7.5.1.4 Aerosols from Dimethyl Sulphide

Dimethyl sulphide produced by phytoplankton is the most abundant form in which the ocean releases gaseous sulphur. Sea-air fluxes of DMS vary by orders of magnitude depending mainly on DMS sea surface concentration and on wind speed. Estimates of the global DMS flux vary widely depending mainly on the DMS sea surface climatology utilised, sea-air exchange parametrization and wind speed data, and range from 16 to 54 Tg yr^–1 of sulphur (see Kettle and Andreae, 2000 for a review). According to model studies (Gondwe et al., 2003; Kloster et al., 2006), 18 to 27% of the DMS is converted into sulphate aerosols. Penner et al. (2001) show a small increase in DMS emissions between 2000 and 2100 (from 26.0 to 27.7 Tg yr^–1 of sulphur) using constant DMS sea surface concentrations together with a constant monthly climatological ice cover. Gabric et al. (2004) predict an increase of the globally integrated DMS flux perturbation of 14% for a tripling of the pre-industrial atmospheric CO₂ concentration.

Bopp et al. (2004) estimate the feedback of DMS to cloud albedo with a coupled atmosphere-ocean-biogeochemical climate model that includes phytoplankton species in the ocean and a sulphur cycle in the atmospheric climate model. They obtain an increase in the sea-air DMS flux of 3% for doubled atmospheric CO₂ conditions, with large spatial heterogeneities (–15 to +30%). The mechanisms affecting those fluxes are marine biology, relative abundance of phytoplankton types and wind intensity. The simulated increase in fluxes causes an increase in sulphate aerosols and, hence, cloud droplets resulting in a radiative perturbation of cloud albedo of –0.05 W m^–2, which represents a small negative climate feedback to global warming.