The role played by concerns about overpopulation in contemporary political discourse is interesting. Though overpopulation was a central concern for earlier waves of environmentalism, today's self-professed environmentalists rarely mention it. You see concern most often raised by people who are liberal but who focus their energies on other causes, as one of their occasional nods to the environmentalist wing of the progressive coalition. Most often, it's used as a rebuttal to nativist complaints that (white) people aren't breeding fast enough, or as a moral-high-ground justification for choosing not to have kids.
The framing of the population issue -- for example in this comic strip -- resembles the ticking time bomb" scenario
used to argue in favor of torture. In both cases, a neat logical setup is created that makes a certain conclusion ("we should torture" or "overpopulation is a problem") rational. But in both cases, the relevance of the hypothetical to the real world situation it analogizes is questionable. Real conterterrorism problems simply do not resemble the ticking time bomb scenario in important ways.
So it's quite true that, given a finite amount of a rivalrous resource, and a fixed minimum per capita demand, it's possible to have simply too many people. The question is, is that a useful way of conceptualizing the environmental problems we actually face? To say that it is, we would have to establish that population size is a primary driver of environmental problems, and that we are now (or will be in the forseeable future) at a population size that cannot be cost-effectively offset by changes in other factors. It's important to note here that population is expected to level off, at least at the global level, sometime this century, meaning that critics of the population bomb have a finite number of humans whose sustainability they have to account for. At the global level and within the developed world, at least, I'm far from convinced that sheer population size is a key problem.
I'll limit my discussion to environmental problems that can be usefully framed as problems of resource inadequacy (while noting in passing that this excludes the two environmental issues that I've researched most extensively, inappropriate fire regimes and brownfield cleanups). This includes both the classic not-enough-to-go-around scarcity problems, as well as situations in which human use of a resource (while sufficient for immediate human purposes) undercuts the sustainability of the environmental system, and also problems of overloaded "sinks." It's tempting here to reason that if there's not enough to go around, the solution is to have fewer people for it to try to go around to.
Yet there are numerous other contributing factors. One huge one is inefficiency. Modern resource use is incredibly wasteful, so that much of our resource base ends up neither used nor conserved. The problem of inefficiency is typically conceptualized as a problem of insufficient technological advancement, and that's often true. But it's also a result of social organization -- society does not provide incentives to make the fullest use of the theoretically available resources. Energy is a good example. Decentralized generation would be much more efficient at using our fuel resources (due to its flexibility and minimization of long-distance power transmission over leaky lines), but our political economy is set up to favor the construction of large, centralized generators. An example of a combined technological-social case would be culturally inappropriate farming technologies exported to the third world by anthropologically ignorant first world researchers and companies.
Perhaps the most common rebuttal to the overpopulation thesis is to point to the role of affluence or overconsumption. The simple overpopulation argument presumes a fixed per capita consumption -- but in reality, consumption levels can vary drastically from one person to another. The modern conception of the good life is overreliant on resource-intensive pursuits. If we had a different view of what made life good, we would greatly reduce the amount of environmental damage done by each person.
When local scarcity does occur, it's quite often the result not of an absolute shortage, but of a maldistribution. Social inequality and the resulting lack of access to resources have the same effect as an absolute shortage, but the causes -- and hence solutions -- are quite different.
The flip side of maldistribution of resources is maldistribution of people. In studies of dryland degradation and deforestation, it has often been found that, insofar as population is a problem, it's a problem of "over-migration." Too many people in the wrong place
is a different story than too many people overall
. The roots of over-migration then go back to the social structures that provide incentives and constraints that drive people into environmentally sensitive areas.
If the kinds of concerns described above were effectively addressed, I think we would find that the current human population, as well as the populations projected to occur in the future in the absence of policies directly motivated by concern for overpopulation, is comfortably within the bounds of what the Earth can support.