What was done
The authors used a "data-plus-model approach" to evaluate the contributions of ship emissions to atmospheric SO2, sulphate and cloud condensation nuclei, as well as the impacts of these factors on the surface radiation balance over the sea.

What was learned
The researchers concluded that ship sulphur emissions are nearly equal to the natural sulphur flux from the ocean to the atmosphere in many areas. Averaged over the surface of the earth, they found that the predicted change in radiative forcing due to ships is currently -0.16 Wm^-2 in the Northern Hemisphere and -0.06 Wm^-2 in the Southern Hemisphere.

What it means
This negative ship-induced surface radiative forcing that tends towards global cooling (the global climatic ramifications of which are described here for the first time in any detail), represents a counterbalance to whatever forces for global warming may currently be active.  Commenting on this aspect of the results, Heubert (1999) states that "it's good for us to realize now and then that we haven't discovered everything of interest, and surprises are still out there."  This is a refreshing attitude, which, if more prevalent, would likely insure that we not act too hastily in (1) assuming we know what has caused the warming of the past century, (2) assuming we know how to combat the assumed causative agent, and (3) assuming we even should do battle against it.  If it truly is CO2, for example, do the many benefits of its historical increase - which are both biological and climatic - outweigh its assumed liabilities?

Reference
Huebert, B.J.  1999.  Sulphur emissions from ships.  Nature 400: 713-714.