Trends in upper-tropospheric humidity

Water vapor is the most radiatively active greenhouse gas, and the process of water‐vapor feedback may significantly amplify global warming induced by anthropogenic greenhouse gasses. Satellite radiance observations from the past 20 years, which are sensitive to the water vapor and temperature of the upper troposphere, provide the first global observations of trends in upper‐tropospheric humidity. These decadal trends are strongly positive in the deep tropics, negative in the Southern Hemisphere subtropics and midlatitudes, and of mixed sign in the Northern Hemisphere subtropics and midlatitudes. The trends are shown to be consistent with atmospheric circulation changes observed in the past 20 years, including a tendency toward more El Niño‐Southern Oscillation warm events and changes in transient eddy activity in the subtropics.