Peoplequake, By Fred Pearce

For 200 years humanity has lived in the long, deep shadow of a gloomy cleric-turned-economist from Surrey called Thomas Robert Malthus, cosily if incongruously known to his peers as "Bob". At the end of the 18th century, and in the first decades of the 19th, Malthus predicted that humanity was bound to out-breed its own food supply and so, sooner or later, must perish messily. Food, he declared, can be increased only arithmetically – by a fixed, absolute amount each year. But population rises exponentially – by a set percentage each year; and exponential growth, unlike arithmetical growth, gets faster and faster.

Now, in what seems the nick of time, comes Fred Pearce with the best news that humanity – or the Earth – have received since our ancestors first began to farm on the large scale about 10,000 years ago. Malthus was wrong – or at least too simple, argues his book about "mass migration, ageing nations and the coming population crash". Population curves of all creatures do rise exponentially for a time, but they tend to level out. After a time, the percentage rate of increase diminishes.

So it is with humans. Absolute numbers continue to rise but over the past 30 years or so the percentage rate of increase has been diminishing. By 2050 the population is on course to reach 9 billion. But by then the percentage annual rise should have fallen to zero, which means that numbers will then stabilise.

If the trend continues the birth rate should then fall below replacement, so total numbers should start to go down. Nine billion is as big as the human population will get. Now we have around 6.8 billion so the final predicted number is only 25 per cent greater. To be sure, we are failing miserably to feed the present population – but only because we feed half of all our staple crops to livestock, and then waste about a third of what we do produce. We already produce enough for nine billion, if only we used it properly. And we don't need to be vegetarian. Cattle and sheep do best on grass and browse, and there is plenty of those.

There is even better news. It isn't war, famine, infection or natural disaster that are cutting us down, although they certainly have their effect in some countries. Indeed, the horrors sometimes advocated to keep us in check are all counter-productive. Numbers bounce back after famines and pogroms, as people rush to fill the space.

The routes to long-term curtailment are all benign. People have fewer babies if the ones that do have survive infancy. Pensions reduce birth-rate – for then people don't need to fill the world with their offspring just to cater for their old age. Women opt to have fewer babies if they have some status and means of fulfillment apart from motherhood: the chance to work outside the home, and take part in their societies.

Fundamentalist religions can be an issue, but are not necessarily the prime cause of "pro-natalism". Italy's population was the first in western Europe go start going down – of the people's own volition. Yet Italy is still Catholic.

Women in South America render unto the Pope what is the Pope's – and then get on with their lives, including contraception, often bravely backed by their priests.

Pro-natalism tends to be political. The women in China now being urged to have only one child are the daughters of women who were told by Mao to have as many as possible, because he wanted a big army. As birth rate diminishes and people live longer the average age of the population increases.

Pearce is sanguine about this, too. Oldies are nice, and although we can't tote barges and lift bails like we used to, we can still do good things, and generally less aggressively. Immigration is good, too. We in Britain should welcome the young Poles and Brazilians and so on to provide the workforce we can't do without until we can adjust our affairs to suit an older population.

So this is a wonderfully upbeat story, excellently told by Pearce, who has proved over the past 30 years to be among the very best commentators on the overall plight of humanity and the Earth. He roots politics in biology which is where, in the end, it is rooted.

Yet I still feel uneasy. There is nothing so good that cannot be messed up by the powers-that-be. For governments and corporations, Malthusian gloom is not an inconvenient truth but an all-too convenient lie. We are still told from on high that even to keep pace with human numbers we need to double food supply in the next few decades – so we need more big-time industrial farming with hi-tech inputs. There is no reason for this – except that it's profitable, and would maintain the political status quo. The future is indeed promising; but if we want to gather its fruits, then we need to find a very different way of running our affairs.

Colin Tudge runs the Campaign for Real Farming