https://phys.org/news/2018-11-tracks-severe-events-pacific-coral.html

When water temperatures rise even slightly, symbiotic algae that live inside the cells of the live coral start to create toxic substances and are ejected by the corals. The algae normally provide the corals with food and energy, as well as their bright colors. Without them, the corals appear to be "bleached" white, then starve and die.

In their study, Cohen's team traveled to Jarvis Island, a tiny, unpopulated coral reef island 1,400 miles south of Hawaii, to study the effects of extreme climate on the corals there. Because Jarvis is both remote and part of a marine protected area, it has been home to stunningly rich coral reefs—but with its location in the middle of the Pacific, it also experiences more extreme heat waves caused by periodic El Nino events than coral reefs elsewhere.