THE increases in the concentration of water vapour constitute the single largest positive feedback in models of global climate warming caused by greenhouse gases1,2. It has been suggested3-5 that sinking air in the regions surrounding deep cumulus clouds will dry the upper troposphere and eliminate or reverse the direction of water vapour feedback. We have now tested this hypothesis by performing an idealized simulation of climate change with two different versions of a climate model. The versions differ in their parameterizations of moist convection and stratiform clouds, but both incorporate the drying due to subsidence of clear air. Despite increased drying of the upper troposphere by cumulus clouds, upper-level humidity increases in the warmer climate because of enhanced upward moisture transport by the general circulation and increased accumulation of water vapour and ice at cumulus cloud tops. The model behaviour is consistent with recent satellite estimates of the water vapour feedback6,7.