Caldeira Lab Research:Energy, Global Carbon Cycle, and Climate/Climate Intervention ('Geoengineering')
Why is there a short‐term increase in global precipitation in response to diminished CO2 forcing?
Long Cao, Govindasamy Bala, & Ken Caldeira
Climates when CO2 concentrations are increasing will tend to have less precipitation than climates when CO2 concentrations are decreasing, even when at the same global mean temperature. Evidence for this hypothesis is found in climate model simulations in which a rapid reduction in CO2 concentrations cause a short-term increase in global precipitation (before global cooling produces reductions in precipitation).
Cao, L., G. Bala, and K. Caldeira, 2011. Why is there a short-term increase in global precipitation in response to diminished CO2 forcing? Geophysical Research Letters, Vol. 38, L06703, 6 PP., 2011 doi:10.1029/2011GL046713.
Recently, it was found that a reduction in atmospheric CO2 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 CO2 concentrations through fast tropospheric adjustment processes. Slow ocean cooling explains the longer-term decrease in precipitation. Increased CO2 tends to suppress evaporation/precipitation whereas increased temperatures tend to increase evaporation/precipitation. When the enhanced CO2 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 CO2-radiative forcing is a consequence of the specific character of CO2 forcing and is not a general feature associated with decreases in radiative forcing.