Atmospheric Dryness Dominates Afternoon Depression of Global Terrestrial Photosynthesis

Satellite observations reveal a widespread afternoon depression of photosynthesis globally. Utilizing satellite observations and eddy covariance tower-based observations worldwide, we investigated the impact of climate factors on the diurnal patterns of ecosystem gross primary production (GPP). Our analysis revealed that the increase in vapor pressure deficit (VPD) shifts the diurnal peak of GPP activity to earlier morning hours, particularly in drylands and areas with short vegetation. After disentangling the strong correlations among VPD, temperature, and soil moisture, we unraveled that VPD emerges as the dominant driver contributing to the widespread afternoon depression of photosynthesis in terrestrial vegetation globally. However, Earth System Models (ESMs) systematically underestimate the significant role of VPD in regulating photosynthesis. Eight out of 10 ESMs exhibited a clear afternoon increase in photosynthesis, which was attributed to temperature. Our findings emphasize the need to enhance the negative effects of VPD on diurnal photosynthesis in ESMs.