There is a project to get transcontinental ships to add emitters of FeCl3 to their exhaust stacks, to catalyze oxidation of methane. It seems to be getting enthusiastic uptake from shipowners for reasons unknown to me. 10,000 ships emitting 100kg a day would be enough to bring atmospheric methane in line.

(Fe seeded to the ocean surface is negligible. This is not a thing to trigger algae production.)

https://en.wikipedia.org/wiki/Atmospheric_methane_removal