EXPLAINER: How warming affects Arctic sea ice, polar bears

Majestic, increasingly hungry and at risk of disappearing, the polar bear is dependent on something melting away on our warming planet: sea ice.

In the harsh and unforgiving Arctic, where frigid cold is not just a way of life but a necessity, the polar bear stands out. But where it lives, where it hunts, where it eats — it's disappearing underfoot in the crucial summertime.

“They have just always been a revered species by people, going back hundreds and hundreds of years,” said longtime government polar bear researcher Steve Amstrup, now chief scientist for Polar Bear International. “There’s just something special about polar bears.”

Scientists and advocates point to polar bears, marked as “threatened” on the endangered species list, as the white-hot warning signal for the rest of the planet — “the canary in the cryosphere.” As world leaders meet in Glasgow, Scotland, to try to ramp up efforts to curb climate change, the specter of polar bears looms over them.

United Nations Environment Program head Inger Andersen used to lead the International Union for the Conservation of Nature, which monitors and classifies species in trouble. She asks: “Do we really want to be the generation that saw the end of the ability of something as majestic as the polar bear to survive?”

THE STATE OF SEA ICE

Arctic sea ice — frozen ocean water — shrinks during the summer as it gets warmer, then forms again in the long winter. How much it shrinks is where global warming kicks in, scientists say. The more the sea ice shrinks in the summer, the thinner the ice is overall, because the ice is weaker first-year ice.

Julienne Stroeve, a University of Manitoba researcher, says summers without sea ice are inevitable. Many other experts agree with her.

Former NASA chief scientist Waleed Abdalati,...