Climate can influence drought through many pathways involving atmosphere, land, and ocean processes as well as their interactions. With global warming, a slow-to-rapid drought transition has been identified in observations and climate projections but the mechanisms for the transition have not been fully explored. Here we investigate the mechanism by which vegetation regulates flash drought and its future change. Ensemble simulations from earth system models show robust increases in flash drought over the northern mid-to-high latitudes where the growing season is lengthening. Although greening might suggest reduced drought stress, it drives precipitation-soil moisture-evapotranspiration decoupling by increasing evapotranspiration partitioning to transpiration. The latter weakens the constraints of concurrent precipitation on evapotranspiration, thus accelerating soil moisture depletion under high evaporative demand, driving a slow-to-rapid drought transition. In contrast to the mid-to-high latitudes, flash drought changes are less obvious in the tropics where greening is also subdued. In the Amazon basin, large increases in vapor pressure deficit are projected in the future particularly during the dry-to-wet transition season, consistent with the delayed onset of rainfall. The drying due to delayed rainfall amplifies the warming in spring, potentially increasing the chance of hot drought which has substantial impacts on the Amazon forest and its role as a carbon sink in the future.