A crucial global warming "tipping point" for the Earth, highlighted only last week by the British Government, has already been passed, with devastating consequences.
Research commissioned by The Independent reveals that the accumulation of greenhouse gases in the atmosphere has now crossed a threshold, set down by scientists from around the world at a conference in Britain last year, beyond which really dangerous climate change is likely to be unstoppable.
The implication is that some of global warming's worst predicted effects, from destruction of ecosystems to increased hunger and water shortages for billions of people, cannot now be avoided, whatever we do. It gives considerable force to the contention by the green guru Professor James Lovelock, put forward last month in The Independent, that climate change is now past the point of no return.
The danger point we are now firmly on course for is a rise in global mean temperatures to 2 degrees above the level before the Industrial Revolution in the late 18th century.
At the moment, global mean temperatures have risen to about 0.6 degrees above the pre-industrial era - and worrying signs of climate change, such as the rapid melting of the Arctic ice in summer, are already increasingly evident. But a rise to 2 degrees would be far more serious.
By that point it is likely that the Greenland ice sheet will already have begun irreversible melting, threatening the world with a sea-level rise of several metres. Agricultural yields will have started to fall, not only in Africa but also in Europe, the US and Russia, putting up to 200 million more people at risk from hunger, and up to 2.8 billion additional people at risk of water shortages for both drinking and irrigation. The Government's conference on Avoiding Dangerous Climate Change, held at the UK Met Office in Exeter a year ago, highlighted a clear threshold in the accumulation of greenhouse gases such as carbon dioxide (CO2) in the atmosphere, which should not be surpassed if the 2 degree point was to be avoided with "relatively high certainty".
This was for the concentration of CO2 and other gases such as methane and nitrous oxide, taken together in their global warming effect, to stay below 400ppm (parts per million) in CO2 terms - or in the jargon, the "equivalent concentration" of CO2 should remain below that level.
The warning was highlighted in the official report of the Exeter conference, published last week. However, an investigation by The Independent has established that the CO2 equivalent concentration, largely unnoticed by the scientific and political communities, has now risen beyond this threshold.
This number is not a familiar one even among climate researchers, and is not readily available. For example, when we put the question to a very senior climate scientist, he said: "I would think it's definitely over 400 - probably about 420." So we asked one of the world's leading experts on the effects of greenhouse gases on climate, Professor Keith Shine, head of the meteorology department at the University of Reading, to calculate it precisely. Using the latest available figures (for 2004), his calculations show the equivalent concentration of C02, taking in the effects of methane and nitrous oxide at 2004 levels, is now 425ppm. This is made up of CO2 itself, at 379ppm; the global warming effect of the methane in the atmosphere, equivalent to another 40ppm of CO2; and the effect of nitrous oxide, equivalent to another 6ppm of CO2.
The tipping point warned about last week by the Government is already behind us.
"The passing of this threshold is of the most enormous significance," said Tom Burke, a former government adviser on the green issues, now visiting professor at Imperial College London. "It means we have actually entered a new era - the era of dangerous climate change. We have passed the point where we can be confident of staying below the 2 degree rise set as the threshold for danger. What this tells us is that we have already reached the point where our children can no longer count on a safe climate."
The scientist who chaired the Exeter conference, Dennis Tirpak, head of the climate change unit of the OECD in Paris, was even more direct. He said: "This means we will hit 2 degrees [as a global mean temperature rise]."
Professor Burke added: "We have very little time to act now. Governments must stop talking and start spending. We already have the technology to allow us to meet our growing need for energy while keeping a stable climate. We must deploy it now. Doing so will cost less than the Iraq war so we know we can afford it."
The 400ppm threshold is based on a paper given at Exeter by Malte Meinhausen of the Swiss Federal Institute of Technology. Dr Meinhausen reviewed a dozen studies of the probability of exceeding the 2 degrees threshold at different CO2 equivalent levels. Taken together they show that only by remaining above 400 is there a very high chance of not doing so.
Some scientists have been reluctant to talk about the overall global warming effect of all the greenhouses gases taken together, because there is another consideration - the fact that the "aerosol", or band of dust in the atmosphere from industrial pollution, actually reduces the warming.
As Professor Shine stresses, there is enormous uncertainty about the degree to which this is happening, so making calculation of the overall warming effect problematic. However, as James Lovelock points out - and Professor Shine and other scientists accept - in the event of an industrial downturn, the aerosol could fall out of the atmosphere in a matter of weeks, and then the effect of all the greenhouse gases taken together would suddenly be fully felt.