"A polar bear's life is tied to the sea ice," writes Krista Wright, the executive director of Polar Bear International, in The New Big 5.

"The bears know the ice will be back soon - they're waiting," says Alysa McCall, from Polar Bears International (PBI).

Alysa and her team are in the Canadian Arctic just outside Churchill, Manitoba, in a tundra buggy - an observation lab on tracks, which allows them to watch bears in safety. "Looking over the last couple of decades, it forms later and later and it breaks up earlier and earlier in spring," Dr Flavio Lehner, of conservation charity PBI, says.