Climate policy is dangling between the cynicism of Donald Trump and the radicalism of Greta Thunberg. The US president has just pulled the world’s second-largest emitter of greenhouse gases out of the Paris climate accord. Ms Thunberg demands significantly more than a 50 per cent cut in global net emissions by 2030. The former is certainly irresponsible. But the latter seems inconceivable.

The exasperation of radical climate activists is understandable. Despite decades of talk, emissions of greenhouse gases and global temperatures continue to rise. If the trend does not alter soon, the chances of avoiding an increase in global average temperatures of more than 1.5C above pre-industrial levels will be zero and those of avoiding a 2C increase will be tiny.

As the IMF notes in its latest Fiscal Monitor, meeting the latter goal requires reducing emissions of greenhouse gases by a third below the baseline, by 2030. To keep below a 1.5C increase, emissions need to be half of the baseline.