Climate change is killing off reefs in two ways. First the ocean temperatures are on average warmer and in shallows, like around the reefs this is even more so. Secomd, about half the carbon emitted into the air is absorbed by the oceans, which changes the chemical nature of he oceans makign them more acidic. Just like carbonated soda rots your teeth, due to the carbon making the soda more acidic, it is harder for coral to survive and be healthy with the oceans acidifying.

The solidification of the oceans are also killing off plankton. That is the bottom of the food chain. Eventually this may trickle up the food chain making for eventual food scarce for those countries an cultures that a highly dependent on the oceans for food. That would eventually result in a kind of famine for hundreds of millions of people.