If you write about the environment you become used to a measure of unfriendly criticism. In the main, it’s pretty innocuous stuff – charges of miserabilism and so on. But since concentrating on the issue of human population growth, I have found the criticism noticeably darkening. The other week, after helping to launch a campaign encouraging couples to “stop at two” (children, that is), I received an email accusing me of “real, hard-hitting fascism” and adding: “The Nazis . . . would be proud of you!” This was nothing, however, compared t0 the hate mail I received when the organisation of which I am a part, the Optimum Population Trust, published a report arguing that, as human beings were the agents of climate change, one way of combating climate change would be to produce fewer new humans.

Population can arouse violent feelings. Much of the hate mail originated from religious groups in the United States. But the more recent message came from an academic address at Oxford. Personally, I find it hard to conceive that an intelligent, acquisitive, expansive, territorial, aggressive and physically large species such as Homo sapiens could increase in numbers from 2.5 billion to 6.8 billion since 1950 and not cause an environmental crisis. Moreover, I cannot see how, on top of the existing 6.8 billion, we can accommodate another 2.4 billion people over the next 40 years (which is what the United Nations says we can expect) without something to go seriously wrong on the earth.

Such views were once widespread but have become less so. After making much of the running on population in the 1960s and early 1970s, green groups, for instance, have become wary of the issue. The UK’s best-known environmentalist, Jonathon Porritt (see page 27), a keen advocate of stopping at two, is among those critical of the green lobby’s neglect of the population growth issue, describing it as gutless, wilfully ignorant and “less than honest”.

There are many who regard the silence of the greens on population as a shameful episode in the history of a movement that has done an enormous amount to change the world for the better. One might cite a number of factors in mitigation, however. The rise of the religious right, particularly in the US, has added to the ranks of those who believe that birth control infringes religious or political liberties – and in the process forged an unlikely holy alliance with Catholicism and Islam. The excesses of state birth control programmes in India and China have left a residue of suspicion – although China’s one-child policy has prevented the addition of 400 million to a population already facing environmental nightmare. The burgeoning human rights agenda has, meanwhile, made all exercises of judgement over the lives of others potentially suspect. So, aid-givers have lapsed into silence on population for fear of being labelled white imperialists.

To an extent, seeing, and experiencing, is believing. In the UK, concern about population was at a peak in the postwar baby-boom decades, when family size was well above the replacement level of 2.1 and the effects of growth were plainly visible. Domestically at least, a quieter demographic era then dawned: below-replacement family sizes, the expectation that the UK population would peak early this century and thereafter decline. This comfortable vision of Britain is now history.

Under the impact of an upward twist in birth rates and record levels of immigration, which now accounts for over two-thirds of population increase, numbers are rising at rates not seen since the baby-boom days. Government statisticians tell us that the UK’s population, six or seven million in 1750, 50 million in 1950 and 61 million today, will reach 85 million in 2081, with no sign of levelling off. And why should it, when we live in a globalised and globally warmed world with potentially millions of environmental refugees heading our way – making the British Isles, as the environmental guru James Lovelock puts it, one of the planet’s lifeboats?

Against this background, it is hardly surprising that the population issue has been reignited, at least at grass-roots level, as millions of us, particularly in the south-east of England, experience crowding and congestion every day and read in our newspapers, as we strap-hang on some packed commuter train, that it is going to get worse. Last September, England was confirmed as the most densely populated of all the larger countries in the EU: only Malta is more crowded. It is also not surprising that, among the political classes, the immigration component of population growth has led to silence on the issue as a whole – after all, who wants to be accused of racism? But never underestimate the power of cognitive dissonance: that human facility, only too familiar in matters of a green nature, to think one thing but do the opposite. In this respect the Daily Mail, which fulminates against higher den­sities, but describes those in favour of limiting family size as green zealots, may be all too representative of Middle England.

Yet if the silence on population has lately begun to crumble somewhat under pressure from below, a larger question lies behind it. How do we know that the world is overpopulated? Common sense might argue there must be a causal link between the loading of an extra four billion people into the biosphere in the second half of the 20th century and the contemporaneous appearance of severe ecological ills. But common sense also argues that there is lots of land left in the world – think Canada, Siberia.

The contemporary environmentalist, meanwhile, will defend his silence on population by arguing that it is not human numbers that are the problem; it is more about how those human beings live. This employs the

I = P x A x T formula popularised by the population ecologist Paul Ehrlich, author of the 1968 classic The Population Bomb. IPAT says human beings’ impact is a product of their population numbers, multiplied by their affluence and their technology. In other words, the more stuff you own and do, the more burdensome you are to Planet Earth.

The Guardian columnist George Monbiot recently argued that as global economic growth, before the credit crunch, was 3.8 per cent and population growth was 1.2 per cent, the affluence or consumption half of the equation bore twice [sic] as much responsibility for environmental damage as the population half.

The truth is far more complex – partly because the figures assume an exact equivalence between economic growth and human impact at variance with the facts. Some of the ingredients of economic growth (oil, mining) have a great deal of environmental impact; some (financial services) have much less. Many human activities do not register in gross national product at all. If I go for a walk in the park – or, for that matter, cut down a wild tree to use as firewood – I will be contributing to impact but not to economic growth.

This is more than scholasticism, however, because we are making value judgements about future human numbers all the time – whether we acknowledge it or not. Faced with sub-replacement birth rates in many countries in the developed world and with talk of a “birth dearth”, for instance, many governments have begun to promote the economic benefit of women having more babies or of higher immigration as a means of paying for our pensions. You hear less of this in the UK since the idea was rubbished by the Pensions Commission, but it is a remarkably durable piece of mythology that carries startling demographic implications. Since new arrivals grow old and then require pensions themselves, you need an ever-growing population to keep the “support ratio” between workers and non-workers the same. To maintain the present support ratio in the UK, for example, would demand a national population of 136 million in 2050 – more than double the current number.

Is that too many? Most of us would think so – including, apparently, the new immigration minister, Phil Woolas, who said last year that Britain required a population policy, and that the government wouldn’t “allow” the population to reach 70 million (we’re on target to hit that in 2028). But explaining why it might be too many is a different matter. It is not easy to determine the “carrying capacity” of a place – whether it be the United Kingdom or the planet. The American population scientist Joel Cohen asked, in his 1995 book of the same title: how many people can the earth support? But he could not answer his own question, though he noted that the carrying capacity of the number of human beings the earth could support had ranged over the past three centuries from half a billion to more than a thousand billion.

Ecological footprinting

Since the publication of Cohen’s book, however, a new methodology – “ecological footprinting” – has emerged and this is providing a higher level of consistency. Ecological footprinting measures national and global biological productive capacity (the stuff we live off) against human demand (the “footprint”). The resulting data takes both population and consumption into account and provides what many regard as the best guide yet to measuring sustainability. It has been reported that, at the current rates of consumption, the world can support only five billion people. This means the planet is already overpopulated by nearly two billion.

Given that the new science of ecological footprinting has borne out what common sense was suggesting as far back as the 1960s, it’s probably a good job we haven’t all waited for proof. In 2007, 69 out of 195 countries had policies to lower population growth, compared with 39 in the mid-1970s.

This included 70 per cent of the less developed countries: 34 out of 53 African states, for example. And there have been some remarkable, and unexpected, success stories – not least Iran, which decided after a census in 1987 that population growth was holding back development and, between 1988 and 2000, reduced its fertility rate from 5.2 children per family to a below-replacement level of two. Thailand cut fertility rates from 6.3 in 1967 to 1.7 in 2003. Many other states have reduced their birth rates at a speed comparable to China but without coercion. They include Costa Rica, Cuba, Mexico, Morocco, South Korea, Sri Lanka, Taiwan, Tunisia, Vietnam and India (the southern states).

Half a century after the first population and family planning programmes began, the ingredients of success are well established: strong government support, often through explicit population policies; partnership with NGOs; an emphasis on women’s status, rights and education; education on sex and relationships; and, above all, the ready availability of contraceptives – supplied in Iran, for example, by a nationwide network of “health houses”.

Yet more than 200 million women worldwide lack access to contraception, and international spending on family planning – partly because of anti-abortion policies adopted by the Bush administration in the US – has recently been in steep decline. Given that even the UN middle-range world population projection of 9.2 billion by 2050 assumes a further drop in birth rates of up to 46 per cent, this is worrying indeed. Without reductions in fertility, the UN says, we could be nearing 12 billion in 2050.

How to make a difference

Oddly, in a world of large populations, small decisions do make a difference. If every woman had half a child less than currently projected, for example, the world population would be 7.8 billion in 2050 – 1.4 billion fewer people, or roughly one China less. In the UK, meanwhile, if the 26 per cent of women currently expected to have three or more children were to limit themselves to two, our mid-century population would be cut by an estimated seven million – enough to return an area the size of Wales back to nature or food production.

Would that be a good thing? If you are concerned about other species or that nebulous but powerful grouping of ideas we label “the wild”, yes. But even from a brutally anthropocentric standpoint, it has a certain logic. Footprint data suggests that, based on current lifestyles, the sustainable population of the UK – the number of people we could feed, fuel and support from our own biological capacity – is about 18 million. There are thus 43 million “too many” of us, all reliant on the outside world for sustenance. In an era of impending shortages – of food, oil, gas, water – does that not seem a little risky?

The UK has no population policy – despite ranking in the top 20 of the most overpopulated countries, judging by the standard above. If we had such a policy, it would need to address immigration as well as birth rates – a good enough reason, cynics might think, for politicians to forget the whole idea. It would also need to address a further vexed issue – what numbers are sustainable and what are desirable?

Environmental orthodoxy treats population and consumption as two factors in an equation, and thus accepts, by implication, that both are important, but concentrates on one (consumption) while ignoring the other (population). This not only compounds errors in analysis with errors of logic: it has had intangible but far-reaching effects, not least in giving environmentalists a reputation as killjoys, forever telling us what not to consume and making calculations of sustainability seem dour technical exercises in survivalism. Both tendencies have damaged the wider green mission. But there is another way of looking at the numbers question, one that goes beyond sustainability and perhaps bears more directly on what it is to be human.

Consider two Planet Earths – one of nine billion people with x amount of “consumption”, the other of one billion with 9x consumption. Bear in mind that the world of nine billion may be more inventive, but also more pressured and stressful, less spacious. Bear in mind particularly that often, by “consumption”, we mean activities which for many people, laudably or not, make life worth living – holidays, hobbies, travel, freedom to choose. In the modern environmentalist’s formulation, both worlds are the same. In practice, they are not; there are choices to be made. Shouldn’t we be making them, and urgently?

www.optimumpopulation.org

Click here for statements from the three main political parties in Britain on population and immigration