Twelve years ago, the American author Bill ­McKibben published a short book entitled Maybe One: A Personal and Environmental Argument for Much Smaller Families. It certainly has its faults: most obviously, it provides a little too much information about the vasectomy ­McKibben decided to have in lieu of a second child. But it isn’t pious or hectoring; if anything, the author tries overly hard to be tentative, emphasising that he isn’t seeking to dictate other people’s choices, and doesn’t think he has all the answers. The “maybe” is right there in the title, after all.

McKibben meant it in the sense of “maybe one child at most”, but it reflects the book’s general tone of modesty and equivocation. Maybe One is a suggestion. It’s something to think about.

He might as well have called for the enforced sterilisation of all men and women of procreating age, along with the outlawing of Father Christmas and the Tooth Fairy. The New York Times called him “sanctimonious” and “holier-than-thou”. The Wall Street Journal labelled him an “extremist” (their specific objection was that he hadn’t mentioned nuclear power as a way to combat global warming, even though Maybe One is a book about parenting). “So much false information, so many bad ideas, in so few pages,” another reviewer fumed. Speaking after publication, McKibben observed that Maybe One’s subject matter was “the last remaining taboo thing to talk about” and in this case the cliché seemed justified.

In 1998, most people weren’t willing to consider any significant lifestyle changes for environmental reasons, let alone cutting back on kids. Much has changed since then, of course, both in terms of the consensus on the threat posed by climate change and our willingness to make sacrifices in the face of it. But one thing has not: you still won’t hear any major environmental campaign group in Britain or the United States arguing that, in addition to flying less and recycling more, middle-class westerners should be having fewer children to save the planet.

Even commentators who warn of the evils of overpopulation, proudly trumpeting their willingness to raise controversial issues in defiance of “political correctness”, only rarely emphasise the notion that we – rather than those in the developing world – might consider doing less of the populating. For several thorny reasons, family size has become the great unmentionable of the campaign for more environmentally friendly lifestyles. And yet, in the end, it may be the only one that really counts.

Trying to understand the debate about population and the climate sometimes feels like peering into a kaleidoscope while drunk. Directly contradictory claims, which can’t both be true at the same time, are advanced as if they were facts. Weird allegiances are created: George Monbiot and American creationists, for example, are roughly equally contemptuous of organisations such as the Optimum Population Trust; supporters of reproductive rights find common cause with anti-abortionists.

You come across nutty-sounding fringe groups like the Voluntary Human Extinction Movement, but then you phone its founder, Les Knight – he’s a supply teacher, based on America’s west coast, and can only talk during breaks between lessons – only to discover that he isn’t nutty at all, but in fact rather sane and self-deprecating. (He simply wants people to choose not to breed. “Eventually we’ll be extinct anyway, but it would be so much nicer if we phased ourselves out through natural attrition,” Knight told me affably. “You know – the way a company reduces its workforce without firing anyone.”)

For all the confusion and sensitivities that ­surround the subject, though, the basic facts are clear. If you live in Britain or the United States in 2010, there is nothing you can do to reduce your impact on the environment that even comes close to the effects of having one fewer child.

This makes intuitive sense: every new human is a new consumer with their own carbon footprint, along with their own potentially limitless chain of descendants. The year before last, two researchers at Oregon State University, Paul Murtaugh and Michael Schlax, set about trying to put a figure on the idea of “carbon legacy”, and last summer their results were published in the journal Global Environmental Change.

Murtaugh and Schlax started from a simple premise. Assume, they said, that if a woman and a man have a baby, they’re each responsible for 50% of that child’s lifetime carbon-dioxide (CO2) emissions; and if that child has its own child, the original two parents each bear 25% of the responsibility for their grandchild’s emissions, and so on down the generations. For how many tonnes, on average, would each original parent end up being responsible?

There are two important obstacles in performing this calculation. The first is that you don’t know what will happen to per capita emission rates in the future: worldwide, they’ll almost certainly rise, but in many ­western countries they’re likely to fall, as energy-efficiency measures kick in. The second is that you don’t know what will happen to fertility rates: you can’t know whether your great-great-granddaughter will give birth to one new carbon-emitter, or two, or six, or zero.

So for fertility rates, Murtaugh and Schlax used UN population predictions. (In the experiment, some of the hypothetical family trees eventually died out; others were stopped after a predetermined time.) And for per-capita emissions, they used three different scenarios: an optimistic one, in which per-capita emissions fell, a pessimistic one in which they rose, and a compromise one, in which they stayed constant.

The headline result was astonishing. Under the constant scenario, an American who forgoes having a child would save 9,441 tonnes of CO2 – almost six times, on average, the amount of CO2 they would emit in their own lifetime, or the equivalent of making around 2,550 return airplane trips between London and New York. If the same American drove a more fuel-efficient car, drastically reduced his or her driving, installed energy-efficient windows, used energy-efficient light bulbs, replaced a household refrigerator, and recycled all household paper, glass and metal, he or she would save fewer than 500 tonnes.

The Oregon study didn’t run the numbers for Britain, where per-capita carbon emissions are already about half as big as in the United States. (This isn’t down to personal virtue: it’s mainly because so many of our power stations use gas instead of coal.) But in every other country they examined – including Japan, where per-capita emissions are similar to Britain’s– the environmental effects of not having a child were similarly vast. Even if every emissions target recommended by the Intergovernmental Panel on Climate Change were to be successfully implemented – the ­“optimistic scenario” – an American could still save 562 tonnes of CO2 by having one fewer child, while a Japanese person could save 233 tonnes.

Leaving aside the complexities of global population issues, then, wouldn’t it make sense for British environmental groups to suggest that well-off westerners might like to consider smaller families? John Sauven, the executive director of Greenpeace UK, concedes that it’s a “no-brainer” that a smaller population would place a smaller burden on the planet. But he’s reluctant to contemplate a Greenpeace campaign; in any case, he says, among environmentally conscious people in his demographic, “my sense is that nearly all of us have had two children or fewer”.

Documentary film-maker Franny Armstrong — who runs the 10:10 campaign to cut UK carbon emissions by 10% this year, which is backed by the Guardian — says the topic came up in the planning stages of the project, but was abandoned. “We did have the discussion. But we decided it couldn’t work, because of the timescale. 10:10 is a short-term campaign about reductions you can make in 2010.”

Besides, a decade after Bill McKibben published Maybe One, we’re apparently still not ready to contemplate its message. “10:10 is a populist campaign. It’s about doing the easy things first,” Armstrong says. “I completely agree that [family size] is the elephant in the room. But we need one of the big thinkers, a George Monbiot or a Naomi Klein, to go first, before anyone else is going to say it. To use that as a message in a populist ­campaign, right now? It would absolutely destroy the whole campaign.”

The fundamental problem with the topic of influencing population levels is that almost everybody – no matter what their politics or other beliefs – has a very good reason to avoid discussing it. If you don’t believe in climate change, it’s yet more irrelevant, busybodyish meddling. If you’re broadly left wing or progressive, as are most people strongly committed to reducing their own environmental impact, it’s awkward, because raising the issue seems to shift responsibility from the developed countries, which bear most historical responsibility for climate damage, to the develop­ing world, where population growth is most rapid.

And for anti-immigration voices on the right, the whole idea seems backwards: they worry that Europe’s population – by which they usually mean its white population – isn’t growing fast enough, so promot­ing smaller families is perverse. Above all, perhaps, there’s the simple fact that family size seems such an intensely personal matter, beyond the legitimate scope of politics or public campaigns. Just mentioning it feels somehow inappropriate.

There’s another awkward truth: historical predictions of catastrophic population explosions have tended to be badly wrong, from Thomas Malthus in the 1700s, to Paul Ehrlich in the 1960s, to the UN Population Fund, which predicted in 1987 that a world population of five billion would mean the world “could degenerate into disaster”. (The number is now well over 6.7 billion.)

Nearly everyone, meanwhile, is troubled by the notion of coercion: China’s “one-child policy”, promoted by Chinese politicians at Copenhagen as a solution to the climate crisis, has resulted in numerous reports of forced sterilisation and abortion, and rumours of infanticide. Supporters of reproductive choice are understandably appalled. Then again, trying to achieve a similar end by voluntary means, by making family planning more widespread, draws fury from the other side of the spectrum: “pro-life” campaigners, who fear a surge in abortions.

A recent study by the Optimum Population Trust (OPT) estimated that saving a tonne of CO2 costs only US$7 if the money is spent on family planning; to achieve the same by means of solar power would cost US$51. The finding paralysed environmental organisations, especially in the United States, where even the hint of increased funding for abortion carries huge political costs.

“I don’t know how to say ‘no comment’ emphatically enough,” David Hamilton, of the US green group the Sierra Club, told the Washington Post. (He had reason to be reticent: the Sierra Club suffered its own encounter with the tangled politics of population in 2004 when a group of population-control advocates tried to stage a takeover. On that occasion, just to confuse matters further, those attempting the takeover were fiercely opposed to immigration, on the ground that immigrants to the United States develop bigger carbon footprints once they arrive.)

Strictly speaking, though, none of this ought to be relevant to the parenting decisions of the average climate-conscious Briton. Perhaps the OPT is a brave voice in the wilderness – “Nobody else wants to put their head above the parapet,” says Simon Ross, an OPT trustee – or perhaps, as Monbiot says, they’re a “congregation of no ones” – a gaggle of post-reproductive white middle-class men trying to shift attention to the one part of the climate problem for which they’re not responsible. Either way, from the point of view of climate change, choosing to have one fewer child – especially if you live in a high-consumption society – remains a Very Good Thing Indeed.

And yet even that more narrowly focused topic seems to provoke a surprising degree of fury. Two years ago, Sarah Irving, then a journalist at Ethical Consumer magazine, was one of several people featured in a Daily Mail article on couples who had taken the small-family idea to its logical conclusion, opting to have no children at all. (The Mail article is inadvertently hilarious, so baffled are its authors by the concept of voluntary childlessness; one woman’s decision to have an abortion on environmental grounds is described as “the reversal of nature” and “the denial of motherhood”.)

“There were people who went to the lengths of finding my personal email address to say things like, ‘Why don’t you just kill yourself?’ ” Irving says, even though she was specifically quoted in the article as saying she’d never dream of telling other people whether or not to have children. “Generally speaking, if you’re talking about having no children at all, you’re still regarded as barmy or selfish. Or you get the patronising, ‘Oh, you know, you’ll change your mind.’ ”

Prejudice remains, too, against the idea of having only one child, even though McKibben’s book is at its strongest in his tour of the research that shows no evidence that a singleton childhood is detrimental: indeed, there are some indications that only children are more sociable and intellectually capable than their peers, because families with more children have to make their time, energy and money spread further.

But the hostility to both childlessness and one-child families explains why the OPT’s campaign targeting British people is called Stop at Two. (The Voluntary Human Extinction Movement objects strongly: “Rather than stop at two, we should stop at once,” says Les Knight.) And even the Stop at Two position caused a minor furore last year when Jonathon Porritt, the veteran environmentalist and then a UK government adviser on sustainability, told an interviewer, “I think we will work our way towards a position that says that having more than two children is irresponsible.”

“This seems to be the same old thing: save the world but kill a human,” said the pro-life campaigner Josephine Quintavalle, following her own unique brand of logic, while Ann Widdecombe, a member of parliament, labelled Porritt “absolutely barmy”.

It is possible that, in Britain at least, the issue will resolve itself naturally, since both no-child and one-child families are becoming much more common: a record one-fifth of all women turning 50 in Britain in 2010 have no children, while the percentage of children without siblings was 26% in 2007, having steadily increased from 18% in 1972. More families already Stop at Two than at any other number of children. Having three or more children is going to become more and more unusual, quite apart from more difficult to justify while claiming to care about the warming planet.

More radical visions persist, though. Alan Weisman‘s 2007 bestseller, The World Without Us, pictures the earth in the hypothetical weeks after humanity vanishes – as weeds and then trees start to break through the pavements and wild animals began to take up residence again in the midst of abandoned cities. It’s a paradisiacal vision, yet also a terrifying one, and Weisman isn’t recommending that we try to bring it about. He reaches a slightly more modest conclusion: the world would easily heal, he argues, if each person brought a maximum of one child into it. (This is intended as a thought-experiment and an inspiration, not a call for coercive policies.) By 2075, the human presence on earth would have been reduced by half.

“At such far-more-manageable numbers … we would have the benefit of all our progress, plus the wisdom to keep our presence under control,” Weisman writes. “That wisdom would come partly from losses and extinctions too late to reverse, but also from the growing joy of watching the world daily become more wonderful. The evidence wouldn’t hide in statistics. It would be outside every human’s window, where refreshed air would fill each season with more birdsong.”



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