Atmospheric Administration, blocked the usual pattern and “allowed cold air from the Arctic to penetrate all the way into Europe, eastern China, and Washington DC”. Nasa reports that the same thing is happening this winter.
Sea ice in the Arctic has two main effects on the weather. Because it’s white, it bounces back heat from the sun, preventing it from entering the sea. It also creates a barrier between the water and the atmosphere, reducing the amount of heat that escapes from the sea into the air. In the autumns of 2009 and 2010 the coverage of Arctic sea ice was much lower than the long-term average: the second smallest, last month, of any recorded November. The open sea, being darker, absorbed more heat from the sun in the warmer, light months. As it remained clear for longer than usual it also bled more heat into the Arctic atmosphere. This caused higher air pressures, reducing the gradient between the Iceland low and the Azores high.
So why wasn’t this predicted by climate scientists? Actually it was, and we missed it. Obsessed by possible changes to ocean circulation (the Gulf Stream grinding to a halt), we overlooked the effects on atmospheric circulation. A link between summer sea ice in the Arctic and winter temperatures in the northern hemisphere was first proposed in 1914. Close mapping of the relationship dates back to 1990, and has been strengthened by detailed modelling since 2006.
Will this become the pattern? It’s not yet clear. Vladimir Petoukhov of the Potsdam Institute says that the effects of shrinking sea ice “could triple the probability of cold winter extremes in Europe and northern Asia”. James Hansen of Nasa counters that seven of the last 10 European winters were warmer than average. There are plenty of other variables: we can’t predict the depth of British winters solely by the extent of sea ice.
I can already hear the howls of execration: now you’re claiming that this cooling is the result of warming! Well, yes, it could be. A global warming trend doesn’t mean that every region becomes warmer every month. That’s what averages are for: they put local events in context. The denial of man-made climate change mutated first into a denial of science in general and then into a denial of basic arithmetic. If it’s snowing in Britain, a thousand websites and quite a few newspapers tell us, the planet can’t be warming.
According to Nasa’s datasets, the world has just experienced the warmest January to November period since the global record began, 131 years ago; 2010 looks likely to be either the hottest or the equal hottest year. This November was the warmest on record.
Sod all that, my correspondents insist: just look out of the window. No explanation of the numbers, no description of the North Atlantic oscillation or the Arctic dipole, no reminder of current temperatures in other parts of the world, can compete with the observation that there’s a foot of snow outside. We are simple, earthy creatures, governed by our senses. What we see and taste and feel overrides analysis. The cold has reason in a deathly grip.