Human history and the sea are inextricably tied. Whether as a source of food and oxygen or a means of generating capital, the ocean has nurtured humankind, provided inspiration, and become the cradle for discovery and tragedy. We interact with the hydrosphere in such a vast number of ways that keeping track of the effects of human activity on aquatic life is a science of its own. Unfortunately, this means that oceanic ecosystems aren’t exempt from the ramifications of climate change.

As I have discussed in a previous article, the public’s understanding of climate change hinges on how the wealth of data behind it is presented to us. Perhaps the most iconic example of how climate change and biodiversity loss manifest in our oceans is coral bleaching, a process that occurs when the conditions in coral reefs become adverse. This has resulted in the death of many coral colonies and the degradation of their adjacent ecosystems. But it’s also an excellent example of how climate change and human activity can negatively influence a single species or ecosystem in a host of different ways. Being an intricate biochemical process, coral bleaching isn’t immediately accessible to the average reader, so as always, let’s take things from the top.

What is coral bleaching?

Corals are living organisms that survive by constantly interacting with their surroundings. Changes in the conditions within the ecosystem - be they in temperature, light, or access to nutrients - often prove stressful to the corals, which react by expelling the symbiotic algae in their tissues and turning completely white. This is because these algae, known as zooxanthellae, are responsible for the beautiful colors found in many corals. 

However, that’s not all zooxanthellae do. The algae are in a mutualistic relationship with their host coral: the former photosynthesize, providing the latter with key ingredients for its metabolic processes. In return, the coral offers zooxanthellae  protected environment and the necessary compounds for photosynthesis. This cooperation is behind the powerhouse dynamics and rapid growth of coral reefs.

When a coral loses its zooxanthellae, this symbiotic cycle comes to a close. It’s a common misconception that corals die during bleaching. While that’s not always the case - corals are known to rejuvenate in time if conditions are favorable - bleaching leaves corals incredibly vulnerable and unable to perform crucial metabolic processes. Granted, this increases their mortality rate, especially if the affected colony is left unchecked. According to the US Ocean Service, half of the country’s coral reefs in the Carribean were eliminated in 2005 after a massive bleaching event that was caused by increased water temperatures. Florida Keys, another area of interest, suffered a coral bleaching event in 2010, this time in response to a drop in water temperature. 

How does climate change affect coral bleaching?

It is now recognized by the international scientific community that climate change poses the greatest threat to coral reefs worldwide. Although that can seem like an oversimplification of what is otherwise an intricate set of problems, the immediacy of that threat becomes apparent when one realizes how deep the influence of climate change runs on the planet. There is hardly an aspect of aquatic life left untouched by the changing climate, and corals, being extremely delicate organisms, bear the brunt of that change. 

For instance, we saw that a leading factor in coral bleaching is an increase in water temperature. Scientific evidence shows that the Earth’s temperature profile is moving to higher temperatures; both the atmosphere and the ocean are becoming hotter, upsetting the balance of marine ecosystems and placing coral reefs under increasing thermal stress. Not only do high temperatures leave corals vulnerable to threats, they are also conducive to disease outbreaks that can devastate entire colonies. 

At the same time, our planet’s landscape and weather are gradually changing as well. Sea-level rise increases the amount of sedimentation in the ocean, while increased precipitation carries more runoff and debris into the sea. In either case, aquatic ecosystems become suffused with particulate matter. This can smother corals and lend itself to the development of algal blooms, nutrient-happy clusters of microorganisms that block out the sunlight and prevent zooxanthellae from photosynthesizing.

More importantly, however, coral reefs suffer from one of the most pressing issues in environmental science: excess CO2. Albeit traditionally associated with air pollution, carbon dioxide is extremely detrimental to the quality of oceanic ecosystems. One of our planet’s saving graces is the fact that the Earth’s oceans are massive CO2 sinks: they absorb excess carbon dioxide from the atmosphere, effectively slowing global warming - but this process disturbs the ocean’s pH balance in a phenomenon called acidification. 

How does ocean acidification affect coral colonies? Corals generate their magnificent geometry through calcification, a process which essentially consists of a series of chemical reactions that sequester carbon dioxide into calcium carbonate, the material corals are made of. The more acidic seawater becomes, the lower its pH balance drops, until it reaches a critical level where calcification is no longer viable. In layman’s terms, this means that although corals have a built-in mechanism for absorbing carbon dioxide, high concentrations compromise their ability to do so. Sadly, acidification also prevents corals from growing, leaving the reef brittle and structurally unsound. 

These are but a few key ways in which climate change drives coral bleaching. Corals are well-loved, and a widely known example of how the Earth’s biodiversity is in distress. However, there are a myriad other vulnerable species exhibiting rapid population decline, many of which have been proved to be under jeopardy due to the current ecological crisis. There is no one-size-fits-all solution for the protection of endangered species; however, the intimate relationship within ecosystems can shed light on new ways of providing security, mitigating, and, hopefully, salvaging the rich natural heritage that was passed down onto us.