Huffington Post: When it comes to global greenhouse gas emissions, population size really does matter. Demographers have some good news for us and some bad. The good news is that human populations will not grow without limit and should reach about nine billion by mid-century. The bad news is the same as the good — human populations will stabilize at about nine billion — a whole heck of a lot of people for our little blue planet, about two Chinas worth on top of today’s population. Of course, …