The reason reefs are dying is human activity. 

In some places, overfishing has wiped out healthy food chains, allowing algae and parasites to overwhelm corals. At other sites, boats dragging anchors and nets — or just scraping along the sea floor — have damaged or destroyed reefs. Pollution from agriculture and runoff from cities can cause disease and kill these creatures as well.

But most importantly, the burning of fossil fuels has warmed the planet. And Earth’s oceans have absorbed the majority of that heat, about 90% of it so far.