Australia: On March 8, 2024, government authorities in Australia acknowledged that extensive coral bleaching brought on by heat stress had affected the Great Barrier Reef. Since 2016, the reef has had five major bleaching events.
The Great Barrier Reef Marine Park Authority, which keeps an eye on the health of the coral, discovered bleaching in shallow water regions throughout two-thirds of the reef during aerial assessments of more than 300 reefs.
According to Roger Beeden, head scientist of the Reef Authority, “the results of the aerial survey and the coral bleaching that we are seeing are consistent with the patterns of heat stress that have been building over the reef over the summer months.”
Off the northeastern coast of Queensland, Australia is home to the Great Barrier Reef. It is among the planet’s richest and most biodiverse natural habitats. Covering an area of 346,000 square kilometres (134,000 square miles), the Coral Sea comprises over 900 islands and 2,500 distinct reefs.
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Ocean temperatures that are consistently higher than normal are the cause of bleaching occurrences. Coral polyps receive their nourishment and vivid colours from zooxanthellae, photosynthetic algae, with which they coexist in a symbiotic relationship. The zooxanthellae are driven out of the corals by heat stress, giving the skeletal structures a “bleached” look.
The number of weeks when sea surface temperatures are above the usual monthly maximum temperature by one degree Celsius is a commonly used statistic to evaluate the risk that high water temperatures provide to coral reefs. Significant coral bleaching may happen after four weeks of high temperatures, and after eight weeks, severe, widespread coral bleaching will probably happen. According to the reef administration, as of early March, certain areas of the reef have seen eight to ten hotter-than-normal weeks.
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On March 4, 2024, anomalies in sea surface temperature off the coast of eastern Australia are seen in the above map. It is based on information from NASA’s Jet Propulsion Laboratory (JPL) Multi-scale Ultra-high Resolution Sea Surface Temperature (MUR SST) project, which combines observations from ships and buoys with sea surface temperature measurements from numerous NASA, NOAA, and international satellites. The anomaly is the difference between the sea surface temperature on March 4, 2024, and the average for that day from 2003 to 2014, rather than the temperature in absolute terms.
The reef has already experienced large-scale bleaching in 1998, 2002, 2016, 2017, 2020, and 2022. “Before these years, there is no evidence of such widespread events in the Great Barrier Reef’s 500-year coral record history,” said the Australian Institute of Marine Science.
El Niño, or warm water from the western Pacific pushing into the eastern Pacific, is a result of the Pacific Ocean’s trade winds weakening in the second half of 2023 and early 2024. For the previous ten months, this trend has combined with climate change brought on by humans to raise average sea surface temperatures to record highs. But the connection between sea surface temperatures off the eastern Australian coast and El Niño is a little more nuanced.
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Climate scientist Josh Willis of JPL said that sea surface temperatures act something like a seesaw during El Niño and La Niña episodes. “The western Pacific usually experiences lower ocean temperatures during an El Niño when the eastern Pacific experiences higher temperatures.”
Studies have revealed that increases in sea surface temperatures and coral bleaching cannot entirely explained by patterns in the large-scale ocean circulation or by changes in climate alone. Instead, Australian summer monsoons have traditionally repressed by weather patterns during El Niño occurrences, leading to lower cloud cover and higher-than-average air temperatures. Temperatures in the Coral Sea and the degree and location of coral bleaching in the Great Barrier Reef significantly influenced by these factors.