Flooding will be costing Britain up to £27bn a year by the end of the century, a twentyfold increase on current damage, according to a high-level investigation by government-appointed scientists.
Global warming, coastal erosion and the practice of building on flood plains will inevitably lead to an increase in the risk of floods that cause loss of life and extensive property damage, the scientists warn in a report published yesterday.
The 60-strong body of experts led by Sir David King, the Government's chief scientist, said that radical measures will need to be taken to fight the sort of floods expected over the next 80 years if sea levels continue to rise and the weather becomes increasingly stormy.
Sir David said that more than £200bn-worth of assets are already at risk around British rivers and coasts and that everyone in towns and cities was vulnerable to major disruptions in power and transport when flooding occurs.
The analysis of the threat by leading climatologists, civil engineers, geographers, environmentalists and economists is the most rigorous undertaken in Britain and probably the world, Sir David says in the report's foreword.
"There are two key messages. First, continuing with existing policies is not an option - in virtually every scenario considered, the risks grow to unacceptable levels," Sir David says. "Second, the risks need to be tackled across a broad front. Reductions in global emissions would reduce the risks substantially. However, this is unlikely to be sufficient in itself. Hard choices need to be taken - we must either invest more in sustainable approaches to flood and coastal management or learn to live with increased flooding," he says.
Flooding can inflict devastation on people's homes and businesses and 1.7 million properties and four million people are already at risk, says Sir David, who has warned that global warming is a greater threat than terrorism.
The scientists, who were drawn together as part of the Government's Foresight initiative, considered four future scenarios that each differed in the degree of economic growth, greenhouse gas emissions and sustainable development over the coming hundred years.
Two scenarios, called "world markets" and "national enterprise", envisage a future of unrestricted economic development and pollution, whereas two other "community-orientated scenarios" - called "local stewardship" and "global sustainability" - envisage a future of more sustainable growth.
In all of them, the risk of flooding increases significantly. However, the report warns that the future costs of flooding will vary widely depending on how much worse global warming gets and how valuable the property in flood-prone areas becomes - the greater the value, the greater the costs when floods occur.
"The two consumerist scenarios contrast with the two community-orientated scenarios, which experience much more modest increases," the report says.
The number of people who could be at high risk from river flooding and coastal erosion could increase from 1.6 million today to between 2.3 and 3.6 million by the 2080s.
In addition to the risk of flooding from rivers and coasts, towns and cities will suffer from localised floods due to old Victorian sewers and drains being overwhelmed. "The potential damage could be huge.... The numbers of properties at high risk of localised flooding could typically increase fourfold under the four future scenarios," the report says.
If climate change increases further and the weather becomes more extreme, then flash floods are likely to overwhelm the drains of major cities. Many of the drains were built nearly 200 years ago.
"Urban floodwaters are invariably mixed with sewage, so future increases in urban flooding would be compounded by the additional risks to health, and higher costs of repair to properties," the report says. In addition to the direct health risks from sewage and foul water, flooding can have serious mental-health consequences as properties become unsaleable.
"The socially disadvantaged will be hardest hit. The poor are less able to afford flooding insurance and less able to pay for expensive repairs," the report says. "People who are ill or who have disabilities will be more vulnerable to the hazard of a flood and to health risks due to polluted floodwaters."
Professor Edward Evans of Glasgow University, who led the scientific team, said that there were a number of measures that could be taken to lessen or minimise the future risks. They ranged from allowing more room for water storage on flood plains to prevent rivers from suddenly bursting their banks, to allowing some coastlines to erode away naturally rather than protecting them with increasingly expensive coastal defences.