The influence of sulphur emissions from ship traffic on clouds and climate

Abstract

The growth of the international ship trade has raised the great concern about the impact of the ship emitted pollutants to the clouds and climate, air quality est. Apart from emitting large extents of sulphur dioxide which is oxidized to sulphate particles in the atmosphere; also ships emit full spectrum range of organics and non organics. These sulphate particles either by their own or in mixture with other emitted organics interrupt the earth’s radiation budget either directly or indirectly (via cloud properties) resulting in a net cooling effect on the atmosphere.

The study used the Community Atmospheric Model version three or CAM3.0 with detailed aerosol and cloud microphysical scheme to investigate the effects of international shipping traffic on clouds and climate. The model simulation results show that the top of atmosphere ship induced indirect effect is highly uncertain and model based. The model calculated global mean value of -0.063Wm-2 to -0.121Wm-2, which contribute to about 3.3% to 6.4% of the total indirect effect of anthropogenic aerosols is three times lower than that of E5/M1-MADE. Particle organic matter (POM) and black carbon from fossil fuels (BC) contribute to about 8% and 16% of the short wave radiative flux due to shipping. Seasonally, the highest short wave radiative flux response due to shipping is on JJA which contribute to about -0.187 Wm-2 to -0.102 Wm-2 or 39% to 40% depending on the inventory. Over the Southern Hemisphere ship induced short wave radiative flux show an increasing trend of about -0.021Wm-2 to -0.065Wm-2 or 17% to 27% of the global mean ship induced short wave cloud forcing. On microphysical properties the simulation results show an increase of only 3% to 5% for cloud droplet number concentrations over the low marine liquid water clouds of Atlantic and Pacific Oceans, where as the percentage changes in liquid water path has shown to be less than 1% both globally and regional. Model results yield a ship induced sulphate burden of 3.0% to 3.8%, and ship contributed BC and POM of 2.0% and 0.5%. For the lower most boundary layer of Atlantic Ocean, the model results reveals an increase of only 9% in particle number concentration of the Aitken mode (less than 0.1μm)), which is accompanied with the decrease in the weighted geometrical mean diameter of about 0.050µm to 0.055µm depending on the inventory, and only 3% increase in the accumulation mode particle number concentration which are approximately five times lower than that of E5/M1-MADE. The study results show a contribution of 9% to 17% of the total indirect radiative forcing (RF) of (-0.7Wm-2) of the year 2005 (IPCC, 2007) and 4% to 7% of the total anthropogenic sulphate AIE (i.e. -1.8Wm-2 for first kind and second kind) calculated by (Kristjansson et al., 2002). For the shipping the study contributes to 15% to 30% of the total indirect RF of (-0.409Wm-2) of the year 2005 (Eyring et al., 2009). Though the model simulations of AIE due to shipping has been affected by uncertainties based on models and shipping emission inventories, but this results show a significant contribution to the cooling effect of the climate. Based on this contribution the study generally conclude that the growing problem of ship induced effects on clouds and climate, which is enhanced by the growth of world sea-trade needs special scientific attention and considerations.