As ocean water heats up, swaths of once-technicolor coral reefs have begun turning white, putting ecosystems across the globe at risk.

Scientists from the National Oceanic and Atmospheric Administration and the International Coral Reef Initiative announced on Monday that the world is undergoing its fourth global coral bleaching event, marking the second such occurrence in the last decade. According to Derek Manzello, coordinator of NOAA’s Coral Reef Watch program, scientists have documented significant coral bleaching across every major ocean since early last year. 

The current bleaching event, caused by long-lasting high ocean temperatures, has hit reefs in more than 53 countries and territories and 54 percent of all areas with reefs. Some places, such the Caribbean, Australia’s Great Barrier Reef, and large parts of the South Pacific, began documenting widespread bleaching in early 2023. Now, with recent reports from countries along the Indian Ocean and the South Atlantic confirming that none of the four major ocean basins have been left untouched, the event has been determined to be truly global, according to NOAA and the International Coral Reef Initiative. 

“Unfortunately, we’re likely to see bleaching events continue to happen, and they’re likely to get worse,” Manzello said. Previous mass bleaching events happened in 1998, 2010, and between 2014 and 2017. According to Manzello, the current event is likely to engulf more reef areas than ever before.

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In February, ocean temperatures hit a record high, and since last June, each month consecutive has been the warmest of that month ever recorded. Although a period of ocean cooling, known as La Niña, is expected to bring relief this summer, the amount of bleached coral reefs will increase by about 1 percent per week while high temperatures persist.

Bleaching occurs when corals become stressed by rising sea temperatures and expel the algae living within their tissues, causing them to turn white. Corals are invertebrate animals and rely on the algae as a symbiotic source of food. If temperatures remain high for too long, bleaching can lead to mass coral death. Even in well-protected areas, like Australia’s Great Barrier Reef, up to half of coral have been killed by warming temperatures.

Die-off of coral threatens not only the health of marine ecosystems, but also the livelihood and food security for people who depend on them. According to some estimates, economic activities that rely on healthy coral reefs are worth $11 trillion dollars every year.

The world’s oceans, which absorb 90 percent of heat trapped in the atmosphere by greenhouse gases, have experienced more frequent and more intense marine heatwaves over the past century. Last year’s first warning sign came from the southern tip of Florida, when an unprecedentedly severe and long-lasting heat wave caused hot tub-like temperatures and bleached corals. In response, NOAA took steps to save reefs, such as moving young corals to deeper, cooler water, and deploying shade to keep reefs out of the sun.

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“This should be seen as a global warning,” Manzello said, adding that cutting back greenhouse gas emissions is an essential part of any solution. “Ocean health is being impacted drastically by climate change, but we still have time to get things right. We still have time to stop this trend.”