HESS Opinions: A planetary boundary on freshwater use is misleading

Abstract

Abstract. In 2009, a group of prominent Earth scientists introduced the "planetary boundaries" (PB) framework: they suggested nine global control variables, and defined corresponding "thresholds which, if crossed, could generate unacceptable environmental change". The concept builds on systems theory, and views Earth as a complex adaptive system in which anthropogenic disturbances may trigger nonlinear, abrupt, and irreversible changes at the global scale, and "push the Earth system outside the stable environmental state of the Holocene". While the idea has been remarkably successful in both science and policy circles, it has also raised fundamental concerns, as the majority of suggested processes and their corresponding planetary boundaries do not operate at the global scale, and thus apparently lack the potential to trigger abrupt planetary changes. This paper picks up the debate with specific regard to the planetary boundary on "global freshwater use". While the bio-physical impacts of excessive water consumption are typically confined to the river basin scale, the PB proponents argue that water-induced environmental disasters could build up to planetary-scale feedbacks and system failures. So far, however, no evidence has been presented to corroborate that hypothesis. Furthermore, no coherent approach has been presented to what extent a planetary threshold value could reflect the risk of regional environmental disaster. To be sure, the PB framework was revised in 2015, extending the planetary freshwater boundary with a set of basin-level boundaries inferred from environmental water flow assumptions. Yet, no new evidence was presented, either with respect to the ability of those basin-level boundaries to reflect the risk of regional regime shifts or with respect to a potential mechanism linking river basins to the planetary scale.So while the idea of a planetary boundary on freshwater use appears intriguing, the line of arguments presented so far remains speculative and implicatory. As long as Earth system science does not present compelling evidence, the exercise of assigning actual numbers to such a boundary is arbitrary, premature, and misleading. Taken as a basis for waterrelated policy and management decisions, though, the idea transforms from misleading to dangerous, as it implies that we can globally offset water-related environmental impacts.A planetary boundary on freshwater use should thus be disapproved and actively refuted by the hydrological and water resources community.

A good life for all within planetary boundaries

“…In general, social performance is most tightly coupled to CO2 emissions and material footprint, and least tightly coupled to eHANPP. The weak relationship between eHANPP and the social indicators is consistent with previous work showing that eHANPP is strongly linked to population density, but not to other socioeconomic factors 40 .…”

“…In general, social performance is most tightly coupled to CO2 emissions and material footprint, and least tightly coupled to eHANPP. The weak relationship between eHANPP and the social indicators is consistent with previous work showing that eHANPP is strongly linked to population density, but not to other socioeconomic factors 40 .The social indicators most tightly coupled to resource use are secondary education, sanitation, access to energy, income, and nutrition. With the exception of education, these are more closely associated with meeting physical needs than with achieving more qualitative goals (e.g.…”

“…This boundary has been debated, both in terms of the level at which it is set 39 , and also in terms of its relevance and scientific rigour, given that the environmental impacts of freshwater use are primarily confined to the river-basin scale 40 .…”

A water cycle for the Anthropocene

“…Third and most fundamentally, misconceptions of water in the Anthropocene extend far beyond popular diagrams of the water cycle. Some of the highest‐profile scientific publications only consider consumptive water use when determining sustainable planetary limits for freshwater (Steffen et al, ), and others present terrestrial evaporation and transpiration as water losses (Schyns, Hoekstra, Booij, Hogeboom, & Mekonnen, ) rather than the primary sources of freshwater for agriculture and ecosystems (Ellison et al, ; Heistermann, ; van Noordwijk & Ellison, ).…”

On the social dynamics of moisture recycling

“…However, in the context of this paper and for the sake of brevity, we will use the phrase moisture recycling to refer specifically to the terrestrial component. Considering the recent debate regarding human impacts on large-scale hydrology (Rockström et al, 2012;Heistermann, 2017), there is a need to clarify and highlight the importance of anthropogenic modification of terrestrial moisture recycling. Moreover, it is incumbent on the scientific community to begin understanding how the constituent components of the Earth system interact with the social.…”