Coral reefs around the world are experiencing global bleaching for the fourth time, top reef scientists declared Monday, a result of warming ocean waters amid human-caused climate change.

Coral reef bleaching across at least 53 countries, territories or local economies has been confirmed from February 2023 to now, scientists from the National Oceanic and Atmospheric Administration and International Coral Reef Initiative said. It happens when stressed coral expel the algae that are their food source and give them their color. If the bleaching is severe and long-lasting, the coral can die.

Coral reefs are important ecosystems that sustain underwater life, protect biodiversity and slow erosion. They also support local economies through tourism.

Bleaching has been happening in various regions for some time. In the world’s largest coral reef ecosystem, Australia’s Great Barrier Reef, bleaching affected 90% of the coral assessed in 2022. The Florida Coral Reef, the third-largest, experienced significant bleaching last year.