we're expected to have 9 billion people in the coming decades.
Humans aren't an endangered species.
These statements are both exceedingly optimistic.
Overpopulation is a condition that is handled quite effectively by nature. Huge populations of immunodepressed people and giant shantytown slums with no medical care and no sanitation apparatus and no clean water mean that we're going to see a plague in this century (very likely soon) that's like nothing we're familiar with.
Swine flu alone might very well rip through the third world like nobody's business.
Sooner or later Malthus always wins. In this case it's to our benefit: Western medicine, barrier protection and superior life support in hospitals mean that we're very likely going to be largely unscathed but Africa and India are going to get a bath. China too, very likely, but that will be more because of population pressures-- their nationalistic dictatorship allows them quarantine measures that are unacceptable to Western societies.
Overpopulation is an argument for education and aid for the third world. Paradoxically, these two things (and especially emancipation for females) result in much smaller rates of population growth.
Believe me, population is the least of our worries (except insofar as it's the single greatest enabling factor for fulminating viral epidemics.) Nature is very powerful, exhibits homeostatic behavior that looks very much like intelligence at times, and takes care of itself.