Climate feedback to stratospheric aerosol forcing: the key role of the pattern effect

Volcanic aerosol forcing has previously been found to cause a weak global mean temperature response, compared to CO2 radiative forcing of equal magnitude: its efficacy is supposedly low, but for reasons which are not fully understood. In order to investigate this, we perform idealized, time-invariant stratospheric sulfate aerosol forcing simulations with the MPI-ESM-1.2 and compare them to 0.5 × CO2 and 2 × CO2 runs. While the early decades of the aerosol forcing simulations are characterized by strong negative feedback (i.e. low efficacy), the feedback weakens on the decadal to centennial time scale. Although this effect is qualitatively also found in CO2-warming simulations, it is more pronounced for stratospheric aerosol forcing. The strong early and weak late cooling feedbacks compensate, leading to an equilibrium efficacy of approximately one in all simulations. The 0.5 × CO2 cooling simulations also exhibit strong feedback changes over time, albeit less than in the idealized aerosol forcing simulations. This suggests that the underlying cause for the feedback change is not exclusively specific to aerosol forcing. One critical region for the feedback differences between simulations with negative and positive radiative forcing is the tropical Indo-Pacific warm pool region (30°S - 30°N, 50°E - 160°W). In the first decades of cooling, the temperature change in this region is stronger than the global average, while it is stronger outside of it for 2 × CO2 warming. In cooling scenarios, this leads to an enhanced activation of the warm pool region’s strongly negative lapse rate feedback