Polar bears (Ursus maritimus) require the relative warmth and stability afforded by snow dens for successful reproduction. Pregnant bears must travel from foraging habitats on the sea ice to land in autumn to establish winter dens. Data of sea ice extent and composition from satellite-acquired passive microwave (PMW) imagery show a reduction in summer sea ice extent throughout the Arctic from 1979-2006. Additionally, General Circulation Models (GCM) predict that Arctic sea ice extent will continue to diminish throughout the 21st century. Greater energetic demands will be placed on pregnant polar bears in the future if they travel greater distances from summer forage habitats to traditional denning habitats on land. We developed an approach for estimating how much these distances may change by modeling autumn movement paths of polar bears using the observational PMW record of sea ice distribution and sea ice projections of 5 GCMs during the 21st century. Over the 1979-2006 PMW record, polar bears returning to Alaska to den have experienced an annual increase in travel of > 6 km/year—an increase of >168 km over the 28 year period. Based on GCM sea ice projections during 2001-2060, the average increase in the distance required to reach traditional Alaskan denning regions was estimated to increase > 16 km/year. Distances traveled, and therefore, energetic demands, will likely vary among the different circumpolar sub-populations of polar bears.
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