Extreme rainfall events - those likely to lead to flooding - have become more frequent and intense over a 40-year period in parts of Britain, particularly in Scotland and the North of England.
Scientists from Newcastle University, who analysed UK weather records from 1961-2000, say the findings provide further evidence of climate change occurring.
They also suggest that the 5 million people who live near to rivers - ten per cent of the UK population - can expect to be flooded with increasing regularity in the future which has implications for the management of flooding and water resources.
Dr. Hayley Fowler, a member of the research team who will speak about the project at the BA Festival of Science in Norwich on September 6*, used records from the UK Met Office and the British Atmospheric Data Centre.
She and colleagues examined four distinct periods classed as 'extreme rainfall events' ie where rain was observed to fall steadily over either one, two, five or ten days. They found the probability of an extreme five or ten day rainfall event during the 1990s, compared to the previous 30 year period, increased by four times in Scotland and by two times in Northern England.
The probability of an extreme rainfall event in South East England over five and ten day events actually decreased by 1.5 times but further analysis showed that this part of the country is experiencing a greater frequency of smaller extreme rainfall events, and a change in the timing of such events, with a greater frequency in autumn months. These still pose a threat in terms of flooding because a greater amount of rain is falling in total.
Additional analysis showed that extreme rainfall events that are expected to happen every 50 years increased in frequency and size in Scotland and Northern England, especially in the autumn.
Dr. Fowler compared the periods 1991-2000 with 1961-1990 for the likelihood of 50-year extreme rainfall events. For an event that would be expected only once every 50 years during 1961-1990, during the 1990s this magnitude of event occurred once every eight years in Eastern Scotland, once every 11 years in Southern Scotland, once every 25 years in Northern Scotland, North West England and North East England. In Central East England and Northern Ireland there has been no change. However, in Southern England this type of event would now be expected only once every 100 years.
The amount of rain falling in the UK for a 50-year event occurring over five days rose by 12 per cent overall during the 40-year period examined by the Newcastle University researchers, from 119.59 mm in 1961-90 to 134.17 mm in 1991-2000.
Yet a further breakdown of the figures for five-day rainfall events showed a stark contrast between locations. In Scotland there was a 29 per cent increase in rainfall, from 131.72 mm in 1961-90 to 166.51 mm in 1991-2000; in Northern England, Northern Wales and Ireland there was a nine per cent increase, from 119.64 in 1961-90 to 130.94 mm in 1991-2000. But in Southern England and Southern Wales there was a two per cent decrease, from 107.42 mm in 1961-90 to 105.06 mm in 1991-2000.
The pattern of change in extremes uncovered by Dr. Fowler and colleagues matches the predictions made in a number of models that estimate the scale and speed of climate change over the next century.
Dr. Fowler, a senior research associate with Newcastle University's School of Civil Engineering and Geosciences, said: "The changes we observed over the 40-year period we studied are consistent with the trend we would expect from global warming.
"If the trend continues, which is likely, this suggests we will have an increase in flooding over the coming years which has major implications for flood risk management."
Extreme rainfall, however, is just one factor contributing to the increasing likelihood of flooding - other causes include increased building on floodplains, changing land management practices and the drainage of upland moorland.
Dr. Fowler added that water companies should consider ways in which they can store water during extreme rainfall events for later use, probably during the summer months which are expected to get drier over the coming years.
"One solution could be to build storage facilities such as small reservoirs close to rivers to catch the excess water following extreme rainfall events," she said.
"This could also help alleviate the potential for flooding as well as solve the water shortage crisis we are likely to experience in the summer months."
Dr. Fowler, who is using the data to carry out an analysis of climate change models and their potential reliability, now intends to delve further back into weather records to observe trends over a longer period of time.
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