(CNN) The impact of the climate crisis is becoming more and more obvious to humans and their animal neighbors. But among all species, polar bears might be some of the hardest hit.

The bears rely on sea ice for nearly every aspect of their survival: It's where they hunt seals, travel, make dens and mate. So the earlier in the season the ice begins to melt , the less time they have to eat and procreate.

The findings were outlined in a new study published in Ecological Applications

Researchers found polar bears are becoming thinner and having fewer cubs, and their declining health was tied to melting sea ice.

Craig says her documentary will take more than a year to film and says she has plans to visit the Russian Arctic and the Northwest Passage (a route between the Atlantic Ocean and the Pacific Ocean around the top of continental North America). "The Arctic feels very remote," she says, "but what's happening in the Arctic is impacting everyone, everywhere."

Craig says her documentary will take more than a year to film and says she has plans to visit the Russian Arctic and the Northwest Passage (a route between the Atlantic Ocean and the Pacific Ocean around the top of continental North America). "The Arctic feels very remote," she says, "but what's happening in the Arctic is impacting everyone, everywhere."

As icebreakers are used to help ships navigate an increasingly open Arctic Ocean, these ships will have a crucial role in the years to come.

As icebreakers are used to help ships navigate an increasingly open Arctic Ocean, these ships will have a crucial role in the years to come.

Crew on deck of the USCG Healy, stationed at Unalaska in the Aleutian Islands, Alaska. The Healy is the only US icebreaker currently operating in the northern hemisphere, and Craig spent time aboard earlier this year with close access to the crew.

Crew on deck of the USCG Healy, stationed at Unalaska in the Aleutian Islands, Alaska. The Healy is the only US icebreaker currently operating in the northern hemisphere, and Craig spent time aboard earlier this year with close access to the crew.

Delbert Pungowiyi, president of the Native Village of Savoonga on St Lawrence Island, part of Alaska in the Bering Strait. Pungowiyi is one of the subjects of Maya Craig's upcoming documentary about how a changing Arctic Ocean is affecting those living and working on its borders.

Delbert Pungowiyi, president of the Native Village of Savoonga on St Lawrence Island, part of Alaska in the Bering Strait. Pungowiyi is one of the subjects of Maya Craig's upcoming documentary about how a changing Arctic Ocean is affecting those living and working on its borders.

Hungarian photographer Esther Horvath has been documenting Arctic Ocean sea ice since 2015. "We live in a time where ... this environment (is) changing so fast, that if you don't do something now it will completely disappear," she says.

Hungarian photographer Esther Horvath has been documenting Arctic Ocean sea ice since 2015. "We live in a time where ... this environment (is) changing so fast, that if you don't do something now it will completely disappear," she says.

Field training for the expedition included sea survival training in Ny-Alesund, Svalbard in April this year.

Field training for the expedition included sea survival training in Ny-Alesund, Svalbard in April this year.

Two polar bears photographed by Horvath at night on October 10. The animals were close to the ship and crew scared them away with the bang of a flare gun, for the safety of crew and the bears.

Two polar bears photographed by Horvath at night on October 10. The animals were close to the ship and crew scared them away with the bang of a flare gun, for the safety of crew and the bears.

A polar bear and her cub spotted close to the Polarstern on October 4. The expedition is traveling with designated polar bear guards who stand sentry while scientists work on the ice.

A polar bear and her cub spotted close to the Polarstern on October 4. The expedition is traveling with designated polar bear guards who stand sentry while scientists work on the ice.

Scientists Gunnar Spreen and Matthew Shupe drill into sea ice on September 30 with the MOSAiC expedition vessel Polarstern in the back on shot.

Scientists Gunnar Spreen and Matthew Shupe drill into sea ice on September 30 with the MOSAiC expedition vessel Polarstern in the back on shot.

Esther Horvath is currently drifting through the Arctic Ocean as part of the MOSAiC expedition, a multinational scientific mission attempting to measure changes in the Arctic.

Esther Horvath is currently drifting through the Arctic Ocean as part of the MOSAiC expedition, a multinational scientific mission attempting to measure changes in the Arctic.

A self-portrait of Martin Hartley with frostbite on his nose. The British photographer has endured numerous dangerous experiences in the Arctic when weather conditions have deteriorated, or help has been unavailable.

A self-portrait of Martin Hartley with frostbite on his nose. The British photographer has endured numerous dangerous experiences in the Arctic when weather conditions have deteriorated, or help has been unavailable.

Canadian Inuit hunter Jimmy Kulak from Resolute Bay, Cornwallis Island photographed by Hartley in 2011. Hartley says many of his Arctic trips pass through indigenous communities to reach the Arctic Ocean. "They all say the same thing, which is the winters are shorter and a lot warmer," he says.

Canadian Inuit hunter Jimmy Kulak from Resolute Bay, Cornwallis Island photographed by Hartley in 2011. Hartley says many of his Arctic trips pass through indigenous communities to reach the Arctic Ocean. "They all say the same thing, which is the winters are shorter and a lot warmer," he says.

Daniels swimming between ice floes on her way to the Geographic North Pole in 2010.

Daniels swimming between ice floes on her way to the Geographic North Pole in 2010.

British polar explorer Ann Daniels negotiating unstable sea ice during a 2009 Arctic expedition. Hartley, who has been exploring the Arctic Ocean since 2002, says his next project is called "The Last Ice Sentinels."

British polar explorer Ann Daniels negotiating unstable sea ice during a 2009 Arctic expedition. Hartley, who has been exploring the Arctic Ocean since 2002, says his next project is called "The Last Ice Sentinels."

Sea ice in the Arctic Ocean is shrinking fast. But what does that look like? (Pictured: Climate journalist and adventurer Bernice Notenboom negotiating sea ice in the Arctic, photographed by Martin Hartley in 2014.)

Sea ice in the Arctic Ocean is shrinking fast. But what does that look like? (Pictured: Climate journalist and adventurer Bernice Notenboom negotiating sea ice in the Arctic, photographed by Martin Hartley in 2014.)

"Climate-induced changes in the Arctic are clearly affecting polar bears," study author Kristin Laidre, professor of aquatic and fishery sciences at the University of Washington, said in a statement. "They are an icon of climate change, but they're also an early indicator of climate change because they are so dependent on sea ice."

Polar bears are considered a vulnerable species, one level below "endangered." If the climate trends continue, their populations will almost certainly deplete even further. Seeing the effect that a shrinking habitat has on polar bears could convince conservationists to take more immediate action

And studying how the melting ice affects them could have broader implications for how it'll eventually affect other species. "These bears inhabit a seasonal ice zone, meaning the sea ice clears out completely in summer and it's open water," she said. "Bears in this area give us a good basis for understanding the implications of sea ice loss."

The bears are spending less time on land and losing weight

Laidre and her colleagues tracked the movements of adult female polar bears in Baffin Bay, a body of water off the west coast of Greenland, over two periods of time in the 1990s and 2010s.

Between 2009 to 2015, the bears spent an average of 30 more days on land than they had in the 1991 to 1997 period. That's because sea ice is melting at a faster rate and earlier in the season now than it did even 23 to 29 years ago.

Sea ice waxes and wanes with the seasons, and when there's less sea ice, the polar bears take up residence on the neighboring Baffin Island. They wait there until there's enough sea ice to hunt seals from.

And the more time they spend on land, closer to the shore and unable to hunt, the thinner they become.

Researchers rated the bears' body conditions on a scale of 1 to 3, 1 meaning thin and 3 meaning fat (fat is good and keeps them warm).

Of the 352 bears they analyzed, not even 50 were considered fat.

The availability of sea ice was to blame: Researchers found that in years with less sea ice, the bears weighed less.

"When the bears are on land, they don't hunt seals and instead rely on fat stores," said Laidre. "They have the ability to fast for extended periods, but over time they get thinner."

Less time on the ice is tied to fewer cubs

The bears' worsening health impacted how big their litters were, too.

Female polar bears had more cubs when sea ice was more widely available and when "spring breakup," the period when ice melts and breaks and more water becomes available, occurred later in the season.

Now that the bears are thinner, they're having fewer cubs. Two-cub litters were once the norm for healthy adult female polar bears but could norm could soon "disappear" if sea ice loss continues, Laidre said.

It's the first time this change in litter size has been observed.

Shrinking habitats push bears further inland

The last count of polar bears in Baffin Bay was conducted between 2012 and 2013 -- then, there were over 2,800 bears in the area, according to the IUCN Polar Bear Specialist Group . But the group hadn't recorded how many were there before the most recent count was conducted, so it's difficult to tell whether it's an increase or a decrease.

Nevertheless, the results of the University of Washington study are not promising.

The future of polar bears, the researchers write in their findings, depends on scientists' ability to predict how climate change will continue to impact the bears. The pace of population changes is already startling, so a conservation response will need to come at an equally quick rate, the researchers say.