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Towards coral-free seas?

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The warming of the oceans is causing the progressive death of corals. Although they usually have some resistance to temperature changes like these, the repetition of heat waves like the ones the planet suffers reduces their ability to recover. Are we heading towards seas without corals?

First of all, what is a coral? To the untrained eye, they might look like beautifully colored and shaped rocks, or perhaps plants with whimsical morphology. And yet they are animals or, rather, colonies of hundreds and thousands of tiny animals called zooids or polyps. Coral reefs can reach enormous dimensions and up to 25% of the world’s marine species depend on them, including turtles.

Corals survived the extinction of the dinosaurs, the Ice Age and other global environmental changes, but it is not clear to scientists that they can survive the current warming process.

The prognosis, in fact, is not at all rosy, since experts have determined that by 2050 (just 28 years from now) nearly 90% of these superecosystems could be gone from the face of the Earth.

And it is that, quite simply, “coral reefs are being boiled alive”, as Gabriel Grimsditch, from the division of marine ecosystems of UNEP (United Nations Environment Program), graphically states. Rising ocean temperatures are killing these organisms.

“They are very sensitive to changes in temperature in the sea. Corals live in symbiosis with microscopic algae that live inside them and give them energy. When the temperature gets too high, this symbiosis that gives life and energy to the coral breaks down. And when it breaks, the same thing happens as with a fever in humans: the coral becomes stressed, weakens, loses its color and bleaches, “explains Grimsditch.

The bleaching of a coral is the anticipation of its death. From then on they may live for a few days, a few weeks or, at most, a few months, but they almost always end up dying.

In 1998 the first global bleaching event took place and around 16% of the world’s corals died. But there have been new episodes, because “since then we have seen more frequent bleaching events, such as the most recent, from 2015-2016,” adds the UN expert.

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In 2016, three-quarters of all coral in the Maldives bleached and two-thirds of its reefs died in northern Australia’s barrier reef.

Mediterranean: worrying situation

What happens to the corals of the Mediterranean? In this sea there are some 200 different species of coral (of the 500 described in Europe and more than 5,600 on the entire planet), where they also play a key role in the underwater ecosystem, comparable in a certain way to that played by Posidonia, although this is a sea plant. In the Mediterranean there are no large coral reefs, as in other parts of the world, but small colonies are formed or they even appear alone.

But what they do have in common with all their other relatives around the globe is the climate threat. And, in fact, this disturbance is more pronounced in the Mare Nostrum than in any other place, because, as scientists and ecologists are warning, Mediterranean waters are warming much faster than the global average.

Just one example: last August the waters of Alicante reached no less than 27.4 ºC, which is a temperature very similar to that of the Caribbean Sea itself. It was the second warmest measurement obtained since these data began to be collected in 1982, says the Center for Environmental Studies of the Mediterranean (CEAM).

Experts such as climatologist Joge Olcina, from the University of Alicante, point out that this has only just begun: «We will still see how the temperature of the sea continues to rise and will reach 28 degrees», predicts.

Since 1982, the waters of this sea have warmed, on average, no less than 1.4º C. This is a much higher rise than that recorded in the atmosphere and also than in other seas on the planet. This exceptional warming (the ‘tropicalization’ of the Mediterranean), apart from killing corals, is also a basic ingredient for the generation of cold drops and other extreme weather events.

Defenseless against heat waves

There are many scientific reports on Mediterranean corals and the unanimous verdict: they have little time left. A study made public a few weeks ago by the Institute of Marine Sciences (ICM-CSIC) and the University of Barcelona reveals that marine heat waves associated with the climate crisis “are leading to the collapse of coral populations in the Mediterraneanin some cases reducing their biomass by between 80% and 90%”, according to a statement from the institution that summarized a study published in this regard in Proceedings of the Royal Society B.

The research warns that the populations of this sea may be unable to recover from the recurring impact of these extreme eventsin which the water temperature reaches very high levels for days and even weeks.

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This was the first study to focus on the long-term resilience of two highly emblematic Mediterranean coral species: the red gorgonian (Paramuricea clavata) and red coral (Corallium rubrum). Both provide essential habitats for many species.

In 2003, due to the very strong heat wave recorded that summer, there was a large mass mortality of corals in the Scandola marine protected area (Corsica). After studying the evolution of these populations over the next 15 years, the researchers found that «far from recovering, all the populations analyzed have tended to collapse and they can be considered practically extinct from the functional point of view”, according to the ICM-CSIC researcher Daniel Gómez.

The consequences of this situation go beyond the affected corals: “The loss of corals is equivalent to the loss of trees in the forests,” says Joaquim Garrabou, also an ICM researcher.

The problem is that water heating is not the only enemy that these marine organisms have to face, since they are also the object of constant attacks by many human activities, such as overfishing, nutrient pollution (fecal and nitrate discharges), coastal modification and illegal harvesting for commercial purposes. .

However, all is not lost. Experts point out that the sooner greenhouse gas emissions are reduced and the sooner the warming of the seas can be stopped, the sooner the death of these ecosystem-organisms can also be stopped. “If we reduce the local impacts of fishing, pollution and coastal development, and if climate change is reduced, corals will have the capacity to restore and return to being healthy, diverse and valuable ecosystems,” says UNEP’s Gabriel Grimsditch.

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