There is one pollutant that oceanic shipping is worse at: SO2. SO2 happens to be a reverse-greenhouse gas, so it actually cancels out most of oceanic shipping's CO2. Because it creates smog and acid rain there is still a huge international push to reduce it, and the level of emissions have fallen by 80% in the past decade or so.
Comparing it to cars is very unfair. Gasoline in the 80s had tons of sulfur, FAR more than oceanic shipping emits today. It was totally eliminated by regulation and so that the sulfur can be sold at a profit. The relative difference is not relevant, because there's virtually no sulfur in car exhaust. Comparing the two tells you nothing about the absolute emissions or the recent and upcoming changes.
> And now a reference to CO2 to muddy the waters.
That's an understatement if anything- Japan and Germany are 3.5% and 2% of global CO2 respectively. It's preying on ignorance to make "6th most polluting country" sound high.
> This (the non-CO2 bit, not the CO2 bit) could be fixed, at relatively small cost to the industry, via changes to fuel and improved filtering, but until someone forces them to fix it it'll stay the same.
They are being forced- by the international maritime organization. The sulfur content has fallen from ~7.5% to an oceanic limit of at most 3.5% in 2012 (actual emissions are lower), falling to .5% this year. In the EU it's has been limited to .1% since 2015. Not least this is happening because China is taking such a hard line on smog. This is an incredible regulatory success, up there with leaded gasoline, iodized salt, and CFC bans.
It's very dangerous and detrimental to talk about oceanic shipping as a CO2 problem. It's essential to third world economies (both buying and selling), for one thing, but more importantly we know what the CO2 problems are: cars and coal. Fix those, then you can worry about natural gas and semi trucks. After that you've solved 80%-90% of the problem.
Seriously, say oceanic shipping globally is 2.5% of CO2 emissions. Light duty road vehicles are 59% of US transportation CO2, which is 29% of US CO2 emissions, which are 14% of of global CO2. Those multiply out to 2.4%. Forget trying to coordinate literally the entire world, you can get the same reductions by going after a single category of emissions from 4% of the world's population.