War is an unethical way to lower the population. Starvation won't do the job either. No, we're just going to have to learn how to reuse all of our resources, probably the hard way. That, and eventually we'll have to expand into space. As is the nature of organisms such as we. Struggle for existence and all.
Fortunately and unfortunately, nature figured out that the more variation in a species the more likely it is to survive. The bigger a species is, the easier it is for it to variate. This helps against diseases, but it also creates population support problems, especially when a species gets as populous as ours.
We need modern medicine to get a good deal more advanced, so that eventually the need to populate to variate to survive will seem less urgent to the genes of the following generations of humanity.
Although, it may take too lng for our genes to slow down fertility rates, and thus, though it is a somewhat controversial idea, future generations might end up having a need to tweak the genes behind fertility a bit.
Please feed the trolls. XD