Climate intervention using marine cloud brightening (MCB) compared with stratospheric aerosol injection (SAI) in the UKESM1 climate model

Abstract

Abstract. The difficulties in using conventional mitigation techniques to maintain global-mean temperatures well below 2 ∘C compared with pre-industrial levels have been well documented, leading to so-called “climate intervention” or “geoengineering” research whereby the planetary albedo is increased to counterbalance global warming and ameliorate some impacts of climate change. In the scientific literature, the most prominent climate intervention proposal is that of stratospheric aerosol injection (SAI), although proposals for marine cloud brightening (MCB) have also received considerable attention. In this study, we design a new MCB experiment (G6MCB) for the UKESM1 Earth-system model which follows the same baseline and cooling scenarios as the well-documented G6sulfur SAI scenario developed by the Geoengineering Model Intercomparison Project (GeoMIP), and we compare the results from G6MCB with those from G6sulfur. The deployment strategy used in G6MCB injects sea-salt aerosol into four cloudy areas of the eastern Pacific. This deployment strategy appears capable of delivering a radiative forcing of up to −1 W m−2 from MCB, but at higher injection rates, much of the radiative effect in G6MCB is found to derive from the direct interaction of the injected sea-salt aerosols with solar radiation, i.e. marine sky brightening (MSB). The results show that while G6MCB can achieve its target in terms of reducing high-end global warming to moderate levels, there are several side effects. Some are common to SAI, including overcooling of the tropics and residual warming of middle and high latitudes. Other side effects specific to the choice of the targeted MCB regions include changes in monsoon precipitation, year-round increases in precipitation over Australia and the maritime continent, and increased sea-level rise around western Australia and the maritime continent; these results are all consistent with a permanent and very strong La Niña-like response being induced in G6MCB. The results emphasize that considerable attention needs to be given to oceanic feedbacks for spatially inhomogeneous MCB radiative forcings. It should be stressed that the results are extremely dependent upon the strategy chosen for MCB deployment. As demonstrated by the development of SAI strategies which can achieve multiple temperature targets and ameliorate some of the residual impacts of climate change, much further work is required in multiple models to obtain a robust understanding of the practical scope, limitations, and pitfalls of any proposed MCB deployment.