Abstract
Volcanic aerosol forcing has been reported in the literature to be less
effective in changing the earth’s surface temperature than CO2 forcing.
This implies a different feedback strength, and therefore different
contributions from individual feedback mechanisms. We employ the CMIP6
version of MPI-ESM to understand the reasons for these apparent
differences in the ability to change the surface air temperature. Using
a highly idealized eruption scenario and comparing it to a doubling and
a halving of CO2 concentration, we identify key reasons for changes in
the magnitude of the feedback parameter. We show that the “efficacy”
[Hansen et al. 2005] of volcanic aerosol forcing depends strongly on
the method and the time scale used to calculate it. We argue that the
seemingly established result of a lower-than-unity efficacy of volcanic
aerosol forcing might only hold under the specific methodological
choices other authors have made, but not in general. Furthermore, we
find qualitative differences between the cooling and warming
simulations, but strong similarities between the 0.5xCO2 and the
idealized eruption cases. This hints towards processes, which are not
forcing agent-specific, but specific to the sign of the forcing. A
pronounced curvature in the N(T) plot (“Gregory plot”) for the cooling
scenarios makes the computation of feedback through regression even more
sensitive to subjective choices than in the 2xCO2 case. We disentangle
the role of ocean heat uptake efficacy and atmospheric feedback
processes in the framework of the pattern effect.