A planetary boundary for green water
Lan Wang-Erlandsson (Stockholm University)
Arne Tobian (Potsdam-Institut für Klimafolgenforschung, Stockholm University)
Ruud J. van der Ent (TU Delft - Water Resources)
Ingo Fetzer (Stockholm University)
Sofie te Wierik (Universiteit van Amsterdam)
Miina Porkka (Aalto University, Royal Swedish Academy of Sciences)
Arie Staal (Stockholm University, Universiteit Utrecht)
Peter Greve (International Institute for Applied Systems Analysis)
Dieter Gerten (Humboldt-Universitat zu Berlin, Potsdam-Institut für Klimafolgenforschung)
Patrick W. Keys (Colorado State University)
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Abstract
Green water — terrestrial precipitation, evaporation and soil moisture — is fundamental to Earth system dynamics and is now extensively perturbed by human pressures at continental to planetary scales. However, green water lacks explicit consideration in the existing planetary boundaries framework that demarcates a global safe operating space for humanity. In this Perspective, we propose a green water planetary boundary and estimate its current status. The green water planetary boundary can be represented by the percentage of ice-free land area on which root-zone soil moisture deviates from Holocene variability for any month of the year. Provisional estimates of departures from Holocene-like conditions, alongside evidence of widespread deterioration in Earth system functioning, indicate that the green water planetary boundary is already transgressed. Moving forward, research needs to address and account for the role of root-zone soil moisture for Earth system resilience in view of ecohydrological, hydroclimatic and sociohydrological interactions.