Global trends in atmospheric dryness dominated by Clausius-Clapeyron scaling

Atmospheric dryness, often quantified by Vapor Pressure Deficit (VPD) or Relative Humidity (RH), is a prominent variable for terrestrial water and carbon cycles. While global warming is widely expected to amplify atmospheric dryness, the physical drivers governing this intensification and its regional variations remain poorly understood. Here we analytically decompose trends in daily maximum VPD and minimum RH into contributions from three key factors: the Clausius-Clapeyron temperature sensitivity of saturation vapor pressure, the diurnal temperature range (reflecting daily heat storage changes in lower atmosphere), and the proximity to saturation of the atmosphere at night (defined as the difference between minimum temperature and the dew point). Applying this framework to long-term observations from FLUXNET and ERA5 reanalysis reveals that Clausius-Clapeyron scaling is the dominant driver of global atmospheric drying trends. In addition, we find that regional variations in drying trends between arid and humid regions primarily come from contrasting trends in nighttime atmospheric dryness. This regionally asymmetric response amplifies dryness trends in arid regions while dampens it in humid regions, aligning with the "dry-gets-drier, wet-gets-wetter" paradigm under future climate change. Our analytical framework helps explain observed spatial heterogeneity in atmospheric drying trends and also offers a new pathway for evaluating its representations in climate models.