“What nonsense! Does anybody realize that the polar bear population has increased from 5,000 in 1972 to 25,000 today! To be put on a threatened or endangered list, shouldn’t the numbers be declining???”

This, from blog reader Vince, was a recurring theme among the hundreds of responses to yesterday’s decision to list the polar bear as a threatened species under the U.S. Endangered Species Act.

I’d seen this number cited before, but I’d never seen any attribution as to its source. After some web research, conversations with polar bear researchers, and some help from a longtime journalist who specializes in animal/wildlife stories, here’s what we could find.

The number has some basis in fact, but is misleading: If polar bears have built in numbers since the 1970’s, it probably had a lot more to do with hunting bans than any aspect of global warming.

Polar Bear habitat covers five nations: The U.S. (Alaska), Russia, Canada, Denmark (Greenland), and Norway (the Spitsbergen and Jan Mayen Islands). Those five nations, along with the International Union for the Conservation of Nature, took a stab at guesstimating worldwide polar bear populations in the early 1970’s. For example, based on observer reports from Arctic villages, ships, and other sources, U.S. researchers came up with an estimate of 18,000 polar bears throughout the Arctic. The Canadian Wildlife Service set the number at 20,000. The Soviet Union submitted the low bid, estimating a worldwide population of 5,000 animals. In late 1973, the five polar bear nations signed the International Agreement on the Conservation of Polar Bears, and agreed to go with the lower Soviet estimates in order to wrap up a wildlife agreement that was unheard-of during the Cold War years.

Whichever of these estimates may have been closer to the real number is still unclear. It’s generally believed that polar bear populations did grow after the treaty was signed — but it had nothing to do with the Arctic climate: The Treaty also set restrictions on trophy hunting for the big bears — outlawing the then-common, controversial practice of hunting polar bears by helicopter. The U.S. had already banned all but some native subsistence hunting the previous year, through the US Marine Mammal Protection Act.

But there’s another revealing number from that year: Merritt Clifton, the editor of Animal People , dug through online newspaper archives and discovered this tidbit: Canada was the last Arctic nation to curtail large-scale hunting of polar bears. According to a 1973 United Press International story, Canada’s Northwest Territories allowed a quota of 422 bears to be killed that year.

So let’s do the math: If the 5,000 number were correct, they authorized the killing of nearly 10% of the world’s polar bear population — In just one part of Canada’s polar bear habitat, and in just one year. It’s very difficult to accept that a global population of only 5,000 could have sustained that rate of loss.

More recent polar bear research suggests that any growth in worldwide population has likely stopped, and that the bears themselves are generally thinner and give birth to fewer young. Here’s the key USGS paper on the bears’ status in the U.S. part of their habitat. The current estimates for global polar bear population is between 22,000 and 25,000 — numbers too big to risk any immediate extinction. But the overwhelming amount of research on polar bear health and future prospects says they’re in for a rough ride in the next fifty years.

One credentialed, dissenting scientist is Dr. Mitchell Taylor, a researcher who did much of his work on behalf of Canada’s Nunavut Territory. Taylor has acknowledged that climate change is impacting polar bears, but he does not see a great risk of collapse of the species. The recently-retired Taylor has researched bear populations in the Davis Strait, between Canada and Greenland. Taylor is due to publish his results later in the year, and in newspaper interviews, he has said that bears in the region are healthy in both size and number. Critics point out that Nunavut, Dr. Taylor’s longtime employers, have a huge stake in what remains of the legal polar bear trophy-hunting business, estimated to be a $2 million business for impoverished Nunavut communities. Yesterday’s US decision to declare the bears as “threatened” will dry up a longstanding loophole: U.S. trophy hunters were barred from killing bears in the U.S. (Alaska), but prior to the ruling, could still obtain permits to hunt polar bears in Canada and import the trophies.

-Peter Dykstra, Executive Producer, CNN Science Tech & Weather

U.S. Interior Secretary Dirk Kempthorne announced that the polar bear will receive protection as a “threatened” species under the U. S. Endangered Species Act. Conservation groups had petitioned the U.S. to give protection to the mammals — citing a rapid decline in Arctic sea ice, and U.S. government studies predicting a rapid decline for the bear population due to loss of habitat. The government was under federal court order to rule on the bears’ status by tomorrow.

CNN.com will have full details soon.

-Peter Dykstra, Executive Producer, CNN Science & Technology