A flash flood in the pan or a rainstorm caused by global warming?

Increasingly severe summer rainstorms like the one which led to the flood that devastated Boscastle are beginning to suggest the influence of global warming, a flood scientist said yesterday.

Increasingly severe summer rainstorms like the one which led to the flood that devastated Boscastle are beginning to suggest the influence of global warming, a flood scientist said yesterday.

Climate change is predicted by computer models to cause more violent summer downpours in Britain, but such is the variability of our climate that no single storm can be taken as conclusive evidence of it. However, the increasing occurrence of such events is now starting to point to global warming as the ultimate cause, said Alan Werrity, professor of physical geography at the University of Dundee and an adviser to the Scottish Executive on flood management.

"One can't point the finger at a particular event and say, this proves it," he said. "However, it is consistent with it. And taking event after event, the more frequent and more intense storms affecting Britain suggest that it is very likely that climate change is beginning to register."

While the popular conception of global warming is of increased heat, it is increased precipitation that may be of much more consequence for Britain. The predictions from computer models such as that run by the Met Office's Hadley Centre for Climate Prediction and Research at Exeter are that as the century progresses, Britain will experience increasing winter rainfall, especially in the North and West, and summer rainstorms of increasing force all across the country.

Britain is in a meteorological "battle zone", where a mass of warm air coming up from the tropics meets a mass of cold air coming down from the North Pole, Professor Werrity said. With global warming, the intensity of the encounter between these two converging air systems is predicted to increase, and more storms will be generated. The warmer the air, the more moisture it can hold - which will eventually fall as rain - and the more dynamic are the atmospheric processes. The result: a soggy future for Britain.

We will often get very wet in the summer, Professor Werrity said, but more seriously, cities which will experience many more intense storms than they have in the past will be unable to cope with the run-off from the streets. "The Victorian sewer system we have wasn't designed to cope with the rainfall intensity we are experiencing today and which will become even more violent in the future," he said. "The urban drainage can't evacuate the run-off from the storms, and in a city if you can't get rid of the water quickly enough through the drains, then the drains back up."

This happened in London two weeks ago, leading to a massive discharge of untreated sewage into the Thames, which killed thousands of fish. But re-engineering Britain's Victorian sewer system would cost billions.