Modification of Cloud Droplet Number Concentration to Reduce Global Warming: The Impact of Marine Cloud Brightening on the Radiative Forcing at the Earth's Surface

Abstract

In the past decade, climate geoengineering by marine cloud brightening (MCB) has been proposed as a possible solution to reduce global warming. This technique relies on seeding marine stratocumulus clouds (MSCs) with roughly monodisperse salt particles to enhance the cloud droplet number concentration N_d. In this study, the impacts of increasing N_d on exemplary MSCs are quantified and analyzed. For a MSC with an optical thickness τ = 8, increasing N_d from 20 to 500 cm-3 enhances surface cooling from -191 to -217 W m-2. For τ = 15, surface cooling is enhanced from -308 to -331 W m-2. The cooling is driven largely by the declines in the solar forcing, while the terrestrial forcing remains unchanged. The existence of more small particles in the cloud increases the number of scattering, and thereby the cloud albedo (Twomey effect). Consequently, the clouds are more reflective reducing the amount of incoming solar radiation at the Earth’s surface. In contrast, changes of N_d have an insignificant impact on terrestrial radiation as long as the cloud temperature is constant. Due to the typical warm temperature of the cloud, most of terrestrial radiation emitted by the Earth’s surface and the atmosphere below the cloud are transmitted to space, and therefore does not contribute to increase surface warming.