Read more: 鈥�Climate change: It鈥檚 even worse than we thought鈥�
Not so long ago, the Arctic Ocean was covered by thick ice several years old. Even at the end of summer, more than half of the sea surface was still shrouded in ice.
As the world has warmed in the past decades, the winter refreeze has stopped compensating for the summer melt. Heat-reflecting white ice has given way to heat-absorbing dark water; snow has melted ever earlier on surrounding lands; more heat-trapping moisture has entered the atmosphere; and bigger waves and storms have assailed weakening ice. Thanks to these feedback processes, the Arctic has begun to warm twice as fast as any other region on the planet.
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By the late 1990s, the extent of sea ice had fallen to . At the end of this summer, only a quarter of the Arctic Ocean was still covered in ice, a record low in modern times, and (see 鈥淩ecord Arctic ice loss鈥�). What鈥檚 left is a thin layer that melts easily.
It wasn鈥檛 supposed to happen this quickly. In 2007, when the Intergovernmental Panel on Climate Change (IPCC) issued , the consensus was that the Arctic would not be ice-free in summer until the end of the century. We might have been unlucky: natural variability might have accelerated ice loss by pushing old, thick ice out of the Arctic. But climate models clearly underestimated the pace of change, too. , such as the melt ponds on the surface of sea ice that absorb more sunlight. The latest models, which include more processes, still suggest it will be . But if current trends are a reliable guide, such summers will happen within a decade.
However long it takes, the continued ice loss will have many knock-on effects. These could include more extreme weather in the northern hemisphere (see 鈥�Climate downgrade: Extreme weather鈥�), faster melting of the Greenland ice sheet (see 鈥�Climate downgrade: Sea level rise鈥�) and (see 鈥�Climate downgrade: Planetary feedbacks鈥�).
Other, even nastier surprises might also lie in store. at the University of Bristol, UK, points out that relatively small changes in Earth鈥檚 state 鈥� orbital changes, shifting ocean currents, and so on 鈥� have in the past produced abrupt climate changes. Some 5500 years ago, for instance, the lush savannahs and wetlands of northern Africa turned into the Sahara desert over centuries, or perhaps just decades. Older climate models produce such dramatic change only in response to big disturbances. It is not yet clear if the newer models are any better in this respect. , says Valdes.
