Planetary boundary for freshwater change: past drivers and future projections

Direct and indirect human pressures have notably changed the global freshwater cycle. The recently updated planetary boundary for freshwater change (PB-FW) illustrates the extent and degree of freshwater change in the Earth system by reporting the share of global land area experiencing anomalous streamflow and soil moisture conditions. However, analysis underlying the new PB-FW remains ambiguous in distinguishing the different drivers of water cycle change and offers no look into how future climate change may affect the PB-FW status. Here, we fill these missing pieces of the recent PB-FW research.

We adopt the methodology introduced in the new PB-FW definition while utilising an updated state-of-the-art global hydrological model ensemble from ISIMIP 3a and 3b simulation rounds. Scenarios in ISIMIP 3a offer a way to separate between direct human forcing (DHF) and climate related forcing (CRF), whereas ISIMIP 3b allows for projecting the status of the PB-FW into the future with respect to Shared Socioeconomic Pathway (SSP) scenarios. Furthermore, we enrich the metrics of PB-FW transgressions by considering also the magnitude of previously identified deviations from stable (unaffected) conditions at the local (grid-based) scale.

Our tentative results show that CRF generally dominates over DHF in determining the PB-FW status at the global scale during the historical period (1901–2019), both for streamflow and soil moisture. However, DHF has a stronger contribution to increasing dry streamflow deviations, which is particularly visible at smaller scales in regions under heavy anthropogenic influences. Quantifying the magnitude of local deviations shows how certain areas, such as Central Africa and the Mediterranean, have experienced the strongest dry local deviations, whereas the strongest wet local deviations locate to the northernmost latitudes and southern South America.

In line with the domination of CRF over DHF in the historical period, analysing future climate projections emphasises the strong dependence of the PB-FW status on climate action. Both for streamflow and soil moisture, the PB-FW transgression plateaus in the low-emission scenario (SSP1-2.6) projections, while high-emission scenarios (SSP3-7.0 and SSP5-8.5) project a continuously increasing trajectory of PB-FW transgression towards the end of the 21st century.

The results are largely in line with the existing PB-FW and related studies on past and projected global water cycle change. By resolving the drivers of PB-FW transgressions with updated scenario simulations and better quantifying PB-FW transgressions by considering the magnitude of local deviations, this study makes the new PB-FW more tangible and actionable.