Future freshwater stress for island populations

Global climate models project large changes in the terrestrial water balance for many regions over this century in response to greenhouse gas emission, but insufficient resolution precludes such knowledge for approximately 18 million people living on small islands scattered across the world ocean. By accounting for evaporative demand a posteriori at 80 island groups distributed among Earth's major ocean basins, we reveal a robust yet spatially variable tendency towards increasing aridity at over 73% of island groups (16 million people) by mid-century. Although about half of the island groups are projected to experience increased rainfall - predominantly in the deep tropics - projected changes in evaporation are more uniform, shifting the global distribution of changes in island freshwater balance towards greater aridity. In many cases, the magnitude of projected drying is comparable to the amplitude of the estimated observed interannual variability, with important consequences for extreme events as well as mean climate. Future freshwater stress, including geographic and seasonal variability, has important implications for climate change adaptation scenarios for vulnerable human populations living on islands across the world ocean.