Modest geoengineering side effects predicted by an emulator

Abstract It is common to speak about side effects of solar radiation management geoengineering considering that the forcing involved is different in nature from the anthropogenic greenhouse forcing. However, we argue that even if side effects are identified in terms of uncontrolled changes of climatic variables, they should scale with the geoengineering forcing magnitude. This would dictate a definition of side effects that takes a reference scenario where the same target is achieved, e.g. limiting the global mean surface temperature to an agreed maximum, without deploying geoengineering, but, say, by CO2 abatement only. We use a response theory based emulator, derived from new simulation output for the Max Planck Institute’s Earth system model, MPI-ESM, as well as the cGENIE carbon cycle model, in order to evaluate regional side effects regarding annual mean surface temperature and total precipitation in this sense. We find that the respective spatial patterns of the side effects bear the features as those known from the G2-type “cancellation” experiments of the Geo-MIP protocol – more so for temperature than precipitation. The former is governed by the effect of polar amplification, for which we provide now a succinct mathematical model, both for the greenhouse and geoengineering forcing, and explain the G2-type temperature pattern.