Coral Reefs as ‘Canaries in a Coal Mine’

“It is a curious situation that the sea, from which life first arose, should be threatened by the activities of one form of that life. But the sea, though changed in a sinister way, will continue to exist; the threat is rather to life itself.” – Rachel Carson, The Sea Around Us, 1950.

If a scuba diver plunged off the back of a boat twenty years ago he/she would find themself submerged in the colorful underwater world of a coral reef. The surrounding coral would be of all different shapes, sizes and textures, serving as home for a diverse array of marine life. The world’s coral reefs have thrived for thousands of years, only periodically disrupted by large storms, volcanic eruptions or bouts of coral disease. However, throughout the past two decades, coral animals and their associated ecosystems have faced sudden and rapid decline. In the year 2016 alone, 29% of the coral in the northern Great Barrier Reef died. As a result, what a scuba diver would see today when submerged below the surface is a colorless environment of rubble and algae, the remains of life that once was. 

In 1998, oceanographers all over the globe recognized an unprecedented phenomenon – the coral was turning white. This occurrence marked the first worldwide coral bleaching event. Since 1998, there have been two more global bleaching events resulting in the overall loss of fifty percent of the world’s corals in 2015. Coral reefs now serve as the oceans’ ‘canaries in a coal mine,’ providing an indication of the health of the oceans and warning humanity of what lies ahead. 

What Exactly is Coral?

A coral is the product of thousands of structures called polyps which form to make a single animal. The coral thrives through its symbiotic relationship with a microalgae called zooxanthellae. The microalgae covers the body of the animal and provides a photosynthesis service, feeding the animal. The coral synthesizes calcium carbonate (limestone) from the process for which it uses to grow its skeleton. Coral reefs are colonies of hundreds to thousands of coral animals. Corals are like the buildings in a city, and home to 25% of all marine life. Some coral reefs grow to be so large that they can be seen from space, such as the Great Barrier Reef. The most interesting fact about coral is that they are essentially immortal; coral will continue to live and develop as long as their environment allows them to. And they have done just this, for thousands of years. However, in the past twenty years, unfavorable environmental conditions have forced them to decline drastically. What is causing this sudden, unnatural and rapid decline?

Why Should You Care About Coral Reefs?

The prospect for coral reefs does not look good. Why should you care? Will the eradication of coral reefs really affect human wellbeing? The answer is yes. Coral reefs provide substantial ecosystem services to humanity. With the loss of coral reefs, the ocean loses 25% of marine life which rely on the reefs as their habitat and food source. Marine life outside of that 25% depend on the fish that will be lost, resulting in their decline as well– and the feedback cycle continues. The loss of coral reefs would result in the eradication of entire species and could result in a complete collapse of ocean biodiversity. Fishing provides an important food source and income to millions of people worldwide, playing a substantial role in the global economy. Additionally, coral reefs provide a breakwater to coastal communities. The reef slows the impact of waves, providing a natural barrier to shorelines from ocean storms and flooding. Further, coral provides the vital service of absorbing, solidifying and storing carbon dioxide in its skeleton. Without coral, more carbon dioxide remains in the atmosphere. Coral reefs are greatly undervalued considering the important ecosystem services they provide humanity. 

The Wrong Mindset

Until the late 1950s, it was widely perceived that human alteration of the environment, atmosphere and oceans would not be problematic until the far distant future, if it was to be problematic at all. In terms of the world’s oceans, the seas were viewed as indestructible and inexhaustible no matter what was taken from or put into them. The oceans are incredibly vast– how could humans possibly alter their nature? This perception still plays itself out in conservation policy, where conservation is framed in terms of conserving resources for ‘future purposes’. However, the rapid decline of coral reef ecosystems is evidence against the temporal distance of change; the death of coral illustrates that there are visible and adverse effects of climate change that are affecting people on the planet in the present day. Because this substantial threat to coral is present, coral conservationists need to outline the present importance of coral ecosystems in their policy efforts. There is a crucial link between the well-being of reef ecosystems and continued socio-economic stability and development. We, as humanity, need to change our perception regarding how far in the future the effects of climate change lay. 

The Decline of Coral & Coral Bleaching Events

The world is in the process of losing coral reefs and their associated ecosystems because of human induced changes to the atmosphere, and oceans. Following from  the first worldwide coral bleaching event in 1998 oceanographers and coral specialists found themselves to be unacquainted with this phenomenon, and as a result, began to engage in research to find out what exactly was causing the corals to turn white. Through a series of tests, scientists were able to rule out many factors which could have contributed to the bleaching, such as coral disease or lack of light. What they found was that when they raised the temperature of the water by 2 degrees Celsius or more, the coral would rid themselves of their microalgae and turn white. Hadthis happened before in coral history and gone unnoticed? Or was this the first event that had ever taken place? To answer this, scientists drilled into the coral skeletons. One can look back in time by analyzing coral skeletons, much in the same way one can look back in time by observing growth rings within trees. When the scientists analyzed the coral skeletons they observed regular growth patterns right up until the 1998 event. Thus, it was ruled that 1998 was the first time ever in coral history that a worldwide bleaching event had taken place. Now understanding that coral bleaching is a completely unnatural phenomenon, scientists sought to understand why ocean temperatures were suddenly rising. Could the rise in sea temperature be caused by global warming? Was climate change revealing itself through the coral reefs? 

Ocean Acidification and Rising Seawater Temperatures

After a decade of studies, scientists ruled that the biggest threat to coral reefs is global climate change driven by emitting carbon dioxide into the atmosphere. Carbon and heat are trapped in the atmosphere due to greenhouse gasses and are transferred to and absorbed by the oceans. The oceans take in a remarkable amount of CO2 and heat. Without this service from the oceans  the average surface temperature of the planet would currently sit around 50 degrees Celsius minimum. Yet, the absorbing services provided by the oceans does not occur without consequences. Increased CO2 levels in seawater causes pH levels to decrease. With lower pH levels, the oceans begin to acidify. Ocean acidification plays a considerable role in the decline of coral because the acid erodes the coral skeleton, making them weaker and less resilient to natural and human-induced stress. 

Further, as stated above, coral need stable sea temperatures or else they will bleach from thermal stress. Bleaching is a stress response in coral much like a fever in the human body. When the water temperature spikes, the microalgae’s ability to photosynthesize, and thus feed the animal host, is impaired. Because the zooxanthellae are no longer serving the animal, the coral purges itself of its symbiotic partner. The purging progress sheds the coral exterior, leaving behind the animal’s naked white skeleton which gives off the appearance of a bleached effect. If water temperature stabilizes, microalgae will reproduce and the coral will repair itself. However, if the water temperature remains warmer than baseline average, then the coral remains without its food source and starves to death. 

Human Activity Effects on Coral Reefs

In addition to the threat of global climate change, coral reefs are put under considerable human-induced stress. Over the past few decades there have been increasing amounts of people moving to coastal areas. Rising population in these areas results in accelerated industrialization and coastal development. Coastal development produces adverse effects on coral reefs. Soil/land erosion and runoff containing excess sediment is washed into the coastal waters where coral flourish. This clouds the water and blocks the sunlight needed for photosynthesis. Further stressors from coastal development include fertilizer contamination and sewage dumping which boosts algae growth around coral reefs. Too much algae starves the water of the nutrients needed for coral growth. Additionally, coral reef territories have become popular destinations for the growing tourist industry. The tourist industry contributes to the decline of coral through pollution of the water via things like chemicals from sunscreens or garbage left behind on the beaches. Cruise ship anchors drag along the bottom of the ocean floor, ripping up reefs that have thrived for hundreds of years. And tourists break off pieces of reefs as souvenirs while scuba-diving. On top of all this, corals face the pressure of unsustainable fishing practices. Reef fish are valuable in global markets, the aquarium trade, and as menu items for local luxury seafood restaurants. Because of their value, reef fish are overfished to the point where supply no longer exists. This causes serious problems for coral as these fish play a vital role in the ecosystem by eating the seaweed around the reef. Without these fish, the seaweed overgrows and smothers the coral, stunting growth.  

What is Happening Now?

Coral bleaching now takes place around the world annually and ocean temperatures continue to rise. People living today could very well see the eradication of coral reefs in their lifetime.. The Intergovernmental Panel of Climate Change (IPCC) predicts continued global warming and anticipates that, as a result, sea surface temperatures will rise in the range of 1.1 to 6.4 degrees Celsius over the next 100 years. Further, it is predicted that the global population will continue to climb. Coastal territories near coral reefs are typically at an economic disadvantage and depend heavily on tourism and fish markets for their economy. With growing populations in these areas, there will be increased human-induced stress on the already at-risk coral reef ecosystems. 

“Even if you never had the chance to see or touch the ocean, the ocean touches you with every breath you take, every drop of water you drink, every bite you consume. Everyone, everywhere is inextricably connected and utterly dependent upon the existence of the sea.” – Silvia A. Earle, The World Is Blue, 2009.

Ways to Protect and Save the World’s Coral Reefs

  • Advocate for Marine Protected Areas – Marine protected areas (MPAs) are an important tool for keeping reefs healthy. Large MPAs protect the Great Barrier Reef and the Northwestern Hawaiian Islands, for example, and in June 2021 Australia created the largest marine reserve network in the world. Smaller ones, managed by local communities, have been very successful in developing countries. 
  • Practice safe and responsible diving and snorkeling, avoid touching reefs or anchoring your boat on the reef. 
  • Take a reef-friendly approach to sun protection with non-toxic sunscreens. 
  • Become an informed consumer and learn how your daily choices such as water use, seafood consumption, vacation spots, etc. can impact the health of coral reefs! 
  • Support initiatives such as Coral Gardening by Coral Gardeners Organization. Coral Gardeners was a project born in 2017 in Mo’orea, the sister island of Tahiti, in French Polynesia. The organization’s team members grow and plant resilient corals to restore dying reefs and bring life back to the ocean. You can also ‘adopt’ a super coral through their website to help aid their continued efforts. To date, they have planted 15,755+ corals, reaching over 170 million people. For more information watch ‘We are Coral Gardeners Short Film.’
Coral Gardeners


  • Arias-Gonzales, Jesus Ernesto, et al. “Scaling Up Models of the Dynamics of Coral Reef Ecosystems: An Approach for Science-Based Management of Global Change.” In Coral Reefs: An Ecosystem in Transit, edited by Zvy Dubinsky and Noga Stambler, 373-391. New York: Springer Publishing, 2011. 
  • Carson, Rachel L. Silent Spring. Boston: Cambridge, Mass: Riverside Press, 1962. 
  • Earle, Silvia A. The World is Blue: How Our Fate and the Ocean’s are One. UK: Simon and Schuster, 2009. 
  • Erez, Jonathan, et al. “Coral Calcification Under Ocean Acidification and Global Change.” In Coral Reefs: An Ecosystem in Transit, edited by Zvy Dubinsky and Noga Stambler, 151-177, New York: Springer Publishing, 2011. 
  • Fong, Peggy and Paul, Valerie J. “Coral Reef Algae.” In Coral Reefs: An Ecosystem in Transit, edited by Zvy Dubinsky and Noga Stambler, 241-273, New York: Springer Publishing, 2011. 
  • Hester, R.E. and Harrison R.M. Ecosystem Services: Issues in Environmental Science and Technology. Cambridge: The Royal Society of Chemistry Publishing, 2010. 
  • Hoegh-Guldberg, Ove. “The Impact of Climate Change on Coral Reef Ecosystems.” In Coral Reefs: An Ecosystem in Transit, edited by Zvy Dubinsky and Noga Stambler, 391-405, New York: Springer Publishing, 2011. 
  • Miles, Edward L. On the Increasing Vulnerability of the Ocean to Multiple Stresses. Annual Review of Environment and Resources, 2009. 
  • Montgomery, W. Linn. “Coral Reef Fishes: Opportunities, Challenges and Concerns.” In Coral Reefs: An Ecosystem in Transit, edited by Zvy Dubinsky and Noga Stambler, 327-347, New York: Springer Publishing, 2011. 
  • Norstrōm AV, et al. Guiding Coral Reef Futures in the Anthropocene. The Ecological Society of America, 2016. 
  • Roberts, Callum. The Unnatural History of the Sea. Washington, DC: Island Press, 2007. 
  • Webster, Michael. Naturally Resilient. Smithsonian Ocean Portal, May 2013. 
Hope Elizabeth Tracey
3 Thoughts on Coral Reefs as ‘Canaries in a Coal Mine’
    30 Jul 2022

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