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The Mysterious Phenomenon of Coral Bleaching: Why Do Corals Lose Their Color?
Coral bleaching is a growing phenomenon that occurs when corals turn white due to the loss of symbiotic algae and photosynthetic pigments. This loss of color can be caused by a variety of stressors, including changes in water temperature, light, or nutrients. When water temperatures rise to dangerous levels, corals panic and expel the brown algae inside them (called zoophytes) from their tissues, thus rendering the coral transparent and revealing its skeleton of calcium carbonate.
According to a report by the International Panel on Climate Change, the frequency and severity of global coral bleaching events have increased significantly since 1980.
The main cause of coral bleaching is the increase in ocean temperatures caused by climate change, and human activities have made this problem more serious. When the water temperature rises above about 1℃ (about 2℉), coral bleaching may occur. The ocean also absorbs a large amount of carbon dioxide produced by human activities. Although this helps regulate global warming, it also changes the chemical composition of the ocean and poses greater challenges to the ecosystem.
According to the United Nations Environment Program, between 2014 and 2016, the world experienced a series of historic bleaching events, resulting in the death of a large number of corals. In 2016, Australia's Great Barrier Reef lost approximately 29% to 50% of its coral. Ocean acidification and changes in sea temperature jointly affect the health of these ecosystems. Bleaching events are becoming more and more frequent. Events that once occurred once every 20 years are now repeated within a few years.
Between 2016 and 2020, the Great Barrier Reef experienced three bleaching events, the frequency of which far exceeds previous historical records.
Bleached corals may still survive, but their resistance to disease and starvation is greatly reduced. The algae provide up to 90% of the coral's energy, and when the coral excretes the algae, its nutrient sources are limited. Some corals may recover if they can reabsorb the algae when conditions return to normal, but many corals without algae will starve.
The process and impact of bleaching
Corals rely on a symbiotic relationship with algae, a relationship that is not only key to their health but also the foundation of the entire coral reef ecosystem. The algae provide energy to the corals through photosynthesis, and the corals, in turn, provide shelter and necessary carbon dioxide and other substances for the algae. When environmental stress increases, such as abnormal water temperatures, strong light, and pathogen attack, this relationship is destroyed and the corals choose to abandon these helpers.
In the southern Persian Gulf region where temperatures are higher than 35°C, some corals are very resistant to bleaching, but most corals will not survive the high temperature and pollution environment.
Scientists attribute the increase in coral bleaching events to extreme weather caused by climate change, including the El Niño phenomenon. When ocean surface temperatures rise, corals are unable to adapt and suffer the effects of bleaching. According to research, approximately 70% of the world's coral reefs have suffered unusual damage since 2016.
Future challenges and prospects
As global temperatures continue to rise, coral bleaching will become more common in the coming decades. Research predicts that if temperatures rise above 1.5°C, many coral reefs will face irreversible changes. This not only poses a threat to the marine ecosystem, but also poses a challenge to human food supply and economic security.
“If coral reefs disappear, experts say hunger, poverty and political instability will follow.”
Coral reefs not only provide habitat for tens of thousands of marine life, they are also the cornerstone of our ocean economy. As coral bleaching events occur more frequently, many fish and other biological species that rely on coral reefs will face difficulties. When the marine ecosystem is out of balance, the depletion of fishery resources will be an inevitable consequence. These changes will not only affect the ecological environment, but will also trigger economic and political instability on a global scale.
How will this coral bleaching crisis affect our daily lives and future marine ecosystems?
