Holistic Assessment of SO 2 Injections Using CESM1(WACCM): Introduction to the Special Issue
We introduce a special issue on a holistic assessment of geoengineering via stratospheric SO 2 injection in the state-of-the-art climate model, Community Earth System Model, version 1 with the Whole Atmosphere Community Climate Model as its atmospheric component (CESM1(WACCM)). This model has numerous complexities that allow it to represent some of the most important nonlinearities associated with stratospheric SO 2 injection, including aerosol microphysical growth and stratospheric chemistry. The studies described herein represent the steps towards and first attempt in a state-of-the-art climate model to use geoengineering to meet multiple simultaneous objectives via SO 2 injection at multiple locations. First, a “matrix” of simulations was carried out to better understand the response of geoengineering to variations in latitude, altitude, and amount of sulfur dioxide injections. Subsequently, in a century long simulation, a feedback algorithm was employed to meet specific objectives and manage uncertainty, wherein the injection amount at each location was adjusted every model year. Most of the analyses contained in this special issue focus on surface climate and stratospheric changes in the matrix and century long simulations. In addition, 20 ensemble members of the feedback simulation have been carried out and provided to the community to expand the scope of analyses to low signal-to-noise ratio fields, including regional effects, impacts assessment, and extreme events. The demonstration provided by these simulations is a step toward understanding the space of achievable climate objectives via geoengineering, or, phrased differently, determining what geoengineering can do and what it cannot do.