Something has changed in the debate on man-made climate change: the US is engaged. But its engagement – or at least the engagement of President George W. Bush – is neither enthusiastic nor unconditional. In particular, at discussions among the heads of governments of the Group of Eight leading countries in Japan, Mr Bush stressed that China and India had to participate. In this, he was right: it will be impossible to tackle the problem without the participation of leading emerging countries. The question is on what terms they do so.