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* Flood costs seen rising to 2-12 bln stg this century
* More deaths in summer, but offset by milder winters
LONDON, Jan 26 (Reuters) - Flooding will be Britain's biggest climate risk this century, with damage set to cost as much as 12 billion pounds ($18 billion) a year by the 2080s if nothing is done to adapt to extreme weather, a report said on Thursday.
British summers are forecast to get hotter, while winters will get milder and wetter.
New government-funded research has identified the top 100 effects of climate change and their expected impact on Britain and magnitude over this century.
The "Climate Change Risk Assessment" found that if no further action is taken to address climate change, annual flood damage to buildings could reach between 2.1 billion and 12 billion pounds, compared to current costs of 1.2 billion pounds.
"If I had to pick one particular issue, the flooding issue is the most dominant," said Bob Watson, chief scientific adviser at the Department for Environment, Food and Rural Affairs and one of report's authors.
Britain is already at risk of extreme weather. In 2007, summer floods cost over 3 billion pounds and disruption from snowfall last year cost 600 million pounds a day over a several-day period.
More intense bursts of rainfall in summer and longer rainfall in winter will cause more floods, worsening damage and disruption to infrastructure and property. Over the longer term rising seas will bring coastal flooding, Watson added.
Annual insurance payouts and premiums will rise significantly, and more properties will find it harder to get insurance and obtain mortgages, the report said.
The government is spending 2.1 billion pounds on flood defences over the next four years, but this represents a cut in funding of around 27 percent, the chairman of the environment agency said when the spending budget was announced in 2010.
Despite this, Environment Secretary Caroline Spelman said the government could still ensure more homes are protected from floods. It hopes to protect 145,000 households by March 2015.
The report identified other risks including hotter summers, which could bring 580 to 5,900 extra deaths a year by the 2050s. The number of days in an average year when temperatures go above 26 degrees Celsius could be 27 to 121 in London by the 2080s, compared to 18 now, the report said.
However, the total number of premature deaths could be offset by milder winters which would avoid some 3,900-24,000 deaths a year by the 2050s.
"Cold spells will not disappear, though on average there will hopefully be less of them," said Watson.
Energy demand for air conditioning will rise in the summer but heating demand will fall in the winter, which could reduce costs by over 1 billion pounds a year by mid-century.
Agriculture and forestry will also be affected. Droughts and some pests and diseases could increase as a result of warmer weather, which could reduce timber yields and quality and drive up timber costs by the 2080s.
On the plus side, sugar beet yields could rise by up to 70 percent and wheat yields by as much as 140 percent by mid-century due to longer growing seasons if water and nutrients are available.
"A warmer climate presents opportunities to grow new crops such as soya, sunflowers, peaches, apricots and grapes, while new markets may open up overseas for British grown produce," the study said.
Peter Mallaburn, climate policy expert at Leicester's De Montfort University, said the report showed Britain's lack of preparedness.
"We need a coherent strategy to sort out this mess. Let's hope that this report acts as a wake-up call," he said.
The government said it will use the study to form the basis of a national adaptation plan, due to be published in 2013.
The full report is available at ccra.defra.gov.uk/ ($1 = 0.6416 British pounds) (Edited by Richard Meares)