Last month, the United Nations announced that the Earth’s population had reached 8 billion. … Most folks would agree that the real worry here is not the sheer numbers, but their collective impact on the environment. We — the planet’s human inhabitants — are clearing land, leveling forests and mountains, mining and drilling minerals and burning fossil fuels in order to sustain ourselves and our lifestyles. That, in turn, is diminishing biodiversity, driving species to extinction and stretching the planet’s carrying capacity to a snapping point, thereby imperiling our own species’ survival. The problems are exacerbated as planet-warming greenhouse gas emissions soar, further diminishing freshwater stores and hurting food production.