If mankind does not put its house in order, temperatures could have risen by 15C (27F) by the year 3000 and sea levels by more than 11 metres (36ft), flooding much of London, the team, from the Tyndall Centre for Climate Change Research, says in a report for the Environment Agency. Abrupt changes could make Britain much hotter, or even — such is the uncertainty of the predictions — first colder and then hotter.

This could happen if the North Atlantic current system collapsed, denying Britain the warming effect of the Gulf Stream. Ocean surface temperatures would fall by 3C (5.4F), but as the Arctic sea ice melted, they would rise again by 8C (14.4F) in an abrupt turnabout over a period of no more than about 20 years.

Climate Change on the Millennial Timescale is the first study to examine comprehensively the impacts of global warming beyond the end of this century. It calls for continued efforts to cut the emission of global-warming gases to prevent the changes from getting out of control.

Baroness Young of Old Scone, the chief executive of the Environment Agency, said: “We are running out of road on decision-making. Unless we dramatically change the use of fossil fuels then we will be committing future generations to the most severe impacts of climate change.”

By the year 3000, the report says:

• Global warming could have more than quadrupled, with temperature rises of as much as 15C, if we continue burning fossil fuels
  
  
• Sea levels will still be rising at the end of this millennium and the total increase could reach 11.4 metres. This dwarfs estimates made by the Intergovernmental Panel on Climate Change that sea levels will rise by between 16cm and 69cm by the 2080s
  
  
• Anything more than a two-metre rise would flood large areas of Bangladesh, Florida and many low-lying cities, and displace hundreds of millions of people
  
  
• Abrupt climate changes are possible even after emissions cease because changes may be set in motion that cannot be stopped
  
  
• The acidity of the oceans will fall significantly, posing a threat to marine organisms such as corals and plankton. That, in turn, would affect the whole marine ecosystem
  
  
• The changes could be even greater than this if the climate turns out to be more sensitive to greenhouse gas emissions than the study assumes.

  The solution, the team says, is to reduce emissions to zero by 2200.

  Tim Lenton, lead author of the study, said: “While most studies stop at year 2100 with temperatures and sea level rising, we explored where they are heading into the next millennium. Only by starting to reduce carbon dioxide emissions now can we avoid dangerous climate change.”

  The message is that the world can afford to burn only about a quarter of its known reserves of fossil fuels. This implies a small increase in global emissions up to 2025, and reducing and eliminating them by 2200.

  “If we follow business-asusual then we will commit future generations to dangerous climate change,” Dr Lenton said. The risk was that of returning the Earth to a hot state it had not been in since 55 million years ago.

  Baroness Young said: “Tough decisions are needed soon. Many of our coastal towns could be in jeopardy and immediate action needs to be taken if we are to avoid many of these impacts.

  “We need to get tough on energy efficiency. This means much tighter standards for buildings and government providing proper incentives for businesses from the transport sector if we are to meet our 2010 target of 20 per cent carbon reduction and tackle more strenuous targets for 2020.”

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