Coral reefs are among the most complex ecosystems on the planet. Often called "rain forests of the sea," coral reefs occupy less than 0.1 percent of the world ocean surface. Their low abundance belies their great importance.

Worldwide about 500 million people have some level of dependence upon coral reefs. They provide economic goods and ecosystem services worth about $375 billion each year.

A study reported by NOAA in October shows the American people assign an estimated total economic value of $33.57 billion to the coral reefs of the main Hawaiian Islands.

In addition to the economic benefits, coral reefs are home to more than 1 million aquatic species, including 4,000 species of fish, 700 species of coral and thousand of other plants and animals down to the smallest single-celled organisms. Reef ecosystems are home to approximately 25 percent of all marine species.

Reefs provide a natural barrier that protects coastal cities and beaches. They hold the potential for treatments of many dangerous illnesses and diseases.

Compounds from coral reefs have yielded treatments for cardiovascular diseases, ulcers, leukemia and skin cancer, and coral’s structure has been used to make advanced forms of bone-grafting materials. More than half of all new cancer drug research focuses on marine organisms.

Coral reefs are also an integral part of Earth’s biogeochemical system, including carbon and calcium storage; and geological formations that facilitate water flow and upwelling, the movement of deep sea water to the surface. Reefs and the geological bodies formed from ancient reefs contain much of the planet’s carbon dioxide in the form of calcium carbonate.

The bad news is that reefs around the world are dying.

A quarter already have disappeared, and 75 percent are at risk today. In Southeast Asia 90 percent are at risk, and in the Florida Keys more than 90 percent have lost their living coral cover since 1975.

Coral reefs respond more quickly to environmental changes than any other ecosystem. They are extremely sensitive to the slightest changes in water temperature. They are considered an indicator or early warning system for the world’s oceans, and perhaps a "canary in the coal mine" for the global ecosystem and subsystems.

Coral polyps are tiny, soft-bodied organisms related to sea anemones and jellyfish. At their base is a hard, protective limestone skeleton that forms the structure of coral reefs.

Reef-building corals exist in a symbiotic relationship with algae called zooxanthellae that live inside the polyps. The algae produce food via photosynthesis that they share with the coral, while the coral provide protection and a platform for access to the light. Zooxanthellae also give color to corals, which become ‘bleached’ when zooxanthellae die off and expose the white calcium carbonate skeletons.

A number of environmental stresses cause bleaching, including temperature, disease, shade, increased UV radiation, sedimentation, pollution and salinity changes.

Coral reefs are under threat from climate change, ocean acidification, blast fishing, cyanide fishing, overuse of reef resources, tourism, reckless diving practices, coral mining and harmful land-use practices, including urban and agricultural runoff and water pollution, which can harm reefs by encouraging excess algae growth.

It is well documented that corals are dying. It has not been proved that climate change is a factor, but whatever the cause, the loss of coral reefs is an environmental tragedy.

Richard Brill is a professor of science at Honolulu Community College. Email questions and comments to rickb@hcc.hawaii.edu.