Notes

They warned decades ago that the coral reefs would be at risk if human society kept burning fossil fuels at a runaway pace, releasing greenhouse gases that warm the ocean. Emissions continued to rise, and now the background ocean temperature is high enough that any temporary spike poses a critical risk to reefs.

“Climate change is not a future threat,” Professor Hughes said. “On the Great Barrier Reef, it’s been happening for 18 years.”

Corals require warm water to thrive, but they are exquisitely sensitive to extra heat. Just two or three degrees Fahrenheit of excess warming can sometimes kill the tiny creatures.

Globally, the ocean has warmed by about 1.5 degrees Fahrenheit since the late 19th century, by a conservative calculation, and a bit more in the tropics, home to many reefs. An additional kick was supplied by an El Niño weather pattern that peaked in 2016 and temporarily warmed much of the surface of the planet, causing the hottest year in a historical record dating to 1880.