Some coral reefs have started to look rather different.
This is called 'coral bleaching'.
To understand this, we need to start by looking at corals.
Corals are animals that make a framework around them that looks like rock.
Coral animals (polyps) have tiny plants - algae - living in their tissues.
The algae provide food to the corals, which they produce by photosynthesis.
Reef-building corals only live in a limited temperature range.
Like porridge, they should be 'not too hot and not too cold'.
Coral reefs are concentrated in a band around the equator, between 30°N and 30°S latitude.
Algae in corals need light
Corals grow in warm, clear, shallow waters that receive plenty of light.
Most corals grow in the warmest water they can stand (about 85° F or 29° C).
This means that slight increases in ocean temperature can harm corals.