They created a computer model to monitor and forecast how long polar bears could survive while fasting.
These models were then applied to a separate set of predictions which investigated at what stage 13 populations of Arctic polar bears - representing around 80 per cent of all polar bears - would reach these thresholds for ice-free days.
Study author Peter Molnar and his colleagues found that, under a high greenhouse gas emissions scenario, the bears' survival would be "unlikely" over much of the Arctic due to reduced sea ice.
However, under a "moderate emissions scenario", more sub-populations could survive this century.
"Ultimately, aggressive greenhouse gas emissions mitigation will be required to save polar bears from extinction," the study said.
Researchers found that cubs would be most at risk if forced to fast, while solitary adult females would be the least affected.
It also found that survival thresholds may have already been reached in several polar bear sub-populations.
The authors said their study was limited by the use of a single "earth systems model" – used to determine how sea ice will be affected – and because of uncertainties and variations in bear behaviour and energy usage among sub-populations.
"Our model captures demographic trends observed during 1979 to 2016, showing that recruitment and survival impact thresholds may already have been exceeded in some (polar bear) sub-populations," the authors said.
"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 sub-populations by 2100."
The study was published in the journal Nature Climate Change.
