A multi-model assessment of the impact of sea spray geoengineering on cloud droplet number
Artificially increasing the albedo of marine boundary layer clouds by the mechanical emission of sea spray aerosol has been proposed as a geoengineering technique to slow the warming caused by anthropogenic greenhouse gases. A previous global model study (Korhonen et al., 2010) found that only modest increases (< 20%) and sometimes even decreases in cloud drop number (CDN) concentrations would result from emission scenarios calculated using a windspeed dependent geoengineering flux parameterisation. Here we extend that work to examine the conditions under which decreases in CDN can occur, and use three independent global models to quantify maximum achievable CDN changes. We find that decreases in CDN can occur when at least three of the following conditions are met: the injected particle number is < 100 cm −3, the injected diameter is > 250–300 nm, the background aerosol loading is large (≥ 150 cm −3) and the in-cloud updraught velocity is low (< 0.2 m s −1). With lower background loadings and/or increased updraught velocity, significant increases in CDN can be achieved. None of the global models predict a decrease in CDN as a result of geoengineering, although there is considerable diversity in the calculated efficiency of geoengineering, which arises from the diversity in the simulated marine aerosol distributions. All three models show a small dependence of geoengineering efficiency on the injected particle size and the geometric standard deviation of the injected mode. However, the achievability of significant cloud drop enhancements is strongly dependent on the cloud updraught speed. With an updraught speed of 0.1 m s −1 a global mean CDN of 375 cm −3 (previously estimated to cancel the forcing caused by CO 2 doubling) is achievable in only about 50% of grid boxes which have > 50% cloud cover, irrespective of the amount of aerosol injected. But at stronger updraft speeds (0.2 m s −1), higher values of CDN are achievable due to the elevated in-cloud supersaturations. Achieving a value of 375 cm −3 in regions dominated by stratocumulus clouds with relatively weak updrafts cannot be attained regardless of the number of injected particles, thereby limiting the efficacy of sea spray geoengineering.