| 13 Nov 2025
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.
This is a nice paper. It does a good job of setting up a new protocol for GeoMIP and provides substantial details on how to do it, what to expect, and some preliminary results from other models.
I have one general comment: some more context is needed. What exactly is this paper doing? It leans very heavily on previous papers to describe aspects of the protocol, but it also provides major aspects of the protocol. It comes across as both suggesting that this experiment be included in GeoMIP as well as implying that it was practically commissioned by GeoMIP. I’d appreciate some clarity of purpose and delineating what this paper adds.
Line 39: its
Line 40: Ahlm et al. (2017) is relevant here. And elsewhere – see below.
Lines 73 and 75: I question your claims about impacts analysis. Given the uncertainties in aerosol-cloud interactions in models, as well as the compromises you make to allow broad model participation (section 2.4), these are still somewhat idealized.
Lines 146ff: What about this simulation setup is specific to the midlatitudes? Did you mean to specify a latitude range? (Or refer to Section 2.2.)
Line 154: Can you be more specific about what “early” means? If this is to be a protocol, either a year range or a forcing level would be useful.
Line 159: It would be useful to talk about Ahlm et al. (2017) here. Simulations can vary widely depending upon how (un)sophisticated the model representations are.
Line 217: Plus at such high rates of injection, the direct effect becomes important.
Line 236: enhancements
Line 236: Is it feasible to reproduce the equations here? Then the paper is more self-contained.
At the end of section 2.4, you might want to talk about the benefits you would gain if models choose to do both cdnc and sea salt.
Lines 309-312: I don’t think your results support these sentences. It’s true for CESM and E3SM (which have a lot of overlap) and not true for UKESM.
Lines 327-328: I promise you, it does. But it may not be enough to affect your results.
Line 367: There’s a typo in here somewhere.
Lines 375: See Kravitz et al. (2016), who talked about a land vs ocean timescale. The point being, there are lots of processes that your system identification simulations are likely failing to pick up. AMOC changes, for example.
Line 476: I strongly advise you to avoid language about policy relevance unless you’re explicit about what policies and how it would be relevant. Trust me – that’s a hard-won lesson. I think your language about cooperative MCB is more than enough justification.