Projections of population into the future that fail to account for the power of changing incentives are intellectually sterile explanations and policies that deny the rational response of individuals to incentives will prove impotent or worse.
Why are market-oriented economists so confident that population will be self-regulating? Well, under a regime of property rights, as we use up any scarce global resources, they will become relatively more scarce. This will put upward pressure on their prices. The first response is for individuals to cut back their consumption, but that's not the most important adjustment (that's only short term, after all).
The more important response is the one Julian Simon pointed out: the increase in resource prices creates an incentive to find more efficient means to use them or to come up with substitutes. Innovation of what we use and how we use it is the best path we know of to sustainability in development. Institutions such as property rights, the family, and, yes, even money are preconditions for aligning incentives with conservation and unleashing systematic resource-saving innovations.
Here both our deepest moral commitments, as well as sound economics, overlap with the professed beliefs of GPSO: certainly women should not have reproductive decisions forced upon them. So the message is not that nothing can go wrong. Absent secure individual rights, individual responsibility, and free markets, quite a bit can: first and foremost for the victims of injustice. However, simply admitting that these problems are real is a long way from endorsing statements like this (from the email I was sent):
the current size and growth of human population [is] a sustainability issue no less crucial than over-consumption in developed nations and all the resultant emissions, habitat loss and toxic pollutants. [emphasis in original]
I want to raise two problems with these sorts of statements. First, admitting that population growth can have adverse consequences is a long way from admitting that anyone has the knowledge to determine the "right" population size, even roughly. Statements like the one above, not to mention the affiliations of some of GPSO's signees--convince me that they believe otherwise. And I'm not arguing that sustainable population size is a difficult calculation to make, I'm arguing that it's meaningless. Sustainability means a balance between what present and future individuals want to do and can do. When human capacities and desires are by their nature heterogenous and changing over time--as in the long run that sustainabilitistas worry about--then what counts as sustainable is simply not knowable unless one knows current and future capacities and desires.
Second, I want to raise the question as to whether a "public discussion addressing the size and growth of human population" is compatible with those individual rights. If governments decide what the right population size is, and the actions of free and responsible individuals give rise to a different population size, either the population target or individual rights must be sacrificed. I believe--I hope--that the GPSO signees would abandon their plan. History has shown too many willing to do the opposite. It is for this reason that, while I obviously do not believe women should be coerced, I cannot sign onto "population justice" as defined by GPSO.
If it is excessive procreation we are worried about, we would do well to remember the words of Henry Simon: "Academic economics is primarily useful, both to the student and to the political leader, as a prophylactic against popular fallacies."