Illuminating water cycle modifications and Earth system resilience in the Anthropocene

Abstract

Fresh water-the bloodstream of the biosphere-is at the center 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. Plain language summaryFreshwater is crucially important for all life on Earth. There is abundant research and evidence on how different processes within the water cycle regulate climate and support ecosystems, and by extension, human societies. Humans are also a major force disturbing those processes and modifying the water cycle. These modifications include, for instance, surface water withdrawals, groundwater pumping, deforestation and other land cover change, and ice melt due to warming climate. As most previous research on human-water interactions focuses on understanding systems at smaller scales, such as a watershed or a nation, comprehensive understanding of what human modifications of the water cycle mean for the stability of the planet is still lacking. In this paper we propose a new framework for analysing and establishing limits to a variety of human modifications of the water cycle,

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

“…Applying the fair shares approach would apportion the water planetary sub-boundaries to the Cienaga wetlands. Using the sub-boundaries tentatively proposed by Gleeson et al (2019Gleeson et al ( , 2020, we see a mixture of relationships between the local safe operating space and fair shares approaches. The surface water and groundwater sub-boundaries have different control variables.…”

“…The fair shares approach is a top-down approach that treats the planetary boundary value of the control variable as a global safe operating space "budget" and then allocates a portion of that global safe operating space Table 1 Proposed Water Planetary Sub-boundaries From Gleeson et al (2019Gleeson et al ( , 2020 Trade-offs between global CO 2 budget and local water availability (Heck, Gerten, et al, 2018) Surface water Hydroecological regulation Aquatic biosphere integrity Watersheds or total river length within environmental flow limits Importance of local aquatic systems to global biodiversity pool (Mace et al, 2014) Groundwater Storage Terrestrial or aquatic biosphere integrity…”

“…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 (2019Gleeson et al ( , 2020 improves the ability of the fair shares approach to accurately represent water's role in Steps to translate water's planetary role in Earth System dynamics to local management and governance. In the harmonized approach, colored circles indicate a water sub-boundary corresponding to a specific store of water and the red-yellow-blue color gradient corresponds with the current position of the control variable with respect to the boundary, as in Figure 1.…”

The water planetary boundary: interrogation and revision

“…Modified from Steffen et al (A), 3 Oki and Kanae (B), 4 and Gleeson et al (C). 5 patterns, modifications of soil moisture by land use and land cover, and water withdrawals from discharge for human use. It was not intended to be an explicit variable implying that water use can or should be aggregated to global scales.…”

“…Earlier discussions have criticized the definition of the freshwater use planetary boundary for a number of reasons, which include (1) scale-water problems are often considered only at local to regional scales, whereas the metric is global which some consider misleading; 52 (2) aggregation-the planetary boundary currently sums streamflow fluxes, but the best way to summarize diverse local impacts to a global metric is not clear; 56 (3) control variable-blue water use is not a biophysical variable representing the complexity of the water cycle and human water modifications; 5,54 (4) mechanism-there is limited evidence of tipping points or connections between water use and processes that would lead to the Earth leaving a Holocene-like state; 52 (5) underestimation of water use-the global consumptive use of freshwater may be higher due to possible additional or larger effects from irrigation and flow regulation, 45,54 although rebutted by Gerten et al; 51 and (6) the planetary boundary tends to disregard conditions of local overuse of water resources and may provoke the thought that all usable water can be accessed. 57 We propose a qualitative evaluation framework with seven criteria for defining a useful water planetary boundary based on the definition and purpose of the planetary boundaries.…”