Polar bears are safe from the effects of climate change — for now. Credit: Jenny E. Ross/Corbis

Some summer sea ice is likely to persist in the Arctic into the next century, providing a last refuge for polar bears, seals and other animals, researchers reported at the American Geophysical Union meeting in San Francisco, California, this week. But both ice and animals still face multiple threats — from oil spills and other pollution to extinction through cross-breeding between distinct animal populations.

Stephanie Pfirman, an environmental scientist at the Lamont-Doherty Earth Observatory in New York, and her colleagues are to present climate models at the meeting tomorrow that predict sea ice will continue to pile up on the northern side of the Canadian Arctic Archipelago and Greenland, where the thickest sea ice exists today. Some of this ice is formed locally, and some is driven in from Siberia by wind and ocean currents. Pfirman estimates that an area of ice perhaps half a million square kilometres in size is likely to persist year-round long into the twenty-first century.

Although the amount of ice that melts each summer is increasing, ice is still forming in the winter, and it is being transported to the Canadian side of the Arctic faster than before because the waters are more open. "If it used to take 8 or 9 years to make the trip, it might now do it in 7 years," says Robert Newton, a geochemist also at Lamont-Doherty.

A paper published in Nature today1 brings more good news for Arctic ice. Steven Amstrup of the US Geological Survey in Anchorage, Alaska, and his colleagues looked at models of future sea ice circulation and found no evidence of a 'tipping point' of warming beyond which the ice will disappear irreversibly. So bringing greenhouse-gas emissions under control, they write, should help to preserve polar-bear habitat and Arctic ecosystems at large.

Refuges under threat

Some spots in the Arctic are predicted to stay ice-covered for longer than their surroundings. Melanie Smith, a landscape ecologist with conservation group Audubon Alaska in Anchorage who has compiled an atlas of Arctic waters but was not at this week's meeting, says that there are two areas of shallow water in the Chukchi Sea between Siberia and Alaska that are protected from melting until late summer each year: Hanna Shoal in US waters and Herald Shoal in Russian waters. Warm currents from the south are diverted around these 10,000-square-kilometre shoals, making them 1–2 ºC cooler than their surrounding waters.

Smith notes that the oil company Shell, based in The Hague, The Netherlands, applied for a permit to drill in the Hanna Shoal area in 2010, although the project has stopped moving forwards since the US government clamped down on offshore oil drilling after the Gulf of Mexico spill in April. Smith doesn't know whether a rig itself would have an impact on wildlife seeking refuge on ice in a shoal, but says that it would be disastrous for an oil spill to occur in the only remaining suitable patch of habitat for local animals such as walruses.

Audubon is "not trying to stop all drilling, but there are absolutely places that shouldn't be touched", says Smith. "Hanna Shoal is on that list."

Dangerous liaisons

Newton and Pfirman say that just taking measures to protect the ultimate last refuges of ice isn't enough — it is also important to ensure that increasing industrial activity in the Arctic does not impede the transport of ice from Asia towards Canada, for example, or deliver pollutants from afar. "The ice that's supporting it comes from a large area," says Newton. The researchers are calling for international collaboration to protect this ice.

Even if pockets of Arctic ice that are suitable for polar bears and seals are preserved, and stay pollution-free, the animals still face another threat: hybridization.

Shrinking and shifting habitats are bringing polar bears that have strayed south into contact with grizzly bears, forcing seals to share smaller areas with each other, and allowing whales from the Atlantic and Pacific oceans to meet in ice-free Arctic waters, says Brendan Kelly, a marine biologist at the US National Oceanic and Atmospheric Administration who is based in Juneau, Alaska. This makes it easier for species and populations to cross-breed2, which could drive some endangered species, including polar bears, to extinction.