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 b y anthropogenic greenhouse gasses. Satellite radiance observations from the past 20 years, which are sensitive to the mater vapor and temperature of t he upper troposphere, provide the first global observations of trends in up per-tropospheric humidity. These decadal trends are strongly positive in th e deep tropics, negative in the Southern Hemisphere subtropics and midlatit udes, and of mixed sign in the Northern Hemisphere subtropics and midlatitu des. The trends are shown to be consistent with atmospheric circulation cha nges observed in the past 20 years, including a tendency toward more EI Nin o-Southern Oscillation warm events and changes in transient eddy activity i n the subtropics.