Federal marine mammal experts in Alaska studying the effects of global warming on walrus, polar bears and ice seals warn there are limit to the protections they can provide.

They can restrict hunters, ship traffic and offshore petroleum activity, but that may not be enough if the animals' basic habitat - sea ice - disappears every summer.

"Ultimately it's beyond my scope," said Joel Garlich-Miller, a walrus expert for the U.S. Fish and Wildlife Service in Anchorage. "I can't make ice cubes out there."

Garlich-Miller said 3,000 to 4,000 mostly young walrus died this year in stampedes on land on the Russian side of the Chukchi Sea, the body of water touching Alaska and Russia just north of the Bering Strait. Instead of spending the summer spread over sea ice, thousands of walruses were stranded on land in unprecedented numbers for up to three months.

Anatoly Kochnev, who conducts walrus research for Russia's Pacific Institute of Fisheries and Oceanography, said the loss of 3,000 to 4,000 animals this year from mostly one demographic could be disastrous.

A walrus rests on sea ice. Experts no fear the diminishing icy landscape of the Arctic landscape could damage walrus populations.

If current ice trends continue, and walrus have to stay on coastlines every summer, they may put too much pressure on nearby foraging areas instead of feeding in the rich waters offshore, said U.S. Geological Survey biologist Tony Fischbach.

Experts on summer sea ice say it's not likely to suddenly reappear. Arctic sea ice this summer plummeted to its lowest levels since satellite measurements began in 1979, according to the National Snow and Ice Data Center at the University of Colorado.

"Certainly we look like we're on a death spiral right now," said Mark Serreze, senior research scientist. "Losing that summer sea ice over by 2030, within some of our lifetimes, is a reasonable expectation."

The U.S. Fish and Wildlife Service will decide within weeks whether to list polar bears as "threatened" under the federal Endangered Species Act because of the loss of sea ice from global warming. Polar bears hunt and breed on sea ice and are poor candidates for survival if they are based on land, where grizzly bears dominate.

Polar bears' primary prey are ringed seals, the only seals that thrive under sea ice. They dig breathing holes with their thick claws and create lairs on top of the ice where they birth their young.

With warming, those lairs collapse earlier in springtime, leaving hairless pups susceptible to freezing, foxes, polar bears and even ravens and gulls.

And then there's the Pacific walrus, which face at least three problems: Their ocean habitat may be changing, they may be forced to shore for long periods, and their weakest members are in danger when crowded on land.

Walruses dive to the ocean bottom to eat clams, snails, crabs, shrimps and worms. Research suggests that diminished sea ice and warmer water may decrease plankton, which are food for creatures on the bottom.

Unlike seals, walruses can't swim indefinitely. Females and their young traditionally use ice as a diving platform, riding it north like a moving sidewalk over offshore foraging areas, first in the northern Bering Sea, then into the Chukchi Sea.

If animals are on shore for three months every summer, they can't reach offshore foraging areas. Chad Jay, chief walrus researcher for the USGS, said there are concerns about how much energy walruses will expend swimming to foraging areas.

An adult walrus can eat 200 pounds of clams in a day. If the walrus population stays within 30 miles of shore in summers, they could overharvest the available clams and other food.

"I suspect they won't do very well as totally shore-based animals," said Vera Alexander, one of three members of the U.S. Marine Mammal Commission.