Far North Science

Photo by USFWS

A 20-year study of Alaska’s polar bears blames the steady disintegration of summer sea ice for the dramatic increase in pregnant and nursing females denning on land — where sows and their cubs may fare worse.

Landward and eastward shift of Alaskan polar bear denning associated with recent sea ice changes, published online in Polar Biology, offers yet one more confirmation that Arctic climate warming has begun to change the lives of Alaska’s polar bears by melting back their Beaufort Sea ice habitat and hunting platform.

Only a few decades ago, sea ice would start freezing close to shore by late September. Now the ice edge might be more than 125 miles out until later in the fall.

“In recent years, Arctic pack ice has formed progressively later, melted earlier and lost much of its older and thicker multi-year component,” said lead author Anthony Fischbach, in a USGS story about the research. “Together, these changes have resulted in pack ice that is a less stable platform on which to give birth and raise new cubs.

“Previous research had already shown that unstable ice can result in failures of on-ice denning attempts. Less ice that is suitable for denning apparently has led to an increased frequency of pregnant polar bears in this region choosing to den on land.”

Sea ice north of Alaska has been declining 7.7 percent per decade since satellite observations began in the late 1970s. With record minimum ice extents seen in recent years, a polar bear habitat larger than Texas has melted from late-summer existence.

For the first time ever, biologists have documented polar bears preying on each other and drowning in the Beaufort Sea. Other studies have also showed the shift to on-shore denning and decreased cub survival in the fall, plus falling weights of bears summering on ice and shore far from the best sealing — all further indications that climate change has already been forcing bears to compromise their hunting strategies.

Partly as a result of these observations (and because of lawsuits brought by conservation groups), the U.S. Fish and Wildlife Service has proposed listing polar bears as threatened under the Endangered Species Act. A decision is expected by next spring.

Photo by USFWS/Scott Schliebe

This newest study searched for causes behind shifts in denning among the estimated 1,500 bears in the Beaufort Sea population, one of 22 across the Arctic.

“USGS researchers Anthony Fischbach, Steve Amstrup and David Douglas used satellite telemetry to evaluate changes in the distribution of denning events by polar bears in the northern Alaska region between 1985 and 2005,” according to a USGS story about the research. “They found that the proportion of dens on sea ice versus coastal land dens declined from 62 percent in 1985-1994 to 37 percent in 1998-2004.”

The scientists then analyzed the results. They looked at three factors: Had polar bear harvests by people driven the bears ashore? Or had the bears been drawn to land to feast on bowhead whale carcasses left by subsistence whalers? Or finally, had the decline in sea ice left bears with poorer habitat and fewer den sites?

The answer eliminated whale carcasses and hunting.

“Our findings suggest that sea ice changes offer the most plausible explanation for the observed shift in maternal denning sites,” Fischbach said

More detail from USGS:

If declines in sea ice availability continue, the authors expect that the proportion of polar bears denning in coastal areas will continue to increase until such time as the autumn ice retreats far enough from shore that it prohibits offshore pregnant females from reaching the Alaska coast in time for denning.

“Right now,” said Amstrup, “pregnant females foraging offshore in summer must wait up to a month longer than they did even 10 years ago for new sea ice to form so they can travel to denning areas on land.