Though Bangladesh will grow 20 times more quickly than Britain this year, those additional Britons will burn more energy. The Bangla-deshis will cause themselves a thousand troubles, of course, with their rapid growth - they'll run short of rice and firewood and classroom space. But they won't do much damage to the planet as a whole. Britain, meanwhile, will absorb its small increase in numbers gracefully; no one will even notice.
Except, of course, for those scientists watching the accumulation of carbon dioxide in the upper atmosphere - watching, that is, as the human race changes the climate of its home planet. If it is global warming at issue, then, England is experiencing a population crisis at least as severe as anyone else's. America, by this definition, is almost certainly the most overpopulated spot on the planet, simply because those who live there are so huge. We could (and should) consume less, but there's little sign of that happening. The US, for instance, despite all the warnings about the greenhouse effect, increased its energy consumpion by 15 per cent this decade.
That in mind, lowering fertility becomes just as important in the First World as in the Third; instead of fretting that we're not reproducing fast enough in western Europe and North America, we should be celebrating our reduced birth rates, and working to make sure they continue low enough that our populations eventually stabilise and even shrink. And since we produce children, not birth rates, the only way that can happen is if more of us decide to stop at one.
There's almost no topic that makes people more uncomfortable - even raising the question violates taboos about intruding on this most personal sphere. But myth and legend colour the thinking of too many on this issue: they see only children as likely to be spoiled, strange, sad; one poll found that the single biggest reason parents gave for having a second child was so their firstborn would not be an "only."
Those ingrown prejudices date from the early days of psychology - America's most famous turn-of-the-century shrink, G. Stanley Hall, went so far as to declare that being an only child was "a disease in and of itself". But hundreds of studies in the last few decades make it overwhelmingly clear that only children differ little from those with siblings. They have as many friends, they're as likely to share, they're every bit as happy. If anything they are likely to do a little better in school, probably because they get more attention from their parents.
If that message spreads - if the truth of the happy only child replaces the superstition of the screwball loner - it should be enough to keep European and American birth rates declining. Eventually that will yield other problems: it complicates pension schemes and gradually raises the age of the population, for instance. But those are not fundamental threats to the planet's ecological stability.
The world will almost certainly pass seven billion souls, and eight as well. Demographers predict that with a bit of luck we'll top out at somewhere north of nine billion sometime in the next century. Those are enormous numbers, since the world already groans to support us. But remember - it's not the total that really counts. It's how big those people are. And we're giants.
Bill McKibben is the author of `Maybe One - a personal and environmental argument for very small families' (Anchor, pounds 7.99)