Caldeira Lab

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 CO 2 forcing? Geophysical Research Letters, 38, 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.


Figure: HadCM3L-simulated changes in global and annual mean precipitation as a function of changes in global and annual mean surface air temperature for (a) CO2_step and (b) Solar_step simulations. The red dots represent the first 7-year simulation period with enhanced CO2 or solar forcing, and the blue dots represent the following 150-year simulation period when the enhanced CO2 or solar forcing is removed. At constant temperature, changes in CO2 have a much larger effect on precipitation than changes in solar intensity with similar radiative forcing.