It is certainly true that "fewer people equals a greener planet" is simplistic. In 2050, 95% of the extra population will be poor and the poorer you are, the less carbon you emit. By today's standards, a cull of Australians or Americans would be at least 60 times as productive as one of Bangladeshis.
Under normal circumstances, it takes perhaps a generation for the birth rate to drop with increasing wealth, whereas carbon emissions go up very quickly. As people get richer, they buy cars, use air conditioning, consume more calories and start to swap their vegetables for meat.
So the richer a country gets, the more pressing the need for it to curb its population. The only nation to have taken steps to do this is China – and the way it went about enforcing the notorious one child policy is one of the reasons the rest of us are so horrified by the notion of state intervention. Yet China now has 300-400 million fewer people. It was certainly the most successful governmental attempt to preserve the world's resources so far.
After all, based on current emissions and life expectancy, one less British child would permit some 30 women in sub-Saharan Africa to have a baby and still leave the planet a cleaner place.
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.
That sounds promising. After all, there are lots of great historical precedents where governments have decided to "cut back the population" for the greater good. Pol Pot & the Khmer Rouge tried it in Cambodia - what could possibly go wrong? As Ann Althouse points out, "Oh, great. Thanks for the warning about cutting back 'population' the hard way. Germany."