The worst thing that you or I can do for the planet is to have children. If they behave as the average person in the rich world does now, they will emit some 11 tonnes of CO ² every year of their lives. In their turn, they are likely to have more carbon-emitting children who will make an even bigger mess. If Britain is to meet the government's target of an 80% reduction in our emissions by 2050, we need to start reversing our rising rate of population growth immediately.

And if that makes sense, why not start cutting population everywhere? Are condoms not the greenest technology of all?

World population is forecast to peak at 9.2bn by 2050. According to a report by the LSE for the Optimum Population Trust, the lobbying body currently asking parents to "Stop at Two", it would cost $220m to provide the family planning that would reduce the 2050 population by half a billion, preventing the emission of 34 gigatonnes of carbon. Introducing low-carbon technology for the same result would cost more than $1 trillion.

So why does population control hardly feature on the agendas of the UN bodies or of the governments now committed to tackling climate change? And why do the development and environmental groups shy away from it? The Guardian's George Monbiot dismisses the topic as a distraction, the obsession largely of "post-reproductive, middle-class white men… a group more responsible for environmental destruction than any other class in history". David King, the government's former chief scientific adviser, argues: "The only way to tackle climate change is to change the way energy is used by those of us that have already been born."

It is certainly true that "fewer people equals a greener planet" is simplistic. In 2050, 95% of the extra population will be poor and the poorer you are, the less carbon you emit. By today's standards, a cull of Australians or Americans would be at least 60 times as productive as one of Bangladeshis.

As a result, NGOs such as Oxfam, for whom I've just written a report on climate change's impact on humans, insist that dealing with consumption in the rich world is much more important than tackling population growth. According to the International Energy Agency, if the whole world moved over to clean electricity, the CO ² savings would offset the emissions of up to 2.8bn poor people, easily accounting for the entire extra population forecast for 2050.

But what if we can't reform the way we produce and use energy? The most worrying of climate change's impacts – food and water shortages, forced migration, health epidemics – are exacerbated by population growth. According to two recent polls, nine out of 10 scientists working in climate change don't believe we will achieve the changes in energy use committed to by the G8 and the EU. If they are right, population is going to start to matter a lot. Don't we need a fallback plan?

The NGOs believe it hypocritical to target the poor for having lots of children. It is one of the universal coping mechanisms of poverty; our own great-grandparents may well have used it. And who made the mess, anyway? As Rachel Baird, who works on climate change for Christian Aid, says: "Often in the countries where the birth rate is highest, emissions are so low that they are not even measurable. Look at Burkina Faso." So why ask them to pay in unborn children for our profligacy?

It's a powerful argument, but it highlights a paradox at the heart of the debate on climate-change adaptation. It is assumed that vulnerable countries will adapt best through economic development. The richer a country, the better it will cope with the shocks. But as countries develop, they emit more carbon. China's per person emissions nearly doubled in the first half of this decade, to 4.6 tonnes.

Under normal circumstances, it takes perhaps a generation for the birth rate to drop with increasing wealth, whereas carbon emissions go up very quickly. As people get richer, they buy cars, use air conditioning, consume more calories and start to swap their vegetables for meat.

So the richer a country gets, the more pressing the need for it to curb its population. The only nation to have taken steps to do this is China – and the way it went about enforcing the notorious one child policy is one of the reasons the rest of us are so horrified by the notion of state intervention. Yet China now has 300-400 million fewer people. It was certainly the most successful governmental attempt to preserve the world's resources so far.

But lowering birth rate need not be so draconian. Experience shows it is most effectively done by ensuring women's equality and improving their education, while providing cheap contraception. Birth rate, gender equality, education and poverty are inextricably linked.

But how do you reduce population in countries where women's rights are already achieved and birth-control methods are freely available? Could children perhaps become part of an adult's personal carbon allowance? Could you offer rewards: have one child only and you may fly to Florida once a year?

After all, based on current emissions and life expectancy, one less British child would permit some 30 women in sub-Saharan Africa to have a baby and still leave the planet a cleaner place.

If you have faith in the rich world's ability to achieve those 80% cuts in emissions in a mere 40 years, you need not concern yourself too much about population. But if you are sceptical, you should be worried. A lot.

Some scientists, the German chancellor's adviser, Hans Joachim Schellnhuber among them, say that if the cuts are not achieved, we will end up with a planet with a "carrying capacity" of just 1bn humans. If so, we need to start cutting back population now with methods that offer a humane choice – before it happens the hard way

• This is an edited version of an article that first appeared in the November issue of Prospect magazine.