Climate Change and the Summer 2007 Floods in the UK
Lane, Stuart N., Geography
We have always bemoaned the wetness of the 'Great British Summer'. The summer of 2007 was, without a doubt, one of the wettest on record, but we have had wet summers before, indeed some quite recently. Analysis of the Meteorological Office's rainfall series for 1914-2007 for the United Kingdom (Figure 1) shows that in about 20% of years the UK's summer rainfall was greater than the median winter and autumn rainfalls. However, the UK's summer rainfall for 2007 (357.1mm) was the second highest on record, and only marginally less than in the wettest year (1956, 358.4mm). But one thing stands out in relation to 2007: the way in which, in many different areas of life, the summer of 2007 has become linked to the belief that our climate is changing. Consider, for instance, Prime Minister's Question Time on 25 July 2007, immediately in the aftermath of the flooding in Central England (Hansard, Volume 463, Part 130, Column 834):
Sir Menzies Campbell: 'The Prime Minister was responsible for the establishment of the Stern review, which he will recall pointed out the severe economic consequences of climate change. Is it not clear from the events of the past few weeks that we cannot afford not to take the necessary steps or indeed, not to spend the necessary money, in order to mitigate the effects of climate change?'
The Prime Minister: 'The right hon. and learned Gentleman is right. The Stern report, which the Treasury commissioned, said that global warming is very likely to intensify the water cycle and increase the risk of floods. It is an accepted part of the Stern recommendations that we have to do more ...'
In this article, I think through the relationship between climate change and the UK floods of 2007 and, in so doing, show that in order to understand the range of scales of climatic variability upon which any kind of climate change signal will be superimposed, we need to take a more historical perspective which has a much stronger grounding in the atmospheric and oceanic drivers of the UK's weather and climate.
The weather 'conveyor belt'
The UK's 'weather' is driven in the main by weather systems originating in the Atlantic. A complex interaction between high and low pressure systems, and the associated 'conveyor belt' of the Circumpolar Vortex, or 'jet stream', acts to determine whether the UK is influenced by 'high pressure', or 'low pressure' systems (Figure 2). The jet stream is a current of fast moving air, thousands of kilometres long, hundreds of kilometres wide and a few kilometres deep. It is a dynamic feature, spatially and temporally, bounding polar air on the north side and cooler air on the equatorward side. There are three primary dynamical aspects:
The strength of the jet stream is primarily a function of the temperature gradient between the equator and the pole. This gradient is weakest in the summer, and therefore the jet stream is commonly weaker in that season. In the winter, the gradient is strongest and the jet stream is commonly stronger. Its average position migrates northwards during spring, to lie north of the UK in a typical summer. It migrates southwards during autumn to lie south of the UK in a typical winter. Its sinuosity varies from weak to strong. When the sinuosity is strong, of particular importance is the position of ridges and troughs (Figure 2a). When a trough is positioned close to the south or south-west of the UK, there is generally divergent air aloft, leading to rising air and low pressure at the surface. This draws in air which tends to rotate counter clockwise, leading to cyclonic weather systems, called extratropical cyclones in the mid-Latitudes (Figure 2b). The development of a cyclone is called cyclogenesis, and this pulls warm tropical air northwards and cool polar air southwards, with the air masses mixing along fronts. As the warm air moves northwards it encounters cooler polar air and is forced to rise, causing condensation and precipitation as it does so, along what is called a warm front. …