Sea fairer: Maritime transport and CO2 emissions

OECD Observer

A clean launch ©Reuters/Nikola Solic

International shipping emits as much CO2 as some of the world's largest countries. What can be done?

Some 90% of world trade in tonnes is carried by ship, and containers represent 70% of total maritime trade by value. Per kilometre, shipping is one of the lowest emitting freight transport options around; at 10-15 grammes per tonne-kilometre, it is lower than rail (19-41g/tkm), trucking (51-91g/tkm) and aviation (673-867g/tkm). But the carbon footprint of the sector as a whole is as large as some major countries.

International maritime activity accounted for 543 Mt of CO2 emissions from fuel combustion in 2005, according to data from the International Energy Agency (IEA), a sister organisation of the OECD and the International Transport Forum. The figure is calculated by the sale of fuel to vessels whose next port-of-call is outside the country, and it places CO2 emissions from international maritime transport in 8th place in the world between Canada and the UK.

The International Maritime Organization, however, has recently updated its own estimate using a detailed bottom-up fleet inventory and has found that international maritime activity emitted a total of 1,120 Mt of CO2–more than twice the IEA estimate for 2005.

This new estimate places international maritime emissions in 6th place in the world between India and Germany and nearly three times that of international aviation. The bulk of shipping’s emissions stem from cargo vessels, with passenger ships representing only 3% of total ship CO2 emissions, a share which is declining.

Total maritime trade has doubled from 1985 to 2007; total containerised trade has grown eight-fold over the same period and currently represents 16% of all maritime trade by weight (and a much larger share by value). Depending on the weight carried per container unit, trade in containers measured by deliveries is projected to more or less triple from 2000 to 2020.

In terms of container movements including transhipments, Drewry Shipping Consultants forecasts a more than six-fold rise from 2000 to 2020. This growth has important GHG repercussions as the average installed power on container vessels is higher than on most other types of vessels and, given the speeds at which container vessels travel, increased container vessel activity will result in greater maritime CO2 emissions than might otherwise have been expected based on past fleet structure. Nonetheless, in aggregate, energy use per tonne delivered by sea has generally followed a decreasing trend from 1985 to 2007 with the advent of more efficient engines, better propellers and larger vessels.

The potential for new technology penetration within the world ship fleet is limited by the long life-span of vessels–up to 30-40 years. Many vessels built today incorporate energy-saving technologies. But as with cars, in many cases, much of these energy savings have been used to buy more power and speed, not to reduce energy use.

This is particularly evident in the fast-growing containership market where new vessels are being built to carry upwards of 10,000 containers and sail at 25 knots, compared with speeds of 21-23 knots in the 1990s. Because a ship must overcome water resistance, changes in energy use and CO2 emissions are particularly sensitive to ship speed–a 4% increase in ship speed entails about a 13% increase in CO2 emissions.

Better hull and propeller design and incremental engine technology improvements can contribute up to 30% energy efficiency improvements for new builds. Future propulsion options based on fuel cells may also be feasible in two or three vessel generations. Advanced sail technology is already being trialled and may also play a limited role in the future.

However, much of the energy saving potential for maritime transport comes through operational changes to the existing fleet which could deliver up to 20% fuel savings. The foremost option is reducing speed. However, this raises operational challenges, especially for container vessels that run on set routes and schedules. Providing the same level of service at slower speeds might require adding vessels to the route, thus eroding some of the fuel savings, or increase costs to owners as vessels arrive at night or on the weekend when stevedoring costs are higher.

Other sources of energy-saving improvements for existing vessels entail shifting in-port power to shore-based auxiliary power units rather than ship engines. Improvements in on-board auxiliary power generation can also be a source of non-negligible fuel savings.



See International Maritime Organization’s website at

See Drewry Shipping Consultants,

For more on International Transport Forum’s work on maritime shipping, contact

©OECD Observer No 267, May-June 2008

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