What is threatening coral reefs according to The Sixth Extinction?

Elizabeth Kolbert argues that carbon dioxide (CO2) changes the acidity of ocean water, which in turn is negatively affecting coral reefs. She looks primarily at Australia’s Great Barrier Reef and the ongoing scientific research based on a nearby island. In addition, she looks back several decades at the background research on CO2, such as that conducted in the experimental Arizona Biosphere. Coral inside that controlled environment, which had high CO2 levels, became eroded. Previously, it was believed that carbon dioxide did not affect coral. As this information has more recently been applied to studying reefs in their natural ocean location, their erosion or bleaching related to rising CO2 levels has been noted, leading some scientists to predict their demise by mid-century.
https://books.google.com/books?id=Ra9RAQAAQBAJ&source=gbs_navlinks_s

https://www.nationalgeographic.com/news/2017/06/coral-reef-bleaching-global-warming-unesco-sites/