Leading article: Act, don't despair, on climate change

It is sometimes difficult to walk the tight-rope of climate change. Focusing on the dire scientific assessments of what could happen in an overheated world can send people tumbling into a pit of despair. Yet making light of the problems caused by rising greenhouse gases can equally generate a false sense of security.

The final part of the fourth report by the Intergovernmental Panel on Climate Change (IPCC), which was released yesterday, makes it clear that there is hope. If governments act - and that is a big "if" - it will be possible to curb man-made emissions of greenhouse gases over the coming decades, and so avert the worst predictions of climate scientists.

The former US vice-president and environmental campaigner Al Gore says that, in making his Oscar-winning film An Inconvenient Truth, it was important at the end of all the pessimism to give people hope. He showed in a simple graphic how the world could be gradually weaned off its addiction to fossil fuels with a series of measures that, collectively, could lead to a dramatic curb on emissions of carbon dioxide.

Individually, each measure - such as renewable energy or improvements in efficiency - is not enough on its own to lower carbon emissions to the point where the climate can be stabilised. Collectively, though, these policies operate like "climate stabilisation wedges" that successively ratchet down future emissions, lowering them sufficiently that global warming can be controlled. At present the level of carbon dioxide in the atmosphere is about 382 parts per million - one third higher than before the Industrial Revolution. The IPCC report suggests that it would be possible to stabilise atmospheric carbon dioxide at between 535 and 590 ppm, and this might be enough to avert catastrophic climate change - if these carbon-curbing measures are enacted.

Central to success will be setting the price for a ton of carbon at the appropriate level. This is the market mechanism that makes it expensive to emit carbon and economical to save it. Effectively, this means putting a real price on the environmental cost of polluting the atmosphere with extra carbon dioxide. The cost is one that we can no longer afford to ignore.

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