Why is there a short‐term increase in global precipitation in response to diminished CO 2 forcing?

Recently, it was found that a reduction in atmospheric CO 2 concentration leads to a temporary increase in global precipitation. We use the Hadley Center coupled atmosphere‐ocean model, HadCM3L, to demonstrate that this precipitation increase is a consequence of precipitation sensitivity to changes in atmospheric CO 2 concentrations through fast tropospheric adjustment processes. Slow ocean cooling explains the longer‐term decrease in precipitation. Increased CO 2 tends to suppress evaporation/precipitation whereas increased temperatures tend to increase evaporation/precipitation. When the enhanced CO 2 forcing is removed, global precipitation increases temporarily, but this increase is not observed when a similar negative radiative forcing is applied as a reduction of solar intensity. Therefore, transient precipitation increase following a reduction in CO 2 ‐radiative forcing is a consequence of the specific character of CO 2 forcing and is not a general feature associated with decreases in radiative forcing.