The way this mechanic works is that you have a warning before the sea level rises. Once that happens, the change is irreversible. If you had flood barriers up, every affected tile within your city's sphere is protected. If you did not have flood barriers up, the tiles flood and become semi-permanently unusable. The tiles are fully pillaged and become impassable. However, you still have a bit of a buffer period. The flood barriers become substantially more expensive (like, as-much-as-a-wonder expensive), but if you build them, they will protect all the tiles within the city, even the flooded ones. You will still have to repair the pillage damage, but you can save your stuff. At some point, I think it's the third sea level rise, you lose the ability to save tiles with new flood barriers.
And of course, this works only one way. If you decide in your benevolence to dump every productive effort your entire civilization can muster into carbon recapture and you drop the global CO2 numbers, it does not reverse the mechanic, nor cause the sea level to fall.