Published online 13 February 2008 | Nature | doi:10.1038/news.2008.574

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New figures bolster calls for shipping to be included in Kyoto Protocol.

Big polluter: ships emit about 4% of the world's carbon dioxide emissions. Getty

The amount of carbon dioxide emitted by the world's shipping industry is much higher than previously thought, according to the latest estimate from the United Nations. The new figures, released on 8 February by the International Maritime Organization (IMO), based in London, show that emissions from cargo ships are about twice those from global air travel.

The discovery has raised the volume of environmentalists' calls for shipping to be included in a successor to the Kyoto Protocol, which expires in 2012. Like commercial aviation, emissions from shipping are not currently included in the Kyoto targets for reducing greenhouse-gas outputs.

According to the new figures1, ocean-going freight tankers belch out 1.12 billion tonnes of CO 2 per year — roughly 4% of the world's overall output. Most previous estimates put the figure at less than 2%.

Global air travel, in contrast, is responsible for around 650 million tonnes of CO 2 each year, despite the fact that aviation has been the target of loud protests from campaigners angered by its exemption from Kyoto.

Overboard

Shipping emits less CO 2 than aviation on a per-vehicle basis, argues Ian Adams, secretary general of the International Bunker Industry Association, an organization based in the United Kingdom that represents the shipping industry. "Cargo aircraft emit 673 grams [CO 2 ] per tonne—kilometre but ships emit between 11 and 42 grams per tonne–kilometre, according to statistics developed by the International Chamber of Shipping and the ECSA [European Community Shipping Association]," he told Nature News.

But attention is increasingly moving towards ocean freight and its possible inclusion in future national carbon targets.

The new figures were calculated with a more detailed method than before. IMO experts divided the world's 90,000 large shipping tankers into 70 categories, based on the number and type of engines, time spent sailing and type of fuel consumed.

This contrasts with the previous approach, which simply estimated the total fuel supplied to ships by oil refineries.

The figures also show that without moves to limit the growth of international shipping, the industry's CO 2 emissions could grow by more than 30% by 2020, potentially reaching 1.475 billion tonnes.

The IMO says that the new figures will act as a wake-up call. "The point is we're trying to do something about emissions so we don't get to that 2020 figure," says a spokesperson.

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Crossing borders

"Shipping is gradually realizing that it can't escape from the need to do something to address climate change," says Peter Bosch, a member of the division of the International Panel on Climate Change that studies ways to reduce the effect of climate change.

However, he says that it will be difficult to incorporate shipping emissions into Kyoto targets because ships travel all over the world. "It is a difficult sector to get hold of politically," he says. "International negotiations have been quite slow going."

Many ships are registered under flags of convenience, and therefore have a different official nationality to the country that owns or profits from them. "Where do you allocate [the carbon emissions]?" Bosch asks. "The country making the money from the shipping, or the area where the emissions are made?"