Coral bleaching facts for kids
Coral bleaching occurs when stony corals turn white.
Stony corals are simple animals that form large reefs. The animals live in a endosymbiotic relationship with single-celled algae. To get sunlight the corals live just below sea level. The algae make food products for the coral polyp by photosynthesis.
Coral polyps are sensitive to changes in their environment. This includes the temperature of the water they live in. Under stress, coral polyps may expel the algae which live inside their tissues. The algae provide up to 90% of the coral's energy. Bleached corals continue to live but begin to starve after bleaching. Some corals recover.
Warmer sea water temperatures caused by global warming is the leading cause of coral bleaching.
The United Nations Environment Programme says that the longest recorded global bleaching events happened between 2014 and 2016. Coral was killed on an unprecedented scale. In 2016, bleaching of coral on the Great Barrier Reef killed between 29 and 50 percent of the reef's coral. In 2017, the bleaching got into the central region of the reef. The interval between bleaching events has halved between 1980 and 2016.
Recent research showed that the coral-algae relationship is much older than was thought. This suggests it has survived many climate changes.
Some corals change color instead of turning white when they are bleached. The colors can be very bright. In May 2020, scientists from the University of Southampton said that the colors act like sunscreen and protect the coral cells from light. The scientists said this means some corals can lose their algae for a while and might have developed bright colors as a way to make it easier for the algae to come back and to stay alive until then.
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Triggers
Coral bleaching may be caused by a number of factors. The large scale coral bleaching events of the recent years have been triggered by global warming. Under increased carbon dioxide concentration expected in the 21st century, corals are expected to becoming increasingly rare on reef systems. Coral reefs located in warm, shallow water with low water flow have been more affected than reefs located in areas with higher water flow.
List of triggers
- increased water temperature (marine heatwaves, most commonly due to global warming), or reduced water temperatures
- increased solar irradiance (photosynthetically active radiation and ultraviolet light)
- increased sedimentation (due to silt runoff)
- bacterial infections
- changes in salinity
- herbicides
- extreme low tide and exposure
- cyanide fishing
- elevated sea levels due to global warming (Watson)
- mineral dust from African dust storms caused by drought
- pollutants such as oxybenzone, butylparaben, octyl methoxycinnamate, or enzacamene: four common sunscreen ingredients that are nonbiodegradable and can wash off of skin
- ocean acidification due to elevated levels of CO2 caused by air pollution
- being exposed to oil or other chemical spills
- changes in water chemistry, particularly an imbalance in the ratio of the macronutrients nitrate and phosphate
Trends due to climate change
The warming ocean surface waters can lead to bleaching of corals which can cause serious damage and coral death.
This problem was already identified in 2007 by the Intergovernmental Panel on Climate Change (IPCC) as the greatest threat to the world's reef systems.
The Great Barrier Reef experienced its first major bleaching event in 1998. Since then, bleaching events have increased in frequency, with three events occurring in the years 2016–2020. Bleaching is predicted to occur three times a decade on the Great Barrier Reef if warming is kept to 1.5°C, increasing to every other year with 2°C.
With the increase of coral bleaching events worldwide, National Geographic noted in 2017, "In the past three years, 25 reefs—which comprise three-fourths of the world's reef systems—experienced severe bleaching events in what scientists concluded was the worst-ever sequence of bleachings to date."
Mass bleaching events
Sixty major episodes of coral bleaching have occurred between 1979 and 1990, with the associated coral mortality affecting reefs in every part of the world. In 2016, the longest coral bleaching event was recorded. The longest and most destructive coral bleaching event was because of the El Niño that occurred from 2014 to 2017. During this time, over 70 percent of the coral reefs around the world have become damaged.
According to Clive Wilkinson of Global Coral Reef Monitoring Network of Townsville, Australia, in 1998 the mass bleaching event that occurred in the Indian Ocean region was due to the rising of sea temperatures by 2 °C coupled with the strong El Niño event in 1997–1998.
In April 2024 a 4th global coral bleaching event was confirmed by NOAA The share of affected coral reefs worldwide by each of the four bleaching events has been estimated to be 20%, 35%, 56% and 54%.
Impacts
Coral bleaching events result in the decline of fish diversity. This, in turn, affects coral reef ecosystems.
Images for kids
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Coral and microscopic algae have a symbiotic relationship. When water temperatures get too high, the algae leave the coral tissue and the coral begins to starve. -
Zooxanthellae, the microscopic algae that lives inside coral, gives it colour and provides it with food through photosynthesis
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Healthy coral at left and bleached, but still living, coral to right
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A major coral bleaching event took place on this part of the Great Barrier Reef in Australia.
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This schematic shows how bleaching can trigger the production of sun-screening pigments that are responsible for the bright colours observed during some bleaching events. In case of a mild or short episode of stress, the protective pigments may help the algal symbionts return to the coral after the stress episode has ended, helping the coral recover and survive the bleaching event.
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The lemon damselfish (Pomacentrus moluccensis) is a coral-associated species that has been shown to decline dramatically following coral bleaching.
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See also
In Spanish: Blanqueo del coral para niños
