Canary in a Coalmine: Coral Reefs and Carbon Dioxide   

By Devon Ledbetter

   The saying “canary in a coal mine” has been used since the 1900s referring to the small bird’s sensitivity to carbon monoxide as a warning to miners to escape a mining cave before it was too late. Today, coral reefs have become the proverbial “canary in a coal mine” when it comes to human’s impact on our planet; their sensitivity to changing ocean chemistry and warming is an accurate indicator of when we have gone dangerously far in harming delicate ecosystems (1). We have seen a staggering decline in the number of healthy corals populating our reefs and scientists have concluded that this decline is primarily due to anthropogenic, or human induced, effects (2). As our cities become bigger and brighter through industrialization, the environment bears the cost. Our carbon footprint has grown exponentially in the last 100 years (3), and this has been detrimental to the health of our oceans. This rise in carbon emissions has been identified as the root of two global stressors that are partly responsible for this decline in healthy coral reefs: coral bleaching and ocean acidification. Scientists estimate that 50% of coral reefs globally have been damaged beyond recovery, with another 90% facing the threat of mortality (4). It is critical to understand the main factors that are threatening the health of our reefs on a global scale if we hope to find solutions. 

Coral biology is an important component in understanding how anthropogenic stressors affect the health of our reefs. The tissue of a coral contains small, symbiotic, photosynthesizing organisms known as zooxanthellae. These tiny symbionts are a type of algae and play a critical role in keeping corals healthy by generating 50-95% of the coral’s total energy budgets (5). This symbiotic relationship is mutually beneficial for both the coral and the zooxanthellae; the coral receives energy, and the coral polyps provide a home and protection for the microscopic zooxanthellae. However, these zooxanthellae are picky tenants; they only “pay rent” during particular ocean conditions. When ocean conditions are outside these thresholds, the coral colony can become stressed and the zooxanthellae are expelled, leaving themselves and the coral vulnerable (5). It is important to understand how this symbiotic relationship between corals and zooxanthellae works and how human stressors are threatening the health of reefs globally.

     One such global threat is a process known as coral bleaching, and it is directly tied to ocean warming from carbon emissions. As cities industrialize and populations grow, humans are releasing more carbon dioxide into the atmosphere than ever before. Greenhouse gasses, like carbon dioxide, become problematic for global climate when their concentration increases beyond the natural state. As global carbon emissions continue to rise, the atmosphere is absorbing more of the excess greenhouse gasses preventing heat from escaping.

This trapped heat causes global warming on land, and in the sea. In the last 100 years, sea surface temperatures have been climbing steadily- coinciding with the rise of industrialization and the increase of carbon emissions (6). These heightened water temperatures exceed the threshold level that corals and their zooxanthellae can tolerate (7). When the temperatures rise beyond a comfortable limit, the zooxanthellae abandon the coral, leaving it vulnerable to other stressors such as disease or predation (7). 

     Without the zooxanthellae, coral can appear stark white due to the loss of tissue pigmentation that is provided by the symbiont. As the zooxanthellae also provides a significant portion of the coral’s energy budget, their absence means that the coral is extremely likely to die (5). Because of widespread global warming, scientists calculate that roughly 75% of the coral reefs in our oceans have experienced a heat-stress event severe enough to trigger bleaching. While it is possible for a coral to recover after a widespread bleaching event, this optimistic outlook assumes the coral does not face any other stressors, which is rarely the case (8). 

     Rising ocean temperatures and coral beaching aren’t the only threat corals face when it comes to increased carbon emissions. The ocean acts as a carbon sink, absorbing ~ 30% of atmospheric CO2 (2). Increased carbon dioxide emissions have resulted in the ocean absorbing an estimated 525 billion tons of CO2 from the atmosphere since the industrial era (9). When this excess carbon dioxide is absorbed into the ocean, it reacts with seawater to produce a compound known as carbonic acid that in turn increases the acidity of the ocean (10). This process is known as ocean acidification and is another stressor for coral reefs (11). Because corals grow by depositing a calcium carbonate exoskeleton on the reef, this imbalance in ocean chemistry makes it difficult for corals to produce the compounds they need to calcify and reduces the success rate of coral spawning (12). This drop in pH is happening at an unprecedented rate too; seawater has become 30% more acidic in the last 200 years (9). Dr. Sylvia Earle, a famous marine biologist, discusses the dangers of ocean acidification: 

     “Ocean acidification - the excess carbon dioxide in the atmosphere that is turning the oceans increasingly acid - is a slow but accelerating impact with consequences that will greatly overshadow all the oil spills put together…” 

When looking at the health of coral reef systems all over the world, it has become clear that the weight of industrialization has fallen onto their shoulders; coral reefs are the “canary in a coal mine” when it comes to the danger of our carbon dioxide emissions. It is estimated that almost a third of reefs have already been lost worldwide, with another 32% seriously threated by human impacts (13). Unless immediate action is taken, we will lose 75% of our reefs by the year 2050 (14).

     The only way to slow the rate of coral mortality on reef systems globally is to create an immediate call for change. It’s time to take responsibility for our actions and work to intentionally reduce reliance on fossil fuels if we hope to keep coral reefs alive in our oceans for future generations, and not just in the pages of textbooks. It’s time we pay attention to this canary in a coalmine, before these ecosystems and their immense value are lost for good.

Images shot by Devon Ledbetter on a Canon PowerShot G7 X Mark II