> Something like 50% of marine fuel usage is shipping fossil fuels around the world

Note that marine shipping is extraordinarily fuel efficient (from a gCO2/(t*km) basis), so I doubt that it adds a lot on a per ton of fuel basis. We just ship a lot of fossil fuels.

This [1] graph looks to be in the right ballpark from what i remember in school 15 years ago, i didn't verify it in depth but +- an order of magnitude better than the next best method is roughly right

https://image2.slideserve.com/4166134/gco-2-t-km-of-freight-...