Comment by jabl

The cooling effect of SO2 was certainly known when the IMO regulations were decided upon. It was decided that the public health advantages outweighed the climate warming. One can certainly quibble whether this decision was correct or not, but it wasn't done in ignorance.

(My take would have been be to allow high sulphur fuel out on the high seas, where there's very few humans around (or flora and fauna in general). )