Global Simulations of Radiative Forcing from Sea Salt Injections into Marine Clouds: The Effect of
Injection Rate and Particle Size
Kari Alterskjær1, Jón Egill Kristjánsson1
1
University of Oslo
Sea salt seeding of low level marine clouds is a suggested technique to counteract or slow global
warming. The injected sea salt is to act as cloud condensation nuclei (CCN) and through the aerosol
indirect effect increase the cloud albedo and therefore the reflection of solar radiation from the
earth-atmosphere system. Using the Norwegian Earth System Model (NorESM) we investigate the
global radiative forcing achieved through sea salt seeding as a function of both (i) emission rate and
(ii) sea salt particle size. The injection rates are uniform and confined over ocean between 30°S and
30°N, and range from 10-9 to 10-11 kg m-2 s-1. The size of the particles ranges from a dry modal radius
of 0.022 µm to 0.13 µm, with geometric standard deviations of 1.59. The study includes aerosol
indirect effects both in the shortwave and the longwave and investigates the direct radiative effect of
the added sea salt particles.
Preliminary results show that increasing emissions of the 0.13 µm sea salt mode leads to an aerosol
indirect effect of between -3.4 Wm-2 and -0.04 Wm-2, depending on emission rate. The maximum
achieved forcing is close to cancelling the positive forcing resulting from a doubling of atmospheric
CO2 concentrations (-3.8 Wm-2). Results also show that the direct effect of the sea salt parties is much
larger than previously assumed, and the total radiative effect and the cooling potential of this geoengineering technique may therefore be much greater than what has been assumed earlier. We find
that ignoring the direct effect of sea salt may lead to serious errors in determining both the
effectiveness of sea salt seeding and possible side effects. The longwave forcing resulting from a
change in cloud emissivity with added sea salt is found to be negligible.
Results also show that the size of the added sea salt is of crucial importance for the achieved
radiative forcing. While adding large sea salt particles leads to a significant negative forcing at the top
of the atmosphere, the opposite is true for particles below a certain size. This happens because the
injected sea salt provide a large surface area for water vapor and gaseous sulfuric acid to condense
on, thereby lowering the maximum cloud supersaturation and suppressing the nucleation of sulfate
particles. Adding particles that are too small to become activated to cloud droplets merely leads to a
reduced supersaturation and sulfate concentration and therefore to a decrease in the overall CCN
concentration. This results in a reduced cloud droplet number concentration and a positive globally
averaged forcing.