As with most anything of beauty found in nature, coral reefs face a number of threats, many of which are borne of human activity, but also a number which are naturally occurring.
Anthropogenic, or human-caused, activities may be the greatest threat to coral reefs. These threats include global warming, coral mining, pollution, selective over-fishing, and blast fishing. Global warming causes the sea level to rise and the water temperature to rise; the increased sea level forces the corals to grow faster, while the increased temperature causes diseases such as coral bleaching. Global warming has increased coral diseases because it has weakened corals in general; in addition to bleaching, diseases such as Black-band disease and White-band disease have become more widespread.
Coral mining is often done for construction purposes, as these rocks can be obtained nearly 50% cheaper than rocks from, for example, a quarry. Coral rocks are ground and mixed with other materials to make cement and concrete.
Pollution leads to poor water quality, which encourages the spread of infectious diseases among corals. Runoff caused by farming or construction can carry soils with added nutrients to coastal areas, causing algae blooms; some algae are toxic and can disrupt the balance in reef communities. Soil and dust can also be blown in from other regions, adding to organic pollutants. Sewage can also be a major pollutant, as many developing countries do not filter their waste before it goes to sea. Non-organic pollutants such as oil, chemicals, fertilizers, and radioactive wastes can also pollute the waters.
Over-fishing promotes the growth of fish and organisms that can damage reefs if they appear in abundance, because the population being caught are the natural predators of the damaging organisms and fish. The sampling of coral reef fish for use in aquariums has also disrupted the environment of the reef; capture of these samples is done using cyanide, which is released into the water as a narcotic, making fish capture easier, but causing remaining fish to die in the field.
Blast fishing is very destructive: explosives are activated, thrown into the water, and explode, killing not only the target fish, but also killing reef animals that are not wanted and corals. Killing the corals eliminates the reef structure, destroying the ecosystem.
There are also a number of natural threats to coral reefs. Chief among these are weather-related threats such as hurricanes and cyclones, whose waves can break apart coral heads. Tidal emersions are also a threat, as low tides can leave coral heads exposed, thus damaging reefs. Reefs also fall victim to predators which may prey on the inner tissues of coral polyps.