The carrying capacity of the earth will be reached whether you have children or not. So the consideration is different: whose descendants will take up that carrying capacity. If you, as a person concerned about the environment, opt to have children, then, statistically speaking, you are biasing the future composition of the human population toward being more concerned about the environment (since your kids share you innate predispositions and will be raised in the familial and cultural milieu that you create for them). And if people who are concerned about the environment systematically choose to not have children because they have become infected by this meme, then you have a situation where the future composition of the human population is becoming more biased toward doing nothing about the environment. So not having kids is a losing strategy.