While many commentators look ahead to 9 billion by 2050 there is a more immediate statistic that 'frightens' the UK government's chief scientist: 1 billion extra people in the next 13 years.

Speaking at a joint WWF and Royal Geographical Society (with IBG) event last week, which looked ahead to the Rio+20 conference in June, John Beddington told an audience that half of that population increase would come from Asia and most of the other half from Africa. Based on the UN's projections, he said Africa's population would grow 'frighteningly fast' from 1 billion today to 1.5 billion by 2025-2030.

He went on to lament the issue of population as 'under thought' and 'our biggest challenge' as it exacerbates existing problems over access to water and other resources.

Much of the population increase in Africa and Asia will see more people living in and migrating to areas of environmental risk, such as coastal cities, said Beddington, which as the recent Foresight report on Migration and Environmental Change points out, will put more at risk from flooding and rising sea levels.

Beddington's protestations are broadly similar to those being made by many others outside government such as Sir David Attenborough, who calls silence over the issue an 'absurd taboo'.

The silence is echoed across many environmental groups and government policymakers. A new paper by philospher Philip Cafaro, 'Climate ethics and population policy', suggests both have been fearful of wading into a host of contentious ethical issues, including family planning, abortion and immigration. The result has been limited progress in tackling ecological limits to growth and a failure to embrace one of the two primary drivers of climate change, along with consumption.

Indeed, when the Ecologist went back to Beddington's officials they clarified his remarks slightly, preferring to suggest population increases would have 'profound implications for the planet' rather than being 'our greatest challenge'.

Of course, it is ethically much easier to talk about how areas of high population growth will be impacted by climate change, as Beddington does, rather than how population growth itself is a cause of climate change and other environmental problems, as Attenborough and others do.

WWF, another group perhaps seeking to avoid controversy, suggests it is an issue for development and humanitarian organisations and instead focuses on the other primary driver of greenhouse gas emissions, overconsumption.

Others such as author Fred Pearce, have argued in the Ecologist that population growth is under control in all but a few exceptions and heading for long-term declines. As such it is a needless distraction from the issue of overconsumption, the major driver of environmental destruction.

Professor Cafaro, from Colorado State University, says both are critically important and that tackling population growth is not a reason for inaction on overconsumption. He cites one paper estimating that slowing population growth could provide 16-29 per cent of emission reductions needed to avoid dangerous climate change.

'What is the greater threat to poor people in Bangladesh, Ethiopia, Niger, Pakistan or India? Global climate change or national population growth?' Professor Cafaro asks.

'Perhaps we need not rank these two threats, since, as the example suggests, they magnify one another's potential harms. More people consuming water and longer, more frequent droughts = water shortages in Niger and Pakistan. More people living on marginal lands and harsher, more frequent storms = more deaths and environmental refugees from Bangladesh and Indonesia. Those worried about alleviating human suffering in the developing world cannot avoid population issues.

'A reasonable approach to environmental risk and a decent respect for human rights argue just as strongly for reining in harmful consumption as they do for avoiding over-population.'

But Fred Pearce has argued that consumption dwarfs population as the main environmental threat.

'Rising consumption today far outstrips the rising headcount as a threat to the planet. And most of the extra consumption has been in rich countries that have long since given up adding substantial numbers to their population, while most of the remaining population growth is in countries with a very small impact on the planet. By almost any measure you choose, a small proportion of the world’s people take the majority of the world’s resources and produce the majority of its pollution.

'The carbon emissions of one American today are equivalent to those of around four Chinese, 20 Indians, 30 Pakistanis, 40 Nigerians or 250 Ethiopians. The truth is that the population bomb is being defused round the world. But the consumption bomb is still primed and ever more dangerous.'

A new debate on population

Professor Cafaro says its time to have a more honest and open debate on population as part of trying to achieve sustainability, the ultimate goal of environmentalists and the Rio+20 conference.

'Cutting consumption is proving a tall order, with a global economy designed to provide ever more. Even amongst environmentalists we largely live like our fellow citizens. I don't know what the answer is there? The goal always seems to be to accommodate more people and more economic activity with fewer carbon emissions.'

He also suggests population decline may be as necessary as a decline in consumption in rich countries. 'Who's to say that 60 or 65 million is the optimum population of the UK, or 315 million is best for the US? It seems to me we have good evidence that those numbers are ecologically unsustainable.'

For Professor Cafaro these limits may even one day mean constraints on population and consumption.

'For many people telling them what kind of car to drive or how many children to have will seem an intolerable infringement of their rights. But then we should move expeditiously to put noncoercive or less coercive incentives in place that achieve the desired ends. If these prove insufficient, then we may have to accept stricter limits on our freedom to consume or to have children.'

With a major study by the Royal Society on population and human wellbeing due to be published in April, the debate looks certain to continue.

Tom Levitt is deputy editor at the Ecologist

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