We're pumping out CO2 to the point of no return. It's time to alter course

Until a few months ago, government targets for cutting greenhouse gases at least had the virtue of being wrong. They were the wrong targets, by the wrong dates, and they bore no relationship to the stated aim of preventing more than 2C of global warming. But they used a methodology that even their sternest critics (myself included) believed could be improved until it delivered the right results: the cuts just needed to be raised and accelerated.

Three papers released earlier this year changed all that. The first, published in Proceedings of the National Academy of Sciences in February, showed that the climate change we cause today will be "largely irreversible for 1,000 years after emissions stop". About 40% of the carbon dioxide produced by humans this century will remain in the atmosphere until at least the year 3000. Moreover, thanks to the peculiar ways in which the oceans absorb heat from the atmosphere, global average temperatures are likely to "remain approximately constant … until the end of the millennium despite zero further emissions".

In other words, governments' hopes about the trajectory of temperature change are ill-founded. Most, including the UK's, are working on the assumption that we can overshoot the desired targets for temperature and atmospheric concentrations of CO2, then watch them settle back later. What this paper shows is that, wherever temperatures peak, that is more or less where they will stay. There is no going back.

The other two papers were published by Nature in April. While governments and the United Nations set targets for cuts by a certain date, these papers measured something quite different: the total volume of carbon dioxide we can produce and still stand a good chance of avoiding more than 2C of warming. One paper, from a team led by Myles Allen, shows that preventing more than 2C means producing a maximum of half a trillion tonnes of carbon (1,830bn tonnes of carbon dioxide) between now and 2500 – and probably much less. The other paper, written by a team led by Malte Meinshausen, proposes that producing 1,000bn tonnes of CO2 between 2000 and 2050 would give a 25% chance of exceeding 2C of warming.

http://www.guardian.co.uk/commentisfree/2009/sep/01/global-warming-emissions-fossil-fuels