Coral reefs have been going through the roughest bleaching phenomenon since 1998. That event appears to have come to an end in 2017. That's the good news.

But scientists at the National Ocean and Atmospheric Administration (NOAA) are concerned that there will not be enough time to allow recovery before the next event.

Coral reefs begin to bleach when they come into contact with water that is especially warm. So you guessed it, this is another symptom of rising ocean temperatures due to global warming. Ocean temperatures rise because they absorb much of the carbon dioxide in the atmosphere. Since humans began the Industrial Age, global carbon dioxide levels have been rising dramatically. Ocean temperatures have been rising as well, which leads to bleaching and destruction of coral reefs.

Coral reefs are the forests of the ocean. They support large amounts of marine life vital to the food chain. When coral reefs begin to die, the crustaceans and small animals that live within the reefs and find shelter from bigger predators begin to die off as well. This causes a chain reaction, and predator fish begin to die off from insufficient food. 

Scientists fear that the bleaching is now going to be the new norm in the world's oceans, affecting the northern and southern hemisphere oceans in alternation.

The most recent bleaching event began in 2014 as warm water spread across the Pacific Ocean. The event continued through 2015 and early 2016 from a strong El Niño weather pattern. The unprecedented warm weather of the past year has made it continue. While coral bleaching was first seen over the past few decades, the most recent event was the longest on record. 

Mark Eakin, head of the Coral Reef Watch program at NOAA, commented on the possibility that the bleaching even could become the norm. "At some point, we're probably going to hit that level [of global warming] where it doesn't go away and it's continuous. The climate models have been saying for well over a decade that we're looking to some time around the 2020s where global bleaching becomes the norm." 

When coral is moderately stressed it begins to change color, a process called "fluorescing" and a sign that the coral is in trouble. Coral will show fluorescing when it is either being affected by cold water or too much warm water. The difference is that it recovers after the cold water passes. But the warm water leads to bleaching, and if not given to time recover, death.