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Risk management has reduced vulnerability to floods and droughts globally1,2, yet their impacts are still increasing3. An improved understanding of the causes of changing impacts is therefore needed, but has been hampered by a lack of empirical data4,5. On the basis of a global dataset of 45 pairs of events that occurred within the same area, we show that risk management generally reduces the impacts of floods and droughts but faces difficulties in reducing the impacts of unprecedented events of a magnitude not previously experienced. If the second event was much more hazardous than the first, its impact was almost always higher. This is because management was not designed to deal with such extreme events: for example, they exceeded the design levels of levees and reservoirs. In two success stories, the impact of the second, more hazardous, event was lower, as a result of improved risk management governance and high investment in integrated management. The observed difficulty of managing unprecedented events is alarming, given that more extreme hydrological events are projected owing to climate change3.

Gravity Recovery and Climate Experiment (GRACE) observations provide information on Total Water Storage Anomaly (TWSA) which is a key variable for drought monitoring and assessment. The so-called Total Water Storage Deficit Index (TWSDI) based on GRACE data has been widely used for characterizing drought events. Here we show that the commonly used TWSDI approach often exhibits significant inconsistencies with meteorological conditions, primarily upon presence of a trend in observations due to anthropogenic water use. In this study, we propose a modified version of TWSDI (termed, MTWSDI) that decomposes the anthropogenic and climatic-driven components of GRACE observations. We applied our approach for drought monitoring over the Ganges-Brahmaputra in India and Markazi basins in Iran. Results show that the newly developed MTWSDI exhibits consistency with meteorological drought indices in both basins. We also propose a deficit-based method for drought monitoring and recovery assessment using GRACE observations, providing useful information about volume of deficit, and minimum and average time for drought recovery. According to the deficit thresholds, water deficits caused by anthropogenic impacts every year in the Ganges-Brahmaputra basin and Markazi basins is almost equal to an abnormally dry condition and a moderate drought condition, receptively. It indicates that unsustainable human water use have led to a form of perpetual and accelerated anthropogenic drought in these basins. Continuation of this trend would deplete the basin and cause significant socio-economic challenges.

Prediction of surface freshwater flux (precipitation or evaporation) in a CO2‐enriched climate is highly uncertain, primarily depending on the hydrological responses to physiological and radiative forcings of CO2 increase. Using the 1pctCO2 (a 1% per year CO2 increase scenario) experiments of 12 CMIP6 models, we first decouple and quantify the magnitude of global hydrological sensitivity to CO2 physiological and radiative forcings. Results show that the direct global hydrological sensitivity (for land plus ocean precipitation) to CO2 increase only is −0.09 ± 0.07% (100 ppm) −1 and to CO2‐induced warming alone is 1.54 ± 0.24% K−1. The latter is about 10% larger than the global apparent hydrological sensitivity (i.e., including all effects, not only direct responses to warming, ηa ${\eta }_{a}$ = 1.39 ± 0.22% K−1). These hydrological sensitivities are relatively stable over transient 2× to 4 × CO2 scenario. The intensification of the global water cycle are dominated by the CO2 radiative effect (79 ± 12%) with a smaller positive contribution from the interaction between the two effects (6 ± 12%), but are reduced by the CO2 physiological effect (−10 ± 8%). This finding underlines the importance of CO2 vegetation physiology in global water cycle projections under a CO2‐enriched and warming climate. Plain Language Summary: Projections of future water resource under rising CO2 and warming scenarios are highly uncertain, which is mainly due to the poor understanding of hydrological responses within the Earth system. We evaluate two major hydrological responses associated with rising CO2 related effects: physiological effect (e.g., decrease in plant stomatal conductance per unit leaf area, increase in leaf area index and net available energy, etc.) and radiative effect (e.g., rising CO2‐induced warming, rapid atmospheric adjustment and surface energy budget change, etc.). Using 12 Earth system models' experiments, we find that the CO2 physiological effect will cause ∼0.1% decline for precipitation or evaporation per 100 ppm CO2 increase at a global scale. The CO2 radiative effect will cause ∼1.5% increase for precipitation or evaporation per Kelvin warming. These changing rates of increase or decline are relatively stable with increasing CO2 forcing. The modeling experiments under high CO2 forcing project that the intensification of the global water cycle is mainly controlled by the CO2 radiative effect, but the CO2 physiological effect and the interaction between the two effects also play important roles. Key Points: Global hydrological responses to forcings from rising CO2‐induced physiological and radiative effects were decoupled from CMIP6 models' simulations with 1% per year CO2 increaseGlobal hydrological sensitivity to CO2 physiological forcing is −0.09 ± 0.07% (100 ppm)−1 and to CO2 radiative forcing is 1.54 ± 0.24% K−1 from transient 4 × CO2 simulationsGlobal hydrological sensitivities to CO2 physiological and radiative forcings have little changes over transient 2 × CO2 to 4 × CO2 scenario [ABSTRACT FROM AUTHOR]

Water scarcity is a critical threat in arid regions in China due to dry climate and rising human water demand. The sustainability of a recent wetter trend and its impact on future water security remain uncertain. This case study focuses on a hotspot region, the North Slope of the Tianshan Mountains (NSTM), to assess water scarcity in the coming decades (2030–2050) under two climate scenarios. To this end, we developed an integrated agro‐hydrological model to simulate historical and future hydrological processes and crop water dynamics in arid regions. Our results indicate nonsignificant increases in precipitation (around 3%) and evident rising temperatures (0.9–1.5°C) in the NSTM compared to the present‐day (2011–2020) climate. This translates to a projected increase in water availability (5.6%–11.2%) during 2030–2050, with slightly larger increases (6.3%–14%) in glacier runoff. However, the spatial mismatch between precipitation increases and water demand makes this potential gain largely offset by rising irrigation water demand (over 7%) if cropland remains constant from 2020 onwards. As a result, the current annual water deficit (3.3 km3) is likely to increase by 5%–11%, with 32% of NSTM basins facing persistent water scarcity. Most croplands are at high risk of groundwater depletion and 17%–34% of basins will experience intensified water scarcity. These findings highlight the urgent need for comprehensive water management strategies, including improved irrigation efficiency and exploration of alternative water sources, to ensure water security and sustainable development in arid China facing a changing climate. Plain Language Summary: Dry regions in China face a water shortage due to low rainfall and rising water use. Recently, even though more rainfall was observed in some arid regions, scientists aren't sure if this will continue. We focus on the hotspot region, the North Slope of the Tianshan Mountains (NSTM), to see how future climate change might affect water scarcity. We found slightly more rain and warmer temperatures in the coming decades (2030–2050) in the NSTM. This could lead to a small increase in available water. However, the need for irrigation water is expected to offset these gains. The current water deficit is likely to worsen, with some areas facing even greater scarcity and increased risk of groundwater depletion. This highlights the need for better irrigation practices and exploring alternative water sources to ensure water security and sustainable development in these regions under a changing climate. Key Points: Slight increases in precipitation and water availability are projected in the NSTMWater scarcity persists due to the spatial mismatch between increased water availability and high irrigation demandThe water deficit is likely to increase by 5%–11% from the current level of 3.3 km3 per year [ABSTRACT FROM AUTHOR]