Huge sections of the Great Barrier Reef, stretching across hundreds of miles of its most pristine northern sector, were recently found to be dead, killed last year by overheated seawater.
Coral reefs are sensitive systems, built by unusual animals. The corals themselves are tiny polyps that act like farmers, capturing colorful single-celled plants called algae that convert sunlight into food. The coral polyps form colonies and build a limestone scaffolding on which to live — a reef.
But when the water near a reef gets too hot, the algae begin producing toxins, and the corals expel them in self-defense, turning ghostly white. If water temperatures drop soon enough, the corals can grow new algae and survive, but if not, they may succumb to starvation or disease.