G6-1.5K-MCB: Marine Cloud Brightening Scenario design for the Geoengineering Model Intercomparison Project (GeoMIP) in CESM2.1, E3SMv2.0, and UKESM1.1

Abstract. We present a protocol for scenario simulations of marine cloud brightening (MCB) solar radiation modification (SRM), which we design for inclusion as a bridge simulation in the Geoengineering Model Intercomparison Project (GeoMIP). This protocol, named G6-1.5K-MCB, parallels the existing G6-1.5K-SAI, but it simulates injecting sea salt aerosol (iSSA) into the lower marine boundary layer to create a MCB scenario. Using information taken from recent modeling studies, we propose to apply MCB iSSA emissions in the midlatitudes, which can produce a surface temperature response that more closely resembles the opposite of the greenhouse gas (GHG) warming pattern without invoking a significant La Niña response that has impacted previous studies. In many ways, this is analogous to the choice of emissions at 30N and 30S for stratospheric aerosol injection (SAI) in G6-1.5K-SAI. Owing to substantial uncertainty in the aerosol-cloud forcing from MCB, we outline recommended benchmark simulations to facilitate similar simulations of cloud brightening across different models. We present simulations of the G6-1.5K-MCB protocol using three Earth System Models (ESMs). All three ESMs show that for an intermediate baseline GHG emission trajectory, midlatitude MCB can maintain global mean surface temperature (GMST) at 2020–2039 temperatures. The iSSA emission rates required to maintain this target vary by a factor of 20 across the ESMs due to differences in the size distribution of the emitted iSSA and in the representations of aerosol-cloud interactions, demonstrating the importance of benchmark simulations for both understanding uncertainties and setting up the scenario simulations. Temperature and precipitation anomalies are greatly reduced relative to the GHG warming background, with most regions experiencing no statistically significant changes relative to the reference period. In some regions, there is a notable seasonal cycle in the residual climate change, though the anomalies are still much smaller than the GHG warming impact. On the basis of the promising results from this three-model testbed, we propose that the G6-1.5K-MCB serve as a basis for future model intercomparison protocols. This will enable further estimation of the structural uncertainties of ESMs in the climate response to MCB and provide a valuable dataset for more detailed analysis of the potential impacts of MCB.