Coral reefs in the Indian Ocean off an island in the Maldives. Sea level rise threatens both the reef and the island itself. Photo by Joseph Litt

Global

There is no doubt that Earth’s climate is changing: our atmosphere is getting warmer and the amount of carbon dioxide in the atmosphere is increasing. Those changes—individually and, more significantly, combined—are challenging the survival of reefs around the globe.

Warming and rising seas

Many corals have a narrow temperature tolerance. Scientists fear that as the ocean warms, coral reefs will not be able to adapt quickly enough to the resulting changing conditions, and bleaching incidents and diseases will increase.

Coral bleaching occurs when corals become stressed, most often when water gets too warm. Corals will “eject” their algae tenants (zooxanthellae), causing the corals’ tissues to turn white—they are then left without nutrition from the photosynthesizing algae. Although corals can survive a bleaching event, this added stress can lead to mortality.

In addition, when the ocean warms, glaciers melt, causing sea level to rise. A predicted impact from sea level rise is slower coral growth. The Intergovernmental Panel on Climate Change has found that sea level is rising at a rate of .12 inches per year—60 percent faster than the .08 inches per year it predicted in 2007. Unfortunately, the vertical growth rate of coral is likely to be slower than this increase. As a result, corals will be deeper, receive less sunlight, and grow more slowly.

Ocean acidification

In the past few decades, the amount of carbon dioxide (CO2) in the air has increased by one-third. As the ocean absorbs carbon dioxide, a greenhouse gas, from the atmosphere, it becomes more acidic and makes it difficult for polyps to create their skeletons. The calcification rates of corals and other reef organisms has already begun to decrease, leading to fears that reef building will not keep pace with climate change. With increased CO2 in the water, coral may form weaker skeletons, making them more vulnerable to storm damage, careless tourists, and destructive fishing practices.

More storms

Other predicted climate change impacts are an increase in the frequency and intensity of tropical storms. As reefs become less robust or die, their ability to buffer and protect coastlines from severe storms is diminished. More frequent and intense storms may also damage reef structure more significantly, and the corals’ regrowth may not be able to keep pace. 

Ozone depletion

The destruction of the ozone layer, which accompanies global warming, is caused by the presence of chlorofluorocarbons (CFCs) and other chemicals in the atmosphere. When the protective ozone is depleted, the intensity and nature of ultraviolet radiation that reaches the earth’s surface increases. Although corals have a natural sunscreen to protect themselves from the tropical sun, most scientists believe that increased levels of ultraviolet radiation damage coral in shallow areas.