The Guardian reports:

Carbon dioxide emissions from shipping are double those of aviation and increasing at an alarming rate which will have a serious impact on global warming, according to research by the industry and European academics.

Separate studies suggest that maritime carbon dioxide emissions are not only higher than previously thought, but could rise by as much as 75% in the next 15 to 20 years if world trade continues to grow and no action is taken. The figures from the oil giant BP, which owns 50 tankers, and researchers at the Institute for Physics and Atmosphere in Wessling, Germany reveal that annual emissions from shipping range between 600 and 800m tonnes of carbon dioxide, or up to 5% of the global total.

Carbon dioxide emissions from ships do not come under the Kyoto
agreement or any proposed European legislation and few studies have
been made of them, even though they are set to increase.