The phenomena known as coral bleaching is a serious threat to coral communities, often connected to long spells of elevated water temperatures at coral reef sites. In a world predicted to see increasingly warmer air and water temperatures due to climate change, scientists want to know if coral bleaching will become more frequent and wide spread over the next century.

In a study published Nature Climate Change researchers used the latest emission scenarios and climate models to show how varying levels of carbon emissions are likely to result in more frequent and severe coral bleaching events, and where and when these events are likely to occur.

In the analysis that assumes carbon emissions stay on the current path, most of the world’s coral reefs (74 percent) are projected to experience coral bleaching conditions annually by 2045.

The rate at which coral reefs begin experiencing annual bleaching does vary over a period of years, with some reefs starting as early as the 2030’s, and others holding off until the 2050’s. However, the study showed that by 2056 all coral reefs would start experiencing annual coral bleaching.                     

The regions that appear to be more susceptible, with a quarter of coral reefs likely to experience annual bleaching events five or more years earlier the average, lie in northwestern Australia, Papau New Guinea, and some equatorial Pacific islands.

Coral reefs in parts of the western Indian Ocean, French Polynesia and the southern Great Barrier Reef, seem to fare better and have been identified as temporary refugia from rising sea surface temperatures. These locations are projected to experience bleaching events annually until five or more years later than the median year of 2040, with one reef location in the Austral Islands of French Polynesia protected from the onset of annual coral bleaching conditions until 2056.

The study also considered reduced carbon emission scenarios, which delayed annual bleaching events by more than two decades in nearly a quarter (23 percent) of the world’s reef areas. Reduced emission scenarios would also delay to some degree the onset of annual bleaching for nearly all coral reef locations.

Scientists aren’t positive if these additional twenty years would “buy” some reefs enough time to improve their capacity to adapt to the projected temperature changes. Some corals have been known to change the type of zooxanthellae they house after a bleaching event, abandoning a more temperature sensitive algae for a more resilient type. Studies have also shown that some corals that are exposed to more variability can be more tolerant of heat stress. However, these possible adaptations would not be a likely response or natural solution to coral reefs globally by 2056.

The researchers involved in the study all concur that projections that combine the threats posed to reefs by increases in sea temperature and ocean acidification will further resolve which coral reef locations will fare better or worse in a world of climate change.