It's official: the heavier rainfall in Britain is being caused by climate change, a major new scientific study will reveal this week, as the country reels from summer downpours of unprecedented ferocity. More intense rainstorms across parts of the northern hemisphere are being generated by man-made global warming, the study has established for the first time ­ an effect which has long been predicted but never before proved. The study's findings will be all the more dramatic for being disclosed as Britain struggles to recover from the phenomenal drenching of the past few days, during which more than a month's worth of rain fell in a few hours in some places, and floods forced thousands from their homes. The "major rainfall event" of last Friday ­ fully predicted as such by the Met Office ­ has given the country a quite exceptional battering, with the Thames still rising. In Gloucester water levels had reached 34 feet, just 12 inches below flood defences ­ the same level as during the flood of 1947 ­ although a police spokesman said last night that the River Severn had stopped rising.

Last night vast areas of the country around Gloucestershire and Worcestershire were still inundated, large numbers of people in temporary accommodation, transport links were widely disrupted, and yet more householders were standing by to be flooded in their turn, in one of the biggest civil emergencies Britain has seen. About 150,000 residents in Gloucestershire were left without drinking water when the Mythe Water Treatment Works in Tewkesbury became inoperable after flooding. Another 200,000 people are at risk of losing their supplies. The water shortages may last until Wednesday and 600 water tanks were being drafted to the area. Panic buying of bottled water was reported, with supermarkets selling out of stocks, and there were contamination problems in south London, where 80,000 households and businesses in the Sutton area were advised to boil their water after rain got into a tank. Yet another potential danger was from car thieves; West Mercia police warned drivers who had abandoned their cars in the floodwater to collect them quickly to prevent theft.

The Great Flood of July is all the more remarkable for following right on from the Great Flood of June, which caused similar havoc in northern towns such as Doncaster and Hull, after a similar series of astonishingly torrential downpours on 24 June. Meteorologists agree that the miserably wet British summer of 2007 has generally been caused by a southward shift towards Britain of the jetstream, the high-level airflow that brings depressions eastwards across the Atlantic. This is fairly normal. But debate is going on about whether climate change may be responsible for the intensity of the two freak rainfall episodes, which have caused flooding the like of which has never been seen in many places. This is because the computer models used to predict the future course of global warming all show heavier rainfall, and indeed, "extreme rainfall events", as one of its principal consequences.

The new study, carried out jointly by several national climate research institutes using their supercomputer climate models, including the Hadley Centre of the UK Met Office, does not prove that any one event, including the rain of the past few days in Britain, is climate-change related. But it certainly supports the idea, by showing that in recent decades rainfall has increased over several areas of the world, including the mid-latitudes of the northern hemisphere, and linking this directly, for the first time, to global warming caused by human emissions of greenhouse gases. The study is being published in the journal Nature on Wednesday, and its details are under embargo and cannot be reported until then. But its main findings have caused a stir, and are being freely discussed by climate scientists in the Met Office, the Hadley Centre and the Department for Environment For Environment, Food and Rural Affairs.

One source familiar with the study's conclusions said: "What this does is establish for the first time that there is a distinct 'human fingerprint' in the changes in precipitation patterns ­ the increases in rainfall ­ observed in the northern hemisphere mid-latitudes, which includes Britain. "That means, it is not just the climate's natural variability which has caused the increases, but there is a detectable human cause ­ climate change, caused by our greenhouse gas emissions. The 'human fingerprint' has been detected before in temperature rises, but never before in rainfall. So this is very significant. "Some people would argue that you can't take a single event and pin that on climate change, but what happened in Britain last Friday fits quite easily with these conclusions. It does seem to have a certain resonance with what they're finding in this research."

The Hadley Centre lead scientist involved with the study was Dr Peter Stott, who specialises in finding "human fingerprints" ­ sometimes referred to as anthropogenic signals ­ on the changing climate. Last September Dr Stott, who was not available for comment yesterday, published research showing that the climate of central England had warmed by a full degree Celsius in the past 40 years, and that this could be directly linked to human causes ­ the first time that man-made climate change had been identified at such a local level. The human fingerprint is detected by making computer simulations of the recent past climate, with and without emissions of greenhouse gases ­ and then comparing the results with what has actually been observed in the real world.

In Dr Stott's research, and in the study to be published on Wednesday, the observed rises in temperature and rainfall could be clearly accounted for by the scenario in which emissions were prominent. The conclusions of the new rainfall study are regarded as all the more robust as they are the joint work of several major national climate research bodies, led by Environment Canada, with each using its own supercomputer climate model. Global warming is likely to lead to higher rainfall because a warming atmosphere contains more water vapour and more energy. Since climate prediction began 20 years ago, heavier rainfall over Britain has been a consistent theme.

(By Michael McCarthy, The Independent, 23/07/2007)