Whether we accept it or not, this will likely be the century that determines what the optimal human population is for our planet. It will come about in one of two ways: Either we decide to manage our own numbers, to avoid a collision of every line on civilization's graph - or nature will do it for us, in the form of famines, thirst, climate chaos, crashing ecosystems, opportunistic disease, and wars over dwindling resources that finally cut us down to size.