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Science  28 Sep 2007:
Vol. 317, Issue 5846, pp. 1834a
DOI: 10.1126/science.317.5846.1834a

The 100,000 or so ships that make up the global commercial and military fleet collectively travel billions of vessel-miles every year, producing a large fraction of the pollution contributed by fossil fuel burning in the transportation sector. In addition to the direct radiative effects of their emissions, caused by the light-scattering properties of the particles themselves, aerosols from the exhaust plumes can produce thin lines of very low clouds in the marine boundary layer, an example of the aerosol indirect effect. It has been shown that the local effects of these clouds can be large, up to 100 W/m2 (for comparison, the average solar flux at the top of the atmosphere is about 340 W/m2), but how large an influence they exert on the global albedo has been an unresolved concern. Schreier et al. analyzed a full year of satellite data derived from ENVISAT AATSR (Environmental Satellite Advanced Along-Track Scanning Radiometer) in order to estimate the size of the radiative forcing caused by ship tracks. They found that, contrary to fears arising from previous global model estimates, the global annual mean radiative forcing from ship tracks was small, 0.4 to 0.6 mW/m2, and negligible compared to estimates of total net anthropogenic radiative forcing, 0.6 to 2.4 W/m2. Thus, it seems that ship tracks are too inconsequential to affect the rate of anthropogenic global warming. — HJS

Geophys. Res. Lett. 34, L17814 (2007).

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