Global model simulations of the impact of ocean-going ships on aerosols, clouds, and the radiation budget

Kurzfassung

International shipping contributes significantly to the fuel consumption of all transport related activities. Specific emissions of pollutants such as sulfur dioxide (SO₂) per kg of fuel emitted are higher than for road transport or aviation. Besides gaseous pollutants, ships also emit various types of particulate matter. The aerosol impacts the Earth's radiation budget directly by scattering and absorbing incoming solar radiation and indirectly by changing cloud properties. Here we use ECHAM5/MESSy1-MADE, a global climate model with detailed aerosol and cloud microphysics, to show that emissions from ships significantly increase the cloud droplet number concentration of low maritime water clouds. Whereas the cloud liquid water content remains nearly unchanged in these simulations, effective radii of cloud droplets decrease, leading to cloud optical thickness increase up to 5-10%. The sensitivity of the results is estimated by using three different emission inventories for present day conditions. The sensitivity analysis reveals that shipping contributes with 2.3% to 3.6% to the total sulfate burden and 0.4% to 1.4% to the total black carbon burden in the year 2000. In addition to changes in aerosol chemical composition, shipping increases the aerosol number concentration, e.g. up to 25% in the size range of the accumulation mode (typically > 0.1 micro-m) over the Atlantic. The total aerosol optical thickness over the Indian Ocean, the Gulf of Mexico and the Northeastern Pacific increases up to 8-10% depending on the emission inventory. Changes in aerosol optical thickness caused by the shipping induced modification of aerosol particle number concentration and chemical composition lead to a change of the net top of the atmosphere (ToA) clear sky radiation of about -0.013 W/m² to -0.036 W/m² on global annual average. The estimated all-sky direct aerosol effect calculated from these changes ranges between -0.009 W/m² and -0.014 W/m². The indirect aerosol effect of ships on climate is found to be far larger than previously estimated. An indirect radiative effect of -0.19 W/m² to -0.6 W/m² (change of the top of the atmosphere shortwave radiative flux) is calculated here, contributing 17% to 39% to the total indirect effect of anthropogenic aerosols. This contribution is high because ship emissions are released in regions with frequent low marine clouds in an otherwise clean environment. In addition, the potential impact of particulate matter on the radiation budget is larger over the dark ocean surface than over polluted regions over land.