Arctic sea ice is melting far faster than climate models predict. Why?
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 CO2’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.