Based on context clues, I'd say water level rising.
The Gorosei are insistent on aquiring as much of the Mother Flame as possible, which implies it's requirement towards enacting their plan for the ''great cleansing''. One use of the Mother Flame on Lulucia was said to cause the sea level to increase by one meter.
So we can at least speculate that they are planning to successively use it across multiple islands, thereby causing the ocean to eventually flood the entire world (Marijoa left unharmed due to its altitude)