One spectre that has long haunted the environmental debate is that of population size. Partly, that controversy seems to derive from some of the extremely dodgy characters who have made it a top concern. Plenty of very ill-informed commentators have based doomsday scenarios around population growth figures. Still, there are reasonable people taking a similar line and it does seem intuitively obvious that fewer human beings would put less strain on limited resources, all else being equal. Particularly among those who want to 'make poverty history' (a noble goal, though only possible when poverty is measured in absolute terms), it seems clear that six billion people simply cannot live at the level of affluence of today's richest, barring some massive change in the way resources are acquired and transformed into goods.
The classic environmental liberal argument says that as people become richer, their family sizes start to fall. This may be because they are better educated and women gain both access to birth control and the knowledge and freedom to use it. It may be because people in relatively undeveloped economies use large families as a strategy to avoid poverty in old age. With the advent of banking, pensions, and the like, the need to do so diminishes. The evidence for slowing population growth is certainly strong, with the UN projecting that the human population will peak sometime around 2050.
For me, the absolute number of people on the planet is obviously far less important than the conditions under which they live. At one point in human history, after the Toba eruption, there may have been as few as 2000 human beings on the planet: living in conditions similar to those of a nuclear winter. Obviously, population size and quality of life are not perfectly correlated. By that metric, population can perhaps best be thought of in terms of the effect it has on people's lives: especially those of women and the poor. The Rawlsian strategy of focusing on the effect on the least advantaged does have an intuitive moral appeal to it.
The great appeal of the 'greater knowledge and empowerment leads to use of birth control and slowing population growth rates' argument is that it serves both the goal of reducing eventual population and the much more immediate goal of helping women to be in control of their reproductive lives, as well as their lives more generally. Given how a hugely disproportionate amount of injustice is directed towards women worldwide, and given the huge inherent dangers in childbirth, even in the rich world, this seems an almost universally appealing kind of development.
One last fallacy should be addressed, in closing, though it's one well covered enough already that I doubt it will be unfamiliar to anyone. It's not the countries that have hundreds of millions of poor people that are using the majority of available resources. Patterns of consumption are not only too high, when it comes to limited resources, but dramatically skewed towards the richest consumers. Each year, humanity as a whole uses as much oil as forms naturally in about 400 years. Taking a look at who is benefitting from that, it is unjust as well as unsustainable.
I suppose the safe, but less than entirely satisfying, conclusion is that we can't take an issue like population and make sweeping generalizations about it, without more cautious consideration of what the important aspects of the situation are and how they relate to moral judgments and non-moral facts. Still, it's not a thing we should shy away from discussing, just because some of the questions and implications are uncomfortable.
This article is also featured on my blog: http://www.sindark.com
Information on the Toba eruption at: http://www.economist.com/surveys/displaystory.cfm?story_id=5299220