Global simulations of the impact on contemporary climate of a perturbation to the sea-to-air flux of dimethylsulfide

Abstract

The sea-to-air flux of the biogenic sulfur (S) compound dimethylsulfide (DMS) is thought to constitute an important radiative impact on climate, especially in remote marine areas. Previous biogeochemical modelling analyses simulate medium to large changes in the sea-to-air flux of DMS in polar regions under warming scenarios. Here we assess the global radiative impact of such a prescribed change in DMS flux on contemporary climate using a low-resolution atmospheric general circulation model. This impact operates through the atmospheric oxidation of DMS to radiatively-active sulfate aerosols, which are known to both reflect incoming short-wave radiation and to affect the microphysical properties of clouds, for example, through an increase in cloud albedo. We use an atmospheric GCM with incorporated sulfur cycle, coupled to a mixed-layer (‘q-flux’) ocean, to estimate the climatic response to a prescribed meridionally-variable change in zonal DMS flux, as simulated in a previous modelling analysis. We compare baseline sulfur emissions (contemporary anthropogenic S and contemporary DMS sea-to-air flux), with contemporary anthropogenic S and a perturbed DMS flux. Our results indicate that the global mean DMS vertically integrated concentration increases by about 41 per cent. The relative increase in DMS annual emission is around 17 per cent in 70–80°N, although the most significant increase is in 50–70°S, up to 70 per cent. However, concentrations of atmospheric SO2 and SO4 2– increase by only about eight per cent. The oxidation of DMS by OH increases by about 20 per cent. Oxidation of SO2 to SO4 2 by H2 O2 increases seven per cent. The oxidation of SO2 by O3 increases around six per cent. Overall sulfur emissions increase globally by around 4.6 per cent. Global mean aerosol optical depth (AOD) increases by 3.5 per cent. Global mean surface temperature decreases by 0.6 K. There is a notable difference between the impacts in the southern and northern hemispheres. In general, most processes and chemical species related to the sulfur cycle show a larger increase in the southern hemisphere, except SO2 and the oxidation of DMS by NO3 . The global mean direct radiative forcing due to the DMS change is –0.05 Wm–2 with total forcing (direct + indirect effects) of –0.48 Wm–2. This perturbation on DMS flux leads to a mean surface temperature decrease in the southern hemisphere of around 0.8 K, compared with a decrease of 0.4 K in the northern hemisphere.