SHIPPING may seem like a clean form of transport. Carrying more than 90% of the world’s trade, ocean-going vessels produce just 3% of its greenhouse-gas emissions. But the industry is dirtier than that makes it sound. By burning heavy fuel oil, just 15 of the biggest ships emit more oxides of nitrogen and sulphur—gases much worse for global warming than carbon dioxide—than all the world’s cars put together. So it is no surprise that shipowners are being forced to clean up their act. But in an industry awash in overcapacity and debt, few have access to the finance they need to improve their vessels. Innovative thinking is trying to change that.