Ah, what you want to say is you only care for the fast death of coral reefs and don't care for things that ensure its death but will take maybe a decade or two?
pH does not fluctuate much, true, but it drops steadily which is just as deadly to coral.
Corals are more likely to survive a slow than a fast change as well. I don't think we know that much about how evolution bottlenecks work, but I'd think it's easier to adapt to a radically different environment in three generations than one.
pH does not fluctuate much, true, but it drops steadily which is just as deadly to coral.