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Global Warming
[This article is extracted from the June 2008 edition of Chiltern News, the magazine of the Chiltern Society.] One element of climate change which is likely to impact most on the Chilterns is rainfall. The emerging message from previous Intergovernmental Panel on Climate Change (IPCC) assessments for south-east England has been ‘wetter winters and drier summers’. Preliminary indications are that further analysis of the latest (2007) IPCC assessment will arrive at the same conclusion. We shall know better towards the end of this year, when the UK Climate Impacts Programme releases the results of its downscaling of the IPCC assessment to regional level (UKCIP08). This mantra has become ingrained in the public awareness and tends to encourage the view, among general public and media alike, that heavier winter rains will be the answer to our problems, sustaining our threatened chalk stream environments and at the same time meeting increased demands on water resources for the public supply. Such impressions lead to complacency and are dangerously misleading: the truth may be very different. There are problems with the simplistic ‘wetter winter’ story: • The IPCC ‘winter’ is the three months of December, January and February. In chalk country autumn rains are equally important for they are needed to overcome end-of-summer soil moisture deficits and to satisfy the demands of vegetation before aquifer replenishment can begin ahead of the main midwinter months. From the water resource point of view we need to look at rainfall over the whole six-month ‘recharge’ period from October to March. This is the water industry ‘winter’. • Rainfall alone is insufficient to specify the availability of water for aquifer recharge. We need to know the ‘effective rainfall’, i.e. that proportion of actual rainfall which penetrates to the aquifer after running the gauntlet of evapotranspiration. As growing seasons lengthen in a warming world more rainfall will be lost to direct evaporation and uptake by plant roots. • UKCIP08 will present the predictions for the remainder of this century as seven overlapping 30-year ‘time slices’. We may expect that the averages for each of these 30-year periods will show an even upward progression, creating the impression of steady increase with time; it will be tempting to draw a straight line to ‘join up these dots’ but British rainfall does not do straight lines – it does zig-zags and wiggles. Inter-annual variability is an enduring and well-recognised characteristic of rainfall in Britain and is likely to remain so. Increasing winter rainfall overall will not preclude multi-seasonal droughts such as we have seen so recently.The story so far
UKCIP predictions will be expressed relative to the standard averaging period 1961-1990. In the same way we can compare post-1990 observed values for the Chilterns with those for the standard period and hence see what has actually happened to date. The table below looks at change for Chilterns East; figures for Chilterns West would not be identical but would show the same general trends. Important points to note are: • Rainfall has increased in all seasons except summer. • A greater increase over the 6-month water industry ‘winter’, mostly attributable to the late-autumn/early-spring ‘transitional’ months where the increase was ~9%. • But only in the midwinter period has percentage effective rainfall increased more than actual rainfall. • Specifically, in the transitional moths (including October and November),in spite of rainfall increasing by 9%, effective rainfall was unchanged. This phenomenon is particularly marked following hotter summers when vegetation demands extend into autumn; after the 2003 record summer, although October/November rainfall was about 90% of average, effective rainfall was only 30% and in 2006 although October/November rainfall was 47% above average, the increase in effective rainfall was a mere 10%. • Most importantly, although there was a general increase in winter period rainfall with time, this 17-year period will be remembered for two serious droughts, 1995-1997 and 2004-2006, attributed to the occurrence of consecutive dry winters. To conclude, the trends witnessed since 1990 are mostly in line with earlier (UKCIP02) predictions, except for autumn rainfall; this has increased against expectations. Nevertheless the observed excess has not been translated into an increase in effective rainfall; the lengthening growing season appears to be narrowing the aquifer recharge window already. Despite any underlying trends, the inherent natural variability of the English climate remains dominant. We should not be lulled into the impression that all winters will be wet. We are not finished with droughts just yet!| Chilterns East | Changes from 1961-1990 to 1991-2007* | ||
| Rainfall | Effective Rainfall | ||
| Percentage change | |||
| Year | Jan-Dec | 3.2 | 2.9 |
| Winter | Oct-Mar | 6.5 | 5.7 |
| Summer | Apr-Sep | -0.1 | -4.3 |
| Midwinter | Dec-Feb | 4.8 | 8.9 |
| Transitional | Oct,Nov,Mar | 9.2 | 0 |