Polar bears may largely die out by 2100 as climate change causes the sea ice to melt around the Arctic.
The sea ice melts and freezes through the year, but has reached record lows in recent years.
Polar bears rely on the sea ice to hunt their prey, but as it disappears, they're forced to survive solely on the land instead.
According to a new Canadian paper published today, all but a few polar bear populations could be wiped out by 2100.
The researchers say the cubs would starve first, with solitary adult females the last to be impacted.
"Showing how imminent the threat is for different polar bear populations is another reminder that we must act now to head off the worst of future problems faced by us all,"zoologist and polar bear expert Dr Steven Amstrup told BBC News.
More than just polar bears will be impacted, the researchers say.
"It also suggests that, with high greenhouse gas emissions, steeply declining reproduction and survival will jeopardise the persistence of all but a few high-Arctic subpopulations by 2100."
The paper looked at polar bears' ability to fast and how much energy they use, to calculate the limits of their endurance.
"The trajectory we're on now is not a good one, but if society gets its act together, we have time to save polar bears," Dr Amstrup says.
"And if we do, we will benefit the rest of life on Earth, including ourselves."
The paper was published in the journal Nature Climate Change today.