Cargo shipping is growing briskly, by about 5 percent a year, according to a 2007 study by the International Council on Clean Transportation. Air pollutants from oceangoing vessels include nitrogen oxide (NOx), sulfur oxides (SOx), and particulate matter, all of which have been linked to various health problems.
Ship emissions of carbon dioxide (CO2), a greenhouse gas that contributes to global warming, make up about 4.5 percent of total global CO2 emissions.
That's more than twice as much as those created by the aviation industry, which has come under heavy public criticism for its emissions, says a report sponsored by some member countries of the International Maritime Organization (IMO).
The report estimates that CO2 emission from oceangoing vessels will increase another 30 percent by 2020 if no action is taken.
The politics of marine fuel
The IMO is the United Nations body responsible for reducing the negative impact of ships on the environment. It's meeting this week to consider tightening standards for ship emissions. Commercial ships are not included in the Kyoto Protocol, the international agreement to cut greenhouse-gas emissions signed by most of the world's major emitters.
"There currently is no international agreement on greenhouse-gas emissions from ships," says Lee Adamson, a spokesman for the IMO. The group is studying CO2 emissions and originally had planned to propose regulations by 2010, but that deadline may be moved up, Mr. Adamson says.
The shipping industry continues to point out that moving freight by cargo ship is the most energy-efficient means of transporting cargo, vastly superior to trucking or rail on a ton-per-mile basis.