Every time I give a public talk, the same thing happens. Someone in the audience stands up and complains that no one ever talks about population: why not, when it is obviously the major environmental issue? That's an awful lot of no ones.
It happened again last Saturday, except that this time the objector - who happened to be the chairman of the Optimum Population Trust - complained that only two out of four of the people on the panel had discussed it, so why did no one ever talk about population? And of those two, they hadn't talked about it enough.
This seemed odd to me, as Sir David King - the former chief scientist and one of the other panellists - had placed population at the centre of his diagram of environmental problems, from which, he contended, all others flowed. But he, like the questioner, is wrong.
As one the graphs King displayed demonstrated, and as the UN and independent scientists predict, the world's population is expected to peak at around 9 billion by 2060 and then to decline to around 8.5 billion by 2100.
Of course the bisophere can ill-afford to carry these numbers, and they will load an extra 40 or 50% of pressure onto every environmental constraint. It's an issue, in other words. But the issue?
Until the recession struck, the global rate of economic growth was 3.8%. The world's governments hope and pray that we'll be back on this track as soon as possible. Population, of course, is one of the components of economic growth, but the global population growth rate is currently 1.2%.
It's responsible, in other words, for one-third of normal economic growth. The rest is supplied by rising consumption. Consumption, on this measure, bears twice as much responsibility for pressure on resources and ecosystems as population growth.
But the comparison is actually much starker than this suggests. Population growth, as King's graph shows, and as he admitted during his presentation, is self-limiting: the entire global population is going or has gone through demographic transition. Consumption is not. It will rise until it causes ecological collapse, which is the problem we are seeking to avoid. By all means let's help people to get access to the contraceptives they want, and assist women to make full and free reproductive choices. But let's not pretend that their fertility is the overwhelming environmental issue.
So why does such a large congregation of no ones keep banging on about this issue? Well I can't help noticing that at least nine out of ten of them are post-reproductive, middle-class white men. They come from a group which is, in other words, more responsible for environmental destruction than any other class in history. Their consumption of just about every known resource outweighs that of most of the world's people put together. There's just one major issue for which they aren't to blame: current increases in population. And - wouldya believe it? - this is the one they want to talk about.