Extensive coral bleaching in the Kimberley region, Western Australia, April 2016 Credit, Morane Le Nohaic
Coral bleaching is caused by higher ocean temperatures, which starve the coral reefs of their main food source, microscopic algae. When the ocean is too warm for the symbiotic algae that live in coral tissues, they leave the corals or die – a process called bleaching. The frequency of coral bleaching is increasing. In the past, coral reefs bleached every 25 to 30 years. Since 2010, that timeframe has shrunk to six years. By early 2018, the Great Barrier Reef alone has bleached four times since 1998.
Some coral reefs can recover if there’s enough time between bleaching events. But if corals can’t adapt quickly enough, “we could be looking at the effective loss of most of the world’s coral reefs,” said Mark Eakin, an oceanographer who is coordinator of the Coral Reef Watch project at the United States National Oceanographic and Atmospheric Administration that looks at loss of world's coral reefs.
You can see images of healthy and unhealthy reefs here.
The climate crisis is affecting coral reefs in a couple of ways.
The Great Barrier reef, the largest coral reef in the world, is in danger because of climate change
People can help coral reefs to recover more quickly from coral bleaching by reducing local stressors.
Some coral reefs recover more quickly than others, for example those around Palmyra Island.
Scientists are experimenting with helping corals to recover that have the best natural chances of survival. And in some places people are coral farming — a process whereby fragments of corals are collected from the local reefs, raised in nurseries on land until mature, and then installed at a site targeted for restoration.
Some corals seem to be preparing for an extinction crisis with “disaster traits” that resemble those from last major extinction crisis 65 million years ago; for example: “increased prevalence of deep-water residing, cosmopolitan distributions, non-symbiotic relationship to algae, solitary or small colonies, and bleaching resistance.”