Disasters At Sea: Mass Coral Bleaching Events
- Emily Eliese Cohen
- 7 days ago
- 5 min read
Introduction
As climate change progresses, marine systems are in increasingly hot water – literally. Water has a high heat capacity and as such our oceans have absorbed 90% of the excess heat created by greenhouse gasses in our atmosphere. These warming waters have led to four major global coral bleaching events. The fourth and most devastating started in 2023 and is ongoing. In this article, we’ll dive into the mechanisms behind coral bleaching, the impacts of bleaching events, and what can be done to protect coral reefs going forward.
The Disaster: Coral Bleaching
To understand what coral bleaching is, it is important to know a bit of the biology of corals. Many people may not be aware that corals are actually animals in the phylum Cnidaria (related to jellyfish and anemones). They are made of hundreds of individual coral polyps in which algae reside. This algae, called zooxanthellae, has a symbiotic relationship with the coral, meaning the two organisms benefit from each other. The coral provides the algae with protection and produces the carbon dioxide algae use for photosynthesis. In return, the algae are the primary energy source for the coral. 90% of the nutrients produced by zooxanthellae are transferred to the coral. Additionally, the algae is what gives coral their vibrant colors.
When corals experience stress however, the coral rejects the algae. There are a number of stressors from pollution to changes in water salinity and temperature. Marine heatwaves — periods of warmer water temperatures — are the most common reason for coral bleaching today and increasingly so due to global warming.
When there’s an increase in water temperature, the zooxanthellae become toxic to the coral, and as such are expelled from the polyps. In doing so, corals lose their color, turning white and appearing bleached. Without the algae, corals lose their primary food source. While it is possible for coral to survive a bleaching event, should stressors persist, bleaching often results in the death of the reef. Those corals that do survive the event often grow more slowly, become more susceptible to diseases, and face lowered reproductive capacity.
Global climate change has increased the ocean’s temperature and made marine heatwaves more common. Because of these temperature shifts, reefs around the world are experiencing bleaching events more frequently. In fact, the large-scale mass coral bleaching events we see today are unprecedented historically.
The Impacts
In the most recent mass bleaching event between 2023 and 2025, 84% of the world’s coral reefs were impacted. It was the most devastating bleaching event ever recorded, with the first three bleaching events affecting 21%, 37%, and 68% of reefs respectively.
Corals are considered a keystone species as they support the structure and biodiversity of their ecosystem. Coral reef ecosystems may have the highest biodiversity — that is, the greatest range of animal species living in one place — of any ecosystem on the planet. One third of all known marine life depends on corals. When coral reefs die out from a bleaching event, the entire ecosystem is impacted. Fish populations often decrease dramatically. What once was a thriving underwater city filled with diverse species suddenly becomes a ghost town.
The loss of a coral reef is felt deeply, not only by marine life, but by humans too. Corals provide both direct and indirect benefits to people and society. As storm barriers they can lessen the impacts of large waves, prevent erosion, and protect shorelines. Since climate change will result in more intense storms, preserving coral reefs will be vital to safeguarding coastal landscapes. Seafood industries also rely on reefs to support fishery populations, and reefs bring in money through eco-tourism. Over one billion people around the world benefit from coral reefs, and reefs contribute an estimate of $10 trillion to the economy through food, jobs, and protection.
Unfortunately, some projections warn that we could lose 90% of all coral reefs by 2050.
Recovery
As the weather begins to warm and we yet enter another hot summer, the future of coral reef ecosystems appears grim. One solution is to slow global warming by decreasing greenhouse gas emissions. However, even if we stop emitting tomorrow, the planet will continue to warm for decades as the climate equilibrates and the oceans will continue to absorb the largest portion of that warmth. It is therefore critical that we work on coral restoration in the face of this future.
Coral restoration includes growing and harvesting coral to repair or create reefs. Scientists are also researching how to produce coral that is more resilient to climate change. Even a rise in temperature of just one or two degrees celsius above the summer maximum for just a couple weeks can trigger a bleaching event. Creating coral that is less sensitive to these temperature shifts is critical.
Conclusion
With 2026 already gearing up to be one of the hottest years on record, and with the fourth major bleaching event still ongoing, it’s important that we prioritize marine environmental work through climate policy, research, and finance. Building resilient reefs and preserving our remaining coral ecosystems is vital to the health of our oceans and our planet as a whole.
Sources
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