The Copenhagen summit on climate change is going to fall short. Does this matter? Yes and no: yes, because the case for action is so strong; no, because the likely agreement would be inadequate. Tackling climate change will be hard. It is crucial that we achieve the goal effectively and efficiently. The likely further delays should be used to achieve just that.

My view that decisive action is justified is contentious. Sceptics offer two counter-arguments: first, that the science underlying climate change is highly uncertain; second, that costs exceed benefits.

Yet it is not enough to argue that the science is uncertain. Given the risks, we have to be quite sure the science is wrong before following the sceptics. By the time we know it is not, it is likely to be too late to act effectively. We cannot repeat experiments with just one planet.