The polar bear, one of the largest land predators since the dinosaurs, was born out of climate change. As the deep freeze of the last ice age set in, polar bears evolved from grizzlies, adapting to live and hunt on the frozen Arctic Sea. Today, climate change is again dramatically affecting the Arctic bears. As winters warm and ice disappears, the polar bears are living on borrowed time, while grizzlies are finding an increasingly abundant world. NATURE offers a close-up look at each species as the tables turn for them when Arctic Bears premieres Sunday, February 17, 2008 at 8 p.m. (ET) on PBS (check local listings). Academy Award-winning actor F. Murray Abraham is series narrator.

From a polar bear giving birth in her snowy den, to fascinating scenes of grizzly and polar bear mothers passing down hunting and survival skills to their cubs, Arctic Bears captures the species from the dark depths of winter to the mild days in June when the sun never fully sets.

But climate change is quickly altering the bears' age-old way of life. Today, the distance in midsummer between the polar ice cap and the shore of northern Canada is 200 miles, when once it was 50. The thaw is an urgent problem for polar bears, for whom less ice means access to fewer seals - the prey that makes up 95 percent of their diet. The widening summer sea also means marathon swims in search of solid surfaces, which cubs need for sleeping and suckling.

Meanwhile, grizzlies are venturing farther north, into the once-icy domain of the polar bear. They are masters at living off the land and can eat almost anything, so the vast new foraging areas work in their favor. But the world of the polar bears is shrinking. They have seemingly forgotten how to be grizzlies and are struggling to adapt to an ancestral world they no longer remember. If the white Arctic world vanishes, so too may the white bears.

Arctic Bears is a co-production of BBC and Thirteen/WNET New York. NATURE is produced by Thirteen/WNET New York for PBS.