
Long-distance swimming puts polar bears at risk of drowning due to fatigue or rough seas. Like humans, polar bears can't close off their nasal passages so they are at risk of drowning in rough water. Cubs are at even greater risk. Their smaller body size and limited body fat leaves them more prone to hypothermia, and they don’t have the energy reserves of an adult bear.
Arctic Continues to Warm
The pattern of Arctic warming could prove disastrous for polar bears and other wildlife in the region. Due to climate change, the Arctic is one of the most rapidly changing regions on earth. Over the last 50 years, the Arctic has warmed at a rate twice the global average. The most dramatic change is the diminishing volume and extent of sea ice. In recent years, the region has experienced record-low ice levels with 2007, 2008 and 2010 being the three lowest on record. 2011 is on track for another record breaking year, with July’s sea ice extent being the lowest every recorded for this time of year. Sea ice volume is now 47% lower than 1979 levels when satellite records began.
The rapid decline in sea ice is particularly disturbing as it’s diminishing faster than projected just a few years ago, and many experts now expect that much of the Arctic will be free of sea ice for extended periods of each summer and fall within decades. This will make polar bear survival exceedingly difficult.
- U.S. Geological Survey, Alaska Science Center.
- Anthony M. Pagano, Kristin S. Simac, George M. Durner, and Geoff S. York. Long-distance swimming events by adult female polar bears in the southern Beaufort and Chukchi seas. U.S. Geological Survey, Alaska Science Center.
- Durner, G.M., J.P. Whiteman, H.J. Harlow, S.C. Amstrup, E.V. Regehr, and M. Ben-David. 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 DOI 10.1007/s00300-010-0953-2.
- WWF US Press Release. "Melting Sea Ice Forcing Polar Bears to Swim Longer Distances, Linked to Increased Mortality of Cubs, Finds New Study." 19 July 2011.
- WWF-Canada Blog » Geoff York
- Arctic section of WWF-US Web site. Includes Arctic Sea Ice Decline and its Impacts: Online Resources. WWF Climate Change blog, 04/11/2011
- Arctic section of WWF International Web site -
- Sommerkorn, M. and Hassol, S.J. (eds.) “Arctic Climate Feedbacks: Global Implications.” WWF International Arctic Programme, Oslo, 2009.
- National Snow and Ice Data Center. "Arctic sea ice falls to third-lowest extent; downward trend persists." 4 Oct. 2010.
- National Snow and Ice Data Center. "Early sea ice melt onset, snow cover retreat presage rapid 2011 summer decline." 18 July 2011.
- University of Washington Polar Science Center. Arctic Sea Ice Volume Anomaly, version 2. Accessed 20 July 2011.
- Catlin Group Limited. Press Release: “Catlin Arctic Survey Data Indicates Arctic Sea Ice Is Thinning Rapidly.” 2009.