Cargo ships are very efficient for moving goods around the world, but they burn large amounts of very polluting oil. The Guardian offers a few numbers.
The world's biggest container ships have 109,000 horsepower engines which weigh 2,300 tons.
Each ship expects to operate 24hrs a day for about 280 days a year
There are 90,000 ocean-going cargo ships
Shipping is responsible for 18-30% of all the world's nitrogen oxide (NOx) pollution and 9% of the global sulphur oxide (SOx) pollution.
One large ship can generate about 5,000 tonnes of sulphur oxide (SOx) pollution in a year
70% of all ship emissions are within 400km of land.
85% of all ship pollution is in the northern hemisphere.
Shipping is responsible for 3.5% to 4% of all climate change emissions