Increased moisture stress and weakened resilience to aridity limit global greening.

The "Greening Earth" and rising aridity are both climate change signatures. We investigate the response of global photosynthesis to moisture stress (higher demand and lower availability of moisture) in current (2000-2021) and future climate scenarios (until 2100). We employ a suite of statistical and machine learning (ML) techniques on satellite remote sensing, reanalyses and climate projection data for robust findings. Remote sensing based high resolution indicators of global photosynthesis in Fraction of Photosynthetically Active Radiation (FPAR), Enhanced Vegetation Index (EVI) and Solar-Induced Fluorescence (SIF) are utilised. Vapour Pressure Deficit (VPD, 39.76%) influences global photosynthesis more than Soil Moisture (SM, 31.44%) and Climatic Water Deficit (CWD, 28.8%), reveals Random Forest (RF). VPD exhibits a direct causal relationship with photosynthesis across biomes and land cover types, unlike SM and CWD. In absence of direct causal association, SM and CWD influence photosynthesis through VPD. Enhanced land (CWD, 16.7%) and atmospheric (VPD, 4.3%) evaporative demands reduce SM (-2.2%) in recent decade (2010-2019) from the previous (2000-2009). Concurrently, global photosynthesis exhibits enhanced cumulative growth rates (CGR), with a slowdown/reversal of global greening (-2.8% CGR), notably in grasses and tropical biomes due to rising moisture stress. Cropland, and temperate and arid biomes exhibit high sensitivity and low resilience to dryness stress. Global photosynthesis has gained resilience against land evaporative (CWD, 6.7%) and atmospheric aridity (VPD, 4.1%), conversely lost resilience against SM drying (-0.5%) in recent decade from previous. In moisture-stressed ecosystems, gain in resilience facilitates photosynthesis and decline in resilience results in slowdown or reversal of greening. This calls for effective land management to enhance the resilience of vulnerable ecosystems to rising moisture stress for ensuring food security and sustainability.