Sensitivity to deliberate sea salt seeding of marine clouds – observations and model simulations 1Department of Geosciences, Meteorology and Oceanography Section, University of Oslo, Oslo, Norway
15 Mar 2012
2Norwegian Meteorological Institute, Oslo, Norway
Received: 05 September 2011 – Published in Atmos. Chem. Phys. Discuss.: 31 October 2011 Abstract. Sea salt seeding of marine clouds to increase their albedo is a proposed
technique to counteract or slow global warming. In this study, we first
investigate the susceptibility of marine clouds to sea salt injections, using
observational data of cloud droplet number concentration, cloud optical
depth, and liquid cloud fraction from the MODIS (Moderate Resolution Imaging
Spectroradiometer) instruments on board the Aqua and Terra satellites. We
then compare the derived susceptibility function to a corresponding estimate
from the Norwegian Earth System Model (NorESM). Results compare well between
simulations and observations, showing that stratocumulus regions off the west
coast of the major continents along with large regions over the Pacific and the
Indian Oceans are susceptible. At low and mid latitudes the signal is dominated by the cloud fraction.
Revised: 28 February 2012 – Accepted: 01 March 2012 – Published: 15 March 2012
We then carry out geo-engineering experiments with a uniform increase over ocean of
10−9 kg m−2 s−1 in emissions of sea salt particles with a dry
modal radius of 0.13 μm, an emission strength and areal coverage much greater than proposed in earlier studies. The increased sea salt concentrations and
the resulting change in marine cloud properties lead to a globally averaged
forcing of −4.8 W m−2 at the top of the atmosphere, more than
cancelling the forcing associated with a doubling of CO2 concentrations. The forcing is large in areas
found to be sensitive by using the susceptibility function, confirming its
usefulness as an indicator of where to inject sea salt for maximum effect.
Results also show that the effectiveness of sea salt seeding is reduced
because the injected sea salt provides a large surface area for water vapor
and gaseous sulphuric acid to condense on, thereby lowering the maximum
supersaturation and suppressing the formation and lifetime of sulphate
particles. In some areas, our simulations show an overall reduction in the
CCN concentration and the number of activated cloud droplets decreases, resulting
in a positive forcing.
Citation: Alterskjær, K., Kristjánsson, J. E., and Seland, Ø.: Sensitivity to deliberate sea salt seeding of marine clouds – observations and model simulations, Atmos. Chem. Phys., 12, 2795-2807, doi:10.5194/acp-12-2795-2012, 2012.