Overpopulation
Population used to be the ultimate environmental issue, with doomsayers from Thomas Malthus to Paul Ehrlich warning that human numbers would eventually choke the planet. In the two decades since Rio+20 conferences began however, population has largely fallen off the environmental agenda, in part because the population control was seen as too coercive and controversial. But the 7 billionth person was born some time last year, and right now, the world is still on track to crack 9 billion by the middle of the century. If those numbers keep rising — and humans keep using more and more of the planet's resources — it could swamp any effort to protect the planet. In the run up to Rio+20, the Global Network of Science Academies released a statement calling for the Earth Summit to take on population, but it remains to be seen what progress can be made.