So combine that with the next sentence:

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 all of the world's cars produce less than 3% of greenhouse gas emissions?

Sounds like we should work at optimizing other parts of the system.

(Eg. When you're trying to improve speed/throughput of a system, you focus on the bigger chunks first, not where < 3% of the work is done.)