Climate change

The prospects for polar bears

Is the polar bear doomed to extinction? Maybe not, according to models of the future extent of Arctic sea ice if greenhouse-gas emissions are curbed. The outlook depends on the ability of policy-makers to act. See Letter p.955

The projected loss of polar bear sea-ice habitat as a result of a warming climate will dramatically reduce the spatial and temporal extent of that habitat by the end of the twenty-first century1. Accordingly, demographic analyses and population-projection models have predicted drastic declines in the polar bear population of the southern Beaufort Sea2. Combined with dire predictions from other regions, these findings led to polar bears being listed as 'Threatened' throughout their range under the US Endangered Species Act. However, the listing process took no account of the potential effects of measures to reduce greenhouse-gas emissions, and so reduce anthropogenic warming. Amstrup et al.3 (page 955) provide such analyses and their results are cause for some optimism — if timely action is taken.

Amstrup and colleagues examined projection models of sea-ice loss based on different greenhouse-gas emission scenarios and found that mitigation could greatly improve the conservation status of polar bears into the next century. Reduced emissions, the lower the better, would yield greater abundance and wider distribution of polar bears than the 'business as usual' emission scenario. Lower levels of warming and sea-ice loss would improve the conservation outlook not just for polar bears, but for other Arctic marine species as well. Nonetheless, most emission scenarios resulted in substantial loss of optimal habitats and a major increase in the ice-free period over the more biologically productive continental shelves. Amstrup et al. also reaffirm earlier findings that emission scenarios with high end-of-century levels of carbon dioxide, a major greenhouse gas, will result in great loss of polar bears.

The best-studied polar bear population is in western Hudson Bay, Canada, and it has shown a slow decline due to the lower survival and reproduction that is correlated with reduced sea-ice duration4. The possible occurrence of 'tipping points' (where rising temperatures trigger a feedback loop that further drives ice loss5), and the summer ice minima6 of 2007, sparked concerns of sudden and irreversible loss of polar bear habitat. Catastrophic shifts have been noted in many different ecosystems7, and in ice-covered Arctic marine ecosystems continued melting of the sea ice could produce episodic changes in predator–prey dynamics and rapid restructuring of the food web8.

Amstrup and colleagues3, however, found no evidence of a critical temperature that resulted in a tipping point. Rather, they found a linear relationship between global mean surface air temperature and sea-ice extent, and that rapid ice loss could partially reverse. But the authors caution that tipping points could occur in the real world and that the situation could still be dire. Although Arctic marine mammals are well adapted to fluctuating environments, and can tolerate substantial inter-annual change, analyses of polar bear energetics show that rapid ice loss could trigger strong nonlinear declines in survival9. Given the low reproductive rates of these animals, even episodic loss of sea ice followed by recovery could have serious effects.

Amstrup et al. also note that the best possible outcomes for polar bears include controlling hunting and other factors in an effort to make populations with the expected lower numbers sustainable. But a ban on hunting would be a serious cultural loss for the Arctic's aboriginal people. There are also other caveats. The authors warn that a lack of data across the full polar bear range (Fig. 1) makes predictions of future abundance tenuous. Further, they stress that the relationships between demographics and sea ice are only partly understood, and that geographical differences in population response to even linear declines in sea ice may vary.

Figure 1: Distribution of the polar bear (Ursus maritimus).
figure1

In this overlay of the Arctic, darker areas denote higher population density, lighter areas lower density. Amstrup and colleagues' model projections3 show that implementation of measures to slow global warming improve the prospects for polar bears. But even with such measures, various factors could still conspire to make their future gloomy.

Increased risk of extinction is often associated with specialization10. So it is not surprising that polar bears — as large predators that rely on sea ice as the platform on which to hunt their seal prey, to mate and to travel — are especially vulnerable. This paper3 provides reason to hope that the previous predictions of declines in polar bear populations can be avoided if concerted efforts are made to reduce greenhouse-gas emissions. The threat posed by climate change to biological diversity has been clear for years, however, and calls for carbon sequestration and reduction of emissions to conserve species11 have largely gone unheeded. Amstrup and colleagues describe polar bears as sentinels of Arctic marine ecosystems and emphasize the importance of sea-ice habitats in the global climate system: both would benefit from greenhouse-gas reduction.

There are few indications, however, that such policies will be implemented in a timely manner. Globally, 25% of mammalian species are threatened with extinction — with habitat loss and degradation being the main causes12. In this context, the plight of polar bears is sadly typical. Their future remains uncertain, but it is now more clearly in the hands of policy-makers. There is cause for optimism, but that requires optimism about our ability to change.

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Derocher, A. The prospects for polar bears. Nature 468, 905–906 (2010). https://doi.org/10.1038/468905a

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