"They're going to drown, they're going to starve, they're going to resort to cannibalism, they're going to become extinct," she said.

As ice recedes, many bears will get stuck on land in summer, where they have virtually no sustainable food source, Siegel said. Some will try and fail to swim to sea ice, she said.

Bears that stay on sea ice will find water beyond the continental shelf to be less productive, she said, and females trying to den on land in the fall will face a long swim.

"It's absolutely horrifying from the polar bear perspective," she said.

Less sea ice also will mean a changing ecosystem for commercial fishermen and marine mammals in the Bering Sea, Overland said.

With sea ice present, many of the nutrients produced in the ocean feed simple plankton that bloom and sink to the ocean floor, providing rich habitat for crabs, clams and the mammals that feed on them, including gray whales and walrus.

"If you don't have the ice around, the productivity stays up closer to the surface of the ocean," Overland said. "You actually have a change in the whole ecosystem from one that depends on the animals that live on the bottom to one that depends on the animals that live in the water column. So you have winners and losers."

That could mean short-term gains for salmon and pollock, he said. But it also could mean that fishermen will have to travel farther north to fish in Alaska's productive waters, and warm-water predators might move north.

The contribution to warming by greenhouse gas emissions likely is set, he said. Emissions stay in the atmosphere for 40 to 50 years before the ocean absorbs them. The amount emitted in the past 20 years and the carbon dioxide put out in the next 20 will linger, Overland said.

"I'm afraid to say, a lot of the images we are going to see in the next 30 to 40 years are pretty much already established," he said.