Human modification of green water flows: Influencing tipping points in ecosystems

Agricultural activities have substantially altered the spatial patterns of global water vapour flows (so called ‘green water ‘or evapotranspiration) over the continents. While irrigation has increased annual global continental vapour flows, deforestation has decreased them by almost the same magnitude. However, the pattern of change is different in different regions across the globe. This spatial redistribution of vapour flows has the potential to alter climate, reduce ecosystem services, and cause ecosystems to cross ‘tipping points.’ These impacts are mostly likely in regions where areas of agriculturally induced changes in vapour flows coincide with ‘hot spots’ for soil moisture-atmosphere coupling. Agricultural activities that affect the flow of water from the soil to the atmosphere can here have an impact on local and regional climate. It is further proposed that agriculture can affect the strength of this coupling, changing the locations of hotspot regions. The resulting climatic change is likely to have an impact on ecosystem function and increasing the potential of this in triggering local to regional scale regime shifts. Regions of particular concern, where there are examples of past regime shifts, or where models illustrate high risk for regime shifts include: the Amazon, the Sahel, mid-US, India and Northern China.  We use several scenarios of agricultural expansion to analyse how agricultural expansion could impact global water flows in the future and consequently impact local and regional hydrology and climate.  Finally, we identify a range of possible knock-on effects of these changes on ecosystem services and in particular discuss the trade-off between agricultural production and the ecosystem service of climate regulation.