The warning could hardly be clearer. The science academies of the G8 nations, including the US National Academy of Sciences and the UK's Royal Society, along with their counterparts in China, India and Brazil, have stated unequivocally that the scientific evidence about man-made climate change is now clear enough for there to be no further excuses about the urgent need for cost-effective steps to reduce greenhouse gas emissions.

The scientific consensus, in other words, has hardened. Global warming is happening. A primary cause is carbon dioxide emissions. Human agency is at the base of it all. The urgent need is to reduce the causes and prepare for the consequences of climate change. While our climate system is too complex to square away some conflicting data, these uncertainties no longer furnish an alibi for inaction.