Widespread coral bleaching is “a sign of a truly momentous global change,” with rapid ocean warming is now a threat to corals in the tropical Indian, Pacific, and Atlantic Oceans.

As part of a recent flurry of research and news reports, Resilience.org reports that this is only the third global coral bleaching event in recorded history.

“Over the past 12 months, the temperatures of the upper layers of the ocean have been running unseasonably warm,” explains Hoegh-Guldberg, director of the Global Change Institute at the University of Queensland.

“Underwater heat waves have torn through these tropical regions over summer, and corals across large areas of reef have lost their colour as the algal partners (or symbionts) that provide much of the food for corals have left their tissues. Bereft, corals are beginning to starve, get diseased, and die.”

Experience shows that temperature increases of 1º to 3ºC above long-term summer averages are enough to cause large-scale coral die-offs. “The key concern here is that corals are not an inconsequential part of the biology of the ocean,” Hoegh-Guldberg notes. “While geographically insignificant (less than 0.1% of the ocean), coral reefs punch well above their weight in terms of their importance to the ecology of the ocean and to humans.” They’re believed to provide shelter to more than a million species, and “an estimated 500 million people derive food, livelihoods, and other benefits from coral reefs throughout the tropics.”

Research attributes rapid ocean warming to a combination of climate change and the intensity of El Niño—but climate change is likely causing strong El Niños to occur more frequently.