KNMI research: Coral on Saba is disappearing at a rapid pace

The coral off Saba has seriously declined in 25 years, from around 30 percent coverage in the 1990s to 8 percent now. The status of the corals around St. Eustatius is comparable to that around Saba.

Healthy coral

In addition to the fact that the coral suffers from climate change, there are also local stressors such as water pollution, erosion, stormwater runoff, overfishing, and diseases. The murkier the water, the less light the coral receives. As a result, the algae, which depend on sunlight, are less able to provide the coral with nutrition. Erosion of the land causes pollution of the water. Restoring the natural vegetation by tackling overgrazing by goats is one of the measures taken to reduce erosion and thus strengthen the health of the coral.

Tropical hurricanes passing by Saba and St. Eustatius also affect the coral reefs. A healthy coral can recover from this in a relatively short period.

Excessively high ocean water temperatures cause coral reefs to die off on a large scale. And that has huge consequences for marine life. Coral reefs have been called the rainforests of the ocean because they are teeming with life. About a quarter of all marine life depends on coral reefs. In many places, the tipping point has already been reached. Scientists expect that by the end of this century there will be hardly any living coral left if global warming rises above 1.5 °C.

Coral depends on algae

Everyone has an image of coral, whether it comes from television, magazines or holidays, but what is it really? Coral is a small, immobile animal that lives in colonies. Together they build an external calcareous skeleton, which slowly forms a coral reef over several generations. We are talking about warm-water coral, which is only found in warm seas. The optimal water temperature for coral is about 25°C.

To survive, corals need the proximity of single-celled algae; They live in symbiosis with each other. It is these algae that give coral its beautiful color. These algae convert oxygen and carbon dioxide into glucose through photosynthesis, just like plants do. Among other things, this glucose serves as food for the coral. The reliance on photosynthesis does mean that this type of coral only occurs in shallow seas, where enough light penetrates.

Coral bleaches when temperatures are too high

When the water gets too warm, coral repels the symbiotic algae. This point can be reached when the water level is 1°C or more above the normal summer maximum. And with the ongoing warming, this is of course happening more and more often. Due to the loss of the algae, the coral loses its color (“coral bleaching”), suffers from a food shortage, and becomes more susceptible to diseases. If this goes on too long or occurs too often, the coral dies.

Coral takes a beating from different angles

Add to that the pollution, lack of oxygen, sea level rise, and the increase in severe storms, and the picture becomes clear: coral reefs are under great pressure. Since the end of the 19th century, about half of the living coral has disappeared, according to IPBES, an international platform that monitors the state of life on Earth on behalf of 94 countries. In particular, large-scale coral bleaching due to excessive temperatures has become increasingly frequent and severe in recent decades.

For about a year now, the seawater temperature in the North Atlantic Ocean has been unprecedentedly warm. Because the seawater has been more than 1°C warmer than normal for such a long period, the coral is currently suffering a lot.

Tipping point has already been reached in many places

On a regional scale, coral reefs have already reached a tipping point. According to a recent review, the critical limit for large-scale coral reef die-off is somewhere between 1 to 1.5°C of global warming, with the best estimate being 1.2°C. That is exactly the extent of warming that has already been achieved in the ten-year period 2014–2023.

Coral reefs that have died once do not come back quickly. In this sense, the tipping point is irreversible on timescales of decades. The death of coral reefs has far-reaching effects on marine life. Fisheries, tourism and coastal protection are also affected.

The forecast is that with a warming of 1.5°C, 70 to 90 percent of the coral reefs will have died. That rises to 99 percent with a warming of 2°C. Marine life would be irreparably damaged.

From the KNMI climate report by Bart Verheggen and Iris Keizer

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