LINDA WERTHEIMER, HOST:
Today is the 40th Anniversary of the Endangered Species Act. The law helped bring back iconic species like the wolf, the grizzly bear, and bald eagle, after hunting, trapping and pesticides almost did those animals in. But these days, a very different kind of threat - global warming - is pushing some species, like the polar bear, towards the brink.
And as NPR's Elizabeth Shogren reports, one government biologist decided the best way he can help save polar bears was to quit his job.
ELIZABETH SHOGREN, BYLINE: Several years ago, the interior secretary was trying to decide whether to put polar bears on the endangered species list. He asked biologist Steve Amstrup to lead a group of scientists assessing the outlook for the great white bear. Amstrup had been heading up the government's polar bear research for many years, but what they figured out surprised even him.
STEVE AMSTRUP: By the middle of this century, we would probably lose two-thirds of the world's polar bears, including those that I had been studying for 30 years in Alaska, if we didn't change our ways.
SHOGREN: Polar bears live on sea ice. They use it as a platform to hunt seals and with global warming, that sea ice is shrinking. The government did put bears on the endangered species list five years ago, but...
AMSTRUP: It was also clear that this was not a typical conservation problem.
SHOGREN: The government couldn't restrict hunting, ban of pesticide or create a reserve. Doing what they normally do wouldn't save the polar bear. Amstrup concluded the bears only chance was for people to reduce their greenhouse gas emissions a lot.
AMSTRUP: So I decided that, you know, I need to get this word out.
SHOGREN: That took leaving the job he loved with the U.S. Geological Survey. For three years he's been chief scientists of Polar Bears International. He still does field research, but his main focus is teaching people about polar bears' precarious position.
AMSTRUP: They live on a habitat that literally melts as temperatures rise and because of that they're a really good symbol of the changes that we're making in our atmosphere and in our world.
SHOGREN: He spends every fall in Northern Canada where polar bears hang out on land waiting for the sea ice to form again.
AMSTRUP: They're getting hungry and they're anxious to get back out on the ice and resume catching seals.
SHOGREN: Amstrup and other scientists use special vehicles to take tourists onto the tundra to see the bears up close.
AMSTRUP: Tundra buggies are these giant sort of school bus-like things with giant tires.
SHOGREN: And the scientists have life webcasts for schoolchildren and anyone else who's curious about polar bears.
AMSTRUP: And all the while polar bears are kind of roaming around in the background.
SHOGREN: This year the ice was frozen longer, so he says the bears seem to be in pretty good shape.
AMSTRUP: But over the last two or three years, my impression has been, man there's a lot of skinny bears out here.
SHOGREN: On average, the sea ice in Hudson's Bay is frozen about a month less than it was 30 years ago. Amstrup says bears don't eat much on land, so they lose about two pounds of body fat every day they're off the ice.
AMSTRUP: They're 60 pounds lighter now than they might have been at this time of year 30 years ago.
SHOGREN: He's disappointed by how little political reaction there's been to all that science has shown about the effects of global warming.
AMSTRUP: You know, we came up with all this compelling evidence. It's really, really clear that we have a problem and yet we see very little action on either the national, or even the international scale.
SHOGREN: The sea ice is melting even faster than scientists' models say it should be. It's not clear how much time the polar bear has. Elizabeth Shogren, NPR News.