Mitsubishi Corporation

Coral Reefs in Crisis

Rising ocean temperatures and coral bleaching due to global warming

Coral bleaching (Photographer: Professor Suzuki, Shizuoka University)
Coral bleaching
(Photographer: Professor Suzuki, Shizuoka University)

In recent years coral damage, thought to be the result of rising ocean temperatures has become a serious problem. The color of coral is actually the color of the zooxanthellae mentioned earlier that live inside polyp cells and without zooxanthellae, coral has no color.

When zooxanthellae, under some kind of stress, either vacate the polyp cells or die inside the coral, the color of the coral turns white—the color of the coral itself. This is the phenomenon known as coral bleaching and rising ocean temperatures due to global warming has been indicated as the cause.

1998 map of areas around the world affected by coral bleaching

1998 map of areas around the world affected by coral bleaching

What is coral bleaching?

Coral bleaching is the phenomenon whereby the color of coral fades to white. This phenomenon is occurring on reefs in many areas around the world including the Indian Ocean, the Red Sea, and the Caribbean Sea. Coral bleaching can also be observed on reefs in Japan, mainly around Okinawa and Kagoshima. Ideal ocean temperatures for coral habitation are 25 to 29°C. Single-celled algae called zooxanthellae live inside the body of corals and photosynthesize. By the ocean temperature rises by a mere 2°C, however, these zooxanthellae die and the coral loses its color. With the zooxanthellae absent, the coral's supply of organic matter is restricted and a majority of the coral dies. During the 1998 El Niño-Southern Oscillation phenomenon, the associated warmer ocean temperatures caused coral bleaching and led to the loss of vast areas of coral reef around the world.

The occurrence of coral bleaching is widespread, affecting many areas in the world and the damage it causes is significant. Yet we still do not perfectly understand the mechanisms at play and there are not preventative measures in place.

Coral devastation by crown-of-thorns starfish outbreaks

The crown-of-thorns starfish is the natural predator of coral, its body is covered with sharp poisonous spikes and growing up to 50 cm in diameter, it is a big starfish. Located on its underside is its mouth, and from its mouth it extrudes its stomach over coral, and the stomach, while still outside the starfish's body, digests and absorbs the polyps from the coral. Up until present, there continue to be many reports of areas in which outbreaks of huge numbers of this starfish occur.

Ocean pollution by development-related sand runoff and human sewage

When a large amount of sand is deposited on coral, the coral cannot remove the sand by itself and is unable to feed or respire, which can lead to it dying. Sand runoff caused by the logging of forests and land development can cause serious damage.

Human sewage also can pollute the ocean, seriously damaging coral. Nutrient levels in the ocean rise causing the population of plankton to increase. With increased plankton, the ocean loses its transparency and if this prevents sunlight from reaching the coral, the coral is at serious risk of dying.