This article was updated on Sept. 21 at 10:12 a.m. ET.
Polar bears have become the fuzzy face of the impacts of climate change, with shrinking sea ice in the Arctic affecting how the bears normally roam and hunt. Now, after a photograph of an emaciated polar bear hobbling on ice went viral online, some people are wondering if global warming is causing these majestic creatures to starve.
Wildlife photographer Kerstin Langenberger snapped the now-famous photo of the gaunt polar bear and wrote a concerned Facebook post questioning the health of polar bear populations. Though it was widely circulated online, it's difficult to say precisely what the circumstance was for this individual, said Karyn Rode, a wildlife biologist at the U.S. Geological Survey in Anchorage, Alaska.
"I think you are always going to have animals in any population [that are] in poor conditions," Rode said. This can be because they have an injury (as may be the case with the polar bear in the photo) or because the animal is old and has lost some of its canines, she said. [In Images: Polar Bears' Shifting Diet]
Steven Amstrup, chief scientist at Polar Bears International, a nonprofit research organization dedicated to studying polar bears, agreed and added that seeing a skinny bear in the wild is not altogether uncommon. "We know that animals in the wild don't live forever," he said. "Polar bears, they don't have natural enemies, so when they die it's of starvation."
There are 19 recognized polar bear subpopulations, but only two have been studied for long enough to show that changes in ice conditions are affecting the livelihood of some polar bears. Temperatures are rising in many regions of the world because high concentrations of greenhouse gases, such as carbon dioxide (CO2), are warming up the Earth's atmosphere. The effects are most notable in the colder regions of the world since a large portion of the ice cover comes from frozen ocean water, or sea ice. Sea ice forms at colder temperatures than freshwater ice, so when things warm up, sea ice is the first thing to melt.
Sea ice is the home of polar bears' major food source, ice seals, so when the sea ice disappears, so does the bears' main way of getting meals. Rising global temperatures are forcing bears to spend more time on land and to go longer between meals. "The climate can only continue to warm as[carbon dioxide] concentrations continue to rise," Amstrup said.
"There is a higher percentage of bears in this situation [starving] now because of sea ice retreat," Amstrup said. "We have documented the populations in Alaska and the western Hudson Bay of Canada. We have shown in both places that we have seen poorer survival rates."
Also, there are several polar bear populations that aren't very well studied and there is currently no data for all sub-populations of polar bears, so it is impossible to know how all bears are responding to ice loss, Rode said. "There has been no study that I know of that said more bears starve specifically as a result of climate change," she added. "There have been models of that, but there has been no empirical data to support that."
Shrinking sea ice is causing polar bears to starve with higher frequency, but "[y]ou can't say that any one individual is starving because of climate change," Amstrup said.
As the climate warms and sea ice continues to decline, the frequency of starving bears is only going to rise — not because of climate change directly, but because the loss of sea ice is taking away their main food source, Amstrup added. However, Rode said, it is also important to note that there is no observational data of polar bear deaths so it is hard to say that starving is the definite root of the higher mortality rate.
Some scientists have speculated that there is a chance polar bears may find alternative food sources on land, but a study published in April in the journal Frontiers in Ecology and the Environment by Rode, Amstrup and others found that the bears' food prospects on land aren't great.
While there are some viable nurtiet sources on land, such as bird eggs, thus far bears are not using these resources in sufficient numbers to offset population level declines in body condition and survival that result from spending more time onshore, Rode explained. Bird eggs are only available during the spring, when birds are breeding, so searching for food on land in other season won't meet polar bears' nutritional needs as easily. Which leaves the bears in a bit of a quandary, unless they can travel northward with sea ice retreat or find ways to hunt bigger game than bird eggs.
Editor's Note: This article was updated to clarify that the uncertainty about how ice loss is affecting polar bear populations comes largely from a lack of observational data on all sub-populations. All changes were based on comments made by Karyn Rode, a widlife biologist at the U.S. Geological Survey.