The Land Wet-Bulb Temperature Increases Faster Than the Sea Surface Temperature

Vapor buildup in the atmosphere and faster warming over land than over ocean are ubiquitous features of climate change. This combination is a threat to society because the associated heat stress may exceed the limit for human survival. The heat stress due to high humidity and high temperature is quantified with the wet-bulb temperature (Tw). A common view is that the land Tw should change at the same rate as the ocean surface temperature (Ts). Using climate model data and atmospheric observations, we show that the land Tw increases 17% faster than Ts. This amplification arises from stronger downward longwave radiation (L down arrow) at the surface in a warmer climate, which causes moist static energy to accumulate in the atmospheric boundary layer. We also find that L down arrow is a better predictor of Tw than Ts at interannual to decadal time scales. These relationships are robust across climates and across model simulations. Humid heat, or combination of high temperature and high humidity, is a serious climate threat to society. A robust understanding of the climatology of humid heat-as measured by the wet-bulb temperature Tw-is needed to inform strategies for heat risk mitigation. Here we perform a comprehensive analysis of Tw using atmospheric observations and climate model calculations, and develop a simple box model for the land humid heat. The relationship between this temperature and the longwave radiation energy can help future prediction of the land humid heat. A box model of moist static energy budget in the atmospheric boundary layer is developed for the land wet-bulb temperature changeWet-bulb temperature over global land changes 17% faster than ocean surface temperatureA robust relationship exists between the wet-bulb temperature and the downward longwave radiation