https://blogs.ei.columbia.edu/2011/06/13/losing-our-coral-reefs/
Many causes, most of them human
“Coral bleaching is caused by global warming, full stop,” said Terry Hughes, lead author of a new studyon coral bleaching. The researchers found that bleaching events have increased from one every 25 to 30 years in the early 1980s to an average of one every six years since 2010. While coral reefs can recover from bleaching if given 10 to 15 years for their algae communities to recover, the increasing frequency of bleaching events means that many reefs may never be able to.
In addition, the 22 million tons of carbon dioxide our oceans absorb every day are changing the chemistry of seawater and increasing acidification. Today, coral reefs are experiencing more acidity than they have at any time in the last 400,000 years. Acidification reduces the water’s carrying capacity for calcium carbonate that corals need to build their skeletons. Even a small decrease in the coral’s ability to construct its skeleton can leave it vulnerable to erosion. Some reefs have already begun to dissolve and it’s estimated that by 2050, only 15 percent of coral reefs will have enough calcium carbonate for adequate growth.
One study showed that ocean acidification profoundly alters coral reef ecosystems. As C02 levels rise and acidification increases, the biodiversity of coral reefs drops, resulting in the elimination of key species needed for healthy reef formation. “The decline of the structurally complex corals means the reef will be much simpler and there will be less habitat for the hundreds of thousands of species we associate with today’s coral reefs,” said Katherina Fabricius, a scientist at the Australian Institute of Marine Science.