Earth's growing human population needs fresh water for drinking and food production. However, fresh water is also needed for the growth of biomass, which acts as a sink of carbon dioxide and thus could help mitigate climate change. Does the Earth have enough freshwater resources to meet these competing demands? Rockström et al. estimate the order of magnitude of freshwater consumption needed to feed a population of 9 billion people by 2050 and the amount of water needed to realize the planet's full biomass carbon sequestration potential. The researchers analyze these uses of freshwater in a framework of “planetary boundaries” within which the Earth system is resilient; beyond the boundaries, abrupt and irreversible change could take place. For instance, river ecosystems can collapse if water levels become too low due to water withdrawal for human uses.