For only the third time on record, scientists have declared a global coral bleaching event. The National Oceanic and Atmospheric Administration warns that we could be witnessing one of the largest ever coral die-offs, as ocean temperatures continue to warm and the weather system El Niño persists. 

While coral can recover from mild bleaching, severe or long-term bleaching, as is being witnessed in every ocean around the world, eventually kills the coral and degrades the reef on which many other species and hundreds of millions of people depend for sustenance and income. It's estimated that 38% of the planet's reefs will have been affected by this event by the end of the year.      

“The coral bleaching and disease, brought on by climate change and coupled with events like the current El Niño, are the largest and most pervasive threats to coral reefs around the world,” explained Mark Eakin, the NOAA’s Coral Reef Watch coordinator, in a statement. “As a result, we are losing huge areas of coral across the U.S., as well as internationally. What really has us concerned is this event has been going on for more than a year and our preliminary model projections indicate it’s likely to last well into 2016.”

^{A four-month prediction for coral bleaching from October 2015 to January 2016 showing the most vulnerable areas, particularly around the Caribbean and Hawaii. NOAA} 

Beginning in the north Pacific last year, the bleaching has spread to the south Pacific and Indian Ocean, with the event expected to continue well into next year. Corals in U.S. waters have been disproportionately hit hard, with the biggest risk currently being to those reefs surrounding Hawaii. This is the second bleaching event in as many years to impact the archipelago, and with many areas still reeling from the first event, large areas have been further devastated this year. Bleaching has also started off the coast of Florida, and is expected to spread south to the Caribbean over the next few months.

Bleaching occurs when the temperature of the surrounding water becomes so high it stresses the coral. When this happens, the tiny little invertebrates called polyps which make up the coral eject the algae with which they partner for photosynthesis.

The coral can survive for short periods of time without the algae, and can even recover if the environment recovers, but if the waters stay warm for too long, the coral eventually dies either due to lack of food or increased susceptibility to disease. The result is a ghostly pale “skeleton” devoid of life.    

^{The outlook for following the four months from February to May 2016 looks even dire, with bleaching spreading south to the Galapagos and to pretty much all of the coral reef regions in the Indian Ocean. NOAA} 

The main driver has been climate change, which has slowly warmed our oceans, as well as increasing the amount of dissolved carbon dioxide, which makes the water more acidic and degrades the corals' “skeleton.” But the NOAA stresses that there are still things people can do to help protect the reefs. “We need to act locally and think globally to address these bleaching events,” said Jennifer Koss, also from the NOAA. “Locally produced threats to coral, such as pollution from the land and unsustainable fishing practices, stress the health of corals and decrease the likelihood that corals can either resist bleaching, or recover from it.”

But ultimately, if we as a species want to stop these sorts of events from happening on an increasingly frequent basis, we’re going to have to get the bottom of the root cause: anthropogenic climate change.