Twenty-five years after its founding by the United Nations, the Intergovernmental Panel on Climate Change has come to the conclusion not only that the phenomenon of global warming is beyond serious scientific doubt, but also that we - human beings - are the dominant cause.
We all knew this, of course. But now, thanks to the efforts of more than 800 scientists from 32 countries, we can say so with certainty.
Curious, then, that in the IPCC's fifth assessment of climate change since 1988, published last Friday, one word is conspicuous by its absence: population.
How times have changed. In 1990, when the IPCC published its first assessment report and there were 5.3 billion people on Earth, the panel's scientists had no qualms about identifying the primary culprit in global warming.
"The predicted population explosion," they reported, "will produce severe impacts on land use and on the demands for energy, fresh water, food and housing." It was "essential that global climate-change strategies take into account the need to deal with the issue of the rate of growth of the world population".
Since then, the population has hit seven billion and is expected to pass nine billion by 2050 - almost twice as many as the IPCC warned that the Earth was struggling to support 25 years ago - and yet the issue of population growth seems to have slipped from the agenda.
So how did population become the elephant in the room?
That was a question posed in 2011 by Sir David Attenborough, the British naturalist, television presenter and patron of the Optimum Population Trust, now known as Population Matters. As the world's population approached seven billion, he pointed out in a speech that the subject had not been mentioned at either of the UN climate-change conferences, in Copenhagen in 2009 or Cancún in 2010.
"Why this strange silence?" he asked. "I meet no one who privately disagrees that population growth is a problem . so why does hardly anyone say so publicly?"
There was, he concluded, a "bizarre taboo around the subject - 'It's not quite nice, not PC, possibly even racist to mention it' . I simply don't understand it. It is all getting too serious for such fastidious niceties".
In 2011, the UN was obliged to revise its earlier estimate that the world's population would peak at 9.1 billion in 2100. That milestone, it said, would now be passed in 2050. By 2100, there would be 10 billion of us.
In a world faced with global warming, increasing water shortages, encroaching desertification and regular food shortages in Africa - the 2011 famine in Somalia alone claimed 250,000 lives - 10 billion seems like an awful lot of people.
So should we panic? Will we run out of space and food, thrusting the world into a bitter fight to the death between the haves and have-nots? Or can we rely on our ingenuity to continue coming up with technological solutions - improved agricultural yields, more efficient use of resources such as oil, electricity and water, and so on?
Two new books that address the issue take dramatically opposing views, demonstrating that, despite all our evolved cleverness, the realities of planetary demographics in a globally interconnected world have become so complex - and so bound up in political ideology - that no two experts seem able to agree on the most fundamental aspect.
"Stop worrying," writes Danny Dorling, until recently professor of human geography at the University of Sheffield, in Population 10 Billion. His book suggests that "the actual number of people on the planet is, to an important extent, incidental to the impact humans have on both the environment and each other".
Looking around at a world still defined most starkly by its inequalities, the optimistic Dorling sees "many signs that we may well collectively be choosing more often to live sustainably, not least in how we are already controlling our numbers".
There is, he writes, "nothing too bizarre when it comes to fears over future human population numbers", and he mocks groups such as the Voluntary Human Extinction Movement, which advocates a moratorium on breeding to "allow Earth's biosphere to return to good health".
Instead, he bangs the drum for "the boring old practical possibilists", bemoaning that "there are no T-shirts . with the slogan 'There probably is enough food for all', or 'Worry less, humans are cooperative'."
The antidote to Dorling's cheerful celebration of the innate goodness of human nature is supplied by Stephen Emmott, head of computational science at Microsoft Research in Cambridge, England, whose polemic 10 Billion was launched at London's Science Museum on July 18.
This is a man who plainly sees the elephant. In his view, "our cleverness, our inventiveness and our activities are now the drivers of every global problem we face - and every one of these problems is accelerating as we continue to grow towards a global population of 10 billion".
We are, says Emmott, facing "an unprecedented planetary emergency . we urgently need to do - and I mean actually do - something to avert global catastrophe. But I don't think we will."
To make his point, he closes his book - a short, sharp, shocking assessment of the state we are in - with the following disturbing anecdote.
"I asked one of the most rational, brightest scientists I know - a young scientist working in this area in my lab - if there was just one thing he had to do about the situation we face, what would it be. His reply? 'Teach my son how to use a gun.'"
So who's right? What should we do? Relax and invest in GM food futures - or arm our children and pack them off to the hills with a lifetime's supply of canned tuna?
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Can there really be no limit to the Earth's capacity to support its most troublesome and destructive species? After all, even allowing for yet-to-be-invented technological wizardry, surely a square metre of soil can yield only a finite amount of calories, sufficient to sustain only a finite number of human beings?
Not an issue, says Dorling, because all the demographic evidence suggests that "human population growth is not just slowing, but is set to stabilise within the current lifetimes of a majority of people on the planet".
But how solid is that evidence?
There are multiple problems when it comes to accurately assessing the current population of the world, let alone predicting what it will be at any point in the future.
On the one hand, says Dorling, speaking by telephone from Oxford, where he has just taken up the post of professor of human geography at the university's Centre for the Environment, "you've got to realise how better at it we are now than we ever were. Around 1950 we really had no idea what the population of the planet was".
On the other hand, he concedes, "very tiny changes in [human] behaviour, like compound interest on money, can have a big impact in a hundred years' time".
All this explains why the Population Division of the UN Department of Economic and Social Affairs hedges its bets and offers three scenarios for the future population of the Earth - the low, medium and high variants - and why its latest predictions, issued last year, venture no further into the future than 2100.
Depending on which of these three scenarios one favours, the population in 2050 could be anything between 8.34 billion and 10.86 billion.
Of course, 2050 is just around the corner and relatively easy to predict. Follow the UN's scenarios to 2100, however, and its numbers range from 6.75 billion to 16.64 billion - a spectrum so wide as to be almost meaningless.
However, these figures conceal crucial possibilities - perhaps the most crucial of which, as Dorling maintains, is that while the absolute number of human beings is still increasing, it is doing so at a rate that is slowing down.
This is what demographers call the total fertility rate - the averaged-out number of children born to each woman. The replacement rate - the number of births required to keep the population stable, allowing for some inevitable child deaths - ranges from about 2.1 babies per woman in the developed world to 3.3 in countries where infant mortality is higher.
Current estimates say there are 111 countries where the fertility rate is 2.1 or above, with the highest rates found in Africa, notably Niger (7.03), Mali (6.25) and Somalia (6.17).
It is easy, of course, to point to the escalating populations in Africa as a cause for concern. But in another 113 nations, mainly in the developed world, the fertility rate is so low that numbers are dwindling. The total fertility rate in Europe, for instance, is just 1.59 babies per woman.
For the world as a whole, the estimated number of total births per woman is 2.45 - a growth rate of 1.095 per cent. That doesn't sound like much, until one applies the maths of compound interest. If this growth rate remained stable, within a century today's population of seven billion would have ballooned to over 20 billion.
But, say demographers, this almost certainly won't happen, because the global fertility rate has been steadily dropping, from a high of 5.02 in the mid-1960s, to today's 2.45. It took us 13 years to add our most recent billion, opposed to just 12 years for the one before that. If the total fertility rate continues to drop as it is, each subsequent billion will come at longer and longer intervals, until the world's population hits about 10 billion and then starts to shrink, in about 2060.
In fact, predicted the International Institute for Applied Systems Analysis in 2008, if the global fertility rate dropped to 1.0 - and it certainly seems to be heading that way in Europe and elsewhere - by 2100 we could be back to just 4.5 billion. No problem, then? Not quite.
For a start, why is the population rate falling? We are not exactly "choosing" to control our numbers, as Dorling asserts. As he concedes elsewhere in his book, this "choice" is being made for us by circumstances - as more and more people migrate from poor but self-sufficient rural environments, where birth rates are high, to consumption-driven cities, where out of necessity fewer children are born.
The revolution in the empowerment of women around the world is also playing a significant part in our declining numbers - education invariably leads to delayed breeding and fewer children, says Dorling.
The UAE provides a good example of how prosperity, education and the empowerment of women lowers fertility rates. Large families are traditional in the Gulf states, and UN figures show that between 1950 and about 1975 women in the UAE had on average more than six children each.
However, the fertility rate among Emiratis began to decline as the UAE became more prosperous, first dropping below that required for replacement between 2005 and 2010, when the average number of births per woman was 1.97. According to UN estimates, that figure is predicted to decline slowly to a low of 1.6 in about 2030 and remain well below the replacement rate for the foreseeable future - as far as 2100.
But with prosperity comes greater consumption - so a smaller, better-off global population could be far more damaging to the planet than a larger, more impoverished one.
Sitting back and waiting for economic evolution to slow birth rates around the world sounds like a good idea, says Simon Ross, the chief executive of Population Matters, but in terms of the impact on planetary resources, it really isn't.
"If you wait until the population of Nigeria, for example, reaches the population of the United States and then becomes rich, that puts an insupportable burden on resources," he says. "It's much better to try to drop the birth rate as soon as you can, rather than wait for everyone to consume a lot."
Nevertheless, Dorling insists it is better to work on changing the behaviour of the affluent than that of the poor: "One person flying to New York and back, on a whim to go on a shopping trip that doesn't even make them happy, that's the kind of thing that's problematic," he says. "You can work out how many dozen people have to be born in the Yemen to use up in their whole lifetime the equivalent of one shopping trip across the Atlantic to New York."
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But what's the prognosis for the planet and its limited resources if we turn African or Yemeni subsistence farmers into globe-trotting super-consumers? "Food demand," writes Emmott, "is accelerating at a far faster rate than population growth . as GDP increases, calorie consumption also increases. As we get richer (or suffer less poverty), we consume more food."
Any way you look at it, a couple who have four children double their impact on the world's resources (the argument goes that parents with only two children preserve the status quo on their death). And when those four children are born to wealthy parents, their impact is much worse than if they had been born into a poor agricultural community. But who are we, having pulled up the ladder behind us, to suggest that the rest of the world should stick to an ecologically more sustainable lifestyle?
This, says Dr Stuart Basten, a research fellow at Oxford University's department of social policy and intervention, is where the population debate finds itself tiptoeing around sensibilities rooted in the colonial attitudes of 19th-century Europe.
He agrees with Dorling's point that "the raw number of people is much less important than how they are going to live their lives", but there is, he says, clearly a difficult relationship between economic development and environmental protection in the developing world, which liberal westerners find hard to confront.
"It seems to come down to the view, 'How dare we?'," admits Basten. "We've done a pretty good job of screwing up the environment for the last 250 years, to get the things that we want, and it becomes quite difficult to start lecturing people about the loss of species such as the slow loris and gum trees."
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All we know for certain right now is that all previous predictions about global population - both its possible size and the maximum number the planet can sustain - have proved incorrect. But regardless of our future numbers, says Emmott, the current reality is that "all of the science points to the inescapable fact that we are in serious trouble. Right now, we are heading into completely uncharted territory as our population continues to grow towards 10 billion . the one thing that is predictable is that things are going to get worse." It's a familiar refrain, although that doesn't necessarily make it wrong.
In 1968, Paul Ehrlich, a professor of population studies at Stanford University, wrote The Population Bomb, a bestselling book later credited with having fired one of the opening salvos in the battle to preserve the environment, which predicted imminent starvation and called for immediate action to limit the growth of population.
In 2009, in an article in the Journal of Sustainable Development, Ehrlich revisited a book he admitted had been "both praised and vilified", but insisted its basic message was "even more important today than it was 40 years ago".
Much of the negative response to the book had been a reaction to that main message, "that it can be a very bad thing to have more than a certain number of people alive at the same time, that Earth has a finite carrying capacity, and that the future of civilisation was in grave doubt".
For the politically far left, wrote Ehrlich, the suggestion that population growth should be limited was immoral; they saw the basic problem "not as overpopulation but as maladministration of resources and worried that the far right would use overpopulation as an excuse to promote births of only the 'right kind'".
The far right, meanwhile, "wed to the idea that free markets could solve any problem, didn't like the idea that population size was a legitimate area for government intervention". Neither camp - nor those opposed to contraception and sex education - "seemed to understand that the fundamental issue was whether an overpopulated society . could avoid collapse". For him, writing in 1998, the contemporary population of 5.5 billion had already "clearly exceeded the capacity of the Earth to sustain it".
Later, Ehrlich redirected the debate away from the heated subject of maximum possible numbers to a more finely nuanced consideration: the optimum population of the Earth, defined by "the biophysical carrying capabilities of the planet", and which could be viable with as few as just 500 people.
This, clearly, was a utopian vision (or dystopian, depending on one's viewpoint), but it came packaged with the observation that "many more human beings could exist if a sustainable population were maintained for thousands to millions of years than if the present population overshoot were further amplified and much of Earth's capacity to support future generations were quickly consumed".
This is an indisputable truth, overlooked in all the debates that focus solely, and selfishly, on the rights and needs only of the human beings currently walking the earth. It should, perhaps, play a greater part in our thinking when we talk grandly of the world we would like our children to inherit.
Jonathan Gornall is a regular contributor to The National.
