First thing coral reefs are not sinks of carbon. Instead, they emit carbon. That means they pollute the environment. But that does not mean the world does not need reefs. They are important for various purpose. Let us leave its importance for a moment and try to understand why they are not sinks of carbon.
According to Global Coral Reef Alliance: Reefs are not carbon sinks
Not only is the entire claim that coral reefs are a CO2 sink completely incorrect, but they are also, in fact, a source of CO2 to the atmosphere even while they remove carbon from the ocean. This has been understood by carbonate chemists for a very long time. But the importance of coral reefs confused people that they are a carbon sink. The only way that reefs could be a CO2 sink would be if they were autotrophic ecosystems that buried most of the algae carbon before it could decompose. But in fact, reef sediments have very low buried organic carbon content, because the organic carbon is almost entirely decomposed. In fact, reefs are not autotrophic at all; they are heterotrophic systems that rely on external organic carbon input from land and oceanic zooplankton. Thus they do not sink of CO2. Read the full article of Global Coral Reef Alliance.
Coral Bleaching Alert System (CBAS): The coral bleaching Alert System (CBAS), a service initiated from INCOIS since February 2011. This model uses the satellite-derived Sea Surface Temperature (SST) to assess the thermal stress accumulated in the coral environs. This information yields in drawing the early signs of the intensity and spatial extents of coral bleaching. This methodology adopted from NOAA reef and tested for the earlier bleaching events of Indian coral environs. The service CBAS disseminated once in three days comprising the Hotspot, Degree of Heating Weeks and time series products.