Deciphering the role of evapotranspiration in declining relative humidity trends over land

In recent decades, relative humidity (RH) over land has declined, driving increases in droughts and wildfires. Previous explanations attribute this trend to insufficient moisture advection from the ocean to sustain RH over land, but this ignores atmospheric moisture supplied from terrestrial evapotranspiration (E). While state-of-the-art climate models underestimate this RH trend, the reason behind this discrepancy remains unclear. Here, we decipher the influence of E on near-surface humidity using observations, reanalysis, and climate simulations. Global E in reanalysis has remained fairly steady in recent decades. Consequently, changes in ocean advection can reproduce observed RH declines without considering changes in E. Conversely, climate simulations estimate significant increases in E in recent decades, leading to model-based underestimation of observed RH declines. These findings suggest E intensifications may be overestimated in current climate models, thus underestimating coupled land-atmosphere drying in model output. We also highlight an upper limit of E change under observed RH trends, which could help benchmark global E trend analyses.