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摘要

"That report is going to scare the wits out of everyone," said Yvo de Boer recently. He is a former United Nations chief climate negotiator and was talking about the forthcoming fifth assessment by the Intergovernmental Panel on Climate Change (ipcc). With two months to go before the assessment is to be published, however, one sign suggests it might be less terrifying than it could have been. The sign in question is about climate sensitivity. This is the measure used by researchers of how much they expect the world's average temperature to increase in response to particular increases in levels of carbon dioxide in the atmosphere. According to one table from the unpublished report, which was seen by The Economist and is reproduced overleaf, at CO_2 concentrations of between 425 parts per million and 485 ppm, temperatures in 2100 would be 1.3-1.7℃ above their pre-industrial levels. That seems lower than the ipcc's previous assessment, made in 2007. Then, it thought concentrations of 445-490 ppm were likely to result in a rise in temperature of 2.0-2.4℃.