Limits of Marine Cloud Brightening as a Technique for Climate Engineering

Climate Engineering is thought to counteract or at least postpone global warming due to anthropogenic emissions. The intentional brightening of cloud albedo is proposed to be one method of climate engineering. The basic idea is to artificially release particles which act as additional CCN, which lead to higher cloud droplet numbers, which increases cloud albedo.

Previous investigations suggested the Southeast Pacific as one of the most favourable region for climate engineering with artificially released sea salt (Korhonen et al., 2010). This region is known to be characterized by a persistent layer of stratocumulus clouds. Their existence is a result of a complex chain of interactions between several processes. Due to the complexity of the system and that processes take place on very small scales atmospheric models encounter difficulties in capturing this cloud layer in a realistic way. Additionally there are still uncertainties in the outcome of this method due to uncertainties in cloud aerosol interactions.

The impact of cloud seeding via additionally emitted sea salt particles is mainly investigated within global circulation models (GCM), or on even smaller grid scales (LES). We investigate the effects of cloud brightening on a regional scale. Therefore we use our model system COSMO-ART (Vogel et al., 2009) to quantify the effect of climate engineering in the South East Pacifical region. Natural sea salt emissions are calculated as a function of wind speed and sea surface temperature. To investigate the effects of climate engineering we added a controlled sea salt emissions flux to the natural emissions as it was done by Korhonen et al. (2010).

We compared our simulation with measurements of the VOCALS-REx field campaign in 2008 without any climate engineering. The simulations showed satisfactory results but with a dependency of vertical and horizontal resolution. Then we conducted a climate engineering simulation and investigated the impact of artificially released sea salt particles on the cloud properties. We found that in principle additionally released particles lead to a reduction of shortwave radiation. On the other hand longwave radiation is increased slightly. In areas with anthropogenic emissions the desired effect is not always achieved, in some areas it is reversed. This is not only due to microphysical effects but also due to chemical effects affecting cloud droplet activation.