First step is countries ban the vessels from coming anywhere near land. I guess that keeps pollution out of sight, out of mind. Next step.... I don't know. But I'm hopeful soon enough the powerplant will be significantly greener. In the short term you can get 1 order of magnitude from regulation. In the long term it's new technology.
The study claims, among other things, that the freighters create a lot of sulphur emisions, and that the EU has currently cleaned up car requirements so that they don't emit much sulphur in exhaust - thus, 15 largest freighters emit as much SOx as all the cars. It assumes that world's cars fit EU emission standards, which is false, and posting in this context seems to assume that the sulphur oxides cause global problems, which is also false - the bad effects of SOx pollution are rather localized, and if we can replace 1 unit of SOx in the middle of a city with 2 units of SOx in the middle of an ocean, then I'd treat that as good thing.
A more appropriate citation (from the same original article, actually) is the following - it still shows some space for improvement, but is significantly diffferent than the untrue alarmist claim in the parent post.
"Shipping is responsible for 18-30% of all the world's nitrogen oxide (NOx) pollution and 9% of the global sulphur oxide (SOx) pollution."
For example, the 15 largest freighters create more pollution as every single car in the world. If this is the case why aren't we trying to fix pollution on these 15 ships instead of all this nonsense with fixing individual cars? Eliminating the pollution from these industrial ships by 10% is akin to taking millions of cars off the road. Yet all we talk about is car pooling, proudest and Teslas, etc.
http://www.industrytap.com/worlds-15-biggest-ships-create-mo...