Polar bears are ideally suited to life in the Arctic: Their hair is without pigment, blending in with the snow; their heavy, strongly curved claws allow them to clamber over blocks of ice and snow and grip their prey securely; and their rough pads keep them from slipping.

The one thing they cannot survive is the disintegration of the ice. They range across the sea ice far from shore to hunt fatty seals, whose blubber sustains them.

Heat-trapping greenhouse gas emissions caused by burning fossil fuel are making the Arctic warm twice as fast as lower latitudes, and Arctic summer sea ice could disappear by 2030, according to climate models.

So a group of activists, zoo officials, lawmakers and scientists have a radical proposal: Increase the number of polar bears in U.S. zoos to help maintain the species’ genetic diversity if the wild population plummets.

In a worst-case scenario, a remnant group of bears would survive in captivity.

That should be good news for the St. Louis Zoo, which designed a $20 million polar bear exhibit with a cooled saltwater pool and concrete cliffs covered in simulated ice and snow for three to five bears. Its goal was to have them there by 2017. But it doesn’t have a bear lined up, because it’s illegal to import them, captive cubs are rare and finding orphaned bears in Alaska is difficult.

The Fish and Wildlife Service could allow the importation of polar bears for public display through future legislative or regulatory changes but has shown no inclination to pursue those options.

Evolved from brown bears tens of thousands of years ago, polar bears have become an iconic species for their majestic size and ability to thrive in the harsh Arctic. Today the image of a mammoth bear clinging to a piece of ice embodies an environment under siege.

Polar bears would prefer to hunt for seals year-round, but the disappearance of sea ice has forced them onto land or far offshore where the ice remains only over deep unproductive water. “Either way, they’re food deprived,” said Steven C. Amstrup, chief scientist for the advocacy group Polar Bears International and an emeritus researcher with the U.S. Geological Survey.

Advocates of the plan to bring more into captivity, including St. Louis Zoo president and chief executive Jeffrey Bonner, say that saving a species whose habitat is disappearing is an immense challenge.

“Polar bears are simply the first species where we have to get it right,” Bonner said. When it comes to research on how to sustain an exotic species through breeding techniques, “that research is only research that can be done in zoos,” he added.

Based on current projections, federal scientists say two-thirds of the world’s polar bears could be extinct by mid-century, though a significant cut in greenhouse gas emissions could help halt that decline. There are roughly 20,000 to 25,000 polar bears worldwide, 3,500 of which live in Alaska and spend part of the year in Canada and Russia.

There are 19 sub-populations of polar bears living in Canada, the United States, Russia, Denmark and Norway, and since scientists fear ice melt could cause some of these to disappear from their historic ranges, the idea would be to preserve enough genetic diversity in captivity to allow them to be repopulated through artificial insemination of wild bears or other methods. Supporters of the plan say researchers are just beginning to experiment with assisted reproduction techniques for polar bears.

Zoological institutions have helped save imperiled species before such as the California condor and the Mexican wolf, which were bred in captivity and reintroduced into the wild.

The American bison’s numbers dropped from the tens of millions to fewer than 1,000 after the 1880s, kept in small private herds in places like the Bronx Zoo. While there are roughly half a million bison now roaming the Great Plains, the International Union for Conservation of Nature estimates at most 7,000 are genetically pure. The Bronx Zoo shipped 15 head to Wichita in 1907, and the herd has grown to 650; this month 71 bison calves were released on the American Prairie Reserve, reintroduced from a herd a member of the Confederated Salish and Kootenai tribes was forced to sell the Canadian government in 1907 when the Flathead Reservation was opened to homesteaders.

“If you don’t build these insurance populations when you have the animals, then it’s too late,” said the Toledo Zoo’s mammals curator Randi Meyerson, chairman of the Association of Zoos and Aquariums’ polar bear species survival program. “We’re planning for something we hope we don’t need.”

The number of captive polar bears in the United States has declined since 1995, when there were about 200. Today 64 bears reside in accredited institutions such as the Maryland Zoo in Baltimore, which houses three. A total of 13 different polar bears lived at different times at the Smithsonian’s National Zoo between 1959 and 1980, but it no longer has one in captivity and has no plans to acquire one because creating the proper habitat would be, in the words of spokeswoman Pamela Baker-Masson, “cost-prohibitive.”

While polar bears have lived for decades in zoos, Ronald Sandler, an associate professor of philosophy at Northeastern University and director of the university’s Ethics Institute, called them “one of the worst candidates for captivity” because they are large carnivores that can roam for thousands of miles in the wild.

“It’s really hard to replicate the conditions in which they live,” he said. “It doesn’t mean they’re miserable. But there’s no sense in which they’d be able to live out the life they’d have in the wild.”

Shrinking ice has put some polar bears into closer contact with humans, especially in Canada, and, in some cases, communities encounter orphaned cubs. Manitoba’s Assiniboine Park Zoo has created an International Polar Bear Conservation Centre, aimed at helping transition cubs into captivity in some instances. The question remains whether these cubs should be available for import into the United States for public display, because as a federally listed threatened species, polar bears are classified as “depleted” under the Marine Mammal Protection Act and can be brought in only for bona fide scientific research or if it enhances the species’ recovery.

The proposal, which would require an interpretation from the Fish and Wildlife Service that polar bear imports comply with federal law, has sparked a fierce debate among scientists, ethicists, policymakers and conservationists.

“If the world cares about polar bears, reducing carbon concentrations in the atmosphere is the only way to preserve polar bears’ habitat,” said Lily Peacock, a research biologist in the U.S. Geological Survey’s polar bear program.

Even the proponents of the zoo plan identify reducing carbon emissions as the top priority for conserving polar bears. Robert Buchanan, president of the advocacy group Polar Bears International, said displaying them in zoos could represent the best way to convince the public to make such cuts.

“The only way at this time to save bears is to have people change their habits, and the way to do that is through zoos and aquariums,” he said. “Polar bears are just ambassadors for their friends in the Arctic.”

The Fish and Wildlife Service allows orphaned cubs from Alaska to be shipped to the lower 48 states for display, like it did with the Louisville Zoo last year. It let in a captive-bred polar bear from Australia in 2006 for Anchorage’s Alaska Zoo, but hasn’t let in any other polar bears since the late 1990s.

Four House Democrats led by Rep. William Lacy Clay (Mo.) — all representing zoos hoping to obtain polar bears — urged Interior Secretary Ken Salazar in a Oct. 27 letter to issue permits for “live rescued polar bears” to be put on display. Eighteen U.S. zoos have either recently renovated polar bear exhibits or built new ones, are in the process of construction, or planning to do so in the future.

Robert Gabel, with the Fish and Wildlife Service’s International Affairs program, said the agency has advised zoo officials “it’s going to be difficult for us to authorize.”

“We’d have to show that an import would either stabilize or increase the wild population of polar bears. It’s difficult to show how an import would accomplish that,” he said, adding that while the law has an exemption for scientific research, “We’ve never allowed that breeding in and of itself is research.”

Dale Jamieson, a New York University professor of environmental studies and philosophy, noted that if you’re facing the prospect of taking an animal out of its natural environment for generations, “we might as well simply be storing genetic material in gene banks.” Zoos, he said “have a huge conflict of interest. This is how they make money.”

But Center for Biological Diversity senior counsel Brendan Cummings, who helped lead the legal fight to list polar bears, said federal officials need to realize they may have to pursue this course if the population closest to humans, in Canada’s southern Hudson Bay, begins to crash: “The most visible and unstable polar bear population in the planet will be in crisis mode caused by our action, greenhouse gas emissions, and there will be pressure to do something about it.”

Fish and Wildlife already has identified 187,000 square miles in Alaska as critical habitat for polar bears and is working on a plan to protect the species through such possible actions as limiting bear hunting and human activity along the coast in polar bear denning areas.

The agency’s Alaska spokesman, Larry Bell, said when it comes to captive breeding, “Right now, it’s not something we’re considering.”

Meanwhile, the World Wildlife Fund and Coca-Cola Co. have launched a campaign to preserve what they call the “Last Ice Area,” 500,000 square miles of polar bear habitat in northern Canada and Greenland, which is likely to remain frozen year-round the longest.

As Meyerson observed, all the genomic banking and artificial insemination techniques in the world have their limits. “This is all given that we have ice to return the animals to,” she said.