This is how much climate change has impacted polar bear populations
Scientists have now quantified how much climate change has drastically reduced the number of polar bears living in Canada's Hudson Bay, the most studied group of polar bears in the world.
The melting of Arctic sea ice in the Hudson Bay is significantly impacting polar bears' ability to hunt, sustain energy and ensure the survival of cubs -- leading to a 50% population decline since the mid-1990s, according to a paper published Thursday in Science.
The melting of the sea ice has shortened the polar bears' feeding season, which has resulted in an energy deficit for the bears for longer stretches of the year, Louise Archer, lead author of the study and international post doctoral fellow at the University of Toronto, Scarborough, told ABC News.
Researchers have known for some time that the population is in trouble, Peter Molnar, senior author of the study and associate professor at the University of Toronto, Scarborough, told ABC News. They created a bioenergetic model that could incorporate the different ways the polar bears have been affected by sea loss, Archer said. Combined with analysis of four decades of research on the Hudson Bay population, the scientists were able to determine the underlying mechanisms driving these changes.
"When we run the numbers, we show declining reproduction size in this region over the last four decades," Archer said.
Sea ice is crucial to polar bear survival because they use it to hunt their main source of food -- seals. Polar bears also use the ice as a platform to catch the seals, Archer said. Without the ice, they are forced to hunt in open water, which is a difficult feat for the large apex predator.
"If a bear tries to catch a seal in open water, the seal will outswim the bear pretty much every single time," Molnar said.
The vast majority of their feeding and energy intake happens when the bears are moving around and roaming across the ice and hunting seals, Archer said.
Bears are spending about three to four weeks less on the sea ice each year, on average, compared to the mid-1980s, Archer said. The sea ice is melting earlier in the spring and freezing later in the winter -- confining polar bears to land for longer periods, Molnar said.
"They're really tightly linked to the sea ice," Archer said. "Their whole lifestyle depends on it."
Mothers and cubs are especially vulnerable to sea ice loss, the models found. When polar bear mothers have a shortened feeding season, they're taking in less energy over the year, which then makes it difficult for them to provide milk for their cubs -- jeopardizing cub survival, Archer said.
In the mid-1990s, there were about 1,200 polar bears in the Hudson Bay population, Molnar said. That number has dwindled to 600 bears, he added.
A study published last year in Communications Earth & Environment showed that when polar bears were forced to find food on land -- typically by foraging berries or eating birds -- they lost the same amount of weight as the bears that simply fasted. This indicates that the food on land does not provide the same amount of nutrients as seals, which are full of fat and blubber.
"Polar bears are the largest bears," Molnar said. "They're enormous. They require a high-fat, high-energy diet."
Sea ice decline is the best explanation for declines in reproduction, declines in survival and declines in population size, Molnar said.
"They are not spending enough time on the sea ice anymore ... because the sea ice is disappearing due to climate change," he said.
The entire food chain in the region relies on the sea ice as well, from the algae that grows in the ice to seals, which rely on the ice to give birth, Archer said. Polar bears are the "bellwether" for what's happening in the ecosystem, but they are not the only species impacted by the loss of sea ice.
Losing the top predator in the ecosystem will have reverberating ramifications throughout the food web, Molnar said.
Based on current greenhouse gas emissions, polar bears in the southern Arctic regions will likely cease to exist if the presence of sea ice continues to wane, Molnar said. Preventing further declines would require drastic mitigation of fossil fuel extraction.
"We, as a society, still have a chance to turn things around," he said. "But in order to do that, we need to act now and not decades from now."
