Wednesday 5 January 2011

It’s a bit cold this winter – has climate change ended?

The appearance of a bit of snow over the Christmas break has brought out the usual questions about whether climate change is actually happening or not. Well, with unusually cold weather, as much with unusually warm weather, a few local and short term events do not alter the clear and increasingly firm evidence of global warming. You can’t draw conclusions, in any direction, from one or two events.

Indeed the Met Office holds this view regarding our recent couple of years of cold winters. This is just normal variation and is nothing to do with global warming. I am sure this is true, but I think it is quite possible to construct an argument linking this to global climate change. Furthermore this is not to suggest that climate change is not happening but on the contrary may actually be one of its effects.

I stress that this is almost certainly untrue and I will say why later but the argument could go as follows:

In 2007 and again in 2010 the arctic lost huge amounts of ice to the sea. You can look at the evidence for this at the brochure on the Met Office web site at:
This ice loss did not fully recover in subsequent years after 2007. This would mean that large amounts of fresh water would have flowed into the arctic sea so reducing the salinity of the sea at that point. This in turn has the effect of weakening the Gulf Stream, and as we all know, it is the Gulf Stream that brings warm weather up to our islands.

So, global warming has melted ice which could now be weakening the Gulf Stream reducing the tendency for warm weather systems to come in from the south-west and allowing cold polar air to penetrate from the north.

In addition, we also know that the poles are warming up faster than average. The average global temperature increase in the last century has been about 0.8 degrees but at the poles it has been nearer 4 degrees. Furthermore we also know that unusually warm air in the stratosphere over the North Pole in 2009/10 had a knock-on effect of reversing the normal direction of wind over the UK – changing it from warm western winds to cold eastern winds.

So again the polar weather system has become more active and as a result could be pushing cold polar air further south.

It’s interesting to look at the weather maps sometimes and try to guess where the “jet stream” (a narrow band of high speed air marking the boundary between polar weather systems and our temperate weather systems) sits. My perception is that it often sits further south than it did in the past. Even in summer it seems that a run of several weeks of good weather suddenly collapses sometime in July as the jet stream, which should sit over Scotland, suddenly shifts south to run over the Bay of Biscay, and we get nothing but rain! Again is this all a possible effect of more active polar air pushing further south?

At the end of all this, however, it is probably far more likely that, as the Met Office suggests, this is just part of natural variation. All these effects do just happen and are not exceptional or part of a trend. For instance melting ice switching off the Gulf Stream has happened in the past, about 10,000 years ago, but it took truly huge amounts of ice when most of North America was covered in an ice sheet which all suddenly fell into the sea at once. And our ice loss, huge though it is, is nothing like on the same scale.

I’ll be interested if these effects keep happening though!

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