Low sensitivity of cloud condensation nuclei to changes in the sea-air flux of dimethyl-sulphide
1Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
2School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK
3Met Office Hadley Centre, Fitzroy Road, Exeter, Devon, EX1 3PB, UK
*now at: Massachusetts Institute of Technology (MIT), 77 Mass. Ave, Cambridge, USA
**now at: ETH Zürich, Institut für Biogeochemie und Schadstoffdynamik, CHN E 28.1, Universitätstrasse 16, 8092 Zürich, Switzerland
Abstract. The emission of dimethylsulphide (DMS) gas by phytoplankton and the subsequent formation of aerosol has long been suggested as an important climate regulation mechanism. The key aerosol quantity is the number concentration of cloud condensation nuclei (CCN), but until recently global models did not include the necessary aerosol physics to quantify CCN. Here we use a global aerosol microphysics model to calculate the sensitivity of CCN to changes in DMS emission using multiple present-day and future sea-surface DMS climatologies. Calculated annual fluxes of DMS to the atmosphere for the five model-derived and one observations based present day climatologies are in the range 15.1 to 32.3 Tg a−1 sulphur. The impact of DMS climatology on surface level CCN concentrations was calculated in terms of summer and winter hemispheric mean values of ΔCCN/ΔFluxDMS, which varied between −51 and +147 cm−3/(mg m−2 day−1 sulphur), with a mean of 56 cm−3/(mg m−2 day−1 sulphur). The range is due to CCN production in the atmosphere being strongly dependent on the spatial distribution of the emitted DMS. The DMS flux from a future globally warmed climatology was 0.2 Tg a−1 sulphur higher than present day with a mean CCN response of 95 cm−3/(mg m−2 day−1 sulphur) relative to present day. The largest CCN response was seen in the southern Ocean, contributing to a Southern Hemisphere mean annual increase of less than 0.2%. We show that the changes in DMS flux and CCN concentration between the present day and global warming scenario are similar to interannual differences due to variability in windspeed. In summary, although DMS makes a significant contribution to global marine CCN concentrations, the sensitivity of CCN to potential future changes in DMS flux is very low. This finding, together with the predicted small changes in future seawater DMS concentrations, suggests that the role of DMS in climate regulation is very weak.