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Ship Tracks -- Summary
Can emissions from ships influence climate?   According to Ferek et al. (1998), two ships that left the mouth of the Columbia River in the early morning of 26 August 1992 (one heading to Pakistan with grain and the other carrying wood chips to Japan) created distinct albedo changes in a uniform layer of marine stratus clouds.  These cloud features were observed in three successive images obtained from Advanced Very High Resolution Radiometers carried on U.S. Government satellites, as well as by in situ measurements obtained during repeated aircraft flights through the tracks.  Chemical analyses of cloud water in and out of the ship tracks showed that cloud condensation nuclei in the effluents from the ships were responsible for the ship tracks.   Furthermore, cloud droplet spectra measured in the ship tracks showed higher number concentrations and lower cloud droplet effective radii than spectra measured in ambient clouds.

So, yes, ships can influence climate.  Natural clouds can be modified by effluents from anthropogenic sources that make them brighter and more persistent, both of which consequences tend to make clouds more reflective of incoming solar radiation.   The net effect of these phenomena has been estimated by Capaldo et al. (1999) to be a mean reduction in radiant energy received at the surface of the earth of 0.16 Wm-2 in the Northern Hemisphere and 0.06 Wm-2 in the Southern Hemisphere, which tends to cool the planet.

References
Capaldo, K., Corbett, J.J., Kasibhatla, P., Fischbeck, P. and Pandis, S.N.   1999.  Effects of ship emissions on sulphur cycling and radiative climate forcing over the ocean.  Nature 400: 743-746.

Ferek, R.J., Hegg, D.A., Hobbs, P.V., Durkee, P. and Nielsen, K.  1998.  Measurements of ship-induced tracks in clouds off the Washington coast.   Journal of Geophysical Research 103: 23,199-23,206.