Many people are frustrated by the fact that any consideration of the impact of human population growth on the environment, sustainable development and climate change seems to be off limits at almost every level of political discourse.
Many people are afraid that any mention of population growth as even a factor in ensuring a sustainable future for the planet risks them being branded as someone denying the right of developing world countries to develop, or ignoring the disproportionate impact of the excessive consumption, waste and pollution of the developed world, or worst still being racist.
So here’s my twopenth worth, flak jacket and helmet in place!
When we consider population growth, the starting point must be the principle that all citizens are entitled to an equal share of the earth’s resources and that those resources (for the sake of future generations) must be exploited sustainably.  This is the principle behind Contraction and Convergence (C&C), which is the model for reducing greenhouse gas emissions favoured by many NGOs. C&C’s starting point is that every citizen has an equal right to emit greenhouse gases within a world limit that keeps emissions below 450 parts per million CO2 equivalent. The model works by the high polluting countries cutting down their emissions until they converge with the emission levels of low polluters which will be allowed to increase to a limited extent to enable economic growth.
The total population of the planet is central to all sustainability issues.  Humans are the only significant living creatures who live without regard to the sustainability of their environment.  This is primarily because of excess consumption by inhabitants of developing countries, but also due to the over-exploitation of immediate natural resources by millions of desperately poor and under-nourished people.  Unsustainable practices take many forms but sustainability can be objectively and scientifically quantified.  The simple fact is that, by definition, unsustainable practices cannot continue indefinitely and the longer we fail to address all such practices, not just CO2 emissions, the more difficult it will be to solve and the greater will be the suffering, particularly for the world’s poorest people.
We should be concerned as much about the quality of life as about economic well-being.  What quality of life is there when every scrap of land is cultivated, probably with G.M monocultures in order to feed our teeming cities? Actually, our abundant planet has enough resources to sustainably give every single person a high quality of life but almost certainly not 6 billion of us and certainly not 9 billion.  A population of 9 billion people will put unbelievable pressure on the earth’s agricultural resources and is likely to involve a last desperate throw of the chemically-induced, water-denuding practices of the so called ‘green revolution’ of the post war era.
The onus is overwhelmingly on the inhabitants of the developed world and their governments to guarantee a quality of life to people in the developing world through measures such as supporting projects that empower women, facilitating fair trade, introducing sustainable energy and agricultural technologies, supporting reforestation initiatives, and so on.  At the same time, the population of the developed world must stabilise or fall and consumption of all resources, especially those that are heavily energy-dependant, must be significantly reduced.
Tackling population growth is not about being for or against certain religions or cultures (though the hierarchies of some are less than helpful), it’s not about curtailing an inalienable right to reproduce; it’s about a decent quality of life for all humans and enough space on the planet for biodiversity to flourish.