A well-fed male polar bear, Chukchi Sea, Alaska. Polar bears eat the fattiest diet of any mammal and store fat for fasting; going without food for 4 months or more is likely to harm their reproduction and survival. USGS.
The Arctic is warming faster than the rest of the world, because melting snow and ice let the region absorb more of the sun's energy. The most visible result is the rapid decline of sea ice over the last 3 decades, which is projected to bring long ice-free summers unless greenhouse gas emissions are substantially reduced.
Polar bears (Ursus maritimus) hunt seals from sea ice over the Arctic Ocean's productive continental shelves. In 2008 the U.S. Fish and Wildlife Service listed them as threatened under the Endangered Species Act because of sea ice loss, the first species listed because of forecast declines from climate change.
A forecasting model
To guide recovery planning, the U.S. Geological Survey updated a model that forecasts polar bears' future in each of the four ecoregions of their circumpolar range. The first version projected a high chance that bears would be lost by mid-century from two ecoregions holding about two-thirds of the world's polar bears.
Polar bear ecoregions. In the seasonal ice ecoregion the ice melts completely in summer and all bears must go ashore; in the divergent ice ecoregion the ice pulls away from the coast, so bears go ashore or follow it north; in the convergent and archipelago ecoregions ice generally lasts through summer. USGS.
The new model (Atwood and others, 2015) weighed many threats and management options separately. Where data were scarce, as for trans-Arctic shipping and disease, it relied on expert opinion; for most threats, including sea ice loss, data informed it. It used the Intergovernmental Panel on Climate Change's sea ice projections under two pathways: stabilised, where new policies and technology reduce emissions, and unabated, where emissions keep rising at current rates.
If carbon dioxide emissions were substantially reduced (RCP 4.5), radiative forcing would stabilise; if they continue unabated (RCP 8.5), so does warming. USGS.
Greenhouse gases remain the main threat
- Outcomes for polar bears worsen through the end of the century under both pathways.
- Under the unabated pathway, populations in two ecoregions reach a "greatly decreased" state about 25 years sooner.
- Under the stabilised pathway, where emissions peak around 2040 and then fall fast, only the archipelago ecoregion never reaches a high probability of "greatly decreased", reinforcing its promise as a refuge for ice-dependent life.
- The strongest drivers of bad outcomes were losses of sea ice and of the marine prey base. Reducing deaths from hunting and defence of life helped only slightly; reducing shipping, oil and gas activity, disease and contaminants made almost no difference.
Forecast outcomes through the 21st century for each ecoregion under the stabilised (4.5) and unabated (8.5) pathways, as defined in the IPCC's Fifth Assessment Report (2014). USGS.
Little resilience to ice loss
The harm grew sharply once the summer ice-free period passed 4 months, which could happen across much of the Arctic basin in the second half of this century on the unabated pathway. Without summer ice, bears must come ashore, where they can rarely reach their main prey, ice seals, and land foods are unlikely to help populations adapt.
A global solution
Managing other threats may slow the decline, but cutting emissions is what matters: keeping human-caused radiative forcing under 4.5 watts per square meter (the stabilised pathway) reduces the threat to the species' survival, and the most optimistic outlook needs immediate cuts to keep it under 3.5.
Sources
- K. Oakley, T. Atwood, D. Douglas, K. Rode and M. Whalen, Changing Arctic Ecosystems—Updated forecast: Reducing carbon dioxide (CO2) emissions required to improve polar bear outlook, U.S. Geological Survey Fact Sheet 2015–3042. https://pubs.usgs.gov/publication/fs20153042
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