Consider this article from an environmental journalist describing the insidious logic concerning some arguments that arise in discussing over population. Among the points addressed they acknowledge the fact that overpopulation has a definitive impact on the environment, but they argue and explain as to why they do not write about it. It outwardly seems nonsensical, but he raises some valid points and concerns.
The measure for impact he uses (and many others use) is fairly simple: Impact = Population x Affluence x Technology (Though, it ought to be noted that this equation really more conceptual than anything, and is very simplified and has various shortcomings). Affluence (A) in the context of this equation (I=PAT) is generally considered through GDP per capita while technology (T), pertains to the costs of the production of affluence (or, another way to think of it is the resource costs of capital). One way of measuring T could be through evaluating greenhouse gas emissions per unit of GDP. Knowing that affluence has a direct observable correlation to consumption, and that technology is interdependent/interrelated to affluence, we can assert that among less affluent populations, there is less impact because there is less consumption.
That said, the UN expects that most of half the growing population up to the end of this century will be relatively poor. Consequently, we can reasonably infer they will utilize significantly less resources. However, by 2100, we can still expect a significant amount of population growth that will definitively lead to greater negative environmental impacts. Working to actively reduce population growth rates is one way to try and address this, and it would likely be more impactful and pragmatic than certain policy proposals / solutions. However, this begs the question as to what would be the best way.
As we’ve seen with the eugenics movement, population control discourse has and can devolve quickly into xenophobia, racism, and pseudoscience. Discussing overpopulation, and more specifically, means of mitigating overpopulation can be a treacherous endeavor exactly because of this. So, what then? What can be done? Well, for starters, family planning, education, and female empowerment has proven a potent solution to reducing population growth rate (and, consequently, the impact that otherwise would be generated from an unchanged population growth rate). The other thing that ought to be addressed is the disproportionate impact that more affluent populations generate. Policies to reduce income inequality and the emergence or maintenance of excessive wealth would lead to less affluence and thus impact and consumption.