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.
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L.W.-E., A.T., I.F., C.S., J.R. and A.S. acknowledge financial support from the European Research Council through the ‘Earth Resilience in the Anthropocene’ project (no. ERC-2016-ADG 743080). R.J.v.d.E. acknowledges funding from the Netherlands Organization for Scientific Research (NWO), project number 016.Veni.181.015. M.P. acknowledges funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (grant agreement 819202).
The authors declare no competing interests.
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Wang-Erlandsson, L., Tobian, A., van der Ent, R.J. et al. A planetary boundary for green water. Nat Rev Earth Environ 3, 380–392 (2022). https://doi.org/10.1038/s43017-022-00287-8
Nature Reviews Earth & Environment (2022)