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.

The planetary boundaries framework defines a safe operating space for humanity. To date, these boundaries have mostly been investigated separately, and it is unclear whether breaching one boundary can lead to the transgression of another. By employing a dynamic global vegetation model, we systematically simulate the strength and direction of the effects of different transgression levels of the climate change boundary (using climate output from ten CMIP6 models for CO2 levels ranging from 350 ppm to 1000 ppm). We focus on climate change-induced shifts of Earth’s major forest biomes, the control variable for the land-system change boundary, both by the end of this century and, to account for the long-term legacy effect, by the end of the millennium. Our simulations show that while staying within the 350 ppm climate change boundary co-stabilizes the land-system change boundary, breaching it (>450 ppm) leads to its critical transgression with greater severity, the higher the ppm level rises and the more time passes. Specifically, this involves a poleward treeline shift, boreal forest dieback (nearly completely within its current area under extreme climate scenarios), competitive expansion of temperate forest into today’s boreal zone, and a slight tropical forest extension. These interacting changes also affect other planetary boundaries (freshwater change and biosphere integrity) and provide feedback to the climate change boundary itself. Our quantitative process-based study highlights the need for interactions to be studied for a systemic operationalization of the framework. 

 The world is faced with the task of addressing rising food insecurities amidst growingpopulation and planetary boundary transgressions. Here we analyze interactions of theplanetary boundaries for climate change and freshwater change (focused on green water,i.e. soil moisture departures from pre-industrial variability ranges) and discuss potentialeffects on food production and security. By forcing the dynamic global vegetation modelLPJmL5.1 with an ensemble of 10 CMIP6 climate models under the RCP7.0 emissionsscenario, we detect significant increases in dry deviations for green water in nearly a third ofthe terrestrial surface area by the end of the century (2071-2100), compared to current levels. This implies a narrowing of the safe operating space, as thetransgression of the climate change boundary adversely affects the status of the green2water boundary. This finding is crucial, as rainfed agriculture, a food production systemexclusively relying on green water, is currently responsible for more than 60% of global foodproduction. We further argue that green water has to be made explicit in the SDG goals andtargets. 

 Anthropogenic modifications and alterations of terrestrial ecosystems by land-usetransformation and intensification are a driving factor of global environmental change in theAnthropocene. The planetary boundary for land-system change (PB-LSC) demarcates ‘safe’planetary conditions for these interferences and is defined by the remaining extent for themajor forest biomes spanning the temperate, tropical and boreal climatic zones. However,agricultural production and its interlinked land-use changes have contributed to thetransgression of several other planetary boundaries, besides PB-LSC. This makes scenariosrepresenting different pressure levels of PB-LSC an essential prerequisite to study how afurther breaching of the boundary would affect other interconnected planetary boundariessuch as those for climate change, biosphere integrity, freshwater change andbiogeochemical cycling. Future policy-relevant projections of land-use change are based onsocioeconomic pathways, yet no scenarios are aligned with the definition provided byPB-LSC. Here, we present a land use reallocation tool that can represent, globally at gridcell scale, the spatial distribution of agricultural land-use to match any statuses andtransgression levels of PB-LSC, e.g. to facilitate systematic assessment of effects ofdifferent afforestation and deforestation scenarios on the status of other boundaries.