A thermodynamic bound on Hadley cell expansion

A long-term widening of the Hadley cell with climate warming is a robust feature of observations and global climate model simulations, but it remains poorly constrained. Here we show that the sensitivity is fundamentally bound by the moist thermodynamics of the tropics, largely independent of more complex extratropical dynamics. By incorporating the 7% per kelvin Clausius-Clapeyron scaling of water vapor concentrations with temperature into a new energetic framework, we predict a modest tropical widening of approximately 20–25 km or 0.7% per kelvin of surface warming that weakens with further temperature rise. Comparing this thermodynamic constraint with atmospheric reanalyses and climate model simulations, we find close agreement over multi-decadal timescales. Our results explain why observed and modeled widening of the tropics is on the lower end of the range reported by some prior studies, and provide a physically grounded bound on shifts in subtropical dry zones with future global warming.