In 2022, the United States Geological Survey released a new diagram of the global hydrological cycle that, for the first time, included people and the social processes affecting freshwater. It was a significant step and also a long-anticipated change. Humans have appropriated more than their share of Earth’s freshwater for decades; thousands of mega-dams block rivers around the world, groundwater is pumped unsustainably in many places, and over half of the world’s lakes are in decline, largely owing to human impacts on local water systems and the planet’s climate (Postel et al. Reference Postel, Daily and Ehrlich1996; Yao et al. Reference Yao, Livneh and Rajagopalan2023). Even so, the update to the hydrological cycle arguably did not go far enough. This is because it is one thing to depict social processes that affect water and quite another to show the scale of anthropogenic impacts on the global water cycle (Abbott et al. Reference Abbott, Bishop and Zarnetske2019). Those impacts connect the planetary boundary for freshwater use to ethical concerns regarding how unequal distributions of water over time and space affect people, other species, and the processes that support life on Earth.

Like other planetary boundaries, calculations for freshwater change have undergone significant shifts since the framework was first proposed. In 2009, a boundary of 4,000 km³ worth of annual freshwater was estimated, with only 419 km³ having been used prior to the industrial era and a current estimate of 2,600 km³ (Rockström et al. Reference Rockström, Steffen and Noone2009). These planetary numbers don’t factor in regional disparities, such as arid versus humid regions or places where human demands for agriculture or industrial extraction already overburden freshwater systems that are the ‘bloodstream of the biosphere’ (Ripl Reference Ripl2003). Even so, the initial estimate for the freshwater boundary was criticized for focusing on what hydrologists call blue water, which is the visible water in lakes, rivers, and taps. This water represents only a fraction of the global water system, with invisible ‘green water’ circulating throughout the atmosphere, soils, and biota in ways also affected by anthropogenic impacts on the environment. Land-use and climate changes, for instance, both significantly impact green water, such when forests are converted to arable or urban land. Further, these changes affect the entire freshwater system in complex ways. Together, these concerns prompted a reassessment of the freshwater boundary to include both blue and green water (Gleeson et al. Reference Gleeson, Wang-Erlandsson and Zipper2020; Wang-Erlandsson et al. Reference Wang-Erlandsson, Tobian and Ent2022). In the 2023 update to the boundary for freshwater change, the assessments for both blue and green water showed significant departures from pre-industrial estimates, with both having been transgressed for some time. The boundary for blue water, on the new model, was passed in 1905 and that for green water in 1929 (Richardson et al. Reference Richardson, Steffen and Lucht2023). Although labelled as ‘preliminary’ findings, the direction of travel for the new boundary on freshwater change showed an increasing risk to a key variable that, in previous assessments, had been characterized as within a ‘safe space for humanity’.

Improved depictions of human impacts on global hydrology and redefining the boundary for freshwater change also raise ethical concerns the two chapters in this part examine. First, Nicole Wilson critically engages the freshwater boundary using the idea of a hydro-social cycle – an approach that does not begin by separating humans from nature, such as by making calculations of ‘natural’ or pre-industrial values from which the freshwater changes are calculated. Instead, hydro-social scholars emphasize how human-water systems are mutually co-constituted (Linton Reference Linton2010). For instance, although ecohydrologists like Carl Folke (Reference Folke2003) have long argued for a coevolutionary view of humans and water, hydro-social scholars prioritize social relations and situate science as one kind of social practice among others, such as those of Indigenous peoples. From this perspective, a ‘boundary’ to freshwater use also requires an ethic that respects the bounded, different ways of knowing and relating to the world. Second, Neelke Doorn considers how the planetary boundaries framework should impact philosophical thought. When it comes to freshwater, a significant ethical concern is how to understand risk, from floods and droughts to engineering decisions. But this is tricky ethical territory, she argues, because the freshwater boundary is global, yet risks are experienced locally. In this context, Doorn considers what science and ethics can offer to one another given the respective aims of each – how can the intrinsic issues of equity that don’t figure into the freshwater planetary boundary be addressed through a focus on justice, for instance? Doorn’s emphasis on disciplinary complementarity contrasts with Wilson’s disciplinary critique in ways that aren’t so much two poles of a wide spectrum as two respective attempts to get to grips with the plurality of approaches to science and ethics affecting freshwater change.