Surging ocean temperatures have triggered a second global coral bleaching event in a decade, threatening crucial marine ecosystems that underpin an estimated US$2.7 trillion a year of goods and services.

The occurrence - only the fourth on record - began in February 2023 and has affected coral reefs in every major ocean basin across 54 nations and territories, the US National Oceanic and Atmospheric Administration said in a statement.

“It is going to take some time to truly understand the impacts from this event,” said Derek Manzello, NOAA’s Coral Reef Watch coordinator. Scientists have already reported a coral mortality rate as high as 93% at reefs off Mexico’s Pacific coast, he said.

The reefs, which are home to about a quarter of all marine life, rely on algae for food and color, but when water temperatures exceed the corals’ heat tolerance, the symbiotic algae turn toxic and are expelled.

Deprived of nutrition, corals can turn white and die unless waters cool. Coral provide food, livelihoods and coastal protection for an estimated one billion people. They also support economic activity worth trillions a year, including US$36 billion in tourism, according to a study by the Global Coral Reef Monitoring Network.

One location to avoid mass bleaching so far is Hawaii, Manzello said.

The transition in the weather pattern known as the El Niño-Southern Oscillation from the current El Niño to a cooler La Niña will “likely bring respite to many different areas and lead to declines in the percentage of reef areas being impacted,” according to Manzello. However, recent ocean temperatures have risen so high that there are risks usual patterns could be upended, he said.

Florida’s coral reefs suffered 100% mortality in some areas after ocean temperatures reached 101F (38.3C) last year, according to Phanor Montoya-Maya, restoration programme manager at the Coral Restoration Foundation, which raises coral in nurseries across the Florida Keys.

Australia last month confirmed its iconic Great Barrier Reef was experiencing a new mass bleaching, the fifth since 2016.

  - Bloomberg