Britain has never had a population policy, but it seems we are well on the way to having one. The population of this country is at present growing at approximately 1,000 people a day and is predicted to reach 77 million in 2050. The immigration minister, Phil Woolas, felt the need to “give assurances to people that that sort of figure is not on the horizon”. Both the Conservatives and the Liberal Democrats have been making similar noises of concern.

Yet the point of a population policy has yet to be made clear. Whether in England, which is now the most crowded large nation in Europe, or at a global level, concern about overpopulation is being expressed ever more freely. But the messages are often mixed, the rationales confused. Many think the real arguments for taking population seriously are green ones. If so, let’s hear them.

Environmental targets are much harder to meet with a rising population. Yet we also need to accept that, on a planet where a growing number of people are buying increasing quantities of stuff, merely stabilising numbers is not going to be enough. Consumer citizens gobble up resources at an alarming rate. If we demand such a lifestyle – and it seems that most of us do – we must also accept the environmental penalty clause. We need a decisive shift away from going for growth, and towards managing decline.

The sooner we get used to this transition, the easier it will be to cope with it. For one of the main facts in this whole debate is that fertility is in free fall in many countries. It has plummeted not just in the UK, but worldwide: in the early 1970s, women had, on average, 4.5 children; today that has fallen to 2.6. And it is still falling. Across the developed world, it averages out at roughly 1.6 (that is, below replacement level). The prediction is that the world’s population will plateau at about nine billion some time in the middle of the 21st century. At this point, small families will be the norm across most of the world.

It is a prediction which relies on the idea that, by 2050, the poverty and social conservatism that force so many women to have large numbers of babies will be history. It sounds like wishful thinking. In fact, it is already happening. In many countries, we are experiencing a transition from population explosion to grey dawn. Japan is seen as the trendsetter here: its population is in steady decline and the number of over-65s is set to rise to one in three by 2025. This is a pattern also now affecting South Korea.

Some countries with shrinking indigenous populations, such as Britain, Germany and Italy, have tried to buck the trend by drawing in young workers from developing countries with still-growing populations, such as India (of the UK’s predicted 77 million people in 2050, 80 per cent will be a product of direct or indirect immigration). But the fertility of many developing nations is already withering: sooner or later they will go the same way as Europe. In 1952, women in India had six babies on average. Today, that figure has halved and is likely to fall further this century. The same is true for much of the developing world (with the exception of large chunks of Africa).

It is time to adapt to this new world and deal with the consequences. The environmental implications of continuing to “go for growth” are certainly pretty scary. Coupled with an exploding rate of consumption, the impact of nine billion people on the planet in 2050 will be profound (today, we have six billion; in 1960, it was three billion; in 1800, one billion). Each Canadian consumes about 6.5 times as much energy as the average Chinese. What happens when 1.3 billion Chinese get anywhere near this? Or when each Haitian has the same environmental impact as each American (at the moment, just one citizen of the United States has the impact of 280 Haitians). It is something of an eco-ditty, but it happens to be true: that even to supply our existing six billion at US levels of consumption, we would need four more Planet Earths (the comparable figure for consumption levels in the UK is just three Planet Earths).

It has been more than 40 years since Paul Ehrlich introduced us to the “population bomb”. Yet Ehrlich’s contention that population growth would lead to mass starvation was wrong. Thanks largely to the intensive use of fertilisers and pesticides, the global population is larger and better fed than at any time in human history. But does that mean population size doesn’t matter? If we approach the issue from a broader environmental perspective – encompassing everything from biodiversity to climate change – then the bomb has already gone off.

The catch-22 of today’s debate is that population slowdown is premised on rising consumption. Wealthier families have fewer children, but such children as they do have enjoy lives of cradle-to-grave consumerism. That pretty much describes my life so far, and it is just as true for most of us in Britain. The Chinese and Indians are heading exactly the same way. The western lifestyle is within reach for billions of people. As such, while efforts to reduce consumption are important, their potential is very limited. Nor is it good enough just to lie back and hope that once world population growth has levelled out, issues of scarcity and environmental degradation will cease to be a problem. They won’t. A smaller world population needs to be actively promoted, its benefits extolled and the problems of models of wealth generation based on labour growth understood.

One of the most absurd modern myths is that societies with lots of old people are destined for poverty. It is worth pointing out that demographic bulges are not permanent. In Britain, we will experience a bulge of people who are aged 70 or above in 20 years, but they will be pushing up the daisies 20 years later. The demographic profile of Britain is not an inverted triangle, but a weird, knobbly thing. Bulges come and go. In the absence of attempts to “go for growth” by inflating the population through immigration, the overall trend would be towards a smaller but then stable population (at least a few million under the present population). This wouldn’t be bad news and it certainly should not be cause for panic.

The message may finally be getting across. A report issued by the Office for National Statistics in December 2008 had the refreshing title Benefits and Challenges of an Ageing Population. So let’s rid ourselves of the mantra that old people are non-productive units. We spend far longer being an unproductive burden on society as infants and young people than we do as broken-down wrecks at the end of our lives. The old are often the main carers for the young and are far better employees. Lurid fears about hordes of welfare vampire wrinklies draining every last penny from the twitching bodies of the overworked young are fantasy.

A society with a declining population has to be an old-age-friendly society. It would also be a place that could combine somewhat reduced rates of consumption with environmental sustainability. In Britain, moreover, it might allow ordinary people to gain access to things that have been priced beyond their reach, such as space and tranquillity. This is an aspirational agenda that the current punitive discourse on population control bypasses.

Last year George Monbiot wrote in the Guar­dian that “most greens will not discuss” overpopulation. Once a favourite liberal-left cause, the whole issue has become taboo for some people. The phrase “population control” still evokes images of enforced sterilisation in Indian villages and draconian sanctions in China. State bullying of the impoverished is nasty stuff. Yet, if I am right, 21st-century population policies should not be about clamping down on the poor but about managing the numerical decline of the rich. A more positive spin on the same argument is to say that 21st-century population policies need to be pro old people. To be honest, there is not much choice. A grey dawn is breaking across the world. On our messed-up planet, it’s a welcome sight. l

Alastair Bonnett is professor of geography at Newcastle University. His latest book is “What Is Geography?” (Sage, £16.99)