Amelioration of global warming via the modification of droplet concentrations in marine stratocumulus clouds

Results are presented from a computational study in which cloud droplet concentrations in regions of marine stratocumulus were enhanced, in order to assess the possibility of ameliorating the effects of global warming via the deliberate modification of the microphysical properties of these clouds, as suggested by Latham (1990, 2002).

This involved the use of HadGAM (the atmospheric component of the UK Met Office Unified Model), to investigate changes in the global radiation balance. In addition, the UK Met Office Large Eddy Model, initialized with atmospheric profiles from the recent VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx), was used to examine the sensitivity of the stratocumulus cloud microphysics.

The results suggest that increasing cloud droplet number concentrations (N) in regions of marine stratocumulus would cause both a decrease in cloud droplet effective radius and an increase in cloud liquid water, i.e., demonstrations of the cloud albedo effect and cloud lifetime effect, respectively. Model output also indicates that enhancing N would lead to a more uniform cloud droplet distribution, causing a reduction in entrainment and an accompanying increase in cloud-top height. These factors would cause an increase in cloud albedo and ultimately lead to a significant cooling of global climate.

If a cloud albedo enhancement geoengineering scheme such as that of Latham (2002) were to be implemented it could potentially yield a negative radiative forcing of –7.97 W/m², i.e., more than twice that required to offset the warming produced by a doubling of carbon dioxide, with the greatest response occurring in the Southern Hemisphere during spring and summer.