Climate Change and El Niño Contribute to Fifth Bleaching Event in Great Barrier ReefGreat Barrier Reef Experiences Record-Breaking Coral Bleaching in 2024. Credit: scitechdaily.com

In March 8, 2024, the Australian government confirmed that the Great Barrier Reef has been affected by widespread coral bleaching due to heat stress. This alarming news marks the fifth time the reef has experienced this phenomenon since 2016.

A recent aerial survey conducted by the Great Barrier Reef Marine Park Authority revealed that two-thirds of the reef's shallow water areas have been affected by bleaching. According to Roger Beeden, chief scientist at the reef authority, this is consistent with the increasing heat stress over the summer months.

Located off the northeast coast of Queensland, Australia, the Great Barrier Reef is a vast natural ecosystem that is known for its rich biodiversity. Covering an area of 346,000 square kilometers (134,000 square miles), it is made up of 2,500 individual reefs and over 900 islands.

Coral bleaching occurs when the ocean temperatures rise continuously. Corals have a symbiotic relationship with photosynthetic algae called zooxanthellae, which provide them with nutrients and their vibrant colors. When the water becomes too warm, the corals expel the zooxanthellae, leaving the skeleton with a bleached appearance.

Experts use a measure to assess the risk of coral reefs due to high ocean temperatures, which is tracking the number of weeks that sea surface temperatures have exceeded the monthly maximum temperature by 1 degree Celsius. Significant bleaching may occur after 4 weeks of elevated temperatures, and severe and widespread bleaching is likely after 8 weeks. As of March, some parts of the Great Barrier Reef have been exposed to 8-10 weeks of above-average temperatures, according to the reef authority.

The map below illustrates the sea surface temperature anomalies along the coast of eastern Australia on March 4, 2024. It is based on data from the MUR SST project by NASA's Jet Propulsion Laboratory, which combines measurements from various satellites, ships, and buoys. The anomaly reflects the difference between the sea surface temperature on March 4, 2024, and the average for that day between 2003 and 2014.

Previous coral bleaching events occurred in 1998, 2002, 2016, 2017, 2020, and 2022. According to the Australian Institute of Marine Science, there is no evidence of such widespread bleaching in the Great Barrier Reef's 500-year coral record history before these years.

The latter half of 2023 and the beginning of 2024 saw relaxed trade winds in the Pacific Ocean, allowing warm water from the western Pacific to flow into the eastern Pacific, known as El Niño. This, combined with the effects of climate change, has resulted in record-high sea surface temperatures for the past 10 months. However, the relationship between El Niño and sea surface temperatures in eastern Australia is complex. Climate scientist Josh Willis explains that during an El Niño event, when ocean temperatures are high in the eastern Pacific, they tend to be lower in the western Pacific.

Research has shown that changes in ocean circulation and climate alone cannot fully explain the trends in sea surface temperatures and coral bleaching in the Coral Sea. Instead, weather patterns during El Niño events have historically suppressed the Australian summer monsoon, leading to reduced cloud cover and higher air temperatures. These conditions significantly impact the temperatures in the Coral Sea and the extent and location of coral bleaching in the Great Barrier Reef.

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