As ships get bigger, the pollution is getting worse. The most staggering statistic of all is that just 16 of the world’s largest ships can produce as much lung-clogging sulphur pollution as all the world’s cars.

Smoke and sulphur are not the only threats from ships’ funnels. Every year they are also belching out almost one billion tons of carbon dioxide. Ships are as big a contributor to global warming as aircraft – but have had much less attention from environmentalists.

Both international shipping and aviation are exempt from the Kyoto Protocol rules on cutting carbon emissions. But green pressure is having its effect on airlines. Ahead of next month’s Copenhagen climate talks, airlines have promised to cut emissions by 50 per cent by 2050.