If we stopped exporting our pollution to them, then their economies would collapse (because the pollution is caused by export-producing industries).

So isn't the North already paying the South for this pollution?

No, because market prices don't account for environmental externalities. Attempts to make them account for environmental externalities just cause industry to move to countries where they don't, which is what already happened.

I sense that no matter what the situation would be, people will always find a way to blame "the North" for everything and anything in the world, and absolve "the South" from everything and anything that they do. It's religion at this point.