http://news.independent.co.uk/environment/article344690.ece
>
> GLOBAL WARMING: Passing the 'tipping point' 
>     Our special investigation reveals that 
>     critical rise in world temperatures
>     is now unavoidable. 
> By Michael McCarthy, Environment Editor 
> Published: 11 February 2006 

>    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. 
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