The fourth-ever global coral bleaching event is underway, scientists have confirmed.
Coral bleaching happens when changing conditions (like warming oceans) make coral eject their colourful algae and turn white.
Bleached coral is more at risk of starvation and disease, which can lead to reefs and ecosystems dying.
Here‘s what that means.
What is coral?
Corals are living organisms that take root on the ocean floor. They form reefs — essential structures that provide habitats and food sources for marine life.
According to NOAA, coral reefs cover less than 1% of the ocean floor but “support an estimated 25% of all known marine species”.
Corals get their colours and energy from microscopic algae called zooxanthellae that live in their tissue, which help them process sunlight.
What is coral bleaching?
Corals can become stressed from changing water conditions caused by factors including changing temperatures or too much sunlight.
This can prompt corals to eject the microscopic algae they need to survive. During this phase, corals change colours before eventually turning white (bleaching).
Bleached coral isn’t dead, but it is more susceptible to death from lack of nutrients. Once corals die, reefs rarely recover.
Global coral bleaching
NOAA said it has identified “significant coral bleaching” in 54 countries and territories since February 2023.
It comes after the EU’s climate change service found that global average sea temperatures reached a record high (21.07°C) in March.
Although coral bleaching is a natural process, NOAA warned the impacts of climate change will lead to “more frequent and severe” bleaching events.
NOAA also said global coral bleaching can put local economies and food security at risk.
Recovery
Reefs that have been severely bleached can take up to 15 years to fully recover.
Coral can recover from a bleaching event if the sea returns to normal conditions with lower temperatures over a long period.
When conditions improve, microscopic algae returns to corals, gradually restoring the reef.
Here are the stages of coral bleaching in the U.S. Virgin Islands (source: NOAA).
Great Barrier Reef
According to aerial surveying by the Reef Authority, over 70% of the Great Barrier Reef is currently bleached.
Last week, the authority recorded water temperatures up to 2°C above average for this time of year.
Since 2016, the Great Barrier Reef has experienced four mass bleaching events.
Two years ago, over 90% of the reef was bleached during a La Niña event.
“A temperature increase of just one degree Celsius for only four weeks can trigger bleaching.”
Great Barrier Reef Foundation
