Illuminating water cycle modifications and Earth System resilience in the Anthropocene

Abstract

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.

Integrating the Water Planetary Boundary With Water Management From Local to Global Scales

“…Using this approach, they calculated monthly environmental flow requirements for all surface water globally, which resulted in a boundary between 1,100 and 4,500 km 3 year −1 . Reframing the water planetary boundary for different stores of water as proposed by Gleeson et al (, ) improves the ability of the fair shares approach to accurately represent water's role in Earth System dynamics, and therefore, we suggest that future work on the fair shares approach should shift to these new control and response variables (Table ).…”

“…For most of these processes, a quantifiable control variable has been suggested, which may cause some response variable to destabilize, either alone or through interactions with other Earth System processes (Figure 1; Rockström et al, ; Steffen et al, ). For effective boundary setting, the control variable should be quantifiable and subject to influence by human actions, while the response variable should describe Earth's stable conditions and be influenced by the control variable (Gleeson et al, ). The boundary value of each control variable is set some distance upstream from departure of the response variable from stable conditions, typically at the lower end of uncertainty due to systemic and/or scientific factors (Figure ).…”

“…Regarding water, which is our focus here, the planetary boundary for freshwater use was originally based on the amount of freshwater that could be withdrawn while maintaining rivers' environmental flow requirements globally (Gerten et al, ; Steffen et al, ). Recently, Gleeson et al (, ) proposed to distinguish six water sub‐boundaries relating to the major stores of freshwater to more holistically represent the various functions of water in maintaining Earth System stability (Table ). Environmental flow requirements are retained in this approach as the surface water sub‐boundary, together with new water planetary sub‐boundaries for frozen water, groundwater, soil moisture, and two sub‐boundaries for different aspects of atmospheric water.…”

“…Environmental flow requirements are retained in this approach as the surface water sub‐boundary, together with new water planetary sub‐boundaries for frozen water, groundwater, soil moisture, and two sub‐boundaries for different aspects of atmospheric water. Gleeson et al (, ) suggested potential control and response variables for these new sub‐boundaries (Table ), but significant work remains to select and evaluate appropriate variables and boundary values. To provide a sound societal relevance for these efforts, it is necessary to first determine whether the water planetary boundary can be meaningfully integrated with existing water management approaches.…”

“…The first step in operationalizing the fair shares approach is defining the safe operating space by determining values for each of the water planetary sub‐boundaries. While recent work has proposed new planetary sub‐boundaries for different stores of water in the Earth System (Gleeson et al, , ), previous efforts to operationalize the planetary boundaries framework using the fair shares approach have all used the control variable and estimated boundary values from Rockström et al () and Steffen et al () (Table S1). In the case of water, this means that the planetary boundary is typically taken as 4,000 km 3 year −1 of consumptive blue water use.…”