Over the past 20,000 years, there has been a strong correlation between the number of polar bears in Greenland and surface water temperatures, the amount of sea ice and the amount of suitable habitat. This shows that polar bears are vulnerable to climate change – but also that they show great resilience when conditions improve. This is written by a large international team of scientists this week Science Advances.
It has been a heated debate for years: are polar bears doomed if the sea ice disappears? Or can they also survive on land, from other food sources? Studies regularly appear that lean more towards one conclusion than the other.
The Arctic heats up about two to four times faster than the rest of the earth. As a result, the ice on the Arctic Ocean is melting rapidly. Space Agency ESA calculated that the Arctic Ocean will be virtually ice-free in summer around the year 2050. That’s a problem for polar bears. They mainly hunt seals. They catch them from the edge of the sea ice, in places where seals rest or feed their newborn young. Without sea ice, there will be no polar bears, which is why it is a frequently heard idea.
Historical climate data
But polar bears also have other ways to get food. They can also catch seals in areas where glacial ice is crumbling into the sea, thus Science in 2022. Moreover, polar bears are opportunists: they quickly switch to new food sources. In Canada and Siberia, for example, groups of polar bears live on land and eat mainly plant foods such as berries, tubers and roots, but also prey such as reindeer and geese, plus carrion and eggs. Polar bears are also no strangers to foraging on rubbish dumps.
But is all that enough to keep the species going when there is no sea ice in the summer? Opinions differ on this. Into the study Science Advances is now making a new contribution. The researchers – from Denmark, Australia, Canada, Hong Kong, Finland, the United Kingdom and Greenland – looked in detail at specific genetic variations among more than a hundred polar bears from East and West Greenland. Using a computer model, they deduced which “evolutionary paths” the ancestors of these bears had followed. These analyzes also show how large the populations were in the past. They correlated all that data with historical climate data.
The authors do not take a clear position on the fate of the polar bears. The article does tend towards a gloomy conclusion, given the strong correlation between the number of polar bears and especially the amount of sea ice. Although the researchers mention the strong resilience of the species, they still see long-term, strong climate change as a major threat.
The arrival of hunters
Maarten Loonen, ecologist at the Arctic Center of the University of Groningen, has been following this issue for years. He also read this new article. “This study is very complex,” he responds. “The authors combine all kinds of things, but some things do not – and I think the conclusions are therefore very strong. For example, wouldn’t it have been nice if they had also looked at changes in seal populations using DNA techniques?”
In addition, Loonen is surprised that the article nowhere mentions the arrival of hunters in Greenland from about 2,500 years ago. “Hunting drives polar bears onto the sea ice,” says Loonen. “We also see that in Spitsbergen. In this Greenland study I think I see an effect on the population size, at a time when the habitat did not change that much.”
With regard to the future perspective, according to Loonen, something else is also relevant: “The biggest question for the polar bear will be whether it will be tolerated if it comes on land more.”