The planetary boundaries framework has proven useful for many global sustainability contexts, but is challenging to apply to freshwater, which is spatially heterogeneous, part of complex socio-ecological systems and often dominated by local dynamics. To date, the planetary boundary for water has been simplistically defined by as the global rate of blue water consumption, functioning as a proxy for water partitioning in the global hydrological cycle, and considering impacts on rivers’ environmental flow requirements. We suggest the current planetary boundary should be replaced since it does not adequately represent the influence of water in critical Earth System functions such as regional climate and biodiversity. We review the core functions of water in the Earth System and set out a roadmap towards a more robust, holistic, and locally applicable water planetary boundary. We propose defining the boundary using four core functions of water (hydroclimatic regulation, hydroecological regulation, storage, and transport) in conjunction with five water stores (surface water, atmospheric water, soil moisture, groundwater and frozen water). Through the functions, the stores are inextricably interconnected with the atmosphere, land, ocean and biosphere. The roadmap outlined here suggests how to move towards setting six new water planetary sub-boundaries. This ambitious scientific and policy Grand Challenge that could substantially improve our understanding and management of water cycle modifications in the Earth System and provide a complementary approach to existing water management tools.

Fresh water – the bloodstream of the biosphere – is at the centre of the planetary drama of the Anthropocene. Water fluxes and stores regulate the Earth’s climate and are essential for thriving aquatic and terrestrial ecosystems, as well as water, food and energy security. But the water cycle is also being modified by humans at an unprecedented scale and rate. A holistic understanding of freshwater’s role for Earth System resilience and the detection and monitoring of anthropogenic water cycle modifications across scales is urgent, yet existing methods and frameworks are not well suited for this. In this paper we highlight four core Earth System functions of water (hydroclimatic regulation, hydroecological regulation, storage, and transport) and key related processes. Building on systems and resilience theory, we review the evidence of regional-scale regime shifts and disruptions of the Earth System functions of water. We then propose a framework for detecting, monitoring, and establishing safe limits to water cycle modifications, and identify four possible spatially explicit methods for their quantification. In sum, this paper presents an ambitious scientific and policy Grand Challenge that could substantially improve our understanding of the role of water in the Earth System and cross-scale management of water cycle modifications that would be a complementary approach to existing water management tools.

The planetary boundaries framework defines the ‘safe operating space for humanity’ represented by nine global processes which can destabilize the Earth System if perturbed. The water planetary boundary attempts to provide a global limit to anthropogenic water cycle modifications, but it has been challenging to translate and apply it to the regional and local scales at which water problems and management typically occur. We develop a cross-scale approach by which the water planetary boundary could guide sustainable water management and governance at sub-global contexts defined by physical features (e.g. watershed or aquifer), political borders (e.g. city, nation, or group of nations), or commercial entities (e.g. a corporation, trade group, or financial institution). The application of the water planetary boundary at these sub-global contexts occurs via two approaches: (i) calculating fair shares, in which local water cycle modifications are compared to that context’s allocation of the global safe operating space, taking into account biophysical, socio-economic, and ethical considerations; and (ii) defining a local safe operating space, in which interactions between water stores and Earth System components are used to define local boundaries required for sustaining the local water system in stable conditions, which we demonstrate with a case study of the Cienaga Grande de Santa Marta wetlands in Colombia. By harmonizing these two approaches, the water planetary boundary can ensure that water cycle modifications remain within both local and global boundaries and complement existing water management and governance approaches.