Britain is facing a future of wildfires, storm surges and crop failures during blisteringly hot summers, according to the most authoritative assessment yet of the threat of climate change.

At the publication of the UK Climate Projections 2009 report yesterday, Hilary Benn, the Environment Secretary, also warned that by 2070 the Thames Barrier may need to be replaced to cope with increased risk of flooding in the Thames Estuary and sea levels up to 68cm (26in) higher than now.

The bleak new government forecasts — the result of 12 years of research by Met Office scientists — indicate that, in one scenario, summer temperatures in London could regularly exceed 40C by 2080.

While cautioning that there were still big uncertainties about exactly how severely Britain will be affected, Mr Benn said that many changes, including an increase in average summer temperatures of 2C by 2040 in southern England, were now virtually guaranteed because of the build-up of stocks of carbon dioxide already present in the atmosphere, which will take 30 years or more to be worked out of the climate system.

He said the report represented a “call to action” for government, businesses and ordinary citizens. It warned that drier, hotter summers could trigger water shortages and Mediterranean-style wildfires, especially in heath and moorland areas such as parts of the Peak District.

Changing patterns in farming were also likely, with some crops unable to survive and cows and sheep dying from heat stress.

More crop damage from storms, pests and diseases was also likely, with the spread of new insects from Southern Europe.

Winters, it predicted, would be characterised by more frequent and intense rainfall with flooding more likely in cities such as Gloucester and Sheffield as well as in coastal areas, where tidal erosion would increase.

“These results are sobering and we know that these changes will affect every aspect of our daily lives,” said Mr Benn, who added: “If there are those who still don’t think climate change is happening and think we don’t need to worry and we can pull up the bed covers and it’s all going to go away, they are profoundly mistaken.

Mr Benn also acknowledged that some benefits were possible. Higher temperatures could mean a boost to tourism in some northern areas, the introduction of new crops and higher yields from others.

Ed Miliband, Secretary for Energy and Climate Change, said that the report bolstered the case for Britain to act vigorously to cut its own carbon emissions and to pursue a global deal with other countries at a UN meeting in Copenhagen in December.

The UK Climate Projections 2009 were produced by the Hadley Centre for Climate Change at the Met Office, funded by the Department for Environment, Food and Rural Affairs. They represent an update of less detailed ones made in 2002. The study sets out a range of changes based on three greenhouse gas emission projections — low, medium and high.

Under the medium forecast, by the 2080s average summer temperatures in the South East will rise by 2C-6C, while sea levels will increase by 36cm.

It also predicts a decrease in average summer rainfall of 22 per cent in Yorkshire and Humber and in the South East — which is already short of water — while the North West would experience an increase of 16 per cent in average winter rainfall.

Under the higher emission projections, London could be up to 12C warmer on the hottest days with temperatures regularly rising above 40C.

Mr Miliband pointed out that while some warming was inevitable, a successful global deal at Copenhagen to reduce emissions could yet avoid the worst outcomes in the projections.

Professor John Beddington, the Government’s chief scientific adviser, said: “We know now that some further climate change over the next two to three decades is unavoidable due to past emissions. But what the projections also show is that strong mitigation action now can start to make a real difference by 2050 and lead to very different outcomes by the 2080s.”

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