That snow outside is what global warming looks like

Unusually cold winters may make you think scientists have got it all wrong. But the data reveal a chilling truth. There is now strong evidence to suggest that the unusually cold winters of the last two years in the UK are the result of heating elsewhere.

With temperatures in some parts of the UK and Ireland set to reach as low as 28°C on Monday night, many have been wondering why this winter has been especially harsh. According to NASA the reason is a natural shift the the location of the Gulf Stream called ‘Negative Arctic Oscillation’.

Writing on their Earth Observatory site NASA describes how the Gulf Stream, which usually bring mild air from the Mexican Gulf to the British Isles, has shifted from its usual path due to a belt of high pressure sitting in the mid-Atlantic. This oscillation has forced the stream further north bathing western Canada and southern Greenland in unusually warm weather while leaving Ireland, the UK and Northern Europe freezing cold.

The Arctic Oscillation is a climate pattern that influences winter weather in the northern hemisphere. It describes the relationship between high pressure in the mid-latitudes and low pressure over the Arctic. When the pressure systems are weak, the difference between them is small, and air from the Arctic flows south, while warmer air seeps north. This is referred to as a negative Arctic Oscillation. Like December 2009, the Arctic Oscillation was negative in early December 2010. Cold air from the Arctic channeled south around a blocking system over Greenland, while Greenland and northern Canada heated up.

The unusual cold brought heavy snow to Northern Europe, stopping flights and trains (right) early in December. Cold temperatures and snow also closed roads and schools in the eastern United States and Canada during the first week of December. The diagonal path of a powerful winter storm is visible as a streak of cold across the Upper Midwest of the United States.

The global temperature maps published by NASA present a striking picture. Last month's shows a deep blue splodge over Iceland, Spitsbergen, Scandanavia and the UK, and another over the western US and the eastern Pacific. Temperatures in these regions were between 0.5C and 4C colder than the November average from 1951 and 1980. But on either side of these cool blue pools are raging fires of orange, red and maroon: the temperatures in western Greenland, northern Canada and Siberia were between 2C and 10C higher than usual. NASA's Arctic oscillations map for 3-10 December (left) shows that parts of Baffin Island and central Greenland were 15C warmer than the average for 2002-9. There was a similar pattern last winter. These anomalies appear to be connected.

The weather in UK winters, for example, is strongly linked to the

The North Atlantic Oscillation also was in a negative phase: i.e. above-normal pressure over Iceland; below-normal pressure over the Azores; in Northern Europe this is usually associated with below-normal temperatures.

contrasting pressure between the Icelandic low and the Azores high. When there's a big pressure difference the winds come in from the south-west, bringing mild damp weather from the Atlantic. When there's a smaller gradient, air is often able to flow down from the Arctic. High pressure in the icy north last winter, according to the US National Oceanic and 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. 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 modeling 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 (right) 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.

Climate change skeptics voice howls of execration at this theory: 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.

Still global warming skeptics say to 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.

Source:
The Guardian, "That snow outside is what global warming looks like", by George Monbiot, accessed December 22, 2010
The Sociable, "NASA explains why there has been so much snow this winter", accessed December 22, 2010