First Simulations of Designing Stratospheric Sulfate Aerosol Geoengineering to Meet Multiple Simultaneous Climate Objectives

We describe the first simulations of stratospheric sulfate aerosol geoengineering using multiple injection locations to meet multiple simultaneous surface temperature objectives. Simulations were performed using CESM1(WACCM), a coupled atmosphere-ocean general circulation model with fully interactive stratospheric chemistry, dynamics (including an internally generated quasi-biennial oscillation), and a sophisticated treatment of sulfate aerosol formation, microphysical growth, and deposition. The objectives are defined as maintaining three temperature features at their 2020 levels against a background of the RCP8.5 scenario over the period 2020-2099. These objectives are met using a feedback mechanism in which the rate of sulfur dioxide injection at each of the four locations is adjusted independently every year of simulation. Even in the presence of uncertainties, nonlinearities, and variability, the objectives are met, predominantly by SO2 injection at 30 degrees N and 30 degrees S. By the last year of simulation, the feedback algorithm calls for a total injection rate of 51Tg SO2 per year. The injections are not in the tropics, which results in a greater degree of linearity of the surface climate response with injection amount than has been found in many previous studies using injection at the equator. Because the objectives are defined in terms of annual mean temperature, the required geongineering results in overcooling during summer and undercooling during winter. The hydrological cycle is also suppressed as compared to the reference values corresponding to the year 2020. The demonstration we describe in this study is an important step toward understanding what geoengineering can do and what it cannot do. Plain Language Summary Understanding what geoengineering can and cannot do is crucial for narrowing uncertainties in the range of potential responses to future climate change. Part of this effort is to demonstrate the capability of meeting specified climate objectives in a climate model in the presence of uncertainty in climate response. Here we provide the first demonstration of a strategy for meeting three simultaneous global temperature objectives via stratospheric sulfate aerosol geoengineering, using a state-of-the-art climate model that represents key processes relevant to the potential climate responses. We show that the objectives can be met successfully using an algorithm we have developed, and we also demonstrate some potential side effects, which could be potential objectives for which we did not control.