Fully bleached reef off Isla Iguana in Panama | Photo: Juan Maté

17/04/2024 | According to scientists from the U.S. National Oceanic and Atmospheric Administration (NOAA) and the International Coral Reef Initiative (ICRI), the world's coral reefs are currently experiencing another global coral bleaching event. Events like this used to be infrequent, but this is now the second of this magnitude in the last 10 years and the fourth since 1982. Dr Sonia Bejarano from the Leibniz Centre for Tropical Marine Research (ZMT) is part of the Global Coral Reef Monitoring Network which reports how this global ecological crisis is unfolding.

Corals are marine animals made up of small polyps that form colonies and live attached to the seabed within beautiful mineral skeletons built by themselves. Corals owe their varied colours and much of the energy they need to live to thousands of microscopic algae that live inside their polyps. When seawater becomes warmer than normal for prolonged periods of time, it becomes impossible for the coral to host these algae. As a last attempt to survive the stressful heat, corals expel their algae at great cost. Although corals may survive this loss for some time, they are left vulnerable to starvation. As corals lose their algae, they also lose their colour and their white coral skeletons become visible through their translucent polyps. Although bleached reefs may appear bright and beautiful, they are in grave danger of dying and eventually eroding away.

The bleaching we observe today is a consequence of the prolonged persistence of abnormally high sea temperatures further intensified by the El Niño Phenomenon. As established by NOAA's Coral Reef Watch (CRW), bleaching has been - and continues to be - extensive in the Atlantic, Pacific and Indian Oceans. "From February 2023 through April 2024, significant coral bleaching has been documented in both the northern and southern hemispheres of each of the major ocean basins," said Dr Derek Manzello, NOAA's CRW coordinator.

Bleaching began advancing across the world's reefs, beginning in early 2023, and has since been confirmed in at least 53 countries and territories, including the United States (Florida), the Caribbean, the Eastern Tropical Pacific (including Mexico, El Salvador, Costa Rica, Panama and Colombia), Australia's Great Barrier Reef, large areas of the South Pacific (including Fiji, Vanuatu, Tuvalu, Kiribati and Samoa), the Red Sea (including the Gulf of Aqaba), the Persian Gulf and the Gulf of Aden. The most recent news also confirms widespread bleaching in several parts of the Indian Ocean, including Tanzania, Mauritius, the Seychelles, Tromelin, Mayotte, and off the west coast of Indonesia.

“Eastern Tropical Pacific reefs have not escaped this heat stress” says Sonia Bejarano, reef ecologist at the Leibniz Centre for Tropical Marine Research in Bremen. “There, we started seeing localised bleaching on Central American reefs since April 2023. All the way from the north of Mexico to the Chilean oceanic islands, we have been working together to keep a close watch on our reefs and it has been an intense year of loss watching entire reef areas bleaching but also with some glimmers of hope as we see other reefs not so badly affected or on their way to recovery” adds the researcher.

The Eastern Tropical Pacific Ocean stretches from the Gulf of California in the North to Ecuador, the Galapagos Islands, and the Chilean oceanic islands of Pascua and Sala y Gomez in the South, and borders the western coast of South and Central America. After the Indo-Pacific and the Caribbean, this region is the third with the highest coral reef development, even though its oceanographic conditions are not optimal for reef growth. The Eastern Tropical Pacific Node of the Global Coral Reef Monitoring Network is a network of 32 Latin-American scientists who have been monitoring the progression of coral bleaching across the entire region.

Since mid-2023 these researchers have examined a total of 72 sites across the Eastern Tropical Pacific Ocean. 38 of these sites show signs of severe bleaching whereas in 20 of them some coral areas have died. Although some reefs have recovered and others are on the path to recovery. “It is still uncertain how reefs will continue to evolve through 2024. Our team will continue studying them closely to understand where are our hotspots of bleaching vulnerability and resilience and to harness this information to design the best possible climate-smart reef conservation and restoration strategies” says Bejarano.

Especially when it occurs globally, coral bleaching strongly impacts the economy, people's livelihoods and countries' food security. But a bleached reef is not a dead reef; it is a reef struggling to survive. If sea temperatures return to normal, corals can recover, and reefs can continue to be valuable sources of food and income for the people who depend on them. This global bleaching requires global action. The International Coral Reef Initiative (ICRI), an alliance of 101 international members, is steadfast in implementing reef management plans based on supporting reef resilience.

In the Eastern Tropical Pacific, reefs are home to a large part of our marine biodiversity. A large number of people depend on coral reefs for food and/or livelihood through fishing or tourism activities. Although many of these reefs are located within Marine Protected Areas such as parks, reserves or natural sanctuaries, many of them allow fishing and other human activities within them, thus generating a deterioration that adds to that caused by natural phenomena. For this reason, various restoration efforts have begun to be implemented. However, there is still a need to invest in basic research to guide all these efforts so that they occur in a strategic and coordinated manner, are as cost-efficient as possible, and do not result in ecological disasters.

Although they occur in all tropical seas of the world, global bleaching events do not harm all reefs equally. It is therefore very important to intensify conservation efforts at local, national and regional levels and to monitor coral reefs constantly and not only during bleaching events. “Coral reef monitoring systems such as those maintained by different universities, government agencies and NGOs are extremely important”, explains Sonia Bejarano.

Further information

On Tuesday, May 14, the International Coral Reef Initiative (ICRI) will host a webinar on the 4th global coral bleaching event and provide information on the causes and consequences and what solutions are within our reach.

Registration at: https://icriforum.org/events/fourth-global-bleaching-event/

Press release NOAA and ICRI: https://icriforum.org/4gbe/

Online platform Coral Bleaching Hub: https://icriforum.org/bleaching-hub/