Long before the industrial revolution
Sea level once stood 6 to 9 meters (20 to 30 ft) higher than today
Gregory F. Fegel
The coral species that build reefs, which form atolls, are dependent on sub-surface sunlight, so the atoll-building occurs in shallow waters that are near to sea level.
During the Eemian interglacial period, between 130,000 and 115,000 years ago, the sea level rose to about 6 to 9 meters (20 to 30 feet) higher than today. Former beaches, reefs, and fossil corals that have been found high above the present tidal zone, in various places around the world, are proof of this. Many published research papers report that from around 7,000 to 6,000 years ago, during the Holocene Climate Optimum, global sea levels peaked at about 2 or 3 meters above today’s sea level. https://notrickszone.com/2m-higher-holocene-sea-levels/
The sea level drops as much as 400 feet during the Ice Age glacial periods, and the atoll-building corals will migrate up and down with the sea level — so we should expect to find the fossils of atoll-building corals down to a depth of 400 feet. The ability of the atoll-building corals to survive the changes caused by the Ice Age cycle — as well as various other environmental challenges during the past 500,000,000 years — shows that the corals are very resilient. In recent years, ocean acidification has been blamed for “coral bleaching”, but vertical exposure due to land uplift, which often happens in volcanic or tectonic zones, could also cause “coral bleaching”.
If major and sustained global cooling occurs, we should expect to see a drop in sea levels, the bleaching of exposed corals, and the downward migration of the atoll-building coral zone.
Earth on the Brink of an Ice Age
by Gregory F. Fegel