A panoramic image of fluorescing corals in New Caledonia; photo by The Ocean Agency / XL Catlin Seaview Survey.

Global Threats

Climate Change

The facts are clear: our world is getting warmer and our oceans are growing more acidic. Ninety-seven percent of publishing climate scientists agree that global climate change is real and a result of human activity. By reducing the human use of fossil fuels, we can curb carbon dioxide emissions (CO2), slow the pace of climate change and give coral reefs the critical time they need to adapt.

Coral Bleaching: Most corals have a narrow temperature tolerance. Coral bleaching occurs when corals become stressed, most often when ocean water gets too warm. Corals will “eject” the symbiotic algae (called zooxanthellae) that live inside them. When corals lose their algae, they not only lose their color (turning white) but also their built-in food source. Scientists have declared three global coral bleaching events: 1998, 2010 and 2014-2017. This most recent event was the longest and most widespread bleaching event ever recorded, killing as much as two-thirds of the corals in the northern part of Australia’s Great Barrier Reef. Although corals can survive a bleaching event, they will eventually die if they are under repeated stress. Outbreaks of coral disease typically follow bleaching events since stressed corals are more susceptible to infection.

Sea level rise: As the planet gets warmer, glaciers melt, causing sea level to rise. As a result, corals are predicted to end up deeper underwater, receive less sunlight and grow more slowly. The Intergovernmental Panel on Climate Change (IPCC) has found that sea level is rising at a rate of 0.12 inches per year—60 percent faster than the 0.08 inches per year that were predicted in 2007.

Stronger storms: Another predicted climate change impact is an increase in the frequency and intensity of tropical storms. Hurricanes, cyclones and typhoons gain their speed and strength from warm ocean temperatures. These storms cause larger and more powerful waves than normal and can break coral branches and overturn coral colonies. Compounding the problem is the fact that heavy rainfall from a storm can cause an increase in runoff from land-based sources, leading to increases in nutrients and sediments.

Ocean Acidification

Since the start of the Industrial Revolution about 150 years, ago, the amount of carbon dioxide (CO2) in the air has increased by one-third. About 25 percent of all of the CO2 emitted is absorbed by the oceans, another 25 percent is absorbed by plants and trees, and the remaining 50 percent stays in the atmosphere. As the oceans absorb CO2, their chemistry changes and they become more acidic. This makes it difficult for corals and other marine organisms to grow their skeletons and shells. The calcification rates of corals and other reef organisms have already begun to decrease. With increased CO2 in the water, coral may form weaker skeletons, making them more vulnerable to disease and destruction by storms.

Ozone depletion

The depletion of the ozone layer is primarily the result of ozone-destroying chemicals called chlorofluorocarbons (CFCs) being released into the atmosphere. Heat-trapping gases like carbon dioxide (CO2) exacerbate the situation by creating atmospheric conditions that lead to ozone loss. When the protective ozone is depleted, the intensity and nature of ultraviolet (UV) radiation that reaches the earth’s surface increases. Although corals have a natural sunscreen to protect themselves from UV radiation, at increased levels, this radiation can damage corals in shallow waters.