Online Stratospheric Aerosol Injection Feedback Control for E3SM

In order to combat anthropogenic climate change, many groups have studied solar radiation modification within climate models. Notable examples include the GLENS [1] and ARISE [2] ensembles, which simulated feedback controlled stratospheric aerosol injection (SAI). These studies restart climate simulations on a yearly basis and the feedback controller performs offline recalculations of the SAI forcing in response to the simulated climate. Yearly updates to an SAI feedback control algorithm can compensate global-mean temperature and interhemispheric temperature gradient, but not without overcooling the tropics and undercooling the poles, leading to undesirable regional and seasonal side effects, including an averaging out of the seasonal behavior of the Arctic sea ice extent.

We propose an online sulfur dioxide (SO2) SAI feedback control algorithm which runs in tandem with E3SMv3 and regularly updates SO2 injection rate according to a feedforward predictive model and a feedback corrective model. Higher update frequency enables seasonally-varying SAI, which is desirable given the seasonally-dependent effectiveness of SAI. This seasonal dependence is exacerbated by Brewer-Dobson circulation, which transports the resulting aerosols toward the winter pole where there is no solar radiation for SAI to mitigate. Sub-annual injection updates pose challenges, notably, how to distinguish forced signal from climate variability, and how to use SAI to target seasonal and regional biases without causing adverse impacts elsewhere in space and time. The higher update frequency (seasonal vs. annual) is made feasible by embedding the feedback controller inside of E3SM, such that E3SM does not have to stop running each season to exchange information with the controller. In this talk we will introduce the mechanics of in situ control, develop a feedback control algorithm, replicate and study the previous annual approaches and their consequences, and discuss the calibration simulations that will enable seasonal-control to avoid this undercooling.

[1] Tilmes, Simone, et al. "CESM1 (WACCM) stratospheric aerosol geoengineering large ensemble project." Bulletin of the American Meteorological Society 99.11 (2018): 2361-2371.
[2] J. H. Richter et al., ‘Assessing Responses and Impacts of Solar climate intervention on the Earth system with stratospheric aerosol injection (ARISE-SAI): protocol and initial results from the first simulations’, Geoscientific Model Development, vol. 15, no. 22, pp. 8221–8243, 2022.