Coral bleaching ends temporarily, but forecasted El Niño sparks fear of recurrence: NOAA

The National Oceanic and Atmospheric Administration (NOAA) announced its view that the fourth global coral bleaching event on record temporarily came to an end around mid-2025. However, an El Niño phenomenon, which raises sea surface temperatures, is expected to develop over the coming months, raising concerns that widespread bleaching could recur.

According to NOAA, mass global coral bleaching events occurred for the first time in 1998, the second time in 2010, and the third time from 2014 to 2017. These events progressed extensively and became severe in alignment with strong El Niño phenomena.

The fourth global bleaching event took place from early 2023 to mid-2025. Temperature rises capable of triggering bleaching affected a staggering 84% of the coral reef areas across the Pacific, Atlantic, and Indian Oceans. Mass bleaching was documented in at least 83 countries and territories. Following severe bleaching in Western Australia early last year, global heat stress subsided, and bleaching reports became sporadic. Based on observations including satellite data, NOAA determined that the global coral bleaching event likely ended around the middle of last year.

However, rising seawater temperatures driven by the El Niño phenomenon expected to develop in the coming months could trigger a widespread recurrence of coral bleaching. The risk is projected to be particularly high during the latter half of this summer across large portions of the North Pacific, including Hawaii, as well as in Florida and the Caribbean. El Niño is a phenomenon characterized by an increase in sea surface temperatures stretching from the equatorial Pacific near the International Date Line to the coast of South America.

Even after the conclusion of the fourth bleaching event, sea surface temperatures remain higher than they were 25 to 30 years ago when mass global coral bleaching first occurred. An NOAA official noted, "We have entered an era where coral reefs experience bleaching almost every year, making it increasingly difficult to define the beginning and end of a global event. To make these determinations, we will continue to advance our tracking of bleaching phenomena." Furthermore, during the fourth bleaching event, not all coral reefs bleached despite being exposed to high water temperatures. Elucidating the factors that provide resistance to bleaching remains a key research priority.

Corals are a type of cnidarian that survive on nutrients produced through photosynthesis by symbiotic algae called zooxanthellae living inside their tissues. The zooxanthellae similarly receive nutrients from the coral, establishing a mutualistic relationship. However, when subjected to environmental stressors such as rising seawater temperatures, the photosynthetic capacity of the zooxanthellae is impaired, causing them to be expelled from the coral and leaving the coral looking white. If the zooxanthellae do not recover, the coral falls into malnutrition and ultimately dies.

NOAA stated: "Coral reefs are a window into the health of our oceans. They protect coastlines from waves, support marine biodiversity, and provide economic opportunities for local communities through tourism and fishing," highlighting the importance of monitoring and protection.