Leading article: The dangers of pessimism in the struggle against global warming

One might be tempted to argue that this is all academic because the Government will not meet its targets anyway

And what it concludes is shocking. According to the report, even by the most optimistic assessments of what humanity is capable of doing to rein in our carbon emissions, the global temperature is destined to rise by 3C within the next century. When we hit this temperature, the carbon dioxide levels in our atmosphere will be roughly double what they were at the time of the industrial revolution. And the earth's climate will be profoundly - and irrevocably - changed.

These findings will have major repercussions for the UK's anti-climate change strategy. Sir David's conclusion that a 3C rise is now unavoidable contradicts what the Department for Environment, Food and Rural Affairs and Downing Street have long stated. We are repeatedly told that the Prime Minister is lobbying around the world for a concerted attempt to stabilise the climate at an increase of no more than 2C. But now his own adviser argues that this is impossible. This suggests that the scientific underpinning of the Government's plans is coming loose.

One might be tempted to argue that this is all academic because the Government is not even likely to meet its targets for cutting Britain's emissions anyway. Others may cite the fact that America, which is responsible for the largest proportion of global fossil fuel emissions, is still showing no signs of taking the problem seriously. Others will point out that China is building a new coal-fired nuclear power station almost every month, cancelling out all our emission cutting in the West. This is all true, but this should be cause for us to redouble our efforts, not give up. Future generations will not forgive us if we do not try at least to prove Sir David's latest report overly pessimistic about what we can achieve by working together.

A temperature rise of 3C over the next century would wreak havoc on the world's ecosystems. Half of all nature reserves would be devastated. A fifth of coastal wetlands would be lost. This would clearly have a profound effect on wildlife. But humans would be just as greatly affected. The report's projections indicate that between 20 million and 400 million tons of global cereal production would be lost. This would put an estimated 400 million people around the world at risk of hunger. Desertification would put up to three billion at risk of water shortages. The developing world would bear the brunt. But there is also a direct risk to the West, including Britain, as sea levels rise.

Sir David was scathing yesterday about those politicians who believe we can rely on new technologies to slow climate change: "There is a difference between optimism and head in the sand." We would agree with Sir David on this point. But we do not agree with his views on nuclear power, which he regards as one of the only realistic ways of substantially cutting our nation's carbon emissions. The deadline for submissions to the Government's consultation on nuclear energy has now closed. Yet there are ominous signs that the Government, with Sir David's support, has already made its mind up to proceed, despite the fact that investing in a new generation of nuclear plants would crowd out investment in renewable technologies.

The debate on nuclear power is far from over. And, in the meantime, this report is yet one more confirmation of the dire consequences for humanity if we continue down this road.