They produce a lot Air pollution of a very specific type because their fuel includes a lot of sulfur. In terms of total pollution ships are actually a small fraction.
Further, releasing sulfur over the ocean does not impact human or animal heath significantly.
- https://www.theguardian.com/environment/2009/apr/09/shipping...
> One giant container ship can emit almost the same amount of cancer and asthma-causing chemicals as 50m cars, study finds
> Confidential data from maritime industry insiders based on engine size and the quality of fuel typically used by ships and cars shows that just 15 of the world's biggest ships may now emit as much pollution as all the world's 760m cars.
The shipping industry needs to get its act together too but it isn't _that_ significant when it comes to climate change. In terms of sulphur based pollutants it's pretty horrifying, though.
Thanks, that certainly clarifies.