A 20 percent increase above current carbon dioxide levels, which could occur within the next two decades, could significantly reduce the ability of corals to build their skeletons and some could become functionally extinct within this timeframe.

In real terms, this does not just mean corals grow more slowly, but also that they will be less able to overcome typical pressures. Tropical coral reefs are constantly engaging in a battle to grow. Many reef dwellers actually break apart pieces of the corals’ skeletons, either to feed upon or to create homes.

This process is known as bioerosion. Even the healthiest reefs are constantly trying to grow faster than they are being eroded. In a high carbon dioxide world, not only is coral growth slower, it is also less robust, so the skeletons that are produced are weaker. Consequently, coral reefs in more acidic conditions may not be able to overcome the typical amount of destruction and may start to shrink. Millions of marine species depend on coral reefs to feed, reproduce, shelter larvae and take refuge from predators in their vast three dimensional networks. If coral reefs disappear, it will threaten the survival of many reef dependent species.

For corals to grow in a healthy manner they require appropriate amounts of aragonite saturation, which will decrease globally due to ocean acidification. Prior to the Industrial Revolution, around 98 percent of coral reefs were surrounded by waters with adequate or optimal aragonite saturation states. However, this has rapidly changed with increasing ocean acidification.

At today’s carbon dioxide concentrations, about 60 percent of coral reefs are surrounded by waters that have less than adequate aragonite saturation states, and if carbon dioxide concentrations increase to 450 ppm, more than 90 percent of coral reefs will be surrounded by such waters.

No corals that exist today will be near waters with adequate saturation states if atmospheric carbon dioxide concentrations are allowed to reach 550 ppm.

According to Dr. Ken Caldeira, “There is at least a reasonable expectation that if current carbon dioxide emission trends continue, corals will not survive this century.”

Recent studies on the growth rates of tropical corals have found that if concentrations of carbon dioxide reach 560 ppm (twice pre-industrial levels) coral reefs all across the globe will have stopped growing and will begin to erode.

On a business-as-usual emissions path, this level of atmospheric carbon dioxide can be expected around the middle of this century.

But this problem is not off in the distant future, we are already beginning to see that impacts of ocean acidification may already be occurring for corals. A recent study expressed a 14% decrease in coral growth in the Great Barrier Reef Australia since 1990, the most significant decrease in coral growth in the last 400 years. 

Alongside threatened tropical coral reefs, cold-water corals are some of the most vulnerable species when it comes to anthropogenic carbon dioxide emissions.

Cold-water reef-forming corals have extremely high biodiversity and provide habitat and nursery areas for many deep-sea organisms, including several commercially important fish species.

There is still much that we do not know about these organisms and yet with the current rate of ocean acidification, we may cause their disappearance before we even fully appreciate their true beauty and importance.

Cold-water corals, sponges and their associated ecosystems have been recognized as important sources of new medical treatments for diseases as varied as cancer, arthritis, Alzheimer’s and skin conditions. For example, bamboo corals, a type of sea fan, have been used to synthesize human bone analogs for grafting and may provide a model for artificial synthesis for collagen.

There are six species of cold-water, reef building, stony corals that create calcium carbonate skeletons out of aragonite. As some of the slowest growing corals on earth, acidification poses a real and immediate threat to these species.