Dependency of the impacts of geoengineering on the stratospheric sulfur injection strategy part 2: How changes in the hydrological cycle depend on injection rates and model?

Abstract. This is the second of two papers where we study the dependency of the impacts of stratospheric sulfur injections on the used model and injection strategy. Here, aerosol optical properties from simulated stratospheric aerosol injections using two aerosol models (modal scheme M7 and sectional scheme SALSA), as described in Part 1, are implemented consistently into EC-Earth, MPI-ESM and CESM Earth System Models to simulate the climate impacts of different injection rates ranging from 2 to 100 Tg(S)yr⁻¹. Two sets of simulations were simulated with the three ESMs: 1) Regression simulations, where abrupt change in CO₂ concentration or stratospheric aerosols over preindustrial conditions were applied to quantify global mean fast temperature independent climate responses and quasi-linear dependence on temperature and 2) equilibrium simulations, where radiative forcing of aerosol injections with various magnitudes compensate the corresponding radiative forcing of CO₂ enhancement to study the dependence of precipitation on the injection magnitude; the latter also allow to explore the regional climatic responses. Large differences in SALSA and M7 simulated radiative forcings in Part 1 translated into large differences in the estimated surface temperature and precipitation changes in ESM simulations: e.g. an injection rate of 20 Tg(S)yr⁻¹ in CESM using M7 simulated aerosols led to only 2.2 K global mean cooling while EC-Earth – SALSA combination produced 5.2 K change. In equilibrium simulation, where aerosol injections were used to compensate for radiative forcing of 500 ppm atmospheric CO₂ concentration, global mean precipitation reduction varied between models from -0.7 to - 2.4 %. These precipitation changes can be explained by the fast precipitation response due to radiation changes caused by the stratospheric aerosols and CO₂ because global mean fast precipitation response is rather negatively correlated with global mean absorbed radiation. Our study shows that estimating the impact of stratospheric aerosol injection on climate is not straightforward. This is because the capability of the sulfate layer to reflect solar radiation and absorb LW radiation is sensitive to the injection rate as well as the aerosol model used to simulate the aerosol field. These findings emphasize the necessity for precise simulation of aerosol microphysics to accurately estimate the climate impacts of stratospheric sulfur intervention. This study also reveals gaps in our understanding and uncertainties that still exist related to these controversial techniques.