Amplified future drying of tropical land constrained by physical theory

Near-surface relative humidity (RH) over land is a key mediator of land-atmosphere interactions, influencing surface energy partitioning, evapotranspiration, wildfire risk, and both temperature and precipitation extremes. Despite its central role in regulating land climate, the response of land RH to climate change remains highly uncertain, with climate models projecting a wide range of historical and future trends. Notably, many models struggle to reproduce the observed decline in land RH over the recent warming period, raising concerns about their representation of land climate processes and future projections. 

Here we develop a simple physical theory to constrain changes in land RH, grounded in an ocean-influence perspective on boundary layer moisture over land. The theory links fractional changes in tropical land RH to the land–ocean warming contrast. As land warms more rapidly than the ocean, the increase in the water-holding capacity of land air outpaces the supply of moisture imported from oceanic regions, leading to a systematic decline in land RH. This mechanism highlights how large-scale land-atmosphere interactions can be regulated by ocean-driven constraints on land boundary layer moisture. 

The theory explains much of the inter-model spread in historical tropical land RH trends, as well as the drying evident in reanalysis data. Combining the theory with observational estimates of the radiatively forced land–ocean warming contrast, we obtain constrained projections of future tropical land RH change (-6.4 %/K and -4.4 %/K) which indicate substantially stronger drying compared to the unconstrained projections (-1.5 %/K). This emergent constraint highlights a systematic underestimation of future land drying by climate models and its physical basis, with important implications for land-climate impacts in a warming world.