There have been disturbing news over and over again in recent years: the acute threat to the enormous and actually strictly protected barrier reef off the Australian east coast, for example, which is being decimated by recurring marine heat waves and pollution.
But the whole dynamic of coral death in the tropical and subtropical oceans is now visible for the first time through the most comprehensive inventory of the international scientific network Global Coral Reef Monitoring Network.
The data set, evaluated by more than 300 marine researchers, takes into account the development of around 12,000 coral areas in 73 countries - a total of more than two million individual observations.
Joachim Müller-Jung
Editor in the features section, responsible for the “Nature and Science” section.
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The research network has been monitoring the health of corals on a regular basis since 1978. However, the researchers have never observed such an acceleration as that since the publication of the last report "Status of Coral Reefs of the World" 13 years ago. Their conclusion: between 2009 and 2018 alone, an estimated 14 percent of the corals on tropical reefs disappeared - around 11,700 square kilometers. This corresponds roughly to the coral area of Australia.
At the same time, the area with strong algae growth has increased by a fifth. No coincidence: where the colorful coral animals with their symbiotic algae can no longer survive, particularly species-rich biotopes for fish, crustaceans and other living beings are disappearing. Coral reefs cover only 0.2 percent of the world's ocean floor, but they are home to around a quarter of the diversity of marine organisms. Since they also live near the coast, they provide the food and livelihood for millions upon millions of people. According to estimates cited in the report, coral reefs provide "ecosystem services" to human beings with a market value of $ 2.7 trillion each year. The diving and tourism industries alone account for $ 36 billion,The estimated six million fishermen in the coral-rich waters along the coast earn six billion dollars.
The most important cause are heat waves
The corals in South Asia and in the Pacific, around the Arabian Peninsula, as well as the corals off the coast of Australia are particularly hard hit by the mass extinction.
In some countries, destructive dynamite fishing is still a cause of widespread coral loss.
The most important cause, however, is now the irregularly recurring heat waves, which can lead to the bleaching of the corals and ultimately to the death of the reefs.
In 1998, an exceptionally hot year globally, which was particularly noticeable in the tropics due to the El Niño climate anomaly, eight percent of all reefs died out in one fell swoop.
The Indian Ocean, East Asia and the Caribbean in particular were severely affected by this mass bleaching.
In fact, a number of the reefs affected at the time have recovered after the temperature peaks had decreased in the following years - at least where the coral animals did not come under further pressure from sewage inflows, sediments or chemicals. In the East Asian coral triangle, for example, which contains a good 30 percent of the global coral population, the reefs have recovered somewhat. Also because global warming does not increase sea temperatures there as it does elsewhere.
An observation that gives the scientists some courage: "We can reverse the losses, but we have to act now," said Inger Andersen, the director of the UN environmental program, about the new findings.
The Prince of Monaco, Albert II, patron of the network report, says: "We know the solutions to protect the corals." In fact, however, the frequency of marine heat waves and the amount of carbon dioxide in the oceans have the drive acidification of the water, which is harmful to corals, has increased almost unchecked in the past decade.