Comparison of climate response to marine cloud brightening and ocean albedo modification: A model study

Solar radiation modification (SRM), an artificial intervention to reduce the amount of solar radiation reaching the surface, has been proposed as a potential option to ameliorate some undesired consequences of global warming. Marine cloud brightening (MCB) and ocean albedo modification (OAM) are two proposed SRM approaches. MCB aims to cool the planet by increasing marine cloud albedo that might be achieved by injecting sea salt into low marine cloud.  OAM aims to cool the planet by increasing surface ocean albedo that might be achieved by using highly reflective microbubbles over ocean. There is speculation that climate effect of OAM and MCB would be similar as forcing is applied only over ocean in both cases.

In this study, we use NCAR CESM model to compare climate response in  these two SRM approaches under the framework of “fast versus slow response”. The term “fast” refers to climate adjustment that is associated with rapid adjustment of the atmosphere and land surface, and “slow” refers to climate feedbacks that are associated with the slow evolution of sea surface temperature.

In our simulation we find that to offset global warming from a doubling of atmospheric CO₂, OAM requires a stronger negative effective radiative forcing than that of MCB, indicating MCB is more effective in producing cooling per unit of radiative forcing. This is mainly associated with differing fast climate adjustment between OAM and MCB forcing. OAM increases upward shortwave radiation from surface and heats the lower atmosphere, causing low-level clouds to dissipate. A reduction in low cloudiness allows more solar radiation to reach the surface, partly offsetting the negative radiative forcing from increase in ocean albedo. At equilibrium state, however, OAM and MCB produces similar pattern of change in temperature and hydrological cycle, but prominent differences in climate response is observed over the tropical ocean where OAM produces larger reduction in precipitation and evaporation than that of MCB. Our results indicate that there is similarity between climate response to marine cloud brightening and ocean albedo increase, but caution should be exercised when using climate response from one to infer the other.