FOOTAGE of a poorly polar bear went viral in December. Emaciated, it stumbled across a green Arctic landscape without a speck of snow or ice in sight (see picture below). Media outlets seized on as an example of how climate change is killing its poster child. But behind the headlines is an awkward question: have climate change activists chosen the wrong mascot?
The International Union for Conservation of Nature has long considered polar bears (Ursus maritimus) 鈥渧ulnerable to extinction鈥�. In May 2008, the US raised its own listing to threatened. The decision made international headlines and helped the polar bear achieve its iconic status in climate change campaigns.
Both listings rested on forecasts that Arctic sea ice would rapidly melt during the first half of the 21st century as a result of greenhouse gas emissions. Polar bears are expected to suffer the consequences. They spend most of the year cruising the fringes of sea ice, hunting ringed seals. The two play a game of Arctic cat and mouse: the seals pierce breathing holes in thin ice; the bears hang around the holes looking for lunch. Even though polar bears can survive on dry land for part of the year, they ultimately depend on the ice to hunt.
Advertisement
The rationale for concern is sound. In the past decade, Arctic temperatures have risen faster than models predicted and the ice has vanished faster. Logically, the polar bear population should have crashed. It hasn鈥檛.
If anything, numbers are up compared with 10 years ago, although it is hard to be sure because each of the 19 subpopulations across the Arctic tells a different story (see Bearing up). The population that hunts on the western side of Canada鈥檚 Hudson Bay, for instance, to have lost a third of its bears in the past 30 years. It is down from 1200 to 800, says Andrew Derocher at the University of Alberta, Canada. In other places, numbers have risen, says Steven Amstrup of Polar Bears International, an NGO in Montana. That is the case in the Canadian Arctic Archipelago, and perhaps also in the Norwegian part of the Barents Sea, where study found a 40 per cent increase in bear numbers over a decade, to almost 1000.
For a lot of populations, the trends are uncertain at best. Counting white bears spread across a vast and still mostly white landscape is tricky. Numbers are disputed even for populations that are regularly tagged, weighed and sampled for blood and teeth. Overall population estimates have to be extrapolated from the regional counts that do exist.
Right now, those estimates suggest that the species is not at immediate risk of extinction. In fact, overall numbers have risen since 1973, when five Arctic nations signed the Agreement on the Conservation of Polar Bears, which brought in hunting bans across the region. Before 1973, bears could legally be shot from planes and ships, and baited with food. Today, only the native Inuit are allowed to hunt them, and they have annual quotas. The known take is between 700 and 800 animals a year, which is thought to be sustainable for now. On top of this, there is evidence of illegal hunting in Russia.
鈥淟ogically, the polar bear population should have crashed. It hasn鈥檛鈥�
Total numbers for the entire Arctic polar bear population are , but they may have been as low as 6000. More than a decade ago, the International Union for Conservation of Nature (IUCN) estimated bear numbers at 22,000. In 2015, it increased that figure to between 22,000 and 31,000. About three-quarters of the gains were in northern Canada. 鈥淲e have no estimates for most of the Russian Arctic,鈥� says Derocher. Nonetheless, on current estimates alone, the poster child of climate change seems to be doing OK.
The question now is whether the treaty鈥檚 success is about to be undone by the disappearance of sea ice. Polar bears can swim for days, so there is little risk of mass drownings. But they face complex decisions about where to find enough seals to eat.
[video_player id=鈥漣tah94TF鈥� access_level=鈥漵ubscriber鈥漖
Here, two conflicting trends are at work. One is the diminishing extent of sea ice. During most Arctic summers, it is 40 per cent smaller than half a century ago. In some areas, like the Hudson Bay, ice is absent for three or four months running. This forces bears to spend ever longer on land, where they fast or graze on berries and birds鈥� eggs 鈥� a natural strategy, but one that has its limitations.
While total loss of ice is bad for bears, a second trend could be a boon. Until recently, a large proportion of Arctic sea ice was typically many metres thick, the result of many years鈥� winter growth. Now, what regrows in winter is predominantly thin annual ice. In March 2016, the average thickness was just 1.2 metres, compared with 3.6 metres in 1975. Polar bears do best in areas with annual ice, because thinner ice makes for a richer ecosystem, says Ian Stirling at the University of Alberta. the water underneath, boosting plankton growth, and this ripples all the way up the food chain to seals.
The two contradictory trends are problematic for forecasts combining an assumption that bear numbers will closely track sea ice extent with forecasts of how fast the ice will melt. Using those assumptions, in 2007, the US Geological Survey predicted a two-thirds decline by mid-century. In 2015, the IUCN on a bear population drop of between 30 and 50 per cent.
鈥淕rolar bears are the offspring of polar bears mating with grizzlies鈥�
Some critics of this approach dismiss the whole idea that polar bears are in trouble. 鈥淭hey are doing just fine,鈥� says Susan Crockford at the University of Victoria in Canada. She says the extinction forecasts are already demonstrably false because the rapid retreat of sea ice hasn鈥檛 caused a crash in bear numbers. has been widely criticised for her links to organised climate sceptic groups, and her blogs have become an irritant for mainstream researchers. , the question remains: how are the bears doing so well on vanishing ice?
One answer could be that they are getting a temporary boost. , Derocher and his colleagues wrote that seal abundance might temporarily increase as multi-year ice is replaced by thin annual ice. But, by the end of the century, longer periods without ice would turn the boom to bust, says Derocher.
As the sea ice diminishes, bears will have two options, says George Durner of the US Geological Survey in Alaska. 鈥淭hey must either follow the sea ice to higher latitudes where the ice may persist, or remain near their traditional foraging ranges by summering on land and fasting.鈥�
Berry hungry
Bears already seem to be using both tactics. By looking at the genetics of different populations, Elizabeth Peacock, a colleague of Durner, that some polar bears have begun moving from southern Canada into the northernmost reaches of the Arctic Archipelago. This is distinct from the random movements of the past, she says, and suggests bears are trying to stay with the ice.
But other bears are taking a different approach. Derocher says some southerly populations that have always spent summers on land are spending longer there, awaiting the return of ice. Biologists are pessimistic about their fate. Bears living off goose eggs, berries and the occasional caribou lose weight, often dramatically. Their short digestive tract isn鈥檛 built to digest vegetation efficiently, so they can鈥檛 do this for long. 鈥淥nce we hit 210 days without ice, things get a lot worse,鈥� says Derocher. He has calculated that a single event like this could kill between 29 and 48 per cent of adult males. Derocher reckons that the Hudson Bay will pass that 210-day threshold by mid-century. If that happens, the local bears could be the first large population .
The evidence seems to support his pessimism. In 2016, the bay鈥檚 southern shores were virtually ice-free for four summer months, more than a month longer than 30 years ago. That is still short enough for bears to survive, but only just. Derocher a decline in their physical condition, especially among pregnant females, whose average weight by about 20 per cent. The number of cubs and their survival rate has also fallen.
But some geneticists are more sanguine about what the future holds for polar bears. The species, they say, has previously survived warm interglacial periods with little sea ice. Matthew Cronin at the University of Alaska Fairbanks says evolved from brown bears within the past two million years, probably when the Arctic gained a relatively stable ice cap, and survived warm eras between glaciations. 鈥淚t seems logical that if polar bears survived previous warmer, ice-free periods, they could survive another.鈥�
Another intriguing aspect is the extent to which the two species of bear have continued to interbreed, thanks to the large amount of DNA they share, says Ceiridwen Edwards of the University of Huddersfield, UK. It isn鈥檛 clear if this helped them survive or whether it simply happened when the two species crossed paths. More recently, researchers have also found polar bears mating with grizzlies. There are reports of 鈥済rolar鈥� or 鈥減izzly鈥� hybrids around Hudson Bay.
It isn鈥檛 too fanciful to imagine that hybrids could thrive as 鈥減ure鈥� polar bears fade away. In the end, the polar bears鈥� route to salvation may be to morph back into the brown bears from which they came.
Chemical cocktail
Guns and greenhouse gases aren鈥檛 the only hazards humans have chucked at polar bears. Organic chemicals, many now banned from use, still travel on ocean currents and winds into the Arctic, where they are slow to break down. The toxins enter the bottom of the marine food chain and work their way up, through fish to top predators, especially in their blubber.
Right at the top, with a long lifespan and a diet of blubbery seals, polar bears accumulate high concentrations of these toxins. Banned pesticides like DDT and dieldrin, industrial chemicals like polychlorinated biphenyls (PCBs) and organic forms of heavy metals, such as methylmercury, have all been found in exceptionally high concentrations in their body fat.
The consequences are hard to disentangle from other environmental factors, but studies have linked the accumulated poisons to immune system disorders, cancers and reproductive problems ranging from hermaphrodite bear cubs to smaller testes and weakened penis bones in adult males.
One recent study found that polar bear populations were smaller in areas where bears had high levels of poisoning. the toxins are weakening bears that are already often out of condition because of hard times out on the ice. It could be the final nail in their coffin.
This article appeared in print under the headline 鈥淭he survivalists鈥�
