Coral reefs are important ocean habitats and offer a compelling case of the risks of climate change. Reefs provide a large fraction of Earth’s biodiversity and have even been called “the rainforests of the seas.” Although coral reefs make up less than 1% of Earth’s surface, scientists estimate that 25% of all marine species live in and around coral reefs, making them some of the most diverse habitats in the world. Reefs also provide both cultural and economic benefits to humans, such as recreational activities, tourism, coastal protection during storms and habitats for commercial fisheries.

Coral reefs are built by and from living animals known as coral polyps. The polyps that make up reefs are mostly a type of coral called stony corals. They create the hard exoskeleton made of calcium carbonate that forms the reefs. These animals live together in a colony that can grow at a rate of 0.3–10 centimeters (about 0.1–4 inches) per year. When a polyp dies, the exoskeleton continues to provide a habitat for other sea life. Corals host a diverse collection of species, including an algae with which they form a type of symbiotic relationship called mutualism. This is a specific type of relationship in which both organisms involved cooperate with and benefit from each other. The algae, called zooxanthellae, live inside the corals, whose tough calcium carbonate exoskeletons provide the algae with shelter. In return for that protection, the algae provide their host with food produced through photosynthesis.

Zooxanthellae also provide corals with their striking colors. These colors are important to many small fish who live in the reefs. These fish use their own bright colors to communicate with other members of their species and attract mates. The colorful coral helps to camouflage the fish from predators. Coral reefs also serve as nurseries for many small fish species because they can easily hide within the coral and feed on algae.

The entire coral reef ecosystem is a balancing act. The reef has a symbiotic relationship with its zooxanthellae, and small fish help control the populations of other algae that might overtake the coral if overgrown. Similarly, small fish find shelter in the reef, but their presence also attracts large predator species, including sharks, that feed on these smaller species. Maintaining this balance helps keep the reef healthy and diverse.

The health and diversity of coral reefs depends strongly on the temperature of the surrounding water. As ocean waters warm due to human-caused climate change, corals expel the zooxanthellae from their tissue, causing them to lose their color and a major source of food. This process is known as “coral bleaching.” Coral bleaching does not always mean the death of a coral reef. Corals can recover their zooxanthellae in time, but the process requires cooler temperatures. 

The Australian Institute of Marine Science has surveyed the world’s largest coral reef—the Great Barrier Reef off the eastern coast of Australia—annually since 1986 and have found a steady decline in coral reef size since they started the survey. These scientists attribute this primarily to increases in ocean temperature associated with climate change. Ocean acidification is another consequence of climate change that can harm reef ecosystems. As carbon dioxide (CO₂) levels rise in the atmosphere from people burning fossil fuels, the ocean absorbs more CO₂—about 30% of the CO₂ emitted into the atmosphere—which alters the ocean’s chemistry and causes it to acidify.

Ocean acidification is making it more difficult for corals to build their hard exoskeletons. When the ocean absorbs CO₂, chemical reactions with the water form an acid that separates into bicarbonate ions and hydrogen ions, making the water more acidic. This lowers the availability of the carbonate ions that the coral need to create their calcium-carbonate skeletons. In 2020, researchers with Woods Hole Oceanographic Institute found that since 1950, ocean acidification has caused a 13% decrease in the skeletal density of one particular kind of coral known as Porites in the Great Barrier Reef. The combination of rising ocean temperatures and increased acidity will likely cause major changes to coral reefs over the next few decades and centuries, meaning climate change is the greatest threat to coral reefs worldwide. New research suggests that corals may begin to dissolve before the end of this century if emissions are not curbed. In 2010, atmospheric carbon dioxide levels were around 390 parts per million, which rose to almost 430 parts per million in 2024. By 2100, scientists expect the ocean to be 150% more acidic than it is today.

Forces outside of climate change also threaten coral reefs’ survival. Pollution, including runoff from land and eutrophication, also harms corals. Toxins, sediments and an overabundance of nutrients can wash off from the land into rivers that ultimately lead to the ocean. Overfishing also presents a threat to coral reefs by disrupting the species diversity and ecosystem balance. In particular, some methods of fishing, such as bottom trawling and blast fishing, pose a danger to coral reefs. Coral reefs are also harmed by people when they step on and break corals, leave trash that pollutes the reef or use sunscreen with chemicals that are toxic to corals.

Fortunately, a number of scientists, volunteers, ocean advocates and others are working diligently to restore coral reefs. Some of these efforts include initiatives to grow and “plant” new reefs or to repair existing reefs. Scientists collect sperm and eggs from the coral, fertilize them, rear the coral larvae in nurseries and then attach them onto reefs. Coral gardening is also an effective tool for restoration. This refers to growing pieces of broken coral in nurseries—much like growing a plant from a clipping—before placing them back in the water. Both methods work because coral can reproduce sexually (with two organisms) and asexually (by itself).

According to a joint report from the United Nations Environment Programme and the International Coral Reef Initiative, coral reef restoration efforts should incorporate climate action and local management to be successful.

One example of this approach is from National Geographic Explorer Nancy Iraba, the co-founder of Aquafarms Organization, a nongovernmental, community-driven organization that restores the coral reef off the coast of Tanzania. The impacts of climate change, pollution, overfishing and tourism have caused coral in this region to decline significantly. Iraba and her team plan to transplant over 10,000 coral onto artificial reefs.

The organization works directly with local fishers, boat captains and tour operators to ensure that ecotourism operations are sustainable and minimize harm to local reefs. They also create opportunities for visitors to participate directly in reef restoration.

“Through the organization which I co-founded, Aquafarms, our main theme is using a community-based approach in all the programs which we conduct, so that is at the heart of every project. How we ensure participation is by engaging the community from the start.” Iraba said. “We have found that [has] tremendous results.”

Critically, Iraba is considering how to make reefs more resilient as global temperatures warm. “We have combined the restoration together with studying climate resilient corals, because we believe in the face of climate change we need to know what species can withstand high temperatures.”

People everywhere can take small actions to protect coral reefs. Individuals can help protect reefs by being more conscientious tourists when visiting reefs, including keeping a safe distance and not touching or trampling coral. It is also best to opt for a reef-safe sunscreen that is free of oxybenzoate and octinoxate, and not leave trash behind. Eating seafood that is caught using methods that are not harmful to reefs can also make a positive difference.

While various efforts to restore coral are needed, the most effective way to save coral is to stop the acceleration of climate change. Ultimately, the survival of coral reefs depends on cooler ocean temperatures and stopping ocean acidification. Lowering global temperatures and the amount of CO₂ absorbed by the ocean by reducing our dependence on fossil fuels will ensure reefs can recover and thrive well into the future.