At Risk! Coral reefs face uncertain future from rising CO2

Shallow water coral reefs are some of the most biologically diverse habitats the world over. They harbor a myriad of life, from tiny microbes to large sharks, turtles and seals. Almost a quarter of all marine species depend on coral reefs at some point in their lives and approximately 4000 different types of fish call coral reefs home. A number of species reliant upon coral reefs are classified as endangered. Not only are coral reef biologically diverse and vital to ocean health, they are also relied upon economically by more than 100 million people, and are vital to even more people for protection from storm surges, tsunamis and coastal erosion. Coral reefs provide $30 to $172 billion annually to the global economy in resources such as food and recreation.

Over 800 species of reef-building corals are known to exist. Unfortunately, many of these species are threatened by many human caused threats including pollution, coastal development and destructive fishing practices. Some species of coral, such as the elkhorn and staghorn corals, are even classified as endangered due to these threats. However, these aren’t the only threats corals and their dependents face. Rising levels of carbon dioxide in the atmosphere due to the continued burning of fossil fuels and land use changes are putting coral reefs in danger of extinction.

The build up of heat in the atmosphere due to excess greenhouse gases is causing ocean temperatures to increase, which can push corals past their temperature thresholds and cause them to expel their symbiotic algae, which provide food and nourishment to the coral animal. These symbiotic algae also provide corals with their vibrant colors so when they are expelled all that is left is the stark white skeleton, hence this phenomenon is known as coral bleaching. Without their algae corals can starve to death, causing massive die offs of entire reef areas. As climate change worsens in the future, bleaching events are likely to become more common. With more frequent bleaching events, corals will have less time between each event to recover and we could see severe permanent die offs due to increasing ocean temperatures.

If that weren’t bad enough, corals are also being threatened by ocean acidification, or the rising of ocean acidity. Carbon dioxide absorbed by the oceans from the atmosphere combines with water to form an acid making the oceans more acidic. A consequence of this acid being created is the decrease in carbonate ions – one of the building blocks of calcium carbonate (limestone) that corals and other marine calcifiers secrete to make their skeletons and shells. As this important building block decreases, these calcifiers may find it more difficult to create their hard structures. Corals use their skeletons for support and protection and may not be able to survive without them.

Coral reefs are predicted to become functionally extinct if CO2 levels continue to rise: at CO2 levels of 450 ppm in the atmosphere calcareous corals will decline; above 500 ppm coral reefs are likely to be crumbling habitats; at 560 ppm (expected to be reached by the middle of the century) reefs will likely be eroding globally. Part of the reason that coral reefs are such biologically diverse habitats is because of the structural complexity that they provide, but as reefs begin to erode in the future this important complexity could begin to disappear, jeopardizing the many species that we know and love to be associated with reefs. 

But it doesn’t have to be all doom and gloom, the reefs are not lost yet, and they are not past the point of no return. We can turn this ship around by moving away from fossil fuels to clean energy alternatives. By reducing our carbon intake we can save the reefs and all that depend on them from the evil twins of ocean acidification and climate change.

For more information on what you can do and other habitats that are threatened by climate change read the report It’s Getting Hot Out There.