sun setting on Mediterranean Sea

The Mediterranean sea is so hot, it’s fizzing carbon dioxide

The Mediterranean sea connects Europe, Africa, and the Middle East. It was a vital route for merchants and travelers throughout history. It’s named after its Mediterranean climate, which is mild and rainy in the winter and warm in the summer. It’s so central, geographically and historically, that although it’s considered part of the Atlantic Ocean, it’s usually referred to as its own independent body of water. So the deep-blue sea has a large audience as it bubbles with carbon dioxide like a fizzy bottle of soda.


The Mediterranean Sea is Releasing Carbon Dioxide

In any deep body of water, sunlight heats the top layer while lower levels stay cool. Carbon dioxide dissolves in lower-temperature saltwater, so the seas on Earth effectively absorb about a quarter of human-made carbon emissions. However, as the Eastern Mediterranean sea heats in the summer, it becomes too hot to absorb CO2. So instead, it releases it. It’s like a bottle of carbonated soda. When it stays cool, the dissolved gasses won’t bubble up. But if it gets warm, the gasses begin to pop up because warmth can’t hold CO2 as well. 

However, bubbling gas bubbles isn’t the only consequence of the new climate. Researchers have found aragonite crystals on the sea floor. Aragonite is one form of calcium carbonate marine creatures like snails use to build their shells. But in the Mediterranean, the aragonite is forming without the aid of living creatures. This process is another indication the water is warm enough to release its carbon dioxide.

In the stable and shallow sea, the top layer of warm water doesn’t mix very much with the colder levels. However, in deep parts of the ocean, the cold water can rise up and cause a reaction, forming crystals. “The conditions are so extreme that we can definitely generate calcium carbonate chemically from these waters, which was kind of a shock for us,” says Or Bialik, a geoscientist at the University of Münster in Germany. “It’s basically like a beaker that sits there for a very long time, and it’s long enough to get these reactions going and start generating these crystals.” [1]

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The Effects on Marine Life

Bialik and his team don’t yet know how the aragonite reaction begins to occur, although they speculate it can start with specks of dust from land; then the layers of aragonite form it into a crystal. Microplastic may also be a reason this process happens in the Mediterranean Sea, which is one of the most microplastic-polluted waters in the world. In 2020, researchers found 2 million particles in a single square meter of the seafloor only 5 centimeters thick. Therefore, another theory is that the aragonite could be forming around microplastic particles.


“They could probably form around any nucleation center,” says Bialik, who co-authored a paper about this process in the journal Scientific Reports. “I suspect that microplastics may also be a possible one. But as scientists love to say, more research is needed.[2]

However these aragonite crystals form, the process releases CO2, and Bialik calculates that these emissions account for about 15% of the carbon dioxide the Mediterranean releases into the air. But what’s even more interesting is that as the sea emits more CO2 its acidity levels decrease. Ocean acidification is a widespread problem occurring as people pour more CO2 into the atmosphere, and the oceans end up absorbing increasing amounts, causing raised acidity levels. The acidification harms marine life like corals, oysters, and snails because it’s more difficult to build shells and exoskeletons out of calcium carbonate in these conditions.

The impact of acidification on corals is especially concerning because they create calcium carbonate structures known as reefs, which house many marine animals. Coral reefs have a vital place in the ecosystem, including food and medicine for humans. [3] 

Meanwhile, the warming Mediterranean is reversing this issue. But the corals, snails, and similar organisms called calcifiers are not saved. “Many of them have specific temperature ranges in which they can build their shells — not too hot, not too cold,” says Bialik. Plus, the overexposure of microplastics is not healthy for them either. It’s also not healthy for humans who consume Mediterranean seafood with microplastic in their tissue. [4]

Read: Worried about Earth’s future? Well, the outlook is worse than even scientists can grasp


Where Else is This Happening?

Bialik and his colleagues came across the crystals by accident in their sediment traps, so it’s unclear if aragonite crystals are appearing in other parts of the ocean. “This is a somewhat unique area with a variety of conditions that have to happen to make this work,” says marine chemist Andrew Dickson of the Scripps Institution of Oceanography. “The question then is, to what degree is that environment really special, or is it common around the oceans? And I don’t have a clear picture of that in my mind.”

Because of the unique conditions of the Mediterranean, Dickson assumes this phenomenon is unique. However, Bialik states that this process could cause climate issues wherever it occurs. Aragonite crystal construction can interfere with the water’s ability to absorb carbon dioxide from the air, which may inhibit one of the Earth’s natural ways to reduce the planet-heating gas from the atmosphere. “I won’t say we fully understand this yet and fully understand what governs it — when it turns on and when it shuts down,” says Bialik. “We didn’t even think this process occurs on this scale in open waters, in normal marine conditions. And so we still have a lot that we need to understand about it.”

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  1. “The Mediterranean Sea Is So Hot, It’s Forming Carbonate Crystals.” Wired. Matt Simon. October 4, 2022
  2. “Role of oceanic abiotic carbonate precipitation in future atmospheric CO2 regulation.Nature. Or M. Bialik. September 24, 2022
  3. “Ocean Acidification.” WHOI
  4. “Microplastics in fillets of Mediterranean seafood. A risk assessment study.Environmental Research. Margherita Ferrante. March 2022