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​Coral Reefs and Climate Change: What to Know and What to Do

By Huntley Penniman

Coral reefs have existed on Earth for 500 million years, providing shelter and food to many generations of ocean life. Although they cover less than 1% of the ocean floor, coral reefs support an estimated 25% of all marine species. Not only do they play an essential role in supporting healthy oceans, they also come in a candy store variety of spectacular shapes, sizes, and colors, from enormous patterned mounds to small, delicate lacy forms.

Abundant hard corals in Palau. © Keoki Stender

Today, coral reefs face a multitude of human-caused threats that include pollution, overfishing and destructive fishing, mismanaged tourism, and the impacts of climate change. In fact, scientists estimate that we’ve already lost as much as half of all coral reefs since 1980, and some fear that we could lose the rest in just 30 years. Fortunately, there are actions we can take to reduce our impacts on corals and the marine life that depends on them, and some corals—super corals—have shown resilience to anthropogenic stressors. Now is the time to become part of the movement to protect these vital ecosystems.

What are coral reefs?

Coral reefs are created when many corals grow together in the same area. There are two main types of corals, hard corals, and soft corals. Stony corals, or hard corals, are reef-building corals. They have hard calcium carbonate skeletons that provide the structure that helps hold coral polyp colonies together. Soft corals on the other hand are soft and bendable, and have a wood-like core and fleshy exterior. They often resemble trees or plants. Though coral reefs are characterized by the presence of many coral growing together, they are home to a very wide diversity of life forms beyond corals that include visually-similar species like anemones, sea fans, sponges, and sea squirts, as well as fish, crustaceans, mollusks, and many, many other species.

Most coral reefs are found in shallow, tropical or semi-tropical habitats where they enjoy warm waters and ample sunlight year-round. In fact, reef-building corals cannot tolerate waters below 64°F (18°C). However, coral reefs are not only found in these areas—there are even coral reefs in the deep sea!

Colorful soft corals in Palau. © Keoki Stender

Are corals animals or plants or rocks?

Although they may look like plants or rocks, corals are colonial animals, with most made up of thousands of polyps. Like sea anemones and jellies, they are cnidarians and have specialized cells that can sting.

Most corals rely on zooxanthellae for survival. Zooxanthellae are algae that have symbiotic relationships with corals and other cnidarians, like sea anemones. They live within the tissues of corals, and come in an array of dazzling colors that give coral reefs their spectacular rainbow hues. Corals provide shelter and nutrients that zooxanthellae need for photosynthesis, and, in turn, zooxanthellae provide the corals with oxygen and carbohydrates that are produced through photosynthesis.

© Coral Reef Image Bank

What is coral bleaching?

Coral bleaching is a process by which corals and anemones lose their zooxanthellae, and with it their color. Bleaching is a stress response that can be triggered due to temperature changes (too hot or too cold), too much or too little light, changes in salinity, or other stressors. Today, the biggest cause of coral bleaching is rising ocean temperatures. When ocean temperatures increase, zooxanthellae can be damaged or expelled. Bleaching is not always fatal, but it can lead to disease and death, because corals and anemones rely on the zooxanthellae to survive.

Why are coral reefs important?

Beyond their intrinsic value and the roles they play in supporting vast numbers of marine species, coral reefs also provide a plethora of functions that benefit humans. They help prevent coastal erosion, offer protection from storm surge, and support jobs ranging from fishing to tourism. It’s estimated that the economic value of coral reefs is at least $30 billion per year, but most likely much higher, and more than 500 million people worldwide rely on coral reefs for food, income, coastal protection, and more.

© Grant Thomas / Coral Reef Image Bank

How does climate change impact coral reefs?

Coral reefs are feeling the heat from climate change. As ocean temperatures rise, the warm waters can cause coral to stress and expel their zooxanthellae (a process known as bleaching, as explained above). Without zooxanthellae, corals are susceptible to illness and death. Recent research has found that the frequency of large-scale coral bleaching events (“mass bleaching”) has increased dramatically in the last 40 years, to the point that corals often have insufficient time to recover between bleaching events. Thankfully, some coral reefs (including those in Raja Ampat, Indonesia) have shown resilience to coral bleaching, but it is unclear what impacts future warming will bring.

In addition to warming waters, excessive amounts of carbon dioxide in the atmosphere can harm coral reefs through a process called ocean acidification. Oceans can absorb about 30% of atmospheric carbon dioxide, but as the amount of carbon dioxide in the atmosphere increases, it can actually lower the pH level of seawater, causing it to become more acidic. Corals, in particular those with calcium carbonate skeletons, require near-perfect conditions to grow and flourish. If the ocean water is too acidic, corals may struggle to build their skeletons and the base structure for many coral reefs.

You can help safeguard coral reefs, no matter where you live

Regardless of where you live, your choices impact coral reefs. The way we get around, how we use energy, our food choices, and many other aspects of our lives directly contribute to greenhouse gas emissions that are warming our planet. Millions of people worldwide are already taking action to fight climate change, and you can join them through a variety of actions ranging from simple lifestyle changes, to how you vote, where you shop, and more. To find out how to join the growing movement of people taking action to fight climate change, visit our Blue Habits guide to reducing your carbon footprint. Your actions truly make a difference, and together we can help coral reefs persist for generations to come.


Huntley Penniman is an Oceanic Society social media specialist and communications strategist. From diving to conservation and environmental communications, her passion lies in learning more about the wildlife that lives under the surface of the ocean. Huntley holds a B.S. in Biology from Boston College and a Master’s in Marine Biodiversity and Conservation from Scripps Institution of Oceanography.


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