Losing Our Coral Reefs

by |June 13, 2011

Editor’s Note (01/26/2018): This post was updated with the latest statistics on coral reefs and climate change impacts.

New Guinea coral reef. Photo credit: Mbz1 at en.wikipedia

Coral reefs, the “rainforests of the sea,” are some of the most biodiverse and productive ecosystems on earth. They occupy less than one percent of the ocean floor, yet are home to more than a quarter of all marine species: crustaceans, reptiles, seaweeds, bacteria, fungi, and over 4000 species of fish make their home in coral reefs. With a global economic value of $375 billion a year, coral reefs provide food and resources for more than 500 million people in over 100 countries and territories. But tragically, coral reefs are in crisis.

Coral reefs are endangered by a variety of factors, including: natural phenomena such as hurricanes, El Niño, and diseases; local threats such as overfishing, destructive fishing techniques, coastal development, pollution, and careless tourism; and the global effects of climate change—warming seas and increasing levels of CO2 in the water. According to Reefs at Risk Revisited, a report by the World Resources Institute, 75 percent of the world’s coral reefs are at risk from local and global stresses. About a quarter of them have already been damaged beyond repair. If we continue with business as usual, 90 percent of coral reefs will be in danger by 2030, and nearly all of them by 2050.

What is coral?


Coral reefs are colonies of individual animals called polyps, which are related to sea anemones. The polyps, which have tentacles to feed on plankton at night, play host to zooxanthellae, symbiotic algae that live within their tissues and give the coral its color. The coral provides CO2 and waste products that the algae need for photosynthesis. In turn, the algae nourish the coral with oxygen and the organic products of photosynthesis. The coral uses these compounds to synthesize calcium carbonate (limestone) with which it constructs its skeleton—the coral reef.

The symbiotic relationship between corals and zooxanthellae can only exist within the narrow band of environmental conditions found in tropical and subtropical waters. The water must be clear and shallow so that the light algae need for photosynthesis can penetrate, and water temperatures must ideally remain between 23˚ and 29˚ C (77˚ to 84˚ F).

The number of coral species in each reef varies: the Great Barrier Reef off Australia has over 600 species of coral while a Caribbean reef has about 65. Today many reefs have 40 to 50 percent less coral than they did just 30 years ago.

Threats to coral reefs

Of local threats to coral reefs, overfishing and damaging fishing techniques such as deep water trawling and the use of explosives and cyanide, are the most destructive. When herbivorous fish that eat seaweed are overfished, uncontrolled seaweed growth can smother coral.

Coastal development results in erosion, and runoff containing the excess sediment can block the light zooxanthellae need. Nutrient-rich fertilizer runoff and sewage effluent can boost algae growth, which starves the water of oxygen, causing eutrophication. Pollution from land, including hot water releases from power plants, pathogens, and trash, and from marine activities, such as fuel leaks and oil spills, also endangers coral reefs. Tourism, while relying on the appeal of coral reefs, can be damaging when careless divers trample on corals or break off pieces as souvenirs. In addition, more and more coral and tropical fish are being harvested for the aquarium trade.

A recent study of 159 reefs in the Pacific found that plastic pollution is killing coral, too. When coral reefs come into contact with plastic waste, the incidence of disease rises 20-fold. The scientists do not know exactly how the plastic causes disease, but they speculate that bacteria on the plastic can infect the coral and plastic can block the needed sunlight. By 2025, they project that 15.7 billion plastic pieces could come into contact with coral reefs.

Natural phenomena that stress coral reefs include predators such as parrotfish, barnacles, crabs and crown-of-thorns starfish, and diseases. Hurricanes or prolonged cold and rainy weather can harm coral reefs. The El Niño weather pattern, which can result in lower sea level, altered salinity due to too much rainfall, and elevated sea-surface temperatures, can also damage coral (oceans absorb 93 percent of climate change heat). When corals overheat, they react to the stress by expelling their algae, which results in coral bleaching.

Bleached staghorn coral. Photo credit: Matt Kieffer

Bleaching leaves corals vulnerable to disease, stunts their growth, affects their reproduction, and can impact other species that depend on the coral communities. Severe bleaching kills them.

The average temperature of tropical oceans has increased by 0.1˚ C over the past century. This, combined with natural fluctuations of warmer ocean temperatures, has resulted in extensive coral bleaching around the globe, involving thousands of square miles of reefs. During the 1997-1998 El Niño, widespread and severe coral reef bleaching occurred in the Indo-Pacific region and the Caribbean, killing 16 percent of the world’s coral reefs in 12 months.

The 2010 El Niño also resulted in massive bleaching around the world. Another coral bleaching event in October 2015 extended into 2017, becoming the longest and most damaging ever recorded. More than 80 percent of the northern part of the Great Barrier Reef has now suffered severe bleaching.

Bleaching at the Great Barrier Reef 2016. Photo: Oregon State University

“Coral bleaching is caused by global warming, full stop,” said Terry Hughes, lead author of a new study on coral bleaching. The researchers found that bleaching events have increased from one every 25 to 30 years in the early 1980s to an average of one every six years since 2010. While coral reefs can recover from bleaching if given 10 to 15 years for their algae communities to recover, the increasing frequency of bleaching events means that many reefs may never be able to.

In addition, the 22 million tons of carbon dioxide our oceans absorb every day are changing the chemistry of seawater and increasing acidification. Today, coral reefs are experiencing more acidity than they have at any time in the last 400,000 years. Acidification reduces the water’s carrying capacity for calcium carbonate that corals need to build their skeletons. Even a small decrease in the coral’s ability to construct its skeleton can leave it vulnerable to erosion. Some reefs have already begun to dissolve and it’s estimated that by 2050, only 15 percent of coral reefs will have enough calcium carbonate for adequate growth.

One study showed that ocean acidification profoundly alters coral reef ecosystems. As C02 levels rise and acidification increases, the biodiversity of coral reefs drops, resulting in the elimination of key species needed for healthy reef formation. “The decline of the structurally complex corals means the reef will be much simpler and there will be less habitat for the hundreds of thousands of species we associate with today’s coral reefs,” said Katherina Fabricius, a scientist at the Australian Institute of Marine Science.

All of these factors act in concert on coral reefs, and complex interactions between the threats leave coral reefs even more vulnerable. Climate change will also bring sea level rise that may result in drowned coral reefs, and more intense storms that produce excessive nutrient or sediment runoff. The overfishing of herbivorous fish and excess nutrients decrease coral’s resilience in the face of increased CO2. Rising ocean acidity lowers the threshold at which corals bleach.

An uncertain future

If we reach 450 parts per million of C02 in the atmosphere (as of January 20, 2018 we were at 408.12ppm), ocean temperatures will rise 2˚ C, calcium carbonate levels in the oceans will decrease, and we will largely destroy all our coral reefs.

Coral reefs provide us with food, construction materials (limestone) and new medicines—more than half of new cancer drug research is focused on marine organisms. Reefs offer shoreline protection and maintain water quality. And they are a draw for tourists, sometimes providing up to 80 percent of a country’s total income. Losing the coral reefs would have profound social and economic impacts on many countries, especially small island nations like Haiti, Fiji, Indonesia, and the Philippines that depend on coral reefs for their livelihoods.

What can be done to save these precious and beautiful ecosystemsReefs at Risk Revisited called for the expansion of Marine Protected Areas where fishing and fishing methods are regulated.

Great Barrier Reef. Photo: Richard Ling

The Great Barrier Reef is the largest and richest coral reef in the world because it has been protected since the early 1970s. After the creation of a marine sanctuary for Apo Island in the Philippines in 1982, the fish population tripled. Reefs at Risk Revisited also recommended curbing unsustainable fishing, managing coastal development better, and reducing both land and marine-based pollution. It also stressed the importance of comprehensive ecosystem management that includes all stakeholders, and the need to educate the public about the importance of coral reefs and investing in scientific research.

Some scientists are studying types of coral that can adapt to warmer ocean temperatures and survive bleaching, and using that information to “train” corals to adjust to warmer acidic water. Their goal is to eventually transplant these more resilient corals into the reefs.

The Coral Restoration Foundation protects and restores coral reefs through creating coral nurseries and transplanting corals into reef restoration sites. Concerned individuals can become citizen scientists and monitor corals at restoration sites, or volunteer to monitor marine sanctuaries, protect marine wildlife or clear ocean debris.  Everyone can help coral reefs by practicing sustainable fishing, and eating only sustainably caught fish. When vacationing near coral reefs, be careful not to touch them and don’t buy souvenirs of coral or other marine species.

And it is crucial, of course, for national and international bodies, and for all of us to address the threats of climate change by curbing carbon emissions.

Students in Columbia’s Earth Institute Center for Environmental Sustainability (EICES) who want to learn more about coral reefs can take EICES’ spring break Coral Reef Ecology course in Bermuda in March 2018. It deals with the biology of corals, reef biodiversity, factors that impact coral reefs, and coral reef conservation and preservation.

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25 thoughts on “Losing Our Coral Reefs

  1. Fly Shop says:

    On a recent fly fishing trip to Mexico I got a VERY good look at what is happening to it. The thing people don’t realize is that when you’re a fly fisherman and you basically live around the water, the rivers and the ocean, that you feel a lot more in touch with the entire flow of life. Ecology is a big part of being an environmental steward. People should realize that if they want their grandkids fishing the same waters and seeing the same beautiful reefs they need to take better care of it. From the fish to the coral.

  2. Kelly says:

    As an avid diver in the Miami area, I also have seen first hand, the slow demise of these beautiful seascapes due to tourists. To make matters worse, there are currently plans to expand ports in the area which will destroy more reefs. I know CRF is doing their best to put a stop to the blasting that will ensue. Chances are slim, but I am keeping fingers crossed.

  3. Some of the corals are losing because I’ve heard in news that there are so many people that take corals, How sad! We should care our corals in the sea. I love mother earth.

  4. I only wish that someday soon the greater public will awake and see that is important to save our planet. The reefs are a beautiful and important aspect to our lives, why on earth would we knowingly harm them?

  5. Adrien says:

    I have seen first hand the same thing in the Great Barrier Reef in Australia.
    The difference between the bleached coral and the live is unbelievable.
    I just hope the coral is still around when my kids are old enough to dive and enjoy the magic under our oceans!

  6. Wilford James says:

    I am sorry to say that at some point, while growing-up, I innocently contributed to the destruction of coral reefs. We have known and have heard for decades how the corals reefs are being destroyed, but the attempt to keep this from happening has been minimal. There are natural phenomenon affecting the growth and preservation of the coral reefs that we can’t control, but on the other hand there are things we can do to help preserve these precious entities. We have to educate the world about the importance of the coral reef and how we can all contribute to keeping them alive for other generation to appreciate. My destruction of the coral reef near my house was out of pure ignorance, no one had talk to me about the importance of the reefs or how to protect them. The same thing I did to destroy the coral reefs are being done today out of pure ignorance. We have to education the world about the preciousness of the coral reef and show them how to protect these living things that some of us sees as rock. The preservation of these coral reefs has to start with education, we already know that they are being destroyed, now we have to concentrate on keeping that from happening.

  7. what will happen in 100 years if this continues?

  8. Ernest Dean Harris says:

    Climate change, global warming from man made carbon our reefs is dying.
    False False False. Coral reefs around the world adjacent to the equator are in warmer waters than Australia’s Great Barrier Reef. These are much healthier in the warmer water. Coral bleaching. Without this our coral reefs would surely die with catastrophic results. Ever wonder why we get such large ocean swells, Cyclones, Hurricanes and extreme weather around the world adjacent to the Equator. Reason is extreme weather conditions in these areas cause huge rough seas and any bleached or dead coral gets pounded into coral sand which lies around most coral reefs by the millions of tons. If water gets acid or alkaline coral sand slightly dissolves and balances PH for coral reef health. Coral sand is used around the world in marine aquariums to stabilise PH.

    The human race is a unique species, everything we touch, do or make creates carbon and in many way’s this is what we have been created for to HELP the planet. Next time you drive through the forest look how many trees are leaning towards what man has made. ROADS for sunlight, moisture and Carbon Monoxide.

    Consider this if humans cut back on carbon emissions and the effect was Global Cooling then YES our coral reefs would die and the effects could be catastrophic.

    Course that will not happen as Man Made Carbon is not the main driver of Climate Change.

    After reading my submission on the truth you will probably call me a “Climate Sceptic” I just call the believers of the flawed religion of Climate Change from Man Made Carbon….. “Climate Synoptics” because just like a flawed weather chart they get it wrong so many times.

  9. Emmalee Brown says:

    We need to save the coral reefs because they are home to millions of water creatures and I am only 12 but I love all animals Lets get our lifes moveing and save the coral reefs and the animals that live within them!!!

  10. Richard Henkle says:

    Ernest I don`t think you are climate change skeptic. I just think you have bought into a policy and a way of speaking which is designed to protect oil, gas, and coal industries. When we adopt unscientific positions it is usually to go with the flow of entrenched power, which is supported by the entrenched industries maintaining that power, even the portions of media designed to support them both. It is a surprisingly USA phenomenon. A free speech right used to protect the main industry and powers they get into voting offices. One sees the worst of this now with how corporations were given unlimited spending rights after the “citizens united” case and now essentiall control our politics. By an unlimited media budget and persons willing to buy into any lie to protect these industries, they just keep getting their way over all of us. The insurance industry sure isn´t buying into the Hoax crowd. Why not?

  11. Susan George says:

    As being nature lover coral relief is one of the beautiful things that are present in the nature and they should be saved. More than that they are home for some of the sea animals, People who are living near coastal areas should be educated how important they are and they should be protected.

  12. nature dude says:

    Ernest Dean is wrong the reefs are dying so to stop it people have get off snap chat or Facebook and move their a** and try to tell people that they are dying

    P.S. I’m thirteen

  13. nature lady says:

    I agree with nature dude Ernest Dean Harris, honestly OPEN YOUR EYES. Tell me. Was the world as unhealthy as it is now before we started polluting it? Tell me, are the glaciers NOT melting? Are sea levels not rising? Also we are making everything worse not just from pollution but all of the trees we are cutting down too. WE. ARE. DESTROYING. OUR. PLANET. And eventually it will catch up with us. We will be over populated, will have run out of all essential resources, and we and our planet will be doomed to extinction because of all of our dumb a** decisions.

    And P.S. I’m 14

  14. m says:

    help the coral reefs!

  15. pam says:

    No more coral mining!!!

  16. m says:

    U R hurting the coral, I Luv coral reefs!

  17. Nature Kid says:

    Stop mining this beautiful coral!!!!!!!!!!!!!!!!!

  18. Nature Kid says:

    Please I beg you! Its so beautiful and has a economic value of $375 billion dollars a year!!!!!!!!!!

  19. Gifts for Men says:

    Coral relief are life line to oceans, if they will vanish the food cycle will be disturbed. It would be really sad if it vanishes.

  20. nature lady says:

    More than sad, a lot of animals could become extinct and if they do it will effect us as well.

  21. Am Ang Zhang says:

    Severe 2010 Cold-Water Event Caused Unprecedented Mortality to Corals of the Florida Reef Tract and Reversed Previous Survivorship Patterns> quoted from> http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0023047

    Why are reefs near Equator suffering less, much less than 18 deg N or S of Equator?

  22. Ernest you’re wrong. I’m an High School student. I’m only fifteen, but I’ve been researching Coral reefs for months. Coral Bleaching is one of the main causes for Coral Reefs dying. “Without this (Coral Bleaching) our coral reefs would surely die with catastrophic results.” No. They are DYING because of this. Coral bleach them selves when they become stressed and exile their zooxanthellae and lose color, and become white and skeletal. Coral can become stressed from, Natural causes: Hurricanes, El Niño, and diseases.
    Local threats: Overfishing, destructive fishing, coastal development, pollution, reckless tourism.
    Global effects: Rising water temperatures and growing levels of CO2 in the water.
    Humans are 90% of the cause to Coral reefs being endangered. They are dying. All over the world. When a coral bleaches itself, it has limited time to go back to a state where they can find new zooxanthellae and regain color, but because of how horrible this planet is, almost 60% of them never do, which causes them to die.

  23. a heart broken girl says:

    Holy cow!

  24. nature lady says:

    Am Ang Zhang are you saying global warming isn’t causing more of our coral reefs to die because some died because it was too cold? Because if so you should know that humans aren’t just causing “global warming” but they are causing climate change. There is a huge difference and you can’t just say oh global warming isn’t a thing or global warming isn’t doing anything because it’s cold because you are wrong. It is a part of climate change and climate change is causing things like extreme weather changes (in Iowa the weather changed from above 60 to low 30s over night) and natural disasters like all of the hurricane people are hearing about, tornadoes, droughts, and even flooding as well. And think about it. If climate change is causing things like hurricanes and extreme temperature changes and these things have been proven to be a HUGE part of killing our coral reefs then how can we say humans and climate change have nothing to do with what is happening to this beautiful thing.

  25. dave says:

    im glad to see people with reef tanks saving the reefs .its like takeing a animal from a shelter careing for it . ALL THE DRILLING OIL AND CHEMICALS ALL IS DESTROYING THE CORAL REEFS

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