Analysis of the GeoMIP G6sulfur experiment with SOCOLv4

Climate change and its associated risks are becoming more and more prominent. Stratospheric solar geoengineering with sulfuric acid aerosols has been put forward as a way to temporarily mitigate some of the risks of climate change and is inspired by the cooling effect of large eruptions of tropical volcanoes. To learn more about the opportunities and dangers associated with stratospheric solar geoengineering, it is important to investigate the strategy beforehand, e.g., by means of climate modelling. To better understand the sources of model uncertainties, the Geoengineering Model Intercomparison Project (GeoMIP) introduced stratospheric solar geoengineering scenarios for an easier comparison of different models. Most models participating in GeoMIP either have no interactive chemistry or simplified aerosol micro-physics. In this study we perform the G6sulfur experiment with SOCOLv4, an atmosphere-ocean-aerosol-chemistry climate model. In the G6 sulfur experiment the aim is to bring the global average temperature of the SSP5-8.5 to the levels of the SSP2-4.5 sceanrio.

For the calibration we ran three different tests in order to analyse the sensitivity of the aerosol burden to the order in which the microphysical processes are simulated at each timestep - nucleation of new particles from H2SO4 vapours and condensation of H2SO4 on pre-existing particles. One experiment had nucleation first, one had condensation first and finally one had nucleation first but with an added subsubstep where coagulation is called again. For all these runs we used an injection of 5 TgS/year. In the run with nucleation first the global stratospheric aerosol burden is 25% bigger than in the run where condensation is called first and 10% bigger than in the run with 2 subsubsteps. This leads to a cooling effect over 2032-2047 which is 1.02 K for nucleation first, 0.95 K for the run with the additional substep and 0.65 K for condensation first. Based on the cooling efficiency of the 5 TgS/year injection, we then derive a time-dependent emission, to keep global mean surface temperatures close to the SSP2-4.5 scenario.

For the G6sulfur experiment we chose the setup of the run with 2 subsubsteps and performed three ensemble members to get a better understanding of the uncertainties within the model. We will discuss the effects on stratospheric aerosol burden, radiative forcing, temperature, ozone and precipitation changes and compare our results to other GeoMIP models. This will work provides useful insights concerning the radiative and climatic impacts of stratospheric aerosols on climate, elucidating the impact of uncertainties in the modelling of microphysical processes.