Background
Emissions of aerosols and their precursors by ships that sail the seas typically result in more cloud condensation nuclei (CCN) being created in the ships' wakes, which causes the mean droplet radius of the marine boundary layer's CCN burden to decline and its droplet number concentration to rise, thus enhancing the albedo of marine stratus clouds.

What was done
In the words of the researchers who conducted the work, an algorithm was developed "to determine ship tracks from satellite data in an automated way," after which "a scene on 10 February 2003 was chosen to extract the optical and microphysical cloud modifications from ship emissions using Terra-MODIS satellite data," in which "a combination of the semi-analytical approach SACURA [Kokhanovsky et al., 2003] with a look-up-table for optically thin clouds [Mayer and Kylling, 2005] was used to calculate cloud properties."

What was learned
Schreier et al. determined that, on average, cloud "optical thickness was increased from 20.7 up to 34.6 and the effective [droplet] radius was decreased from 13.2 µm to 10.1 µm," while "the calculated average droplet number concentration increased from 79 up to 210 cm^-3." Using these numbers to calculate changes in the radiative energy budget above and below the marine stratus clouds that were influenced by the ship emissions, they found "a decrease of 43.2 Wm^-2 for the surface radiation below the ship tracks and an increase of 40.8 Wm^-2 for the increased reflectivity at the top of the atmosphere." In addition, they write that "if the whole low-cloud area with 6.7% ship-track-pixels is taken into account, a decrease in the radiation at the surface of 2.1 Wm^-2 and an increase of 2.0 Wm^-2 in backscattered solar radiation [is] found."

What it means
The eight German researchers concluded that "modifications of clouds by international shipping can be an important contributor to climate on a local scale," and that "further studies are needed to assess the global impact of ship-track formation on climate," which obviously is to create a tendency for cooling the planet.

References
Kokhanovsky, A.A., Rozanov, V.V., Zege, E.P., Bovensmann, H. and Burrows, J.P. 2003. A semi-analytical cloud retrieval algorithm using backscattered radiation in 0.4-2.4 micrometers spectral range. Journal of Geophysical Research 108: 10.1029/2001JD001543.

Mayer, B. and Kylling, A. 2005. The libRadtran software package for radiative transfer calculations, description and examples of use. Atmospheric Chemistry and Physics 5: 1855-1877.