Intensity of extreme rainfall in the UK may increase
9 March 2009
Extreme rainfall - the type that only occurs once every decade or so - is likely to become more intense across the UK, according to new research.
A shopper is caught in a torrential downpour.
This could lead to more flooding in autumn, winter and spring by the 2080s particularly in the north and west.
As temperatures rise, the global water cycle is changing - warmer air can hold more moisture. Pinning down what this means for countries around the world is a priority for climate scientists and governments.
Climate researchers Dr Hayley Fowler from Newcastle University and Dr Marie Ekstrom from the University of Exeter investigated two types of heavy rainfall: one-day downpours that can cause flash flooding; and heavy rainfall over five days that quickly saturates the ground and may lead to widespread flooding. They wanted to see how these two types of rainfall are projected to change in intensity towards the end of the century, between 2070 and 2100.
The scientists, funded by the Natural Environment Research Council, looked at how 13 different climate models simulated these two types of extreme rainfall across nine regions of Britain. They compared the model results to actual rainfall measurements between 1961 and 1990 over the four seasons.
Models that could replicate measured rainfall patterns across the nine regions were then used to project future changes, and the results compared.
The results indicate that if global greenhouse gas emissions follow a for a 'medium-high' trajectory, that is, some action is taken to reduce emissions but action falls short of the large-scale cuts, then the type of heavy rainfall the UK only sees on average once every five years and once every 25 years will become more intense.
'The major finding is that in winter, spring and autumn almost all models project increases in the intensity of extreme rainfall,' says Fowler who points out they did not examine if extreme rainfall would become more frequent.
The researchers could not detect a common pattern for extreme rainfall in summer. This is probably because the grid cells in the climate models' are too large (50-kilometre-by-50-kilometre) to reproduce the much smaller thunderstorms common in summer.
'The models indicate the north-west could see very large increases in extreme rainfall intensity in autumn - up to 60 per cent,' says Fowler, who publishes the results in The International Journal of Climatology.
Across the UK, the amount of rain falling during one of these extreme events is likely to increase by up to 30 per cent by 2080.
In autumn and winter, the intensity of one day downpours is predicted to increase in all regions, with Scotland and northern England seeing changes of more than 20 per cent.
'The south and east may not see such large changes but it is still a significant 10-20 per cent increase,' added Fowler.
'We have least confidence in the models' predictions for the summer months and it is still highly uncertain how summer flash flooding such as the Hull, Hereford and Worcester floods in 2007 will change.'
The climate models the team used cover the whole of Europe. But only the UK with its comprehensive records stretching back to 1961 could provide the kind of regional detail the scientists required.
Reliable regional climate predictions, particularly regarding rainfall, are notoriously difficult to produce. But they are essential for governments and local authorities who need to plan for change. In the coming years, as scientists improve climate models, and have more access to supercomputers to run their models, they expect to improve the accuracy of predictions. In February, the Natural Environment Research Council launched the £10 million changing water cycle programme to help reduce uncertainty
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