Larger increase in future global terrestrial water availability than projected by CMIP6 models

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<p>Climate change and increasing water use associated with socio-economic growth have exacerbated the water crisis in most of the world. However, terrestrial water availability (precipitation minus evapotranspiration, PME) is inadequately represented within Earth System Models, directly affecting the reliability and consistency of future projections. To address this issue, we employ twenty Coupled Model Intercomparison Project Phase 6 (CMIP6) models and multiple observational PME datasets to produce more robust future projections. Most models underestimate the increase in PME over 1982–2011, while over 80% of the models project a continued rise in future PME. We thus constrain future changes in global mean PME (ΔPME) under medium and high greenhouse gas emission scenarios (SSP2-4.5 and SSP5-8.5) using the hierarchical emergent constraint (HEC) approach. The constrained PME shows a significant increase by 33.81 ± 13.72mm, or 13% ± 5%, during the period <styled-content style-type="number">2070</styled-content>–<styled-content style-type="number">2099</styled-content> relative to 1982–2011 under SSP5-8.5. Implementation of the HEC method reduces the uncertainty range by 24% and shows the ΔPME of the raw CMIP6 was underestimated by 37%. The PME change trends are most evident in low-latitude regions, with constrained PME increasing by 75.69 ± 24.35 mm, or 28% ± 9%, in the Northern Hemisphere’s low latitudes and by 34.56 ± 38.91mm, or 8% ± 10% in the Southern Hemisphere’s low latitudes by <styled-content style-type="number">2070</styled-content>–<styled-content style-type="number">2099</styled-content> under SSP5-8.5. Our findings highlight the importance of accurate model representation of precipitation to produce reliable projections of global terrestrial water availability. The observationally-constrained ΔPME is valuable for sustainable management of water resources.</p>