The simplest truths are sometimes the hardest to recognise. This month, according to the UN, world population will reach 6.7 billion, en route to a newly revised global total of 9.2 billion by 2050. The latest housing forecasts for England predict that we will need about 5m more homes in the next two decades. The economist Jeffrey Sachs devoted this spring's Reith lectures to a planet "bursting at the seams". And the most recent Social Trends analysis from the Office for National Statistics painted a picture of a Britain driven mad by overcrowding. Meanwhile, Gaia scientist James Lovelock has been warning about ecological collapse and world resources able to support only 500 million people, with many extra millions driven to take refuge in the UK.
In the midst of all these alarms is a very quiet place where the green lobby should be talking about human population growth. Today has been designated World Population Day by the UN, but you will not see any of the big environment and development groups mounting a campaign on population. Indeed, you will be lucky if they even mention the P-word. Earlier this year, Nafis Sadik, former director of the UN's population fund, berated such non-governmental organisations for being more concerned with fundraising than advocacy. Their silence on population, she observed, was "deafening".
So why isn't the green movement talking about population any more? In its early days, back in the 60s and 70s, population growth was a mainstream concern. Groups including Greenpeace, Friends of the Earth (FoE), WWF and Oxfam took well-publicised positions on population issues - endorsing the Stop at Two (children) slogan, supporting zero population growth and publishing reports with titles such as Already Too Many (Oxfam). These days, Greenpeace declares that population is "not an issue for us" and describes it as "a factor [in] but not one of the drivers of" environmental problems.
FOE last year tried to answer some "common questions" on the subject, including: "Why isn't Friends of the Earth tackling population growth?" Oxfam, which as recently as 1994 published a report entitled World Population: The Biggest Problem of All, now does not list it among the dozen or so "issues we work on", and nor does it figure in the "What you can do" section of WWF's One Planet Living campaign.
The green lobby's main argument is that numbers do not matter so much - it is how we live and consume that counts. FoE even remarks that "it is unhelpful to enter into a debate about numbers. The key issue is the need for the government to implement policies that respect environmental limits, whatever the population of the UK". It is a statement that seems to treat population and environmental limits as entirely separate subjects.
There are two powerful counter-arguments to this. One is common sense: that consumption and numbers matter and that if a consumer is absent - that is, unborn - then so is his or her consumption. The second is the weight of evidence. Sir David King, the government's chief scientist, told a parliamentary inquiry last year: "It is self-evident that the massive growth in the human population through the 20th century has had more impact on biodiversity than any other single factor."
The increase in global population over the next 40 years, for example, is roughly what the entire world population was in 1950. The UK, currently around 61 million people, is on course for 71 million by 2074, by which time England's densities will have outstripped those of South Korea, which, by some measures, is currently the world's second most crowded country - second only to Bangladesh.
The Optimum Population Trust today publishes a new report, Youthquake, that warns - echoing Lovelock - that environmental degradation caused by the number of humans may force more governments to follow China's lead and introduce compulsory limits on family size.
Ironically, the world now views climate change as the greatest environmental threat but sees the solution in primarily technical terms. Yet expert bodies routinely identify human numbers as one of the main engines of climate change. Of the various social and technical factors involved, for example, the UN says: "The link to population is clearest: the more people there are, the higher emissions are likely to be."
Many suspect other motives for the green lobby's neglect of the population issue. It is a sensitive subject, bound up with issues on which the progressive left, which most environmental groups identify with, has developed a defensive intellectual reflex. These include race and immigration - the latter accounts for more than 80% of forecast UK population growth, for example - reproductive choice, human rights and gender equality. Calls for population restraint can easily be portrayed as "anti-people" - surely people are part of "the solution"? It is far easier to ignore the whole subject; let somebody else - or nobody - deal with it.
This often involves intriguing verbal contortions. The 70s organisation Population Countdown, having morphed into Population Concern, in 2003 rechristened itself as Interact Worldwide - under its former name, consultants told it, its funders, and future, would dry up.
Faced with escalating forecasts of housing need - one recent government projection says we will need 11m more households in the UK by 2050, an increase of over 40% - the Campaign to Protect Rural England (CPRE) proclaims itself in favour of "development that protects the countryside and the environment" and ignores the fact that the main cause of forecast housing growth, responsible for 59% of the total, is population increase.
So why does the CPRE not campaign on the issue that poses the greatest threat to rural England? "If we did," says Shaun Spiers, CPRE's chief executive, "it appears unlikely that our actions would have any effect on population growth, and that would lay us open to the charge of misusing our charitable funds."
How to categorise such reactions? Pragmatism? Cowardice? Sensible tactics? Or an overdose of organisational self-preservation? Whatever the reason, it is infectious - the media (and politicians) take many of their awareness cues from NGOs so the silence on population becomes society-wide. As a result, family size is seen as an exercise in individual lifestyle choice: few people consider the consequences for the planet of their fertility decisions. That means fertility rates in the UK rise, and the population keeps on growing.
It was Mark Twain who observed that those who refused to share vital information with others were guilty of a "silent lie". The green movement needs to start telling us the truth.
· David Nicholson-Lord is an environmental writer and research associate for the Optimum Population Trust. The Youthquake report is available at optimumpopulation.org
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