Abstract
Abstract. 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 CO2 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.
Highlights
Several geoengineering options have been proposed to slow the rate of warming due to the anthropogenic increases in greenhouse gases, including the modification of stratospheric aerosol (Crutzen, 2006) and artificially increasing the surface albedo (Akbari et al, 2009). Latham and Smith (1990) proposed that climate warming could be slowed by increasing the albedo of marine stratocumulus clouds through the injection of sea spray aerosol
Marine stratocumulus updraughts are generally quite low, for example Guo et al (2008) present PDFs of vertical velocity in marine stratocumulus clouds measured during the Marine Stratus/Stratocumulus Experiment (MASE), they found that characteristic updraughts (w) were always < 0.2 m s−1 at the middle and base of the cloud, but found slightly higher updraughts close to the cloud top
In summary all models perform reasonably compared to large-scale observations but GLOMAP and EMAC tend to overestimate polluted marine concentrations and ECHAM-HAM tends to underestimate in these regions
Summary
Several geoengineering options have been proposed to slow the rate of warming due to the anthropogenic increases in greenhouse gases, including the modification of stratospheric aerosol (Crutzen, 2006) and artificially increasing the surface albedo (Akbari et al, 2009). Latham and Smith (1990) proposed that climate warming could be slowed by increasing the albedo of marine stratocumulus clouds through the injection of sea spray aerosol. The Korhonen et al (2010) study was the first to consider geoengineering from an online windspeed dependent emission rate through to the change in CDN and they found that the calculated emission rates resulted in a regional average change in CDN of ≤ 20 %, and in some areas it even resulted in a decrease in CDN (because the increased competition for water vapour between the activated aerosol suppressed the in-cloud maximum supersaturation) This decrease in CDN is in line with the finding of Ghan et al (1998) who found that the additon of sea spray aerosol can decrease CDN when the updraft is low and there is a large background concentration of sulfate particles. Understanding the limiting factors helps to quantify the maximum possible increase in CDN (for macrophysically identical clouds) which is useful for studies that calculate the potential radiative cooling arising from sea-spray geoengineering
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