Sea ice is a polar bear conveyor belt

The changeable face of sea ice / Courtesy NOAA
The changeable face of sea ice / Courtesy NOAA

“It’s very difficult to observe polar bears directly in their environment.” “They travel widely,” George Durner, research zoologist with the United States Geological Survey Alaska Science Center, explained the bears travel “Hundreds of thousands of square kilometers in the course of the year across the sea ice environment.”

To enter the polar bear world Durner and other scientists use small aircraft, particularly helicopters, to fly over the sea ice off Alaska’s coast. Bears are located, tranquilized, and given temporary collars and tags that track their movement. Polar bear capture “Gives us the biggest bang for the buck for data and information on the status of the population of polar bears in the Beaufort Sea,” Durner said.

The method doesn’t let scientists easily witness natural polar bear behavior. Researchers are “At a disadvantage,” Durner said, “Because the polar bear we see most often is a polar bear running from our aircraft,” responding to the unusual noise and sight of a helicopter. That caution makes sense. “In a rare case you get a few that run to the aircraft out of wonderment,” Durner related.

That’s my favorite part: a sense of wonderment. Wonderment and curiosity are experiences bears and humans share. Bears are, after all, a top predator on their food chain so there are few threats they need to flee from. You might even say that an adventurous spirit is necessary for the bears since their environment is ever-changing.

You and I live in places with maps and landmarks. A polar bear’s sea ice habitat is in constant flux. The ice platforms they travel to hunt are always moving, drifting. “Polar bears are essentially on a conveyor belt,” Durner described, “Especially in the Beaufort Sea,” where ocean currents force constant motion. The Beaufort “Has a current phenomena called the Beaufort gyre; it’s a clockwise current and it moves relatively fast across the northern Alaska coast. But that’s some of the best polar bear habitat in Northern Alaska. So there is an energetic consequence from using this platform.” That is, the polar bears expend energy as they travel in response to their changeable habitat.

A polar bear rests atop ice / Courtesy NOAA
A polar bear rests atop ice / Courtesy NOAA

Sea ice grows toward Alaska’s northern coast in cold months then retreats from the coast during warm months. New ice is thin, while multi-year ice (ice that did not melt away during the summer) is meters thick. Durner described: “The actions of winds and currents move the sea ice around, creating leads that are important for polar bears and the seals they depend on, or it can converge ice such that ice floes collide.” Leads are fractures in the ice where open water can be reached. In contrast places where ice floes collide form pressure ridges, thick and high.

Polar bears “Like to stay on this ice that is relatively high concentration– 50% sea ice or greater– because it affords them stability, security,” Durner said. Ideally the stable ice they traverse will be close by to fractured ice where seals surface and crawl ashore to molt and to give birth to pups; from the stable ice pack the polar bears “Venture into these other areas right on the edge of ice where seal hunting might be most productive for them.”

However, Durner noted “The degree of melting has changed in the past decade.” “The retraction of sea ice has been rather extensive and it continues to be so in most years. 2012 set a new record of sea ice melt.” Extensive sea ice melt makes the ice pack retreat further north from Alaska’s shore, removing bears and the seals they hunt from their preferred habitat and taking them out over deep ocean waters. The bears must travel further to stay on ice floes in order to hunt their preferred prey, either trekking on the changeable ice surface or swimming to reach the ice when it’s far from shore.

Data from one polar bear’s collar showed she “Swam basically 2 miles an hour for 9 days straight. She never really varied her rate of speed and just plugged North,” swimming to reach sea ice far offshore. Durner stated “All told she swam 425 miles over the 9 day period.” It impressed the scientists. “In years past … probably up until the mid-90’s when they had to swim from land to ice the distance was not that far: maybe 100 miles but not 400 miles.” Traveling long distances requires a high energy cost for polar bears looking for their next meal.

Durner forecast that knowledge like this can help us in planning future steps: “What we learn from polar bears is important for making decisions– for making informed decisions– and so it’s important for directing policy that might affect the Arctic.”

Laura Nielsen 2015

Frontier Scientists: presenting scientific discovery in the Arctic and beyond