Climate change has reduced sea ice in the Arctic to record lows in the last year and Dr Ian Stirling, who has studied the bears for almost 40 years and examined the animal, said the lack of ice forced the bear into ranging far and wide in an ultimately unsuccessful search for food.
"From his lying position in death the bear appears to simply have starved and died where he dropped," Stirling said. "He had no external suggestion of any remaining fat, having been reduced to little more than skin and bone."
The bear had been examined by scientists from the Norwegian Polar Institute in April in the southern part of Svalbard, an Arctic island archipelago, and appeared healthy. The same bear had been captured in the same area in previous years, suggesting that the discovery of its body, 250km away in northern Svalbard in July, represented an unusual movement away from its normal range. The bear probably followed the fjords inland as it trekked north, meaning it may have walked double or treble that distance.
Polar bears feed almost exclusively on seals and need sea ice to capture their prey. But 2012 saw the lowest level of sea ice in the Arctic on record. Prond Robertson, at the Norwegian Meteorological Institute, said: "The sea ice break up around Svalbard in 2013 was both fast and very early." He said recent years had been poor for ice around the islands: "Warm water entered the western fjords in 2005-06 and since then has not shifted."
Stirling, now at Polar Bears International and previously at the University of Alberta and the Canadian Wildlife Service, said: "Most of the fjords and inter-island channels in Svalbard did not freeze normally last winter and so many potential areas known to that bear for hunting seals in spring do not appear to have been as productive as in a normal winter. As a result the bear likely went looking for food in another area but appears to have been unsuccessful."
Research published in May showed that loss of sea ice was harming the health, breeding success and population size of the polar bears of Hudson Bay, Canada, as they spent longer on land waiting for the sea to refreeze. Other work has shown polar bear weights are declining. In February a panel of polar bear experts published a paper stating that rapid ice loss meant options such the feeding of starving bears by humans needed to be considered to protect the 20,000-25,000 animals thought to remain.
The International Union for the Conservation of Nature, the world's largest professional conservation network, states that of the 19 populations of polar bear around the Arctic, data is available for 12. Of those, eight are declining, three are stable and one is increasing.
The IUCN predicts that increasing ice loss will mean between one-third and a half of polar bears will be lost in the next three generations, about 45 years. But the US and Russian governments said in March that faster-than-expected ice losses could mean two-thirds are lost.
Attributing a single incident to climate change can be controversial, but Douglas Richardson, head of living collections at the Highland Wildlife Park near Kingussie, said: "It's not just one bear though. There are an increasing number of bears in this condition: they are just not putting down enough fat to survive their summer fast. This particular polar bear is the latest bit of evidence of the impact of climate change."
Ice loss due to climate change is "absolutely, categorically and without question" the cause of falling polar bear populations, said Richardson, who cares for the UK's only publicly kept polar bears. He said 16 years was not particularly old for a wild male polar bear, which usually live into their early 20s. "There may have been some underlying disease, but I would be surprised if this was anything other than starvation," he said. "Once polar bears reach adulthood they are normally nigh on indestructible, they are hard as nails."
Jeff Flocken, at the International Fund for Animal Welfare, said: "While it is difficult to ascribe a single death or act to climate change it couldn't be clearer that drastic and long-term changes in their Arctic habitat threaten the survival of the polar bear. The threat of habitat loss from climate change, exacerbated by unsustainable killing for commercial trade in Canada, could lead to the demise of one of the world's most iconic animals, and this would be a true tragedy.