While emissions from aircraft have received quite a bit of attention recently, particularly because much of it is at high altitude, there have been several recent studies of shipping emissions as well. Those studies by the Institute of Atmospheric Physics of the German Aerospace Center DLR and by the College of Marine and Earth Studies of the University of Delaware are indicating that CO2 emissions from shipping are comparable to those from aircraft and some other emissions are much higher.
The graph above is plotted on a logarithmic scale, so each line on the vertical axis represents an increase of a factor of ten. CO2 emissions for shipping and aviation are at a similar order of magnitude while road traffic is 5-7 times higher. Emissions of NOx for shipping are not far behind road transport and almost ten times greater than aircraft, and sulfur dioxide emissions are far higher for shipping than either of the others. With most manufacturing being moved from current developed countries to lower-cost sites like China, India and other countries, the number of container ships plying the oceans is constantly climbing. That means that by 2050 the emissions of CO2 and SO2 could double by 2050 and NOx emissions could exceed road transport.
[Source: Institute of Atmospheric Physics]