The new planetary boundary for freshwater change: key findings and their potential to guide water management and policy

The recent third planetary boundary (PB) assessment replaced the original PB for ‘freshwater use’ with a new PB for ‘freshwater change’. The new PB is defined by the percentage of global land area experiencing streamflow (blue water component of the PB) and root-zone soil moisture (green water) deviations from pre-industrial baseline conditions. Here, we first present the spatiotemporally explicit results of the comprehensive analysis underlying the new PB, and then discuss possible applications of the approach and the challenges related to providing meaningful guidance for water management and policy across scales. We find a clear transgression of both the blue and green water components of the freshwater change PB already during the first half of the 20th century. Our spatiotemporally explicit analysis reveals a general pattern of drying across a significant portion of the tropics and subtropics, contrasting with wetting in temperate and subpolar regions as well as numerous highland areas. This overall pattern is likely attributed to alterations in precipitation patterns associated with global warming. Significant increases in streamflow and soil moisture deviations are also found in regions facing the highest direct human pressures, such as irrigation, flow regulation, and land use change. In many cases, both streamflow and soil moisture deviations have increased – underlining the influence of human impacts on the freshwater cycle as a whole. While our analysis highlights regions undergoing the most substantial freshwater changes and their potential drivers, using the results to guide water policy and management remains challenging. Key knowledge gaps include our limited understanding of the (quantitative) driver–freshwater change–Earth system response relationships, and the mismatches between spatiotemporal scales of 1) human drivers of freshwater change, 2) the Earth system impacts of freshwater change, and 3) water management and governance institutions. We conclude our presentation by proposing a research agenda to bridge these gaps, with a goal to provide policy-relevant information on freshwater change that would enable a stronger adoption of an  Earth system perspective in water management and governance.