Here's a column in The Times, imagining what the world might look like if the UN's low-fertilty scenario comes true.
The peak is in sight. Even as the population passes seven billion, the growth rate of the world population has halved since the 1960s. The United Nations Population Division issues high, medium and low forecasts. Inevitably the high one (fifteen billion people by 2100) gets more attention than the low one (six billion and falling). But given that the forecasts have generally proved too high for the past few decades, let us imagine for a moment what might happen if that proves true again.
Africa is currently the continent with the highest birth rates, but it also has the fastest economic growth. The past decade has seen Asian-tiger-style growth all across Africa. HIV is in retreat, malaria in decline. When child mortality fell and economic growth boomed like this in Europe, Latin America and Asia, the result was a rapid fall in the birth rate. For fertility to fall, contraception provides the means, but economic growth and public health provide the motive. So the current slow decline in Africa's birth rate may turn into a plummet.
If that happens, the low UN estimate could prove more accurate with the world population peaking a little above eight billion and falling to a billion less than today by the end of the century.
Imagine too that agricultural productivity continues to rise. Abundant gas drives down fertiliser prices. Demand from China ensures that new varieties of seeds, better storage and cheap fertiliser reach more African farmers. Perhaps even EU tariff barriers against African produce are lifted and America's crazy policy of diverting food into motor fuel is reversed.
Let us dream, too, that Europe's dogmatic objection to genetic modification is quietly dropped, releasing a plethora of crops that are drought-resistant, salt-tolerant, better at utilising nitrogen fertiliser and better protected against pests. Let us suppose that the efficiency with which chickens, pigs and cows convert plants into meat continues to rocket upwards, thanks to selective breeding, without worsening cruelty.
If even half of these things happen, feeding six or even nine billion in 2100 would take far less land than feeding seven billion requires today. A greater proportion will live in cities, freeing still more land. And with more people able to afford fossil fuels, fewer will depend on forests for cooking fuel (or bushmeat), freeing still more land from human pressure. If they wear synthetic fleeces instead of wool and live in steel and concrete buildings instead of wooden ones, the footprint of their lives will shrink. Even their carbon footprint will fall as gas replaces coal and oil. Imagine, too, that water use grows steadily more efficient with the spread of drip irrigation (where water is delivered straight to plant roots from a tube, with only 10 per cent wastage), and that fish farming provides a greater part of our protein, taking pressure off wild fish stocks.
It is quite possible that your great grandchildren will not only be fewer in number, but will live in a world with huge nature reserves, vast forests and rich seas. Of course, none of this may happen. The most fundamentalist groups within Christianity, Judaism, Islam and Mormonism are furiously encouraging their followers to have big families; they may yet reverse the fall in global birth rates. The greens may win the argument for renewable energy and demand vast acreages for their expensive toys - Renewistan, as the inventor Saul Griffith calls it. The Luddites may prevent innovation from raising food yields and drive us back to land-hungry organic farming. My crazy optimism about the management of the oceans may well prove misplaced.
And even if these dreams come true, we will not have reached Utopia. The rapidly ageing population will be a huge burden. Not just Germans and Japanese, but Brazilians, Indians and perhaps even Nigerians will find that too few young workers are supporting too many elderly dependants with unaffordable pensions and expensive healthcare needs.
None the less, let us briefly dwell on what could go right, if only to encourage us to achieve it: by the end of the century, a smaller population, with higher living standards and a better environment.