Journal cover Journal topic
The Cryosphere An interactive open-access journal of the European Geosciences Union
The Cryosphere, 10, 2027-2041, 2016
http://www.the-cryosphere.net/10/2027/2016/
doi:10.5194/tc-10-2027-2016
© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
14 Sep 2016
Sea-ice indicators of polar bear habitat
Harry L. Stern1 and Kristin L. Laidre1,2 1Polar Science Center, Applied Physics Laboratory, University of Washington, 1013 NE 40th Street, Seattle, WA 98105, USA
2Greenland Institute of Natural Resources, Box 570, 3900 Nuuk, Greenland
Abstract. Nineteen subpopulations of polar bears (Ursus maritimus) are found throughout the circumpolar Arctic, and in all regions they depend on sea ice as a platform for traveling, hunting, and breeding. Therefore polar bear phenology – the cycle of biological events – is linked to the timing of sea-ice retreat in spring and advance in fall. We analyzed the dates of sea-ice retreat and advance in all 19 polar bear subpopulation regions from 1979 to 2014, using daily sea-ice concentration data from satellite passive microwave instruments. We define the dates of sea-ice retreat and advance in a region as the dates when the area of sea ice drops below a certain threshold (retreat) on its way to the summer minimum or rises above the threshold (advance) on its way to the winter maximum. The threshold is chosen to be halfway between the historical (1979–2014) mean September and mean March sea-ice areas. In all 19 regions there is a trend toward earlier sea-ice retreat and later sea-ice advance. Trends generally range from −3 to −9 days decade−1 in spring and from +3 to +9 days decade−1 in fall, with larger trends in the Barents Sea and central Arctic Basin. The trends are not sensitive to the threshold. We also calculated the number of days per year that the sea-ice area exceeded the threshold (termed ice-covered days) and the average sea-ice concentration from 1 June through 31 October. The number of ice-covered days is declining in all regions at the rate of −7 to −19 days decade−1, with larger trends in the Barents Sea and central Arctic Basin. The June–October sea-ice concentration is declining in all regions at rates ranging from −1 to −9 percent decade−1. These sea-ice metrics (or indicators of habitat change) were designed to be useful for management agencies and for comparative purposes among subpopulations. We recommend that the National Climate Assessment include the timing of sea-ice retreat and advance in future reports.

Citation: Stern, H. L. and Laidre, K. L.: Sea-ice indicators of polar bear habitat, The Cryosphere, 10, 2027-2041, doi:10.5194/tc-10-2027-2016, 2016.
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Short summary
Polar bears, found in 19 distinct regions of the Arctic, depend on sea ice as a platform for hunting. Using satellite data of sea ice for the period 1979–2014, we found that the date of sea-ice retreat in spring is arriving weeks earlier, and the date of sea-ice advance in fall is arriving weeks later, in all 19 regions. We calculated several other measures of sea ice, which all show declines. These measures were designed to be useful for management agencies to assess polar bear habitat.
Polar bears, found in 19 distinct regions of the Arctic, depend on sea ice as a platform for...
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