Change in negative emission burden between an overshoot versus peak-shaved Stratospheric Aerosol Injections pathway

Abstract. Stratospheric Aerosol Injection geoengineering (SAI) is being investigated as a potential means of temporarily reducing the impact of global warming, allowing additional time for the implementation of conventional climate mitigation strategies. SAI operates by intervening in the radiative energy balance of the Earth system, exerting a temporary direct cooling effect on the climate. However, SAI also indirectly affects global temperature through its impact on atmospheric CO2 levels by influencing the natural carbon uptake efficiency. Most previous research on the carbon cycle under SAI suggests that continuous injections enhance the uptake of carbon, implying a larger amount of allowable emissions for a given temperature target relative to a simulation without SAI. However, there are considerable uncertainties regarding the extent and timeline of facilitation or inhibition of atmospheric carbon removal under SAI. In this study, we evaluate the extent of change in negative emission burden over the entire trajectory of a peak-shaving SAI deployment (SSP534-sulfur) compared to the baseline overshoot pathway (SSP534-over) that does not involve SAI. We run the SSP534-over scenario on the CNRM-ESM2-1 Earth System Model from 2015 to 2249 and compare it to the simulation where, under SSP534-over conditions, SAI is used to maintain 1.5 °C warming (ssp534-sulfur). The results indicate that the carbon benefit associated with SAI evolves over time: While the increase in carbon uptake during SAI phase-in confirms prior studies and supports the concept of buying time during ramp up of SAI, later stages of SAI show the carbon benefit reducing and turning into an additional obstacle making a phase-out of SAI more difficult and potentially less desirable.