It must rank among the most remarkable events in recent human history. In just 60 years, the global average number of children each woman bears has fallen from 6 to 2.5. This is an astonishing triumph for women's empowerment, and whatever your position on population growth, it is something we should celebrate.

But this decline in fertility, according to the United Natinos report published on Wednesday, is not the end of the story. It has also raised its estimate of global population growth. Rather than peaking at about 9 billion in the middle of this century, the UN says that human numbers will reach some 10 billion by 2100, and continue growing beyond that point.

That's the middle scenario. The highest of its range of estimates is an astonishing 15.8 billion by 2100. If this were correct, population would be a much greater problem – for both the environment and human development – than we had assumed. It would oblige me to change my views on yet another subject. But fortunately for my peace of mind, and, rather more importantly, for the prospects of everyone on earth, it is almost certainly baloney.

Writing in the journal Nature in May, Fred Pearce pointed out that the UN's revision arose not from any scientific research or analysis, but from what appeared to be an arbitrary decision to change one of the inputs it fed into its model.

Its previous analysis was based on the assumption that the average number of children per woman would fall to 1.85 worldwide by 2100. But this year it changed the assumption to 2.1. This happens to be the population replacement rate: the point at which reproduction contributes to neither a fall nor a rise in the number of people.

The UN failed to explain this changed assumption, which appears to fly in the face of current trends, or to show why fertility decline should suddenly stop when it hit replacement level, rather than continuing beyond that point, as has happened to date in all such populations. I expected yesterday's report to contain the explanation, but I was wrong: it appears to have plucked its fertility figure out of the air.

Even so, even if we're to assume that the old figures are more realistic than the new ones, there's a problem. As the new report points out, "the escape from poverty and hunger is made more difficult by rapid population growth". It also adds to the pressure on the biosphere. But how big a problem is it?

If you believe the rich, elderly white men who dominate the population debate, it is the biggest one of all. In 2009 for example, a group of US billionaires met to decide which threat to the planet most urgently required their attention. Who'd have guessed? These men, who probably each consume as many of the world's resources in half an hour as the average African consumes in a lifetime, decided that it was population.

Population is the issue you blame if you can't admit to your own impacts: it's not us consuming, it's those brown people reproducing. It seems to be a reliable rule of environmental politics that the richer you are, the more likely you are to place population growth close to the top of the list of crimes against the planet.

The new report, inflated though its figures seem to be, will gravely disappoint the population obsessives. It cites Paul Murtaugh of Oregon State University, whose research shows that:

"An extra child born today in the United States, would, down the generations, produce an eventual carbon footprint seven times that of an extra child in China, 55 times that of an Indian child or 86 times that of a Nigerian child."

And it draws on a paper published in the Proceedings of the National Academy of Sciences, which makes the first comprehensive assessment of how changes in population affect carbon dioxide emissions. It concludes:

"Slowing population growth could provide 16-19% of the emissions reductions suggested to be necessary by 2050 to avoid dangerous climate change."

In other words, it can make a contribution. But the other 81-84% will have to come from reducing consumption and changing technologies. The UN report concludes that "even if zero population growth were achieved, that would barely touch the climate problem".

This should not prevent us from strongly supporting the policies which will cause population to peak sooner rather than later. Sex education, the report shows, is crucial, as is access to contraception and the recognition of women's rights and improvement in their social status.

All these have been important factors in the demographic transition the world has seen so far. We should also press for a better distribution of wealth: escaping from grinding poverty is another of the factors which have allowed women to have fewer children. The highly unequal system sustained by the rich white men who fulminate about population is one of the major reasons for population growth.

All this puts conservatives in a difficult position. They want to blame the poor for the environmental crisis by attributing it to population growth. Yet some of them oppose all the measures – better and earlier sex education, universal access to contraception (for teenagers among others), stronger rights for women, the redistribution of wealth – that are likely to reduce it.

And beyond these interventions, what do they intend to do about population growth? As the UN report points out:

"Considerable population growth continues today because of the high numbers of births in the 1950s and 1960s, which have resulted in larger base populations with millions of young people reaching their reproductive years over succeeding generations."

In other words, it's a hangover from an earlier period. It has been compounded by another astonishing transformation: since the 1950s, global life expectancy has risen from 48 to 68.

What this means is that even if all the measures I've mentioned here – education, contraception, rights, redistribution – were widely deployed today, there will still be a population bulge, as a result of the momentum generated 60 years ago. So what do they propose? Compulsory sterilisation? Mass killing? If not, they had better explain their programme.

Yes, population growth contributes to environmental problems. No, it is not the decisive factor. Even the availability of grain is affected more by rising livestock numbers and the use of biofuels – driven, again by consumption – than by human population growth.

Of course we should demand that governments help women regain control over their bodies. But beyond that there's little that can be done. We must instead decide how best to accommodate human numbers which will, at least for the next four decades, continue to rise.

(By George Monbiot, The Guardian, 27/10/2011)