Human Modification of Global Water Vapor Flows from the Land Surface

It is well documented that human modification of the hydrological cycle has profoundly affected the flow of liquid water across the Earth's land surface. Alteration of water vapor flows through land-use changes has received comparatively less attention, despite compelling evidence that such alteration can influence the functioning of the Earth System. We show that deforestation is as large a driving force as irrigation in terms of changes in the hydrological cycle. Deforestation has decreased global vapor flows from land by 4% (3,000 km(3)/yr), a decrease that is quantitatively as large as the increased vapor flow caused by irrigation (2,600 km(3)/yr). Although the net change in global vapor flows is close to zero, the spatial distributions of deforestation and irrigation are different, leading to major regional transformations of vapor-flow patterns. We analyze these changes in the light of future land-use-change projections that suggest widespread deforestation in sub-Saharan Africa and intensification of agricultural production in the Asian monsoon region. Furthermore, significant modification of vapor flows in the lands around the Indian Ocean basin will increase the risk for changes in the behavior of the Asian monsoon system. This analysis suggests that the need to increase food production in one region may affect the capability to increase food production in another. At the scale of the Earth as a whole, our results emphasize the need for climate models to take land-use change, in both land cover and irrigation, into account.