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  • Rachel Larson
    • Apr 14, 2023
    • 4 min read

Coral Reefs: The Impacts of Coral Bleaching


Photo by LI FEI on Unsplash


By as soon as 2040, scientists estimate that 70-90% of the world’s coral reefs may disappear as a result of rising temperatures in ocean waters, ocean acidification and pollution (Forbes, 2020). While known for their beauty, coral reefs play an integral role in marine ecosystems as well as economies. The below piece will provide an overview of how anthropogenic climate change causes coral bleaching and highlight why losing coral reefs is harmful for our planet and societies.


What is coral bleaching?

Coral bleaching is a phenomenon that occurs from prolonged increases in seawater temperatures.


The shallow water coral reefs are typically vibrant with colors due to microscopic, photosynthetic algae called zooxanthellae. The algae plays an integral role in the coral’s survival, providing it with essential resources for survival such as food and oxygen. When the ocean water temperature rises, the coral undergoes extreme stress and expels the algae causing the coral to lose its color and appear bleached (WWF, 2019).


If the warmer water temperature is sustained for a period of time, the coral will be unable to intake and reintroduce the algae to its system and will eventually die-off. This is the genesis of coral bleaching.


Why are ocean waters warming?

The warming of ocean waters is known to be a direct, man-made result of the effects of greenhouse gas (GHG) emissions and subsequent climate change. GHG traps heat in the Earth’s atmosphere and since 1955, the ocean has absorbed over 90% of this extra heat (EPA, 2022). Additionally, the increase of temperatures in the sea’s surface has been higher over the past three decades than in any other period since observations began in the late 1800s. The UN Environment Programme has warned that if the ocean continues to warm, we could see all of the world’s coral reefs bleached by the end of this century.


The importance of coral reefs

Coral reefs are essential to healthy ecosystems, coastlines and economies. To examine just how integral these systems are, let’s consider the following statistics.


Around 25% of fish in the ocean depend on sustained coral reefs for shelter, food and reproduction (NOAA, 2019). Without corals serving as a stable habitat, reef-dependent marine life could see a significant decrease in population, disrupting the broader ecosystem.


Coral reefs protect coastlines from storms, currents and erosion, as healthy reefs can “absorb up to 97 percent of a wave’s energy” (NOAA Coast, 2023). The function of reefs is similar to low-crested breakwaters. Through dissipating a wave’s energy, coral reefs have proven time after time that they are capable of reducing erosion and coastal flooding. Without them, coastal communities will be left vulnerable.


Local economies are heavily supported by coral reefs. Using the Great Barrier Reef as an example, in 2021, tourism in the reef contributed $6.4bn to Australia’s economy and provided 64,000 jobs (Australian Government, 2023). Losing coral reefs would take a drastic toll on the economy of all ocean-facing civilizations.


Case: The Great Barrier Reef

Known as the world’s largest reef system, Australia’s Great Barrier Reef is among the most biodiverse ecosystems. 2022 marked the sixth occasion of a mass bleaching event in the Great Barrier Reef since 1998. Over 90% of the reef was impacted, exhibiting some degree of coral bleaching. Water temperatures in certain areas of the reef were recorded up to 4 degrees Celsius above the March average. Prior to this, in 2016, the Reef lost 50% of its shallow water corals due to a mass bleaching event. (Barrier Reef, 2022).


While this happened to be the fourth mass bleaching event in the Reef since 2016, scientists were especially concerned that it occurred during a La Nina year - a weather event that typically results in cooler conditions and more cloud cover. The 2022 incident was the first mass bleaching to ever occur during a La Nina weather event (AIMS, 2023). This only increases worry for what could happen during an El Nino weather event, where there is less cloud cover and warmer temperatures.


Mitigating the Impacts

Scientists have long studied the impacts of coral bleaching and researched methods to reduce the harmful effects. The most effective way to stop coral bleaching is to drastically reduce climate change - the root cause - and rapidly reduce global GHG emissions. A significant reduction in global emissions would allow for limiting the rise of ocean temperatures to hopefully manageable levels. While this would be by far the most effective method, it feels less feasible based on the direction we are heading. We should also focus our energy and efforts on acute and actionable pinpoints that can be managed at a common level across societies.


Researchers have looked at ways to increase tolerance and resilience of coral reefs. Reducing local stressors can help build up the tolerance of corals to bleaching. NOAA has found that “removing chronic local stressors caused by intensive tourism use, water pollution, or over-fishing can increase coral reef health and lipid levels” (Managing for Mass Coral Bleaching, p.123). Coral reefs that have higher energy reserves have a greater chance at surviving coral bleaching events.


More specifically, coral reefs are often physically injured by reckless boat anchoring, snorkeling, and diving. Stressed corals are less likely to recover if injured physically. Removing stressors will increase the ability of coral reefs to survive. While there is much more that needs to be done, limiting the damage to coral reefs caused by recreation and intensive tourism can promote their survival in the case of a mass bleaching event.


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