Using Feedback Control to Regulate Global Mean Temperature with Marine Cloud Brightening in CESM2

Climate intervention via the artificial brightening of low marine clouds, or Marine Cloud Brightening (MCB), has been proposed as a method of reflecting sunlight to cool the earth and reduce the impacts of global warming. The impacts of MCB will depend on, among other things, the size and location of the intervention, and evaluating and comparing different "strategies" is therefore necessary to fully understand what MCB could or could not accomplish. The application of feedback control theory has been used in climate models in conjunction with Stratospheric Aerosol Injection (SAI), another proposed mode of climate intervention; rather than using prescribed aerosol injection rates, a climate "goal" (such as a constant global mean temperature) is specified, and the feedback control algorithm chooses the injection rates necessary to meet that goal. With enough information about how the intervention affects the climate system, multiple climate goals, including different temperature patterns, precipitation distribution, and Arctic sea ice extent, can be targeted simultaneously. We apply this methodology to MCB in version 2 of the Community Earth System Model (CESM2), first by conducting "system ID" simulations to quantify the impacts of different MCB strategies, and then by writing feedback control algorithms and simulating feedback-regulated MCB to meet a global mean temperature target. Our simulations demonstrate that by "ramping up" the scope of an MCB intervention by increasing the area of the ocean in which MCB is deployed, we can eliminate most of the warming above the specified target (1.5°C above pre-industrial) in CESM2 under the SSP2-4.5 scenario in the 2035-2069 period. This methodology can be expanded to individual interventions in different areas of the ocean to allow for multiple degrees of freedom, with the eventual goal of managing global mean temperature (T₀), the inter-hemispheric temperature gradient (T₁), and the equator-to-pole temperature gradient (T₂) independently, analogous to the GLENS (Geoengineering Large Ensemble) and ARISE-SAI-1.5 (Assessing Responses and Impacts of Solar climate intervention on the Earth system) simulations.