Polar bears are one of the animals most effected by seasonal and year-to-year decline in the Arctic sea ice extent, mainly due to their reliance on sea ice for essential activities such as hunting, traveling and breeding.

A new study by University of Washington researchers found a trend toward earlier sea ice melt in the spring and later ice growth in the fall across all 19 polar bear subpopulations. This negatively impacts the feeding and breeding capabilities of the bears. The paper, published on Sept. 14 in "The Cryosphere," is the first to quantify the sea ice changes in each polar bear subpopulation across the entire Arctic region.

"This study shows declining sea ice for all subpopulations of polar bears," said co-author Harry Stern, a researcher working at the University of Washington's Polar Science Center in Seattle. The analysis shows that the critical timing of the sea ice break-up and sea ice freeze-up is changing in all areas in a direction that is harmful for polar bears.

“Other researchers have used the satellite-derived sea ice data to look at how the sea ice extent in a particular place is changing in a particular month. But for us the important thing was the timing of the retreat of sea ice in the spring and its advance in the fall, for all 19 polar bear subpopulations,” Stern said.

Nineteen separate polar bear subpopulations live across the Arctic, spending their winters and springs roaming. The bears have evolved to predominantly eat seals, which provide necessary fats and nutrients during the harsh Arctic winter. Polar bears can't outswim their prey, so instead they perch on the ice as a platform and ambush seals at breathing holes or break through the ice to access their dens.

“Sea ice really is their platform for life," said co-author Kristin Laidre, a researcher at the UW's Polar Science Center. "They are capable of existing on land for part of the year, but the sea ice is where they obtain their main prey."

The center also reports each fall the yearly minimum low for Arctic sea ice. This August was the fourth lowest in the satellite record and the September minimum extent is likely headed to be the second lowest level in the record.

In 17 of the 19 polar bear subpopulations, the researchers found that the total number of ice-covered days declined at the rate of seven to 19 days per decade between 1979 and 2014. The decline was greater in the Barents Sea and the Arctic basin. Sea ice concentration during the summer months also declined in all regions, by one percent to nine percent per decade.

The most striking result, researchers found, was the consistent trend across all polar bear regions for an earlier spring ice melt and a later fall freeze-up. Arctic sea ice retreats in the springtime as daylight reappears and temperatures warm. In the fall months the ice sheets build again as temperatures drop.

"These spring and fall transitions bound the period when there is good ice habitat available for bears to feed," Laidre said. "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."

"We expect that if the trends continue, compared with today, polar bears will experience another six to seven weeks of ice-free periods by mid-century, the trend appears to be linear and isn't accelerating or leveling off, Stern said.

The study's results currently are used by the Polar Bear Specialist Group, part of the International Union for Conservation of Nature's Species Survival Commission. The Polar Bear Specialist Group used the sea ice metric as a measure of polar bear habitat in the IUCN Red List assessment of polar bears, which assesses the conservation status of polar bears. The researchers plan to update their findings each year as new ice coverage data are available.