Assessment of near-future policy instruments for oceangoing shipping: impact on atmospheric aerosol burdens and the earth's radiation budget.

We apply the global climate model ECHAM5/MESSy1-MADE with detailed aerosol and cloud microphysics to study the impact of shipping on tropospheric aerosol burdens, clouds, and the radiation budget for four near-future ship emission policy scenarios for the year 2012. We compare a "No Control" scenario with global sulfur limits and regionally applied reductions. We show that, if no control measures are taken, near surface sulfate increases by about 10-20% over the main transoceanic shipping routes from 2002 to 2012. A reduction of the maximum fuel sulfur (S) content allowed within 200 nautical miles of coastal areas ("global emission control areas") to 0.5% or 0.1% (5000 or 1000 ppm S, respectively) results in a distinctive reduction in near surface sulfate from shipping in coastal regions compared with the year 2002. The model results also show that if emissions of nitrogen oxides (NO(x)) remain unabated, a reduction of the fuel sulfur content favors a strong increase in aerosol nitrate (NO3) which could counteract up to 20% of the decrease in sulfate mass achieved by sulfur emission reductions. The most important impact of shipping on the radiation budget is related to the modification of low maritime stratus clouds resulting in an increased reflectivity and enhanced shortwave cloud forcing. The direct aerosol effect from shipping is small. Our study shows that one can expect a less negative (less cooling) radiative forcing due to reductions in the current fuel sulfur content of ocean-going ships. The global annual average net cloud forcings due to shipping (year 2012) are in the range of -0.27 to -0.58 W/m2 with regional cooling occurring most over the remote oceans.