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Coral Reefs Under Stress

Peter J. Mumby and Rosanna Griffith-Mumby


Coral reefs are more critical to an ocean’s health than you might think, and the effects of climate change and ocean acidification are effectively damaging and destroying most reefs in the world. Coral reefs provide many important ecosystem services, including:

  • providing food, shelter, and meeting places for thousands of animals
  • anchoring sand for recreational beaches
  • supplying building materials for remote peoples

October 2009


A healthy coral reef in the Red Sea, Port Ghalib, Egypt. Photo: J. Hutsch.

Coral reef ecosystems are fragile.

A magical ecosystem

If you have ever dived or snorkelled on a coral reef, you will know the experience is truly magical. It is a world of bewildering color and diversity that instills a sense of profound tranquillity. Yet, these beautiful ecosystems are fragile, and they are now possibly the most threatened ecosystem on Earth. To understand why reefs are in such a threatened state, we need to understand how they function.

A coral reef is a formation that is created by the external skeletons of coral plants in shallow ocean waters.

Corals are the backbones of the reef.
  • Corals are, quite literally, the backbones of the reef system.

  • Corals build limestone skeletons that create the largest living structures on Earth—so large that they are clearly visible from space.

  • The reef itself provides the habitat for almost all other life in the ecosystem—it provides food, shelter, and meeting places for thousands of animals.

  • Corals are an evolutionary success story, and they build the most diverse and structurally complex marine habitats on the planet.

Corals can’t live without zooxanthellae.

Their success is largely attributable to an intimate symbiotic relationship with tiny plant cells that live in the coral’s body (host). These plant cells, known as zooxanthellae,1 harness the sun’s energy like any other plant cells, and these provide an important source of energy to their coral hosts. In return, the zooxanthellae receive a predictable and safe place to live, and a steady supply of nutrients to enable them to grow. The energy acquired from the zooxanthellae helps the coral convert carbonate ions found in the surrounding seawater into solid calcium carbonate—a form of limestone. Corals use the limestone skeleton to support themselves and provide shelter; consequently, they, build layer upon layer of limestone reef.

Coral reefs and climate change

Greenhouse gases are affecting oceans.

The 2007 report of the Intergovernmental Panel on Climate Change is one of many studies that illustrate scientific evidence for the current rise in global average temperatures; it attributes the rise to an increase in concentrations of anthropogenic (from human activity) greenhouse-gases.2 Carbon dioxide (CO2) is one of a number of greenhouse gases that are responsible for global warming. Since the industrial revolution in the 1700s, human activities such as burning fossil fuels, industrial manufacturing, and deforestation have increased the amount of CO2 in the atmosphere by 36%.3

We sometimes overlook the fact that CO2 is not an atmospheric issue only; it can affect water resources and their ecosystems, too (see Figure 1). You do not have to be a scientist to see the effects of climate change on life in the oceans. Recreational divers are alarmed by the dramatic changes in coral reefs. Oceans absorb CO2, and its rapid rise is causing two quite distinct—yet catastrophic—impacts on the world’s coral reefs: coral bleaching, and ocean acidification.


Figure 1

Ocean carbon cycle. (2009). In UNEP/GRID-Arendal Maps and Graphics Library. Retrieved 04:00, October 21, 2009 from Source: IPCC, 2001. Cartographer: Riccardo Pravettoni, UNEP/GRID-Arendal.

Coral bleaching

Higher sea temperatures affect zooxanthellae.

During the 20th century, sea temperatures rose on average of 0.74ºC (1.33°F) and sea levels rose by 17 cm (6.7 in).3 Elevated sea temperatures cause problems for the zooxanthellae when they attempt to photosynthesize inside the coral. Photosynthesis is the plant cell’s way of harnessing the sun’s energy to convert CO2 and water to sugar; however, the plant cells are sensitive to the amount of sunlight they absorb. As noon arrives, there is usually excess light, and the coral has various ways of channelling the extra light energy into non-harmful pathways safely. When sea temperatures are unusually high—sometimes only 1°C (1.8°F) above the normal summer maximum—part of the photosynthetic machinery is damaged, and dangerous oxygen free radicals are formed. As a result, the coral host begins to expel the zooxanthellae. Under severe temperature conditions, the majority of zooxanthellae are expelled, and the coral’s body is left as a colorless, translucent layer sitting upon its limestone skeleton. This often causes the coral to die. Given that the skeleton is a brilliant white without its plants and animals, the overall effect resembles a bleaching of the entire reef; and it is arguably one of the most poignant natural distress calls of any ecosystem.

Mass coral bleaching has occurred recently.

In 1998, unusually warm sea temperatures caused massive coral bleaching throughout the world, and approximately 16% of all reefs4 suffered a significant loss of corals within a few months. While many reefs have experienced good recovery, a troubling number have shown little recovery in the decade following this event, and indeed, some of the corals that died may be as large as a garage or small house—these may take centuries to regenerate. Other mass coral-bleaching events have occurred since 1998—the worst took place in the eastern Caribbean in 2005, which caused extensive damage to reefs in the Virgin Islands.5 With rising global temperatures, coral bleaching events could occur every few years in the future, and the likelihood that corals can recover between such frequent catastrophes is slim.6

Ocean acidification

CO2 forms an acid in oceans.

The oceans have absorbed approximately one-third of the CO2 pumped into the atmosphere by humans.7 In preindustrial times, the oceans had a pH of around 8.2, which is mildly alkaline.8 Unfortunately, when CO2 is absorbed by the ocean’s surface, it forms a mild acid. Since the industrial revolution, the absorption of CO2 into the oceans has reduced its pH by 0.1 pH unit,8 which may not sound impressive. However, this constitutes a 30% increase in the concentration of hydrogen ions, which are the basis of acidity in fluids. Corals build their own skeleton out of calcium carbonate. The problem with an increase in acidity, therefore, is that the extra hydrogen ions react with dissolved carbonate ions in the water and form bicarbonate. As this happens, the availability of free carbonate ions in the water plummets, which makes it harder for corals to access the carbonate they need to build their skeletons.


Pillar coral (Dendrogyra cylindricus) in the Florida Keys National Marine Sanctuary. Photo: Commander William Harrigan, NOAA Corps (ret.), NOAA’s The Coral Kingdom Collection.

The effects of ocean acidification on corals are only just beginning to emerge. As corals struggle to sequester carbonate from the water for conversion to limestone, their skeletons may become increasingly brittle, and the growth rate of the coral may fall quite dramatically (figures of up to 30% have been reported already). Scientists are concerned about these coral growth changes because these changes undermine one of the most important aspects of coral reefs—building reef structures. These structures house millions of plants and animals, and they form a natural breakwater along thousands of coastlines.

Corals lose building abilities under stress.

To put the problem into a human context, consider the landscape of a city like New York City. Like a coral reef, New York City is a complex structure with hundreds of skyscrapers—capable of housing a huge number and diversity of people. Now, consider if there had been a shortage of concrete during the construction of this city, and that the builders used inferior products during the city’s construction. Presume these products failed to protect the buildings from wear and tear; consequently, the buildings deteriorated quickly. Under this scenario, New York City would have probably resembled the more modest structures of Newark, NJ, and it would have housed far fewer people. This mirrors the fate of coral reefs because climate change and ocean acidification depletes the coral’s building materials. Corals will suffer frequent bleaching events as well as short-lived periods of growth between bleaching events; and they will become less and less constructive—quite literally.

How losing healthy reefs affects us

Unless we take action to reduce the effects of climate change and CO2 emissions, scientists expect a bleak future for the world’s coral reefs.8 If we continue to allow reefs to deteriorate, the damage will affect people throughout the world—often in surprising ways, including:

  • eroding beaches;
  • losing favorite recreational activities;
  • decreasing fish choices for aquarium hobbyists;
  • depleting fisheries that harvest coral-reef fish; and
  • losing building materials in some parts of the world.
Corals preserve our beaches.

For many people living thousands of miles from a coral reef, the allure of reefs is often associated with luxury vacations that include palm-fringed beaches, exotic fare, and warm, turquoise seas. Indeed, relaxing on the soft white sand of the Caribbean is one of life’s great pleasures. The sand we take for granted is made of limestone, however, and much of it has coral origins. Perhaps more importantly, coral reefs provide a natural breakwater that prevents sand from being washed away by storms. Losing the reef means that the rate of sand erosion will increase dramatically, and this could change the sand in the idyllic beach in the holiday brochures to barren, hard pavement. The common approach to dealing with this problem is to dredge more sand from behind the reef and to deposit it back onto the beach. Unfortunately, this process can create so much sand in the water that it smothers the remaining corals, and actually, this accelerates the cause of the problem—dead corals.

Millions of people visit coral reefs to fish, scuba dive, or snorkel. Imagine your disappointment if you were to encounter a brown seascape of seaweed and jellyfish—instead of a vibrant cityscape of riotous color and sizzling fish action. It has already happened to many coral reefs, and this will continue to worsen—unless climate change is controlled.

Fish lose habitat when corals die.

Some of the first casualties of a dead coral reef are the brilliantly hued fish that either feed on the coral or take shelter in its branches. Now, many of these fish support a multibillion-dollar aquarium trade that supports thousands of anglers in some of the world’s poorest countries. Clearly, a loss of reef health not only threatens many aquarium hobbyists, but certainly, the effects of climate change and ocean acidification on coral reefs are potentially devastating in many parts of the tropical world. Tens of millions of people depend on coral reefs for their main source of proteins, building materials, and their livelihoods.

If temperatures continue to rise, we will have climate refugees.

Under a changing climate, poverty is likely to worsen, and the coastal zone will become a more difficult place to live with limited coastal protection from storms, faster beach erosion, and dwindling incomes. In time, this may lead to a mass migration of people away from coastal areas, and even among continents—particularly where entire nations currently live at, or near, sea level. People who move because of environmental disruptions are sometimes referred to as environmental refugees, and researchers have proposed adding the term climate refugees to encompass human migration caused by a changing climate.

What we can do

Reducing emissions will alleviate damage.

We still have time to take the steps necessary to reduce greenhouse-gas emissions and to reduce the costly consequences to the environment. Most people are aware of the practical steps they can take to protect the environment—examples include avoiding the use of a car to get to work, switching to more energy efficient products, and so on. Much can be done to avert the problems facing coral reefs directly, as well. It is more important than ever that we try to reduce the local human-induced stresses imposed on reefs.


A male Bicolor Parrotfish (Cetoscarus bicolor) is at home on Indo-Pacific coral reefs, North Horn, Osprey Reef, Australia. Photo: Richard Ling.

To achieve this we can:

  • avoid catching too many fish;

  • use less fertilizer on farms in coastal regions;

  • stop clearing foliage from hillsides—a practice that causes massive amounts of soil to flood the reef each time it rains;

  • be conservation minded when vacationing in areas that offer coral reef recreation;

  • cease dining on coral reef fish that are considered at risk, such as most grouper species,9 or species that perform essential tasks on the reef, such as parrotfish, which are considered a delicacy in countries of the Pacific and the Caribbean10 (parrotfish are herbivores, and they play an important role in keeping the levels of seaweed under control, which helps corals grow and recover after bleaching events).

It makes economic sense to protect coral reefs.

Overall, the plight of reefs will improve if we see value in sustaining them. One practical step that visitors can take is to support local forms of tourism so that a larger proportion of the coastal community benefits from tourism; therefore, the community has an economic incentive to maintain an inviting, healthy environment for all. To continue to enjoy both the economic and the aesthetic values of these beautiful and fragile environments, we must work together to prevent its demise—directly and indirectly.

Peter J. Mumby, Ph.D, is a professor and researcher at the Marine Spatial Ecology Lab, School of BioSciences Hatherly Laboratory, at the University of Exeter, Exeter, U.K. He is also an adjunct associate professor for the Rosenstiel School of Marine and Atmospheric Sciences, at the University of Miami, Miami, FL. Mumby is the chair of the World Bank/Global Environmental Facility Targeted Research Group on Coral Reef Remote Sensing. He has published journal articles on coral population biology, remote sensing of coral reefs, seagrass beds, mangroves, coastal management, and parrotfish ecology. Rosanna Griffith-Mumby, MSc Environment and Developmental Education, University of South Bank, London, focuses on environmental education with specialisation in tropical ecosystems and issues. She is project administrator and educator, Future of Reefs in a Changing Environment (FORCE), an EU-funded project at the University of Exeter, UK.

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Reef Videos

View video footage of a variety of reef-related phenomena during research trips taken by the author of this article—it is free to use and edit for educational purposes.

Coral reef facts, pictures, and more

Find information using the menu choices, from the Nature Conservancy.

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Reefs at Risk

Read/download the report, or view a slideshow that evaluate human pressure on coral reefs worldwide. Click on the interactive map and other links in the menu to learn more.

Zooxanthellae… What’s That?

For more about zooxanthellae, view the National Oceanic and Atmospheric Administration’s (NOAA) page dedicated to these amazing plant cells.

Tips for Tourists

Coral friendly guidelines for tourists! (snorkelling) (diving)

Underwater Cleanup Tips

Taking your students on a beach trip? Organizing a community beautification group? Diving with friends in the ocean? Consider cleaning up what you see under the water.

For Reef Managers

Reefbase Pacific has collated reference manuals, guidebooks and tools into the Reef Manager ToolBox to assist managers from the Pacific region in the effective monitoring and management of reef resources.

Reef Ed

A site devoted to education about coral reefs, focusing mainly on the Great Barrier Reef—it includes videos and podcasts.

Coral Reef Outreach and Education

Resources for students interested in learning more about reefs as well as lesson plans and other useful information for educators.

  1. Any of various yellow-green algae that live symbiotically within the cells of other organisms, such as those of certain radiolarians and marine invertebrates. Zooxanthellae definition. (accessed September 12, 2009).
  2. Intergovernmental Panel on Climate Change (IPCC). 2007. Climate Change 2007: Synthesis Report, Summary for Policy Makers, IPCC’s Fourth Assessment Report (AR4). p. 5. (accessed September 12, 2009).
  3. Wikipedia. 2009. Global warming definition. 2009. (accessed September 12, 2009).
  4. Kallman, M. 2008. Climate Change Putting Coral Reefs at Risk. World Resource Institute. (accessed September 12, 2009).
  5. National Oceanic and Atmospheric Administration (NOAA). 2005. 2005 Caribbean Basin Bleaching Event. (accessed September 12, 2009).
  6. Silverman, J., B. Lazar, L. Cao, K. Caldeira, and J. Erez. 2009. Coral reefs may start dissolving when atmospheric CO2 doubles. Geophys. Res. Lett. 36: L05606, doi:10.1029/2008GL036282. (accessed September 12, 2009).
  7. The Ocean Acidification Network. 2007. How is Ocean Acidity Changing? (accessed September 12, 2009).
  8. Zeebe, R. E., Zachos, J. C., Caldeira, K., and Tyrrell, T. 2008. Oceans. Carbon emissions and acidification. Science 321(5885):51–52.
  9. Commiskey, B. 2006. The Habitats, Behaviors, and Importance of Groupers (Epinephelus/Mycteroperca) in Coral Reef Ecosystems (accessed September 12, 2009).
  10. Science Daily. Nov. 5, 2007. Parrotfish Critical To Coral Reefs: Permanent Damage Likely Unless Urgent Action Taken, Scientists Warn (accessed September 12, 2009).

General Reference

Hoegh-Guldberg, O., Mumby. P. J, Hooten, A. J., Steneck, R.S., Greenfield, P., Gomez, E., Harvell, C. D., Sale, P.F., Edwards, A. J., Caldeira, K., Knowlton, N., Eakin, C. M., Iglesias-Prieto, R., Muthiga, N., Bradbury, R.H., Dubi, A., and Hatziolos, M. E. 2007. Coral reefs under rapid climate change and ocean acidification. Science 318: 1737–1742


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