Coral Bleaching: The Death of an Ecosystem
The science behind how climate change has lead to a mass bleaching of coral and the devastation of coral reef ecosystems
What is so important about coral reefs?
Coral reefs are among the most biologically productive ecosystems on Earth. The symbiotic algae that live in corals are responsible for around 50 to 70 percent of the total primary production in most reef systems. This energy-rich environment attracts many lifeforms, with an estimated 25 percent of all marine life dependent on coral reefs at some point in their lives. For this reason, coral reefs are often likened to rainforests. Yet, despite covering only about 5 percent of the area covered by rainforests — amounting to less than 0.1% of the Earth’s surface — they are likely the most diverse marine habitat of them all, with the only possible contender being the deep sea.
The positive impact of coral reefs is not limited to the oceans. Coral reefs form the foundation for thousands of islands. They also support coastal infrastructure and form a protective barrier around the land, preventing loss of life from tsunamis, storms, floods, and erosion. Additionally, their brilliant biodiversity provides tourist hotspots that bring billions to local economies around reef systems.
What exactly are corals?
What we typically think of as coral is actually a colony of genetically identical small animals called corals. Individual corals are generally only a few millimeters to a few centimeters in size. Their bodies are soft polyps with a central mouth and tentacles. Corals with hard, bony structures are called stony corals. The polyps of stony corals excrete calcium carbonate underneath their bodies, which over time builds up to form the hard skeleton that constitutes the main body of a reef. For this reason, they are often called reef-builders.
Most stony corals that live in shallow water have a symbiotic relationship with a type of algae called zooxanthellae. The zooxanthellae reside inside the cells of the polyps. They account for 1 to 10 percent of the total biomass of the coral on average. Zooxanthellae produce carbon compounds through photosynthesis and share them with the coral. These compounds can account for up to 60 percent of nutrients received by the coral through food. Overall, the algae account for 50 to 70 percent of the total primary production of most reefs.
What is coral bleaching?
Coral bleaching is a phenomenon that occurs when the zooxanthellae are expelled from the coral. Since the algae contribute significantly to the color of the coral polyps, they appear translucent after the algae have left. This process leaves only the white calcium carbonate skeleton visible, creating a “bleached” appearance.
A study by researchers Barbara E. Brown and John C. Ogden titled “Coral Bleaching” published in the journal Scientific American states that “although several factors can cause the process [of coral bleaching] — including disease, excess shade, increased ultraviolet radiation, sedimentation, pollution and changes in salinity — the episodes of the past decade have consistently been correlated with abnormally high seawater temperatures” (64).
Temperatures rising one degree Celsius above the average can cause bleaching. Because corals rely heavily on the algae for nutrients, severe or long-lasting bleaching events can lead to death. If the bleaching event is shorter and less severe and the corals have some time to rebuild their algae populations, they may recover.
How does bleaching impact coral reef ecosystems?
A single, severe bleaching event can be devastating. In a ten-month period between 1982 and 1983, the El Niño Southern Oscillation (ENSO) winds brought forth an unusually severe El Niño warming phase, with water temperatures in the eastern Pacific ocean rising 3 to 4 degrees above the seasonal average. This led to massive damage — with 70 to 90 percent of corals in Panama and Costa Rica and 95 percent of corals in the Galapagos having died as a result of bleaching.
This event caused the extinction of one species of fire coral, loss in population sizes of species that inhabited the coral reefs, and a complete disruption of the ecosystem from the bottom up. Benthic algae began to colonize the dead coral, attracting sea urchins who scrape at the skeletons of the reefs when feeding on the algae, thereby contributing to the erosion of the structures.
The exact changes that happen will depend on the specific ecosystem. Still, major disruptions in the food chain can be expected, potentially leading to population decline, local extinction, and an overall loss of biodiversity.
While this was a particularly dramatic warming event, it demonstrated one of the worst scenarios of what could happen to reef ecosystems if warming is not controlled. It also gives us insight as to what is occurring already as coral reefs are being threatened by rising global average ocean temperatures. As mentioned previously, an increase by one degree Celsius over a few months can cause coral bleaching and death.
These “worst scenario” events are becoming more of a reality with each passing year. In 2016, there were bleaching events so severe that the corals were cooked in the ocean. These extremes occurred around the world, from the Great Barrier Reef in Australia to the Caribbean.
What does the future look like for coral reefs?
Oceans have already taken the brunt of global warming. The upper oceans (0 to 700 meters deep) account for approximately 63 percent of the warming between 1971 and 2010.
As a result, there has been a 0.74 degree Celsius rise in average ocean temperatures within that period. Across the tropics, there has been anywhere from a 0.4 to 1 degree Celsius rise in temperatures since the early 1970s.
Bleaching is not the only negative impact of warm waters on coral reefs. A study published in 2007 found that the higher temperatures were affecting the growth rates of reef-building coral. In the Red Sea, the growth rate of a principle reef-building coral, Diploastrea heliopora, had already declined by 30 percent since 1998. For corals, fast growth rates are essential for survival. The mortality rate of corals is highest for those that fall between 1 and 10 centimeters. Large coral colonies have a higher rate of injury, but the lowest rate of whole-colony mortality.
Thus, a slower growth rate will likely mean a higher mortality rate for individual corals, which may slow the overall expansion rate even more. According to climate model simulations and the data from the 2007 study, the researchers predicted that if the warming trend continues, Diploastrea helipora will cease growth altogether in the Red Sea. Given the predicted 80 percent decrease in coral calcification and the increased erosion of corals due to ocean acidification, corals will not be able to keep up and will likely begin to experience a net loss in mass by 2055.
Combined with the observed growth of the frequency of bleaching events in the past few decades, the future of coral reefs looks grim. Under the low emission B1 scenario developed by the Intergovernmental Panel on Climate Change, global temperatures would increase to 1.5 degrees Celsius above today’s average, and in the high emissions model, they could increase by 4 degrees Celsius in the same timeframe. Meanwhile, corals are already being pushed to their limit with the current conditions.
One study projected three scenarios based on the concentration of CO2 in the atmosphere to predict how rising ocean temperatures and ocean acidification will impact coral reefs. The study predicted that if CO2 was stabilized at 380 parts per million (ppm), coral reefs in most areas would remain coral-dominated and carbonate accreting — that is, the coral would still have an annual net growth — while continuing to change. In this scenario, local factors such as pollution would be the primary determinants of reef health. This study was published in 2007, and at that point, the CO2 levels were at 380 ppm. In 2021, we are at 416 ppm. So, we have already failed to meet the criteria for the first scenario.
In the second scenario, which deals with carbon levels between 450–500ppm, reef erosion will exceed calcification, leading to a net loss of corals. The density and diversity of corals would likely decline in a given reef, leading to losses in habitat diversity — meaning a decline in the overall biodiversity of all plants and animals in the ecosystem. The loss of corals would also create space in the reefs for colonization by macroalgae, which both compete with coral for resources and release anti-fouling compounds which deter settlement.
The third scenario models what could happen if CO2 levels surpass 500ppm. It would entail a two-degree increase in average ocean temperatures compared to modern-day and a significant drop in carbonate ion concentrations (which are used by reef-building corals to construct their skeletons). According to the study, reefs would become “rapidly eroding rubble banks” such as those seen in some areas of the Great Barrier Reef in Australia today. Coral-associating fauna would be driven out or go extinct. Chances of recovery would be slim, as the unstable climate would likely impede successful colonization by more tolerant corals.
For reference, CO2 levels increased at a rate of 2.3 ppm per year in the period between 2009 and 2018. From 2018 to 2019, the levels increased by 2.5 ppm. If the rate plateaus at 2.5ppm per year, CO2 levels would reach 450ppm by 2034, the threshold for the second scenario. By 2054, we would reach 500ppm, landing us in the third scenario where coral reefs are irreparable.
To prevent this from happening, we need to take immediate action to mitigate climate change. This involves a transition to clean, renewable energy in all sectors including transportation, a large-scale shift to a largely plant-based consumption, the expansion of protected regions and reforestation of land, and changes in infrastructure and home design.
We have a chance to save one of the most unique and diverse ecosystems on the planet if we act quickly.
