We have already reached one such point in that we cannot go back to a system with no climate change. We know that carbon dioxide continues to act climatically for about 100 years, so if we went back to the Stone Age tomorrow we would still experience another century of warming.

But there are further tipping points where subsystems break down; for example, the Gulf Stream slowing because less cold water is sinking at high latitudes. Eventually a point is reached where the whole climate-regulating system breaks down, producing changes that can never be reversed.

Just recently, for instance, it was realised that as carbon dioxide levels rise the oceans become more acidic and that this will destroy plankton, especially plankton with shells, which absorb and reduce carbon levels. The Intergovernmental Panel on Climate Change (IPCC) concluded in 2001 that the rate of global warming is about one third faster than previously suspected.

It predicted that by the end of this century there will be an average warming of between 2C (3.6F) and 5C, with devastating consequences.

The next IPCC assessment is due, and the new fear is that runaway change could soon be upon us, whatever we do.

Peter Wadhams is Professor of Ocean Physics at Cambridge University

It says there is only a small chance of greenhouse gas emissions being kept below "dangerous" levels.

And it says it fears the Greenland ice sheet is likely to melt, leading sea levels to rise by 7 metres over 1,000 years.

The report, "Avoiding Dangerous Climate Change", collates evidence presented by scientists at a conference hosted by the UK Meteorological Office in February 2005.
NASA greenhouse gas expert says US tried to silence him

NASA's top climate scientist says the Bush administration has tried to stop him from speaking out on the threat of greenhouse gases.

The director of NASA's Goddard Institute for Space Studies, James Hansen, made the claim in an interview with the New York Times newspaper.

He's quoted as saying officials at the space agency's headquarters ordered a review of his lectures, papers, and website postings.

It followed his lecture in December calling for prompt reductions in emissions of greenhouse gases.

A NASA spokesperson has denied any effort to silence Mr Hansen, but says the restrictions apply to all staff that could be seen speaking for the agency.