On the Range of Marine Cloud Brightening Radiative Forcing across Earth System Models

Abstract. Marine cloud brightening (MCB) is a solar radiation modification method that involves using injections of sea salt aerosol (iSSA) particles to increase cloud albedo and cool surface climate. Earth system models (ESMs) are crucial tools for projecting the potential climate response to MCB, but they must represent many key aerosol and cloud processes using parameterizations that are often poorly constrained and can vary substantially. Here, we conduct simulations in six ESMs, plus four alternate modified parameterizations versions, to compute effective radiative forcing (ERF) from MCB iSSA emissions in three subtropical regions. We a find large range in ERF across the nine models with appropriate aerosol activation, with iSSA mass emissions to get -2 Wm⁻² ranging from 7 Tg yr⁻¹ to 93 Tg yr⁻¹ in models. The emitted iSSA size distribution drives much of this spread, as the ESMs use median modal radii ranging from 28.1 nm to 72.5 nm, resulting in major differences in CCN concentration. Forcing from aerosol-cloud interactions drives the majority of the total ERF below about -2 Wm⁻² across seven of ten models. At high emission rates, the aerosol-cloud forcing saturates, but the direct aerosol forcing continues to increase sufficiently quickly such that there is no regime in which additional appropriately-sized iSSA emissions would cause warming in any of the ESMs. After accounting for iSSA size distribution, the spread in aerosol-cloud interaction forcing is mainly due to differences in cloud adjustments at low emission rates and mainly due to cloud droplet activation saturation at high emission rates.