As climate change continues to heat up the Arctic, there’s rarely good news these days for the polar bears who call it home. And now, a broad new study charting nearly four decades of changes in Arctic sea ice has again underscored the animal’s plight.
The study, just out in the journal The Cryosphere, finds that in every Arctic region where polar bears live, there’s been a decline in the total number of ice-covered days the bears have at their disposal. That’s a serious problem because polar bears use sea ice as their prime hunting grounds, waiting near the edge and snatching up seals as the marine mammals surface for air.
Previous research has already suggested that as less ice is available, polar bears are forced to spend more time on land, where they have to scavenge or hunt smaller prey to survive. As this behavior becomes more frequent, scientists worry the bears could become malnourished or even starve. Earlier this week, reports of polar bears stalking a Russian research outpost drove this point home again.
While there’s great concern about the future of the species, there’s some debate about how the bears are faring now. Scientists estimate there are currently about 25,000 bears in all, divided into 19 subpopulations throughout the Arctic. For the new study, the researchers decided to investigate sea ice trends in each of these subpopulation regions.
The researchers relied on satellite data from 1979 through 2014. Every year, for each of the 19 regions, they calculated the date sea ice began to melt in the spring; the date it began to refreeze in the fall; the amount of sea ice present during the summer months (when the ice concentration is typically at its lowest); and the total number of ice-covered days per year.
They found that in 17 out of the 19 subpopulation regions, there are significant trends toward earlier sea ice retreat in the spring, generally by about three-to-nine days per decade. And in 16 of the regions, there were significant trends toward a later fall sea ice advance, again by about three-to-nine days per decade. These trends were slightly more pronounced in some regions, such as the Barents Sea off the northern coasts of Norway and Russia, and the central Arctic Basin.
An earlier spring melt was associated with a later fall freeze pretty much every time — and the researchers have attributed this pattern to a special kind of climate feedback system.
“Sunlight bounces off ice, but it’s absorbed by water,” explained Harry Stern, a mathematician and sea ice researcher at the University of Washington and one of the study’s two authors. So when sea ice melts, it opens up large swaths of water where sunlight can be absorbed and heat can be trapped. The earlier the ice melts, the more heat goes into the ocean. In the fall, when temperatures start to drop again, all of this heat has to be released before the sea ice can refreeze. But if there’s more heat stored in the water than usual, the process takes longer and the freeze is delayed.
Overall, the researchers found that the length of time between spring sea ice retreat and fall sea ice advance has increased by between three and nine weeks since 1979. The biggest single increase of any polar bear subpopulation region was in the Barents Sea, where the interval lengthened by a whopping 17 weeks.
Additionally, the researchers found the concentrations of summer sea ice is declining by anywhere from one to nine percent per decade in most regions. And the total number of ice-covered days per year has also significantly declined in every subpopulation region, generally by between seven and 19 days per decade. Again, these trends were slightly larger in the Barents Sea.
The study’s results are already being used by conservationists, said Kristin Laidre, a principal scientist and polar bear expert at the University of Washington’s Polar Science Center and the study’s other author. The International Union for the Conservation of Nature (IUCN) has incorporated the sea ice data into its Red List assessment of polar bears (which is part of a global assessment of the endangered or threatened status of wildlife all over the world) and into its public status information on the 19 subpopulations.
As for the future of the bears’ habitat, most projections suggest things are looking pretty grim. Models have predicted that we could be seeing ice-free Arctic summers by mid-century.
In that light, the new study “highlights the clear changes we’re seeing around the globe, especially in the Arctic,” Laidre said, adding that she hopes the results might influence high-level decision-making about climate policy and greenhouse gas emissions moving forward.
Assuming the sea ice trends are irreversible, though — at least in the short term — she said the results could also be used to “compare and contrast subpopulations of polar bears across the Arctic and identify areas where we might expect to see more negative changes occurring.”
In this way, increased knowledge of sea ice trends in each of the subpopulation regions could help inform management decisions and future conservation effort. As for the bears themselves, continued monitoring will be necessary to find out how they respond to the ongoing ice losses.
For the time being, nine of the populations don’t have enough data for scientists to make an assessment of how they’re doing, while three are known to be declining, six are considered stable and just one is growing. How these patterns change in the future remains to be seen. But given the bears’ dependence on the sea ice, the researchers have concluded in their paper that “climate warming poses the single more important threat to their persistence.”
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