Coral Bleaching, Explained

Staghorn coral is one of the species that bleached in the Florida Keys, during last year's record-breaking marine heatwave. "Bleached Staghorn Coral" by mattk1979 is licensed under CC BY-SA 2.0.

Thanks to global warming, coral reefs across the world are turning white — and thanks to the Circle, here’s your guide to coral bleaching.

When it comes to the natural beauty of the ocean, coral reefs are the perfect poster child. They're colorful, beautiful and flourishing with marine life — a quarter of the world's marine life, to be exact.

But like everything else, they too just cannot escape the heavy hand of the ongoing climate crisis, and no other impact of the crisis is more striking for corals than bleaching. In Australia, the Great Barrier Reef is in the midst of its seventh mass bleaching event. In the U.S., researchers found that live staghorn and elkhorn corals were a rare sight after the effects of last year's marine heatwave when ocean temperatures in the Florida Keys were the hottest ever on record.

How does bleaching happen, why is it significant and what can we do about it?

What is coral bleaching?

Contrary to popular belief, corals are not plants; in fact, they are animals called polyps that form calcium carbonate structures, which we otherwise know as coral reefs. Coral bleaching occurs at the individual level, which can then spread throughout a reef. In terms of the bleaching process itself, you can think of it as the biological version of a breakup. 

Corals benefit from a symbiotic relationship with an algae called zooxanthellae — the algae provide food and energy for the coral via photosynthesis, while also having a place to call home. But when corals become stressed, they expel the algae and turn white, since zooxanthellae is their source of color. Bleached corals are not dead, but they are left more vulnerable to both disease and mortality. 

What causes coral bleaching?

Corals can get stressed out when exposed to air while in shallow tides or storm runoff pollution. But for coral bleaching in particular, global warming remains the leading cause. The ocean holds 90% of the Earth's excess heat produced from greenhouse gas emissions, and it is running a high fever that has only gotten worse. Last year, the ocean was hotter than at any other time in the past six decades, according to the World Meteorological Organization's State of the Global Climate 2023 report.

Moreover, El Niño conditions over the Pacific Ocean make matters worse. As this periodic climate pattern enters its dry phase, increased sunny days and decreased sea level heights will escalate the exposure of corals to warmth and stress, both of which will likely lead to more bleaching. At the same time, ocean acidification, which arises due to high levels of carbon dioxide in saltwater, makes it that much harder for corals to construct their skeletons and rebuild reefs. 

How does coral bleaching affect living things?

Coral bleaching threatens the livelihoods of all creatures big and small that depend on coral reef ecosystems. As the ocean's version of major metropolitan areas, coral reefs are teeming with life, from sponges and lobsters to seahorses and sea turtles.

Up to one billion people worldwide rely on coral reefs for the 4,000 species of fish that serve as a source of food, while tourists flock to see corals annually, bringing in $36 billion per year to the tourism industry. Coral reefs also aid humankind in adapting to the impacts of climate change, since they act as natural seawalls that protect against coastal erosion and sea level rise.

What is the future outlook for coral bleaching?

Coral bleaching is poised to take the world by storm in the near future. According to the 2020 Status of Coral Reefs of the World report, almost all the world's regions have experienced a reduction in average coral cover since 2010. A bleaching event on the global scale may transpire this year, with the majority of coral reefs in the Southern Hemisphere expected to bleach in the next four months. 

That's not to say corals aren't resilient. Some corals can switch out heat-vulnerable algae for kinds better able to resist warming, which allows them to tolerate up to 1.5 degrees Celsius above the threshold for bleaching. But even if global warming levels are kept below 2 degrees Celsius, there is a very high confidence that almost all reefs will degrade — and already, the average global temperature has risen by 1.1 degrees Celsius.

How can we work to mitigate coral bleaching?

Marine Protected Areas, such as those in Hawaii and Florida, can safeguard coral reefs. This is especially the case for those that are 'no take' MPAs, which prohibit taking any natural resources and allow for a higher total coral cover. But climate change knows no boundaries, and bleaching is still expected to occur, whether or not coral is protected through a designated status. 

In response, researchers are stepping in to help through a variety of techniques. For instance, coral tree nurseries replant reefs under the sea, cryobanks on land deep freeze corals to preserve biological diversity for the future and genetic engineers crossbreed species to develop coral that are more resistant to heat. For all the non-scientists out there, the Environmental Protection Agency recommends visiting reefs responsibly and engaging in the typical eco-friendly activities. 

But the best long-term solution still comes down to addressing the core cause: climate change. Once we do that, these rainforests of the sea can grow healthy and strong for years to come.