ST. LOUIS (AP) — Work crews began tearing out the old bear pits last week. They trucked in giant yellow excavators, strung caution tape along walkways and started ?ripping into the 100-year-old concrete.Soon, icy blue pools, rocky outcrops and 4-inch-thick glass walls will rise ?from the debris. The St. Louis Zoo's newest exhibit, Polar Bear Point, is scheduled to open in 2015, the St. Louis Post-Dispatch reports.The only thing missing?The polar bears.St. Louis has gone without the giant white hunters since 2009, when staff euthanized Hope, the last of its three, after discovering a tumor on her liver. Now the zoo wants polar bears back.But the institution is stepping back into a market that has vastly changed over the past decade.Polar bears are no longer just a cute attraction. They have become poster children for climate change.The effects of global warming on other species can be more subtle, more difficult to notice, said Steven Amstrup, chief scientist at Polar Bears International, one of the leading bear conservation advocates. But polar bears live on the arctic ice."Their situation is much simpler," Amstrup said. "Polar bears rely on a habitat that literally melts as the temperature rises."That predicament — coupled with the bears' innate and fuzzy charisma — has fueled a boom in popularity over the last several years. Zoos across the country have renovated or rebuilt enclosures complete with long, deep pools, rock pedestals, snow machines and inches-thick glass, behind which visitors can gaze at their beloved creatures.The St. Louis Zoo made a new polar bear exhibit a priority in 2008. It mounted a $120 million fundraising campaign and set aside $15 million for Polar Bear Point.The new space, alongside Penguin & Puffin Coast, will give the bears a rocky moraine, grassy tundra, a deep pool, a shallow pool, and will offer visitors at least two spots for underwater viewing.Here, then is where the problems arise.All those zoos — including St. Louis — want polar bears.And there just aren't very many to go around.By the end of the 1990s, zoos were generally getting rid of their polar bear exhibits. The animals, ?large terrestrial carnivores, were hard to care for and expensive to maintain. New government regulations made new habitats costly.In 1995, there were about 200 polar bears in 81 institutions, according to the North American Regional Polar Bear Studbook, which tracks zoo bears' lives and deaths.By 2006, both numbers had been cut in half.Then, in 2008, President George W. Bush's administration added the polar bear to the list of threatened species under the Endangered Species Act.Practically, the decision did two things. It made polar bears the flagship species for global warming. And it made importing foreign bears prohibitively difficult.It's unclear whether the polar bear population had begun to shrink. Polar bears are notoriously difficult to count. They wander sea ice in the farthest reaches of Alaska, Canada, Russia, Norway and Denmark. Some of the groups are so far away that no credible counts have been logged.Most scientists agree that there are more bears now — 20,000 to 25,000 — than 50 years ago, when overhunting ravaged populations.It was the projected melting of sea ice that led the Bush administration to list the bears as threatened.Arctic bears need huge quantities of fat to survive. To get this fat, they hunt ringed and bearded seals. And to hunt, they rely on some very specific physical adaptations — and sea ice.The nose of a polar bear is so sensitive it can smell a seal's breath from a mile away. Bears travel to seals' breathing holes, wait, and when the seal surfaces, plunge their arms into the water, hook their curved claws into the seal and pull it onto the ice.Less ice means harder hunting, and fewer seals.Amstrup, the wildlife biologist, says scientists are already seeing some consequences: Skinnier bears, fewer births and more dead cubs.Zoos agreed that polar bears needed help. For those with the means, the decision was a no-brainer: The public loves polar bears.Polar bear curators now count 10 relatively new exhibits in North America, with five more planned in the next five years.John Kemper, vice president of the St. Louis-based PGAV Destinations, has seen the appeal firsthand. He and his colleagues designed exhibits in Louisville, Chicago and Kansas City. They are also the architects in St. Louis.Unlike the old bear pits, which typically separate visitors from bears via moats, the new exhibits boast platforms with overhead viewing, glass walls that let visitors peer underwater and steel netting that puts bears next to humans."We as designers want to tell that story," Kemper said. "They swim, play, dive. What of that can we show the public?"Kansas City's new exhibit has nearly single-handedly saved its zoo, according to Executive Director Randy Wisthoff.The zoo had off-loaded its polar bears and was "really struggling" when he arrived in 2003, he said.In 2004, city voters passed a bond issue. The zoo used $10 million to build a new polar bear exhibit — on spec. It didn't know if it would find a bear to call its own, Wisthoff said. Maybe, he figured, they'd put grizzlies there instead.But in 2010, the Toledo Zoo sent a 2-year-old male to Kansas City. The exhibit opened that summer.And within two years, the zoo doubled its annual attendance to more than 800,000.Now the zoo has two bears, Nikita and Berlin. They mated this spring. Wisthoff should know this winter whether Berlin is pregnant.No zoo has built a new facility but been denied a bear, said Randi Meyerson, assistant director in Toledo and coordinator of the Association of Zoos and Aquariums' Polar Bear Species Survival Plan.No one has made any promises. St. Louis has to prove it's ready first, both Meyerson and Van Norman said.Steve Bircher, the carnivore curator at the St. Louis Zoo, is aware of the demand, but says he isn't worried."We never wanted to get out of the polar bear business," he said. "There just weren't any bears available."New St. Louis Zoo exhibit still needs polar bears