Coral reefs are facing extinction

Coral bleaching

This post is part two of a four-part series. Here I will look at how climate change is jeopardizing the existence of coral reefs across the world’s oceans. In the third part of the series, I will explore coral reefs’ overexploitation, and in the fourth, I will explore some unknown causes of coral reef mortality.)
(Here is the first post of the series, which covers the general dismal situation of coral reefs and their lack of official protection from trade, plus the incredible importance of these species for the survival of marine life and, in turn, human life.)

How climate change is threatening coral reefs

Ocean acidification

Carbon dioxide (CO2), as we know, plays a huge role in warming up our planet and causing climate change. What you might not know is that one-third of the planet’s carbon dioxide is absorbed by the ocean, and the more CO2 the ocean absorbs, the greater the waters’ acidity. The effects of ocean acidification, as this phenomenon is called, remain partly unpredictable.
Scientists do know, however, that worsening ocean acidification harms coral reefs, crustaceans, and shellfish because it weakens these species’ calcium carbonate shells. These animals thus become unable to build thick, protective shells for themselves and turn into easier prey for predators and pollution.
Coral reefs located near the poles will suffer these effects more sharply than those in warmer waters for two reasons: first, colder water absorbs more CO2 than warm water, and second, coral reefs in cold water grow at a slower pace than other coral reefs.

Bleached corals

Coral bleaching and disease

Warming waters also cause the bleaching of coral reefs. Naturally occurring, beneficial bacteria living on coral turns begins to disappear as temperatures rise, facilitating the onslaught of pathogenic bacteria that cause bleaching and other types of disease.
Moreover, the type of pathogenic bacteria that take over coral reefs sticks even if the temperature drops low enough to provoke the return of beneficial bacteria. By this time, the coral is too sick to recover and dies.
For those interested, here is an article on a recently created mathematical model that explains how coral reefs die due to warming waters.
Here is the U.S.’s National Oceanic and Atmospheric Administration (NOAA) explanation:

“Because many corals live in water which is already near their upper temperature limit, a water temperature increase of only a few degrees can be deadly. As water temperatures rise, corals become increasingly stressed. When stress levels get too high, corals expel the symbiotic algae, or zooxanthellae (tiny one-celled plants) which live within the thin layer of live coral tissue. Zooxanthellae are important because they turn sunlight into food for their coral hosts. They also facilitate the formation of the coral skeleton — the main structural component of coral reefs. Because zooxanthellae give corals their various rich colors, a coral without zooxanthellae appears bleached. Corals can not thrive without zooxanthellae. For coral reefs that are already stressed due to poor water quality, destructive fishing, or frequent interactions with irresponsible divers and snorkellers, increased water temperatures could become the proverbial straw that breaks the camel’s back.”