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—they have been called “the rainforests of the seas.” Scientists estimate that 25 percent of all marine species live in and around coral reefs, making them one of the most diverse habitats in the world.

Paulo Maurin, education and fellowship coordinator for NOAA’s Coral Reef Conservation Program, says the reefs are invaluable to our planet’s biodiversity.

“They act as productive nurseries to many fish species, giving the small fish a home and a chance to grow,” he says. “Coral reefs’ diversity is so rich that we do not have a firm count on all the species that live within it and every year discover new species.”

Reefs provide a variety of economic benefits, including recreational activities, tourism, coastal protection, habitat for commercial fisheries, and preservation of marine ecosystems.

“Corals are important to us for many reasons,” Maurin says. “From a practical point of view, they can help protect coastlines from storm events, for instance, and help maintain fisheries that are essential to a lot of people. And complex compounds found in coral reefs hold promises in modern medicine. These are what we call ecosystem services that would be very difficult and expensive to replace.

“They also have a unique ability to inspire us to explore and visit the ocean. Can you think of any other invertebrate that people would come from afar just to see?”

Corals live with algae in a type of relationship called symbiosis. This means the organisms cooperate with each other. The algae, called zooxanthellae, live inside the corals, which provide a tough outer shell made from calcium carbonate. In return for that protection, the algae provide their host with food produced through photosynthesis. Zooxanthellae also provide corals with their striking colors.

This symbiotic relationship is strongly dependent on the temperature of the surrounding water. As the water warms, zooxanthellae are expelled from a coral’s tissue, causing it to lose its 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.

Warmer ocean water also becomes more acidic. Ocean acidification is making it more difficult for corals to build their hard exoskeletons. In Australia’s Great Barrier Reef, coral calcification has declined 14.2 percent since 1990—a large, rapid decline that hasn’t been seen for 400 years.

Ocean acidification also occurs because of rising carbon dioxide (CO₂) levels. The ocean absorbs carbon dioxide released into the atmosphere. Carbon dioxide alters the chemistry of seawater by reducing pH, a measure of acidity. Water that has a lower pH is more acidic.

“When the pH of seawater is lowered as a result of CO₂, the availability of carbonate ions—one of the main building blocks in their calcium-carbonate skeletons—is reduced, and corals have a tougher time building up, or even maintaining, their skeleton,” Maurin says.

The combination of rising ocean temperatures and increased acidity will likely cause major changes to coral reefs over the next few decades and centuries. New research suggests that corals may begin to dissolve at atmospheric CO₂ concentrations as low as 560 parts per million, which could be reached by the middle of this century if emissions are not curbed. In 2010, atmospheric carbon dioxide levels were around 390 parts per million.

Maurin believes there are several ways people can help preserve these valuable resources.

“Over the long term, we need to reduce the amount of CO₂ that is up in the atmosphere that is causing both increased bleaching and acidification,” he says. “But in the more immediate time, there are other ways to help. By understanding that bleaching and acidification stress corals, we can help by building up what we call ‘reef resiliency.’ That is, making sure that reefs have this capacity to bounce back.

“For instance, ensuring that there is less pollution entering the ocean can help far-away corals. Also, people can help by making sure that the seafood consumed is sustainable and not contributing to a depletion of fish species that keep algae in check, following fishing regulations when fishing, as well as supporting marine protected areas in key conservation sites.”