Regionally divergent drivers behind transgressions of the freshwater change planetary boundary

Human-driven freshwater change relates to elevated Earth system risks, which motivates analysis to better understand its global characteristics. Because freshwater is integral to the functioning and stability of the Earth system (in terms of ecosystems and climatic processes, for instance), disruptions to freshwater cycle dynamics can contribute to a situation where human activities both depend on and undermine a stable Earth system. This interplay creates a strong need to assess and understand freshwater change at the global scale, including its spatial patterns and drivers.

Building on the newly updated planetary boundary for freshwater change (PB-FW), we analysed global and regional patterns of anomalous conditions and their drivers in blue water (streamflow) and green water (soil moisture). We used a large ensemble of global hydrological model simulations covering the years 1901–2019 from the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) simulation round 3a experiments. We first determined local deviations at the grid cell scale and then aggregated land areas affected by those deviations, following the approach of the previous PB-FW estimate. Here, however, we extend its timeline by 15 years (from 2005 to 2019) and decompose the historical contributions of climate-related forcing (CRF) and direct human forcing (DHF; encompassing land and water use changes) to PB-FW transgression at global and regional scales.

During the late 20th and early 21st century, PB-FW transgression has increased markedly across its blue and green water components. In 2010–2019, local deviations in streamflow and soil moisture affected 22–23% of the global ice-free land area, notably exceeding the PB-FW, which places at 12–13%. Approximately half of the total transgression has occurred since 1990. CRF has increasingly become the dominant global influence on dry and wet streamflow and soil moisture deviations from preindustrial-like baseline conditions, while DHF amplifies dry deviations. Regionally, streamflow and soil moisture deviation occurrence varies widely; CRF dominates both dry and wet deviations across broad regions, whereas DHF exerts stronger influence at more confined scales, particularly by intensifying dry deviations. Additionally, the strongest DHF contributions to local deviations appear to be associated with human pressures on ecosystems, pointing to prospects for further studying freshwater change and vulnerabilities to its impacts in specific regions.

Our coherent unpacking of the global PB-FW transgression into regional components and their main drivers is a substantial advance in the use of the PB-FW. By linking the globally defined boundaries to regionally specific trajectories of freshwater change, we show how the new PB-FW can improve understanding of the extent, degree and drivers of global freshwater change. Similar applications and appraisals of other PBs could aid broader efforts of using the framework to inform sustainable environmental governance and Earth system stewardship, and to better connect global-scale approaches with more actionable, regional-scale knowledge on the drivers and impacts of freshwater change.