ON SEPTEMBER 23rd 120-odd presidents and prime ministers will gather in New York for a UN meeting on climate change. It is the first time the subject has brought so many leaders together since the ill-fated Copenhagen summit of 2009. Now, as then, they will assert that reining in global warming is a political priority. Some may commit their governments to policies aimed at reducing greenhouse-gas emissions. What few will say is how many tonnes of carbon dioxide these will save—because they almost never do.