Sea – coral bleaching: When the underwater world fades

The corals in the sea water shine spookily. They are their life partners, the unicellular dinoflagellate algae that give them their characteristic colors. Without them, the lime skeletons protrude naked, and with the bright colors, life itself – fish, seafares, hermit crayfish and other animals – also disappears in an intact reef.

According to Mathilde Godefroid, marine biologist at the Max Planck Institute for Marine Microbiology Bremen, the current coral bleach is “the largest mass bleach that has ever existed”: As reports by NOAA CORAL Reef Watch, around 84 percent of all tropical and subtropical coral reefs in at least 83 countries have been picked up from her, Including the largest and best known of all, the Great Barrier Reef off the Australian coast. In the past nine years, this has already experienced five mass bapes in the summer months. In 2016 and 2017, at least half of all reef -forming corals in flat – five to ten meters deep – died, as shown in the science magazine “Nature” in August 2024.

Kippement of the Erdklimas

As one now knows, it is primarily to blame for the rapidly increasing temperatures of the surface water of the oceans and long -lasting heat waves. The years 2023 and 2024 have been the hottest since the beginning of the weather records, and the trend continues in the current year. In water temperatures around 30 degrees Celsius but the microalgae fabrics that are toxic for their life partners, in whose outer skin they live. To protect yourself from it, the corals emit them. But the price for this is high, the Dinoflagellaten provide you with 90 percent of your food by means of photosynthesis.

The world climate (IPCC) classifies the tropical and subtropical coral reefs as one of the central tipping elements of the earth’s climate. If the global average temperature increases permanently or at least over longer periods of 1.5 degrees Celsius compared to pre -industrial times, according to its forecast, 70 to 90 percent of these unique habitats will disappear. At two degrees it would be almost all at 99 percent. We are currently heading for temperatures by the end of the century, which are even three degrees above those in front of the fossil age. In 2024, the 1.5-degree threshold was exceeded for the first time worldwide.

However, if the coral reefs disappear in the lower latitudes, the profound consequences for nature and human beings: in their diversity of species, they resemble the ecosystem of the tropical rainforests. They accommodate a quarter of all known marine life. Fish are dependent on them as a spawning square and as a nursery and find shelter and food there. The reefs also protect the coasts from storms and high waves and represent millions of people who live on fishing or tourism.

But do coral bleaching, like the current one, pledge the beginning of the end of these magical underwater worlds? “Coral bleaching is a clear sign of stress,” explains the coral researcher Godefroid. Whether they come back or die again depends on the frequency in which such bleed events occur and how long they last. In fact, bleaching appear more and more and continue to grab you. “So the corals don’t find enough time to recover,” says Godefroid.

Coral reefs of the same tropical rainforests in their abundance of species. Her disappearance would have profound consequences for nature and humans.

A long -term study published in the publicly accessible magazine “Diversity” shows that the net lesson of a still largely untouched reef off the coast of the Sudan has decreased significantly in the past 30 years compared to the period from 1980 to 1991. First author Sarah Abdelhamid and her colleagues also associate this with the coral bleaching that have been piling since the early 1980s.

Not all types are equally affected by the development: the scientists found that the more robust Pocillopora, the so -called Katzenpfötchen-Coralle, increasingly displaced the offspring of the Acropora, the more sensitive antlers. »More resistant species then establish themselves more successfully. The picture that we have from the reefs in the Red Sea today continues to change, «explains Reinicke, who worked as a head of the study. The same can be observed in Southeast Asia or at the Great Barrier Reef. “If a species spreads like a field of nettles, it displaces other types and reduces the biodiversity,” warns Reinicke. “Without two corals, it becomes more difficult for all types that live on them.” Often there are very close symbioses, such as between large fish and the so -called cleaning fishing. If one is missing, others disappear. In any case, it is certain that the reef landscapes of tomorrow will look different than today.

The other twin

But it is not only the increasing temperature of the water surface that creates the tropical corals, but also the acidification of the world’s oceans. In science one speaks of the “Evil Twins” (evil twins). The oceans take up about a quarter of our annual carbon dioxide emissions. In the water, the CO dissolves₂ And reduces its pH value. The deeper it falls, the more energy need stone corals, but also snails, mussels, sea urchins and seafare to form their skeletons and shells. The ocean acidification also weakens existing reefs, hinders their growth, makes them fragile or even lets them collapse. This is added to ocean heating and other stress factors such as fishing, acceptance of the salinity in the tropical waters, storms and pollution of the world’s oceans.

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A study published in April 2025 in the specialist magazine “Global Change Biology” comes to the conclusion that another planetary border has already been exceeded with the progression of the ocean acidification. This reduced the habitable habitats for the tropical and subtropical corals by 43 percent. Even more dramatic is the location for wing snails in the Arctic Ocean: because cold water is more CO₂ As a warmer, the pH value drops even faster there than in the tropical surface waters. According to the study, the snails even lost over 60 percent of their habitats.

The cold water corals also suffer from the consequences of climate change. They do not bleach because they do not live in symbiosis with microalgae like their tropical relatives. Rather, they filter the water after food like mussels. But there are also many types among them that build their aragonite skeletons, the more soluble form of lime. And they too only thrive within a clearly contrasted temperature window. So you know from the widespread cold water coral Desmophyllum pertusum that it has problems forming lime at over 13 degrees water temperature.

Open questions about cold water corals

But because there is very little monitoring in the deep sea, so far there have been less reliable knowledge about how cold water coral reefs, the hotspots of the biodiversity of the deep sea, have been dealing with the current climate change. “Where there are actually the limit values in which the reefs collapse, that is not yet sufficiently understood,” assures Jacek Raddatz, Paleooceanographer at the Geomar-Helmholtz Center for Ocean Research. Because unlike the tropical reefs, it not only needs diving equipment, but with water depths of 400 or 1000 meters of robots or even a submarine. It is cold there, dark and there is often a strong current.

As a paleooceanographer, Raddatz knows that cold water coral reefs have shifted to the south or north in the course of changing cold and warm periods. About 5000 years ago, all reefs were collapsed from Norway, possibly since the water chemistry had changed. After that, they formed again. “But these were natural fluctuations. The corals have not seen such a quick increase in temperature and the dissolved carbon in the water as now. You have to go back to massive volcanic eruptions geologically, «says Raddatz.

The attempt by scientists to collect samples of corals that have survived the blade events well in the laboratory shows how desperate the situation is, in the laboratory. The researchers agree that it is urgently time to drastically reduce the anthropogenic greenhouse gas emissions and to eliminate any pressure on the sensitive ecosystems. This is the only way we can at least receive a small part of it.