a healthy coral reef
Elizabeth Kolbert wrote in National Geographic, “Coral reefs are already threatened by a wide array of forces. Rising water temperatures are producing more frequent "bleaching" events, when corals turn a stark white and often die. Overfishing removes grazers that keep reefs from being overgrown with algae. Agricultural runoff fertilizes algae, further upsetting reef ecology. In the Caribbean some formerly abundant coral species have been devastated by an infection that leaves behind a white band of dead tissue. Probably owing to all these factors, coral cover in the Caribbean declined by around 80 percent between 1977 and 2001. [Source: Elizabeth Kolbert, National Geographic, April 2011]

According to the Environmental Defense Fund, reefs in 93 of the 109 countries where reef are found have been damaged by human activity. A report by the United Nations estimated that 30 percent of the world's reefs have been damaged by human activity. Another report put the figure at 58 percent. Others sources have estimated that 10 percent of three world's reefs have been wiped out. A forth are regarded as degraded beyond recovery.

According to some pessimistic estimates if the pace of damage continues half of the world’s coral reefs might be lost by 2050 and another 30 percent could be seriously depleted. Optimists point out that coral reefs have survived more than 250 million years and disturbance and renewal is common among reef ecosystems and they are so vast and scattered they will survive.

In the Caribbean the amount of reef surface covered by live coral has fallen about 80 percent in the last three decades according to the Global Coral Reef Monitoring Network. In the Pacific Ocean between Hawaii and Indonesia, reefs have lost about 1 percent of their coral coverage annually over the last 25 years.

Some reefs corals that were bright purple are now dull brown. Groupers living there are 60 percent smaller than they were 50 years ago and their population has declined by 95 percent. As much as 20 percent of the world’s coral reefs have effectively been destroyed by pollution, overfishing, diseases and bleaching. In some places 90 percent of the coral reefs have been lost. A survey published in Science in 2008 found that half the coral reefs in U.S. territory are in fair or poor condition.

Sources: "Reefs at Risk", a report by the World Resources Institute; the International Coral Reef Action Network; the Global Coral Reef Monitoring Network.

Websites and Resources: National Oceanic and Atmospheric Administration ; Smithsonian Oceans Portal ; Ocean World ; Woods Hole Oceanographic Institute ; Cousteau Society ; Montery Bay Aquarium

Websites and Resources on Fish and Marine Life: MarineBio ; Census of Marine Life ; Marine Life Images ; Marine Species Gallery

Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) ; International Coral Reef Initiative ; Wikipedia article Wikipedia ; Coral Reef Alliance ; Global Coral reef Alliance ; Coral Reef Pictures ; The Global Coral Reef Monitoring Network; the International Coral Reef Action Network.

Coral Reefs and Climate Change

Elizabeth Weise wrote in USA Today: Coral are vital to the health of the oceans. Although they cover only 0.2 percent of the ocean floor, they are home to at least a quarter of all marine species. They provide safety for juvenile fish and are home to the small organisms and fish which provide food for larger fish. Scientists estimate that the reefs account for 25 percent of fish caught in developing countries. [Source: Elizabeth Weise, USA Today, April 7, 2022]

Coral reefs can survive within only a relatively narrow temperature band. The coral that build them get much of their food from algae living in their tissues. When the seawater is too warm, the coral’s stress response is to expel algae, causing the coral to turn white. The process is called coral bleaching, and if it lasts too long, the coral can starve – turning a thriving ecosystem into a cemetery of dead shells.

A report released in 2021 showed that almost 15 percent of the planet's reefs have vanished since 2009, primarily because of climate change. “They’re being cooked to death,” said Dutton, a MacArthur Genius Award winner, who studies the deep history of the oceans. “The frequency at which we’re seeing these bleaching events is astounding to those of us who study them,” she said. “It’s going to have a huge domino effect on marine systems and on humans.”

Hurricanes and Other Natural Threats to Coral Reefs

20110307-NOAA coral damage fb_bottom_100.jpg
storm damaged coral
Corals and reefs are very sensitive. Water that is too muddy, warm, salty or fresh will kill them. Natural threats to reefs include warm waters produced by the El Niño climate phenomena, storms such as typhoons and hurricanes, too much plankton, polyp-eating predators like the crown of thorn starfish and diseases.

Researchers from NOAA have suggested that hurricanes and typhoons may actually be beneficial to coral reefs in the face of global warming as they long as they don’t strike the reef directly with heavy damaging waves. The logic behind the idea is that winds mix cold deep water and warm surface water, reducing the temperature of the damaging water around the reef that is too warm. NOAA researchers studied two bleached coral reefs — one in the Florida Keys and another in the Virgin Island. The one in the Florida Keys rebounded after the passing of Hurricanes Rita and Wilma in 2005, while the one in the Virgin Islands, which was not affected by any hurricanes, did not.

Coral Bleaching

Coral bleaching is a phenomena characterized by the changing of coral color from brown, green and pink to bleached white caused by the death of algae within the coral. If algae starts covering the coral, the coral is considered dead.

Reefs all around the world are plagued by coral bleaching. Damage has been particularly bad in the Indian Ocean, where 70 percent of the coral reefs whitened or died in 1998 and 1999. The year 1998 was particularly bad for coral bleaching. About 16 percent of the world’s coral became bleached or died. Not coincidently 1998 was the second warmest year on record up to that time. In 1998 and 2002 the Great Barrier Reef suffered major bleaching that was blamed on El Nino-related warming. Though much of damaged coral recovered about five percent was permanently damaged.

Coral bleaching generally occurs in waters that become warmer than 30 degrees C (85 degrees F). When waters get too hot the coral polyps become stressed and expel its algae. A temperature rise of 1.8 to 3.6 degrees F can trigger coral bleaching in many places.

Causes of Coral Bleaching

healthy coral and bleached coral
The whitening is caused by loss of symbiotic zooxanthellare (a kind of algae) from the tissues of the coral polyps. Sometimes healthy corals can become completely white in two or three weeks. If algae returns to the coral within a month the white coral can revive itself. But if the conditions continues for more than two months most of the coral dies.

Coral bleaching occurs because stress has prompts the polyps to expel its dark algae symbionts. Bleaching is not only bad in its own right it also destroys important fisheries and contributes to global warming. Algae in coral and the sea absorb a lot of carbon dioxide.

Among the causes of high water temperatures are the El Niño effect and an absence of storms. Abnormal salt densities can also cause algae to die. Storms like hurricanes and typhoons help keep the water cool by churning up the water so that cooler subsurface layers of water mix with the warmer surface layers.

Many scientists believe that bleaching is a natural phenomena. Others believe it is caused at least in part by human activities. They blame global warming, overfishing and pollution for contributing to the problem.

Recovery from Coral Bleaching Caused by El Nino

Describing what he saw after a sudden increase in water temperatures around Kanton Island in the Phoenix Islands, a tiny archipelago in the central Pacific five days by boat from Fiji, Gregory Stone, wrote in National Geographic, During the El Niño of 2002-03, a body of water more than 1̊C (1.8̊F) warmer than usual had stalled for six months around the Phoenix Islands. We'd heard that the hot spot had severely bleached the region's corals. [Source: Gregory Stone, National Geographic, January 2011]

Settling down beside the reef, I saw dead coral everywhere. What had been flourishing, overlapping, overflowing brown and auburn plates of corals were now ghostly, broken reminders of their former beauty. When I'd first visited the Phoenix Islands a decade ago, these reefs had supported numerous species of hard corals, as well as giant clams, sea anemones, nudibranchs, and great populations of fish, from blacktip reef sharks to parrotfish to bohar snappers. Because the islands have remained undisturbed for so long, they'd largely avoided overfishing, pollution, and other harmful impacts of modern civilization. But they hadn't been able to avoid climate change, which most scientists believe amplifies El Niños.

Not ready to accept this setback, I was heartened to see lots of reef fish and vibrant corals growing up through the rubble — early signs of recovery. Was it possible that the reefs of the Phoenix Islands, like their mythical namesake, were rising from the ashes of a terrible warming?

In 2010 Stone returned to Kanton. He wrote: “The bleaching had killed all the coral on the lagoon floor, but almost half appeared to be growing back — the fastest recovery any of us had ever seen. The reason seemed clear: abundant fish. When coral bleaches, seaweed can grow out of control, stifling reef recovery. But fish eat the algae, keeping it from smothering the coral. Because fish populations had been protected here, the reefs remained surprisingly resilient even after suffering one of the worst bleaching events ever recorded.

Affects of Ocean Acidification on Coral Reefs


Global warming is causing ocean temperatures as well as air temperatures to rise. An increase of carbon dioxide in seawater cause acidification in the oceans The mixture of carbon dioxide and seawater creates carbonic acid, the weak acid in carbonated drinks. The increased acidity reduces the abundance of carbonate ions and other chemicals necessary to form calcium carbonate used make sea shells and coral skeletons. To get an idea what acid can due to shells remember back to high school chemistry classes when acid was added to calcium carbonate, causing it to fizz.

Elizabeth Kolbert wrote in National Geographic, “Ocean acidification adds yet another threat, one that may be less immediate but ultimately more devastating to hard, reef-building corals. It undermines their basic, ancient structure — the stony skeleton that's secreted by millions upon millions of coral polyps over thousands of years....To make calcium carbonate, corals need two ingredients: calcium ions and carbonate ions. Acids react with carbonate ions, in effect tying them up. So as atmospheric CO2 levels rise, carbonate ions become scarcer in the water, and corals have to expend more energy to collect them. Under lab conditions coral skeleton growth has been shown to decline pretty much linearly as the carbonate concentration drops off. [Source: Elizabeth Kolbert, National Geographic, April 2011]

Slow growth may not matter much in the lab. Out in the ocean, though, reefs are constantly being picked at by other organisms, both large and small. (When I went snorkeling off One Tree Island, I could hear parrotfish chomping away at the reef.) "A reef is like a city," said Ove Hoegh-Guldberg, who used to direct the One Tree Island Research Station and now heads the Global Change Institute at Australia's University of Queensland. "You've got construction firms and you've got demolition firms. By restricting the building materials that go to the construction firms, you tip the balance toward destruction, which is going on all the time, even on a healthy reef. In the end you wind up with a city that destroys itself."

By comparing measurements made in the 1970s with those taken more recently, Caldeira's team found that at one location on the northern tip of the reef, calcification had declined by 40 percent. (The team was at One Tree to repeat this study at the southern tip of the reef.) A different team using a different method has found that the growth of Porites corals, which form massive, boulderlike clumps, declined 14 percent on the Great Barrier Reef between 1990 and 2005.

bleached corals
Ocean acidification seems to affect corals' ability to produce new colonies as well. Corals can, in effect, clone themselves, and an entire colony is likely to be made up of genetically identical polyps. But once a year, in summer, many species of coral also engage in "mass spawning," a kind of synchronized group sex. Each polyp produces a beadlike pink sac that contains both eggs and sperm. On the night of the spawning all the polyps release their sacs into the water. So many sacs are bobbing around that the waves seem to be covered in a veil of mauve.

Selina Ward, a researcher at the University of Queensland, has been studying coral reproduction on Heron Island, has found through her research that lower pH leads to declines in fertilization, in larval development, and also in settlement — the stage at which the coral larvae drop out of the water column, attach themselves to something solid, and start producing new colonies. "And if any of those steps doesn't work, you're not going to get replacement corals coming into your system," Ward said.

Once a reef can no longer grow fast enough to keep up with erosion, this community will crumble. "Coral reefs will lose their ecological functionality," Jack Silverman, a member of Caldeira's team at One Tree, told me. "They won't be able to maintain their framework. And if you don't have a building, where are the tenants going to live?" That moment could come by 2050. Under the business-as-usual emissions scenario, CO2 concentrations in the atmosphere will be roughly double what they were in preindustrial times. Many experiments suggest that coral reefs will then start to disintegrate.”

Reefs Could Perish by 2100 Experts Warn

Oa-buoy, which measures temperatures in the sea
In July 2009, Reuters reported: Increasingly acidic oceans and warming water temperatures due to carbon dioxide emissions could kill off the world's ocean reefs by the end of this century, experts told a meeting in London, saying the predicted pace of emissions means a level of 450 parts per million of carbon dioxide (CO2) in the atmosphere will be reached by 2050, putting corals on a path to extinction in the following decades. The experts were comprised of two dozen coral reef specialists and climate change experts representing universities, government research offices and the Intergovernmental Panel on Climate Change. [Source: Michael Kahn, Reuters, July 6, 2009]

"The kitchen is on fire and it's spreading around the house," Alex Rogers of the Zoological Society of London and the International Program on the State of the Ocean, said in a statement. "If we act quickly and decisively we may be able to put it out before the damage becomes irreversible."

"If CO2 is allowed to reach 450 ppm, as is currently widely regarded as being the most optimistic threshold target for world leaders to agree at Copenhagen, we will have put the world's reefs on a path to major degradation and ultimate extinction," John Veron, the former chief scientist of the Australian Institute of Marine Science, told the meeting. "Such a catastrophe poses a dire threat to the future wellbeing of all humanity."

The scientists agreed that governments should strive for a level of 320 parts per million of carbon dioxide, saying 360 was a breaking point for reefs to survive. At the current level of 387 parts per million of carbon dioxide, reefs are in serious decline, they said. This will have a future knock-effect that threatens other marine and coastal ecosystems.

Image Source: National Oceanic and Atmospheric Administration (NOAA) and Wikimedia Commons, except giant jellyfish from Hector Garcia blog

Text Sources: Mostly National Geographic articles. Also the New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Natural History magazine, Discover magazine, Times of London, The New Yorker, Time, Newsweek, Reuters, AP, AFP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

Last updated April 2022

This site contains copyrighted material the use of which has not always been authorized by the copyright owner. Such material is made available in an effort to advance understanding of country or topic discussed in the article. This constitutes 'fair use' of any such copyrighted material as provided for in section 107 of the US Copyright Law. In accordance with Title 17 U.S.C. Section 107, the material on this site is distributed without profit. If you wish to use copyrighted material from this site for purposes of your own that go beyond 'fair use', you must obtain permission from the copyright owner. If you are the copyright owner and would like this content removed from, please contact me.