Climate More Responsive to Marine Cloud Brightening Than Ocean Albedo Modification: A Model Study
Mengying Zhao, Long Cao, Lei Duan, Govindasamy Bala, Ken Caldeira
Abstract
Abstract Solar radiation modification has been suggested as a backup option to reduce anthropogenic warming. Marine cloud brightening (MCB) and ocean albedo modification (OAM) are two proposed approaches to intentionally reflect sunlight back to space over oceanic regions. Using the NCAR Community Earth System Model, we compare climate response to MCB and OAM under the framework of fast adjustment and slow feedback. We implement MCB and OAM uniformly over the global ocean to offset CO 2 ‐induced warming. We find that to offset 3.3 K global mean warming from a doubling of CO 2 , diagnosed effective radiative forcing is −4.8 and −3.6 W m −2 for OAM and MCB, respectively. Correspondingly, radiative forcing efficacy of OAM is about 70% of MCB. Fast climate adjustment differs in response to MCB and OAM forcing. MCB cools the lower atmosphere by reflecting sunlight from cloud, causing a reduction in sunlight absorption in the atmosphere. In contrast, OAM, by reflecting more sunlight from surface, increases shortwave heating of the lower atmosphere, leading to a decrease in low marine clouds and hence a positive cloudy‐sky shortwave forcing that partly compensates the negative clear‐sky shortwave forcing. The slow climate response and pattern of equilibrium climate change are similar between MCB and OAM. As for hydrological cycle, relative to the climate under a doubling of CO 2 , both MCB and OAM produce an increase in precipitation and runoff over tropical land.