Atmospheric drought dominates changes in global water use efficiency

Water use efficiency (defined as the ratio of gross primary productivity to plant transpiration, WUE T ) describes the tradeoff between ecosystem carbon uptake and water loss. However, a comprehensive understanding of the impact of soil and atmospheric moisture deficits on WUE T across large regions remains incomplete. Solar-induced chlorophyll fluorescence (SIF) serves as an effective signal for measuring both terrestrial vegetation photosynthesis and transpiration, thereby enabling a rapid response to changes in the physiological status of plants under water stress. The objectives of this study were to: 1) mechanistically calculate WUE T using top-of-canopy SIF data and meteorological information by using the revised mechanistic light response model and the Penman-Monteith equation; 2) analyze the effects of atmospheric and soil water deficits on SIF -based WUE T by using decoupled soil water content (SWC) and vapor pressure deficit (VPD); 3) evaluate estimated SIF -based WUE T against data from 28 eddy covariance (EC) flux sites representing eight different vegetation types. Results indicated that the model performed well in ecosystems with dense canopies, explaining 56 % of the daily variability in EC tower-based WUE T . For the years 2019 - 2020, the global average WUE T derived from SIF was 3.49 g C/kg H 2 O. Notably, this value exceeded 4 g C/kg H 2 O in tropical rainforest regions near the equator and went beyond 5 g C/kg H 2 O in the high -latitude regions of the Northern Hemisphere. We found that SIF-based WUE T was primarily influenced by VPD rather than SWC in over 90 % of the global vegetated area. The model used in this study increased our ability to mechanistically estimate WUE T with SIF at the global scale, thereby highlighting the significance of the global response of SIF-based WUE T to water stress, and also enhancing our understanding of the water-carbon cycle in terrestrial ecosystems.