Abstract
Abstract. In this study, we employ the global aerosol-climate model ECHAM-HAM to globally assess aerosol indirect effects (AIEs) resulting from shipping emissions of aerosols and aerosol precursor gases. We implement shipping emissions of sulphur dioxide (SO2), black carbon (BC) and particulate organic matter (POM) for the year 2000 into the model and quantify the model's sensitivity towards uncertainties associated with the emission parameterisation as well as with the shipping emissions themselves. Sensitivity experiments are designed to investigate (i) the uncertainty in the size distribution of emitted particles, (ii) the uncertainty associated with the total amount of emissions, and (iii) the impact of reducing carbonaceous emissions from ships. We use the results from one sensitivity experiment for a detailed discussion of shipping-induced changes in the global aerosol system as well as the resulting impact on cloud properties. From all sensitivity experiments, we find AIEs from shipping emissions to range from −0.32 ± 0.01 W m−2 to −0.07 ± 0.01 W m−2 (global mean value and inter-annual variability as a standard deviation). The magnitude of the AIEs depends much more on the assumed emission size distribution and subsequent aerosol microphysical interactions than on the magnitude of the emissions themselves. It is important to note that although the strongest estimate of AIEs from shipping emissions in this study is relatively large, still much larger estimates have been reported in the literature before on the basis of modelling studies. We find that omitting just carbonaceous particle emissions from ships favours new particle formation in the boundary layer. These newly formed particles contribute just about as much to the CCN budget as the carbonaceous particles would, leaving the globally averaged AIEs nearly unaltered compared to a simulation including carbonaceous particle emissions from ships.
Highlights
Ship tracks are widely seen as one of the most prominent manifestations of anthropogenic aerosol indirect effects (AIEs), or the change in cloud properties by anthropogenic aerosols serving as cloud condensation nuclei
We illustrated that the AIE of shipping emissions as calculated by an aerosol-climate model such as ECHAM-HAM strongly depends on the chosen emission parametrisation, we investigate a possibly more obvious AIE-determinant : the uncertainty associated with total shipping emissions
We used the shipping emissions inventory from Behrens (2006) and designed the experiments to investigate the uncertainty of the derived radiative forcing (RF) associated with the uncertainty in the shipping emissions themselves
Summary
Ship tracks are widely seen as one of the most prominent manifestations of anthropogenic aerosol indirect effects (AIEs), or the change in cloud properties by anthropogenic aerosols serving as cloud condensation nuclei. In recent years, modelling the impact of shipping emissions on the Earth System on climate relevant scales has received increasing attention Most of these modelling studies focus on changes related to atmospheric chemistry and composition (Eyring et al, 2010, and references therein) and assessing global AIEs from shipping emissions has to date just been performed with two distinct models. As the present estimate of the total greenhouse gas (GHG) RF, as given by the IPCC, is about +3 W m−2 (Forster et al, 2007), the above mentioned model results suggest that AIEs and DREs from shipping might mask a significant portion of the GHG induced radiative forcing This masking may be reduced due to shipping emission regulations (e.g. Lauer et al, 2009), but even without those policy regulations, the estimated current cooling effect of shipping emissions will switch to a long-term warming
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