Losing Our Coral Reefs

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

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.

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.

“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

According to the IPCC’s Global Warming of 1.5˚ C report, if ocean temperatures rise 1.5˚ C, coral reefs are projected to decline 70 to 90 percent more; at 2˚C, we would 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 ecosystems? Reefs at Risk Revisited called for the expansion of Marine Protected Areas where fishing and fishing methods are regulated.

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.

Correction: This post was updated on 12/7/19 to correct an error about ocean temperatures reaching 2˚C.