Scientists have long known that Arctic climate change is bad news for bears, but University of Washington researchers quantify just how bad it is in a study published today.
The study in The Cryosphere, a journal of the European Geosciences Union, is said to be the first to assess the impact of sea ice changes for 19 different populations of polar bears across the entire Arctic region, using the metrics that are most relevant to polar bear biology.
The analysis draws upon 35 years’ worth of satellite data showing daily sea-ice concentration in the Arctic. There’s a consistent trend toward earlier thawing in the spring, and later freezing in the winter. Between 1979 and 2014, the total number of ice-covered days declined at the rate of 7 to 19 days per decade. Over the course of 35 years, seven weeks of good sea-ice habitat were lost.
The researchers found that sea-ice coverage during the summer months declined in all regions at a rate of 1 to 9 percent per decade.
All this is really bad news for bears, because the bears subsist primarily on a diet of seals. The predators can’t outswim their prey, so they have to hunt from the ice – either by ambushing the seals when they surface or breaking through to their dens.
“Sea ice really is their platform for life,” UW researcher Kristin Laidre said. “They are capable of existing on land for part of the year, but the sea ice is where they obtain their main prey.”
The fact that sea ice is being lost earlier in the spring and coming back later in the fall is bad news for another reason. “Those periods are also tied to the breeding season when bears find mates, and when females come out of their maternity dens with very small cubs and haven’t eaten for months,” Laidre said.
The researchers say that if the current trend continues, polar bears will lose another six to seven weeks of sea-ice time by the 2050s. The sea-ice analysis will be updated every year, and will be factored into polar bear species assessments conducted by the International Union for Conservation of Nature. The researchers also want the data factored into future editions of the National Climate Assessment.
Stern and Laidre are co-authors of “Sea-Ice Indicators of Polar Bear Habitat,” a study funded by NASA and the Greenland Institute of Natural Resources.