Speaker: Andrew Chingos (St Andrews) Title: Amplified decreases in relative humidity over tropical land constrained by physical theory Abstract: Near-surface relative humidity is a fundamental metric of land climate modulating temperature, precipitation, evapotranspiration, wildfires, and heat stress in a warming world. The response of land relative humidity to climate change is substantial but uncertain, with models projecting a wide range of future changes. Previous work showed that models fail to reproduce observed historical decreases in land relative humidity, raising doubts about future projections. Here, to constrain projections of land humidity in the tropics, we derive a simple physical theory — grounded in an ocean‐influence perspective on moisture over land — that links fractional changes in relative humidity to the land–ocean warming contrast. According to this theory, stronger land warming leads to an increase in air’s water-holding capacity that outpaces moisture supplied from oceanic regions, driving a decrease in relative humidity. The ocean‐influence theory explains much of the spread across models in historical land relative humidity trends as well as the trend in reanalysis data. Combining the theory with estimates of the historical radiatively-forced land-ocean warming contrast, we obtain constrained estimates of future tropical land relative humidity change (-7.1 %/K assuming a climate-invariant warming contrast; -4.8 %/K when changes in warming contrast across climate states are accounted for). The constrained estimates suggest systematically stronger decreases in tropical relative humidity compared to the unconstrained multi-model median change (-1.5 %/K). This emergent constraint highlights a systematic underestimation of future land relative humidity decreases by models and reveals its physical basis, with important implications for projecting the impacts of climate change in tropical regions.

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