The optimistic scenario is that world population will level out at around 10 billion some time around the middle of this century. But I wouldn't bank on it. The birth rate equation is fertility multiplied by fecundity, where fertility is the number of women of childbearing age and fecundity is their propensity to have children. Unprecedented disasters apart, fertility is very easily predictable in the medium term and is, of course, rising steadily. Fecundity has proved very unpredictable, the consequence of a myriad of factors including religion and fashion. In any case, the embourgeoisement thesis suggests that people will trade babies for cars and central heating. Which means, of course, that if man-made climate change is the problem which it is now assumed to be, there is nothing we can do about it unless we can attack not just overpopulation in the abstract, but the level of increase which is currently considered inevitable.

The idea that more people is a bad thing is very difficult to accommodate in moral philosophy. I drafted this essay with one granddaughter pottering in the background, playing and talking to herself in a way that I can only describe as endearing and another leaning on my knee and smiling up at me in a way I can only describe as lovable. The idea that I might sacrifice their existence for the benefit of some unknown members of my species in a hundred or a thousand years time is absurd. I do not care about the long-term future of my species, but about the two little girls: "caring" at that level of abstraction is neither rational nor rooted in natural sympathy.

So, here is the solution: a strain of viral pneumonia which spreads aggressively and wipes out 80 per cent of the population, leaving us at Ehrlich's "acceptable" level of 1.5 billion people. Like an elephant cull it tends to follow genetic patterns of immunity, wiping out whole families, but leaving other families intact. It tends to hit the weak, in Darwinian terms. Go with it? Open the container? Nobody said saving the planet was going to be nice!