Quantifying the Impact of Internal Variability on the CESM2 ARISE-SAI-1.5 Control Algorithm

Earth system models are a powerful tool to simulate the response of the climate to the hypothetical climate intervention strategies, such as stratospheric aerosol injection (SAI). Recent simulations of SAI implement tools from control theory, called “controllers”, to determine how much aerosol to inject into the stratosphere to reach or maintain specified global temperature targets, such as limiting global warming to 1.5oC above pre-industrial temperatures. In this work we explore how internal (unforced) climate variability can impact controller-determined injection amounts in the Assessing Responses and Impacts of Solar climate intervention on the Earth system with Stratospheric Aerosol Injection (ARISE-SAI) simulations. Since the ARISE-SAI controller determines injection amounts by comparing annual global-mean temperatures to the predetermined temperature targets, any internal variability that impacts temperatures can impact the injection total amount as well. Using data from both the ARISE-SAI CESM2 simulations and the 100-member CESM2 large ensemble, we investigate how internal climate variability and volcanic eruptions impact the injection amounts determined by the ARISE-SAI controller. The data are decomposed into a climate warming component and an internal variability component and then combined in a variety of ways to create the controller inputs. These inputs are then fed through an offline version of the ARISE-SAI controller to quantify the sensitivity to the input patterns. While idealized, this approach allows for the investigation of a large variety of climate states without additional simulations and can be used to attribute controller sensitivities to specific modes of internal variability.