Sea ice decline is hitting all 19 subpopulations of polar bears in the Arctic during the spring and fall, the times of year when they're most vulnerable, a new study found.

This study is the first to examine dates of spring sea ice retreat and fall ice advance across polar bear habitats around the Arctic. Previous studies had either examined sea ice loss in general or seasonal timing shifts among specific subpopulations.

The new research, published in The Cryosphere on Wednesday, found that polar bears are being forced onto land earlier in the spring than they were just a few decades ago, thereby depriving them of their main food source, travel route and breeding grounds for a longer time period. In addition, polar bears are waiting a longer period of time before they can venture back onto the ice in the fall.

Global warming has propelled a sharp decline in Arctic sea ice since the late 1970s. This year, for example, is expected to be the second-lowest sea ice extent on record, eclipsing 2007 but coming in second to the record melt year of 2012.

"We wanted to look at a measure

(or metric) of the sea ice that had some relevance to polar bears and other marine mammals that depend on sea ice," said study co-author Harry Stern, a researcher in the Polar Science Center at the University of Washington, in an email to Mashable.

Polar bears depend on sea ice for hunting seals, which are their main prey and food source. In areas where the sea ice retreats during spring and summer and reforms in the fall, polar bears spend time on land, where they are known to fast until they're able to get back onto the ice again.

Broken sea ice seen in July 2016. Image: nasa

"Sea ice really is their platform for life," study co-author Kristin Laidre, a researcher at the UW's Polar Science Center, said in a statement. "They are capable of existing on land for part of the year, but the sea ice is where they obtain their main prey."

The time they're being forced onto the land is growing, the study found, which can be problematic for these iconic creatures.

"We found that there has been a consistent and large loss of habitat for polar bears across the Arctic," the study states.

"These spring and fall transitions bound the period when there is good ice habitat available for bears to feed," Laidre said.

"Those periods are also tied to the breeding season when bears find mates, and when females come out of their maternity dens with very small cubs and haven't eaten for months."

The study found that sea ice throughout the Arctic is retreating about three to nine days earlier per decade when examined over the 1979 to 2014 period. Trends in fall ice advance, the study found, have shown that sea ice is forming about three to nine days later per decade in the fall than it used to.

This works out to about a 3.5 week shift in the spring melt season and a 3.5 week shift in the fall freeze-up, which adds up to about seven weeks of lost sea ice habitat for polar bears since 1979, the study found.

Sea ice concentration in the Arctic on Sept. 13, 2016. Image: University of bremen

"We found that in all 19 sub-population regions, spring sea-ice retreat is trending earlier, and fall sea-ice advance is

trending later," Stern said.

In addition, the number of ice-covered days during the period from June 1 through October 31 is declining significantly, with a rate of about seven to 19 fewer ice-covered days per decade.

"We expect that if the trends continue, compared with today, polar bears will experience another six to seven weeks of ice-free periods by mid-century," Stern said in a press release.

Right now, polar bears across the Arctic are waiting and watching for the ice to reform and thicken up enough for them to venture back out to get a good meal again.

If this year is anything like recent ones, they may be forced to wait a few weeks longer.