ON SEPTEMBER 9th, at the height of its summertime shrinkage, ice covered 4.33m square km, or 1.67m square miles, of the Arctic Ocean, according to America's National Snow and Ice Data Centre (NSIDC). That is not a record low—not quite. But the actual record, 4.17m square km in 2007, was the product of an unusual combination of sunny days, cloudless skies and warm currents flowing up from mid-latitudes. This year has seen no such opposite of a perfect storm, yet the summer sea-ice minimum is a mere 4% bigger than that record. Add in the fact that the thickness of the ice, which is much harder to measure, is estimated to have fallen by half since 1979, when satellite records began, and there is probably less ice floating on the Arctic Ocean now than at any time since a particularly warm period 8,000 years ago, soon after the last ice age.

That Arctic sea ice is disappearing has been known for decades. The underlying cause is believed by all but a handful of climatologists to be global warming brought about by greenhouse-gas emissions. Yet the rate the ice is vanishing confounds these climatologists' models. These predict that if the level of carbon dioxide, methane and so on in the atmosphere continues to rise, then the Arctic Ocean will be free of floating summer ice by the end of the century. At current rates of shrinkage, by contrast, this looks likely to happen some time between 2020 and 2050.

The reason is that Arctic air is warming twice as fast as the atmosphere as a whole. Some of the causes of this are understood, but some are not. The darkness of land and water compared with the reflectiveness of snow and ice means that when the latter melt to reveal the former, the area exposed absorbs more heat from the sun and reflects less of it back into space. The result is a feedback loop that accelerates local warming. Such feedback, though, does not completely explain what is happening. Hence the search for other things that might assist the ice's rapid disappearance.

Forcing the issue

One is physical change in the ice itself. Formerly a solid mass that melted and refroze at its edges, it is now thinner, more fractured, and so more liable to melt. But that is (literally and figuratively) a marginal effect. Filling the gap between model and reality may need something besides this.

The latest candidates are “short-term climate forcings”. These are pollutants, particularly ozone and soot, that do not hang around in the atmosphere as carbon dioxide does, but have to be renewed continually if they are to have a lasting effect. If they are so renewed, though, their impact may be as big as CO 2 's.

At the moment, most eyes are on soot (or “black carbon”, as jargon-loving researchers refer to it). In the Arctic, soot is a double whammy. First, when released into the air as a result of incomplete combustion (from sources as varied as badly serviced diesel engines and forest fires), soot particles absorb sunlight, and so warm up the atmosphere. Then, when snow or rain wash them onto an ice floe, they darken its surface and thus cause it to melt faster.

Reducing soot (and also ozone, an industrial pollutant that acts as a greenhouse gas) would not stop the summer sea ice disappearing, but it might delay the process by a decade or two. According to a recent report by the United Nations Environment Programme, reducing black carbon and ozone in the lower part of the atmosphere, especially in the Arctic countries of America, Canada, Russia and Scandinavia, could cut warming in the Arctic by two-thirds over the next three decades. Indeed, the report suggests, if such measures—preventing crop burning and forest fires, cleaning up diesel engines and wood stoves, and so on—were adopted everywhere they could halve the wider rate of warming by 2050. Without corresponding measures to cut CO 2 emissions, this would be but a temporary fix. Nonetheless, it is an attractive idea because it would have other benefits (soot is bad for people's lungs) and would not require the wholesale rejigging of energy production which reducing CO 2 emissions implies. Not everyone agrees it would work, though. Gunnar Myhre of the Centre for International Climate and Environmental Research in Oslo, for example, notes that the amount of black carbon in the Arctic is small and has been falling in recent decades. He does not believe it is the missing factor in the models. Carbon dioxide, in his view, is the main culprit. Black carbon deposited on the Arctic snow and ice, he says, will have only a minimal effect on its reflectivity. The rapid melting of the Arctic sea ice, then, illuminates the difficulty of modelling the climate—but not in a way that brings much comfort to those who hope that fears about the future climate might prove exaggerated. When reality is changing faster than theory suggests it should, a certain amount of nervousness is a reasonable response. It's an ill wind…

The direct consequences of changes in the Arctic are mixed. They should not bring much rise in the sea level, since floating ice obeys Archimedes's principle and displaces its own mass of water. A darker—and so more heat-absorbent—Arctic, though, will surely accelerate global warming and may thus encourage melting of the land-bound Greenland ice sheet. That certainly would raise sea levels (though not as quickly as News Corporation's cartographers suggest in the latest edition of the best-selling “Times Atlas”, which claims that 15% of the Greenland sheet has melted in the past 12 years; the true figure is more like 0.05%). Wildlife will also suffer. Polar bears, which hunt for seals along the ice's edge, and walruses, which fish there, will both be hard-hit.

Watch our animation of the receding Arctic ice-shelf and the shipping routes it could unlock

The effects on the wider climate are tricky to assess. Some meteorologists suspect unseasonal snow storms off the east coast of America in 2010 were partly caused by Arctic warming shifting wind patterns. One feedback loop that does seems certain, though, is that the melting Arctic will enable the extraction of more fossil fuel, with all that that implies for greenhouse-gas emissions.

The Arctic is reckoned to hold around 15% of the world's undiscovered oil reserves and 30% of those of natural gas. Hence a growing polar enthusiasm among energy companies—as witnessed last month in an Arctic tie-up between Exxon Mobil, of America, and Rosneft, Russia's state-controlled oil giant. Recent plankton blooms suggest a warmer Arctic will provide a boost to fisheries there, too. And the vanishing ice has begun to allow a trickle of shipping across the Arctic's generally frozen north-west and north-east passages, thus linking the Atlantic and Pacific oceans. In August a Russian supertanker, the Vladimir Tokohonov, aided by two nuclear icebreakers, became the first such vessel to cross the north-east route (or, as Russians refer to it, the northern sea route), hugging the Siberian coast.

So far, despite some posturing by Canada and Russia, there are few territorial disputes in the region and the Arctic Council, the club of Arctic nations, has functioned reasonably well. Whether the interests of these countries coincide with those of the wider world, though, is moot. A warming Arctic will bring local benefits to some. The rest of the world may pay the cost.