The Ends of the World: Volcanic Apocalypses, Lethal Oceans, and Our Quest to Understand Earth's Past Mass Extinctions

by Peter Brannen

Additionally, corals are exquisitely sensitive to temperature changes: many species cannot live in the cold, but they are also subject to life-threatening episodes of bleaching when the water gets too warm. Microorganisms called zooxanthellae live on reef-building corals (the corals first recruited these symbionts in the Triassic), and the corals rely on them to photosynthesize their food. When episodes of unusually warm water hit, zooxanthellae literally start poisoning this relationship and the corals, it’s thought, expel them out of desperation. This is called “bleaching” for a good reason: to visit a reef after a bleaching event is to visit a panorama of calcium carbonate as white as desert bone. Bleaching is a medical emergency for corals, and the rare colony that does survive a bleaching event is often left an exhausted shade of its former splendor and even more vulnerable to future crises. Cores of centuries-old coral colonies show that the waves of global bleaching events that have wiped out corals in the past few decades—like the horrific bleaching that pummeled the Florida Reef and the Hawaiian Islands in 2015—are unprecedented for at least the past several thousand years.