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 rain forests 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.”

Coral Reefs
Coral reefs are home to a wide array of aquatic organisms, but are at risk of bleaching due to climate change.
acid
Noun

chemical compound that reacts with a base to form a salt. Acids can corrode some natural materials. Acids have pH levels lower than 7.

adapt
Verb

to adjust to new surroundings or a new situation.

algae
Plural Noun

(singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.

alter
Verb

to change.

Noun

all the different kinds of living organisms within a given area.

calcification
Noun

process by which calcium or calcium salts build up in organic tissue.

calcium carbonate
Noun

chemical compound (CaCO3) found in most shells and many rocks.

carbon dioxide
Noun

greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.

century
Noun

100 years.

Noun

gradual changes in all the interconnected weather elements on our planet.

coastline
Noun

outer boundary of a shore.

compelling
Adjective

forceful or persuasive.

complex
Adjective

complicated.

Noun

management of a natural resource to prevent exploitation, destruction, or neglect.

coral bleaching
Noun

loss of symbiotic algae (zooxanthellae) in corals, leading to a loss of pigmentation.

coral reef
Noun

rocky ocean features made up of millions of coral skeletons.

decade
Noun

10 years.

deplete
Verb

to use up.

developing world
Noun

nations with low per-capita income, little infrastructure, and a small middle class.

dissolve
Verb

to break up or disintegrate.

diverse
Adjective

varied or having many different types.

economic
Adjective

having to do with money.

Noun

community and interactions of living and nonliving things in an area.

emission
Noun

discharge or release.

estimate
Verb

to guess based on knowledge of the situation or object.

exoskeleton
Noun

the hard external shell or covering of some animals.

expel
Verb

to eject or force out.

expensive
Adjective

very costly.

fertilizer
Noun

nutrient-rich chemical substance (natural or manmade) applied to soil to encourage plant growth.

fishery
Noun

industry or occupation of harvesting fish, either in the wild or through aquaculture.

Noun

material, usually of plant or animal origin, that living organisms use to obtain nutrients.

fraction
Noun

number expressed as a ration, such as a/b.

Great Barrier Reef
Noun

large coral reef off the northeast coast of Australia.

gross domestic product (GDP)
Noun

value of the goods and services produced by a single country during a single year.

Noun

environment where an organism lives throughout the year or for shorter periods of time.

invertebrate
Noun

animal without a spine.

ion
Noun

electrically charged atom or group of atoms, formed by the atom having gained or lost an electron.

marine
Adjective

having to do with the ocean.

marine protected area (MPA)
Noun

area of the ocean where a government has placed limits on human activity.

medicine
Noun

substance used for treating illness or disease.

Noun

large body of salt water that covers most of the Earth.

ocean acidification
Noun

decrease in the ocean's pH levels, caused primarily by increased carbon dioxide. Ocean acidification threatens corals and shellfish.

pH
Noun

measure of a substance's acid or basic composition. Distilled water is neutral, a 7 on the pH scale. Acids are below 7, and bases are above.

Noun

process by which plants turn water, sunlight, and carbon dioxide into water, oxygen, and simple sugars.

physical environment
Noun

exterior features of a specific place or region.

Noun

introduction of harmful materials into the environment.

Noun

area of tall, mostly evergreen trees and a high amount of rainfall.

recreational
Adjective

having to do with activities done for enjoyment.

regulation
Noun

rule or law.

resilient
Adjective

able to recover.

seafood
Noun

fish and shellfish consumed by humans.

seawater
Noun

salty water from an ocean or sea.

site
Noun

specific place where something is located.

storm
Noun

severe weather indicating a disturbed state of the atmosphere resulting from uplifted air.

subsistence fishing
Noun

harvesting seafood to meet the nutritional needs of an individual or family.

sustainable
Adjective

able to be continued at the same rate for a long period of time.

symbiosis
Noun

two or more distinct organisms living together for the benefit of one or both.

symbiotic
Adjective

associating with another organism, not always to the mutual benefit of either species.

tourism
Noun

the industry (including food, hotels, and entertainment) of traveling for pleasure.

unprecedented
Adjective

never before known or experienced.

vulnerable
Adjective

capable of being hurt.