If you have faith in the rich world's ability to achieve those 80% cuts in emissions in a mere 40 years, you need not concern yourself too much about population. But if you are sceptical, you should be worried. A lot.

Some scientists, the German chancellor's adviser, Hans Joachim Schellnhuber among them, say that if the cuts are not achieved, we will end up with a planet with a "carrying capacity" of just 1bn humans. If so, we need to start cutting back population now with methods that offer a humane choice – before it happens the hard way.