In 2023, more than three-quarters of the world’s coral reefs were exposed to ocean temperatures that can cause coral bleaching, researchers with the U.S. National Oceanic and Atmospheric Administration (NOAA) report.
Roughly 90% of the excess heat from anthropogenic climate change is absorbed by the oceans, making them hotter now than at any point in modern history. That excess heat is a killer for the symbiotic algae that live inside corals, providing them with nutrients and their bright colors. When water becomes too warm, the corals expel their algae, causing them to become weak and susceptible to disease. The result is coral bleaching, leaving reefs ghostly white.
Scientists at NOAA regularly track ocean temperatures to see what percentage of the world’s coral are experiencing bleaching-level heat stress. In areas they suspect bleaching might be happening, they then reach out to local scientists and colleagues for verification.
The NOAA researchers have now found that 77% of the world’s reef areas have experienced bleaching-level heat stress since 2023. “That is record-setting,” Derek Manzello, coordinator of the NOAA Coral Reef Watch, told Mongabay in a phone call.
“We’re up to 74 countries and territories where we have confirmed mass bleaching has happened since February of 2023,” Manzello added.
This is the fourth global bleaching event since 1998; the previous record peaked at roughly 66% of the world’s coral between 2014 and 2017.
Some corals like those in Florida and the Caribbean are very sensitive to heat. “There have been a few examples where temperatures have spiked so quickly, corals basically just experience acute heat shock and die,” Manzello said.
More often, corals become weak, vulnerable to disease and die a few years after the bleaching event.
Other corals may develop something Manzello calls “environmental memory,” that is, corals that survive bleaching one year sometimes show an increased tolerance to a similar subsequent event.
Some corals are heat-tolerant and can survive extreme temperatures, so reefs with more heat-resistant species tend to suffer less mortality. However, sensitive corals will likely die off, leaving behind less diverse reefs and reducing ocean biodiversity.
Roughly 25% of all marine species are associated with coral reefs at some point in their lives. Many have developed relationships with certain species of coral or other organisms that depend on corals.
“So, things like clownfish, if their anemone bleaches and dies, then they’re basically going to get eaten. So, there are these cascading ecosystem effects that are happening, and we are losing biodiversity.”
Manzello said climate change and ocean heat are the fundamental problem for corals, but humans can also reduce secondary stressors like runoff and water pollution, sediments in the water and overfishing.
“I think we just need to accept the reality that reefs are going to change and figure out how to best manage those changing reefs so that they’re still potentially providing some of the ecological functions,” Manzello said.
Banner image: Courtesy of NOAA.
