Sea-Salt Injection to Cool the Arctic : an Earth System Model Comparison.

As we reach dangerous levels of warming, Sunlight Reflection Methods (SRM) may be used to cool the Earth either regionally or globally. Sea-salt injection has been proposed as a method to either increase the brightness of clouds (Marine Cloud Brightening, MCB), or directly increase the brightness of the sky (Marine Sky Brightening, MSB). By reflecting a small amount of extra sunlight, these would cool the Earth's surface and reduce some of the impacts of global warming. In this study, we analyse three sets of simulations of regional sea-salt injection aimed at cooling the Arctic using the UKESM1, CESM2, and E3SM Earth System Models. Specifically, the target is to maintain the Arctic surface air temperature at a constant level in a moderate greenhouse gas emissions scenario. While sea-salt injection successfully keeps the Arctic temperature constant in CESM2 and E3SM, UKESM1 is 2K above the target by the end of the simulations and requires 10 times more injection of sea-salt relative to CESM2. This difference in sensitivity between the two models was also found for sea-salt injections in the Pacific Ocean in previous work. Moreover, the cooling in UKESM1 results from MSB, whereas the cooling in CESM2 results mostly from MCB. We also find that cooling the Arctic has limited remote impacts and successfully maintains Arctic sea ice and the Atlantic Meridional Overturning Circulation. Finally, we emphasize that these are highly idealised simulations where sea-salt is injected in all open ocean Arctic grid cells, hence questions around feasibility are side-stepped in this study.