The humidity paradox: how drier conditions and fewer clouds amplify terrestrial warming due to global climate change

Abstract. Clouds are a controlling factor determining the planetary albedo and thus strongly contribute to establishing the radiation balance at the top of the atmosphere as well as the energy balance at the surface. Terrestrial warming rates, divergent to oceans, have accelerated substantially since around 1980, along with significant changes in humidity and cloud cover. We analyse spatiotemporal changes in warming rates compared to changes in Earth’s radiation and humidity, considering land globally for the period 1979–2023, using reanalysis and satellite data. We find statistically significant increases in top of the atmosphere net solar radiation and surface net terrestrial radiation, causing a net warming, together with decreasing cloud cover. These changes coincide with drier land surface conditions and are associated with the humidity paradox: insufficient supply of water vapour causing a decrease in relative humidity. Reduced evaporative cooling of the land surface is an additional positive feedback and has likely contributed to the ocean-land warming contrast. The oceans have, on the other hand, effectively unlimited water to evaporate and can therefore cool in a warming climate by evaporating more and more water. Inhibited cloud formation over land and an increase in solar radiation provide an amplifying feedback loop for the observed rapid terrestrial warming in recent decades due to a CO2-driven humidity deficit. A decrease in precipitation over land regionally is a strong indication of perturbed surface water balance that is driven by increases in absorbed infrared and solar radiation.