An exceptionally strong El Niño Southern Oscillation (ENSO) in the Pacific is leading to massive coral bleaching this year. Climate change is expected to increase the frequency of such events and corals are living already close to their temperature maximum.
When exposed to warming corals expell the algae-like organisms (zooxanthellae) that live in their tissue and give them their colour. The coral will turn white, hence the term "bleaching". Prolonged warming kills the coral. That destroys not only the corals but also the habitat of fish and other organisms forming the coral ecosystem. Other stress factors can also lead to bleaching. After short-term stress corals can recover.
The El Niño this year is particularly freaky and feared to become the most severe since scientific observations began. A massive warming is building up in the Equatorial Pacific Ocean.
Quirin Schiermeier reports in a news feature on 20 October 2015 in Nature:
"Big El Niños can turn climate conditions in the Pacific upside down and disrupt weather around the globe. The impacts of this one have already been felt. Indonesia has suffered through a withering drought that has intensified fires in forests and agricultural land, and Pacific corals are experiencing one of the worst bleaching events on record. Peru has declared a state of emergency in some regions in expectation of flooding, and farmers in Australia have been put on alert for expected drought."