Air passing over vegetation produces about twice as much rain as air passing over sparse vegetation according to a report in Nature this week. These findings highlight the importance of tropical forests in maintaining regional rainfall patterns. The study also suggests that current rates of deforestation of the Amazon will lead to large reductions in regional rainfall.

When rain falls, some of it is returned to the atmosphere via evaporation and transpiration (the release of water vapour from plants). This precipitation recycling has long been thought to be an important part of the overall precipitation budget in the tropics; however, most evidence has come from modelling studies and the implications remain uncertain. Dominick Spracklen and colleagues use satellite observations and atmospheric modelling to demonstrate that the presence of tropical forests leads to more rain downwind. They show that evapotranspiration from vegetation enables forests to maintain atmospheric moisture, which can return to land as rainfall downwind.

Modelling current rates of Amazonian deforestation, the authors estimate that this activity results in a 12% reduction in rainfall during the wet season, and a 21% reduction in the dry season. Thus, the authors conclude that recent efforts to curb deforestation must be maintained if large-scale clearance of the Amazon and the resulting impacts on regional rainfall are to be avoided.