The solution to over population is advanced education for everyone. But it won't happen; my prediction is that the solution will be left to nature and nature's solutions are usually quite painful.
Have fun -- Dick
PS A billion is way over the correct answer. With modern technology I think the human race would do fine with a population around a hundred million and the optimum population could be as low as twenty million. That would be enough to provide a sufficient number of people to carry on every activity we currently enjoy plus enough people to support those efforts and, if the population were that low we wouldn't have to worry about much.