Population growth is as potentially catastrophic as climate change. So why aren’t we talking about it?

Two colossal challenges face the Earth in the 21st century, and threaten its very habitability by human beings, yet widespread concern focuses only on one of them. This anomalous, not to say crazy situation has in the past fortnight been made clearer than ever before.

The first challenge is that of climate change, the dire nature of which was formally recognised at the climate summit at the UN in New York last week by 120 world leaders, including Barack Obama and David Cameron (joined by the Hollywood star Leonardo DiCaprio). They were spurred on to do so by the climate protest marches which took place in more than 160 countries a few days beforehand, with more than half a million marchers taking part, including 300,000 in New York and 40,000 in London. And more scientific backing was given to the case for action by a new report from leading scientists, published on the day of the marches, warning that the point of no return for combating global warming would be reached within thirty years.

This report, from the Global Carbon Project, an academic coalition which monitors the soaring carbon dioxide emissions causing the atmosphere to warm, led by Professor Corinne Le Quéré from the University of East Anglia, was published on 21 September, in Nature, one of the world’s two leading scientific journals. Its concerns were widely noticed by all those making their voices heard on the grim dangers of an overheating world.

Three days earlier, on 18 September, an equally significant report on the dangers facing the Earth in the coming century was published in the other leading scientific journal, Science; yet its conclusions were on the lips of few if any of the world leaders in New York, or of the climate marchers in cities around the world. That is because its subject matter was what has become the Great Unsayable, the Truth Which Dare Not Speak Its Name: the threat to the planet from the uncontrollable increase in human numbers.

The Science report, from a team led by Patrick Gerland of the UN Population Division in New York and Professor Adrian Raftery of the University of Washington in Seattle, was groundbreaking, in that it overturned the principal assumption of world population policy of the past 20 years: that human numbers, now up to 7.2bn, from 3bn in 1960, would rise to a peak of about 9bn by mid-century, then level off or even decline, as fertility levels fell in the developing world.

This decline is not, after all, going to happen, the report says. Its headline is stark: “World population stabilisation unlikely this century.” Instead, it says, there is an 80 per cent probability that the world population will increase to between 9.6 and 12.3bn by 2100. Most of the increase will be in sub-Saharan Africa, the reason being that fertility rates there are remaining very high, partly because of the lack of women’s education, and partly because of lack of access to contraception services.

The Gerland/Raftery report is studiously restrained, as a scientific paper has to be, in its comment: “Because rapid population increase in high-fertility countries can create challenges ranging from depletion of natural resources to unemployment to social unrest, the results of this study have important policy implications.” But it is prefiguring a catastrophe, not least because of the other great threat, climate change itself; climate destabilisation and soaring human numbers will not be separate phenomena. They will combine, and act upon, and reinforce each other. One of the biggest threats of global warming is thought to be to agricultural production in sub-Saharan Africa; think how that threat will be intensified when the mouths to feed there are five times more numerous than they are today, as the new report predicts for 2100.

Jonathon Porritt, one of Britain’s leading environmentalists, recognises the threat. “The simple truth is that continuing population growth is a multiplier of every one of today’s converging sustainability pressures,” he says. Other leading green figures, including David Attenborough and Professor Jim Lovelock, have begun to agree with him openly; but from the green movement as a whole, the silence on the subject is deafening. No marches about population growth; the very idea seems unthinkable.

The projected vast increase in human numbers is a threat to the Earth’s life-support systems, to its “carrying capacity”, every bit as much as climate change is, and although it is a difficult issue, it is ludicrous simply to ignore it and pretend it is not there. There are two great dangers facing the planet, not one. And not to recognise that is crazy indeed.