Apr 16, 2024 - Energy & Environment

Global coral bleaching a warning, "not a death sentence"

Map showing in bright colors the maximum heat stress from coral bleaching during 2023-2024.

Map showing in bright colors the maximum heat stress from coral bleaching during 2023-2024. Image: NOAA

The NOAA's finding that the fourth global coral bleaching event is underway — the second in the past decade, indicates global warming's ocean impact is worsening, scientists say. But all hope is not lost.

Why it matters: At stake is the fate of everything from the health of national economies to the availability of experimental treatments for cancer.

How it works: Warm water corals have narrow temperature ranges in which they can survive.

  • When water is too warm, corals expel algae that live in their tissues and give them their vibrant colors. This process causes them to turn a ghostly white.
  • Bleached corals are vulnerable to disease and mortality, though they can eventually recover from a bleaching episode.
  • But if the heat stress remains high for long enough, it can lead to coral die-offs.
  • In parts of the world, this has already taken place. Scientists have noted unprecedented levels of heat stress in parts of the Great Barrier Reef during the past few months, and expect to see high levels of mortality when surveys are completed.

Zoom in: According to Derek Manzello, coordinator of NOAA Coral Reef Watch, corals contribute $3 billion annually to the U.S. economy alone.

  • Globally that figure is closer to $2.7 trillion annually.
  • Coral reef health also connects humans to the sea, and the vast knowledge and beauty that lay beneath the surface.
  • For marine scientists, tourists and others, coral bleaching and mortality can be emotional, with a period of mourning for the suffering or loss of some of the most colorful communities in the sea, once teeming with life.

Threat level: Currently, it's difficult to find a region that isn't affected by coral bleaching, with vast areas of the Atlantic, Pacific and Indian Ocean Basins affected in both hemispheres.

  • Since the winter of 2023, bleaching has already been confirmed across the world. Florida, the Caribbean, Brazil, Australia's Great Barrier Reef, huge swaths of the South Pacific, the Persian Gulf and Africa have also been among the affected.
  • Manzello expects bleaching to further impact the Indian Ocean, and corals off the coast of Brazil in the next few weeks.
  • He is also concerned about record warm ocean temperatures in the tropical North Atlantic entering the warm season, which could cause early-onset bleaching in the Caribbean.

What they're saying: "I think this is a huge global warning," Manzello said. "We're seeing severe mass coral bleaching taking place simultaneously in all three major ocean basins," he tells Axios in an interview.

By the numbers: The global bleaching event index, which takes into account the area of the planet's coral coverage affected by bleaching, has spiked.

  • The index reached 54%, and is increasing by about 1% each week, Manzello said. The trend is being driven by more than a year of record warm global ocean temperatures associated with a strong El Niño in the tropical Pacific, plus long-term, human-caused climate change.
  • The record high for this metric was 56.1%, set during the longest global bleaching event yet observed, which lasted from 2014-2017. The current event is likely to surpass that, he said.

The intrigue: Scientists have resorted to assisted evolution to try to save some coral species. They plucked coral from the Florida Keys last summer, putting them in cooler water tanks on land until the seas cooled sufficiently.

  • But such desperate measures may not work in the long run as oceans continue to warm.
  • Scientists are trying to identify coral species that better tolerate heat stress, and in some cases, breed them or otherwise encourage them to grow.

Yes, but: La Niña, which features cooler-than-average temperatures in the equatorial tropical Pacific Ocean, may provide some relief to bleached reefs in parts of the world.

  • Manzello said that La Niña is not a panacea, noting that as the planet has continued to warm, large coral bleaching events have been observed even during La Niña years.
  • "Ocean temperatures maybe have increased to the point where large-scale bleaching events are now happening during any ENSO phase," he said, using the term for the broader climate cycle of which El Niño and La Niña are a part.

The bottom line: A global coral bleaching event designation does not mean certain death for all corals, far from it.

  • "People need to be aware that all is not lost," Manzello said. He noted that in past bleaching events, reefs have exhibited variability in damage from one area to another.

Go deeper:

"Widespread" coral bleaching threatens iconic reefs, NOAA warns

"Red Alert to the world": Record warmth in global oceans hits one-year mark

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