Cold weather: surely this proves the skeptics are right?...er...no

The AGW (man-made global warming) deniers are getting excited about the present cold weather, which is affecting not just Europe, but also the whole Northern hemisphere.

The BBCs David Shukman has an excellent video on what is happening. If you are, like me, impatient with video information, move the slider to 1:06 into the video, where you will see the cold air mass lying around the northern hemisphere.

The north polar jet stream lies at the boundary of the cold arctic air mass and the warmer air masses further south. Between 1979 and 2001 this jet stream has been moving slowly northwards, but in 2007, 2008 and 2009 it has slipped to the south, often positioning itself over the English Channel at 50*N, instead of its customary position of 60*N, north of Scotland.

The current hemispheric cold weather could be the result of two possibilities: either
a) the Arctic is unusually cold, so its cold air has spread south, or
b) the coldness that should be contained in the arctic is "leaking" southwards, which would result in less sea ice, which has a global warming effect through loss of its albedo (reflectivity).

This data seems to support (b), showing that Greenland is up to 7*C warmer than average for this time of year, and parts of Russia up to 8*C colder.

Either way, as Shukman says, there is local or regional weather and there is global climate, and we have to make a distinction between the two.

We will have to wait until all the global temperature observations for 2009 are in before making pronouncements one way or the other about 2009.

Among all the complexities of the debate over climate change, what is clear is that there has been an historic rise in observed global temperatures over the last 150 years, with recent temperatures higher than for many centuries. It is also clear that there has been a slight cooling trend over the past decade, which excites the deniers greatly, since it is reasonably attributable to the our being on the downward side of the sunspot cycle.

The essence of the AGW deniers' argument is to take any single climatic fact or fancy that does not depend on CO2, and claim that this "disproves" the theory that CO2 is the only determinant of global temperatures. However, no climate scientist has ever claimed that CO2 is the only determinant of global temperatures. The temperatures are the result of many factors: solar variation, volcanoes, aerosols, ocean current oscillations, and greenhouse gases. It is a fact that global temperatures over the past 150 years can only be explained by factoring in man-made greenhouse gases.

The real debate should be about the values we should give to these various factors. Climate scientists put a greater value on greenhouse gases, and a lesser value on solar variation. It is asinine to claim "It is all solar", as some deniers do, just as it would be to claim "It is all greenhouse gases", if anyone did, which they do not.

It is devoutly to be hoped that solar variation has been underestimated by the climate models, because that will give us more time to decarbonise the world economy.