Global warming and climate change: when carbon dioxide is emitted from the sea

The sea absorbs a lot of man-made carbon dioxide. Thus mitigating climate change. But this process has its limits: in some areas, the Mediterranean Sea actually releases more carbon dioxide than it absorbs.

It’s an interaction: the ocean is swallowing our carbon dioxide, and not very little. According to the research report, it represents a good quarter of all emissions this year Global carbon budget 2022. As a result, the sea also moderates climate change to some extent.

However, climate change itself ensures that the sea can no longer absorb as much carbon dioxide as it once could. It is already four percent lower. “The uptake of carbon dioxide by the oceans is not decreasing, but it would be greater without climate change,” says Judith Hook. She is a biogeochemist at the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research in Bremerhaven, and is one of the authors of the Global Carbon Budget.

The sea can emit carbon dioxide

In extreme cases, the warmer temperatures resulting from climate change mean that the sea can not only absorb more carbon dioxide — it can also become a greenhouse gas emitter, explains marine geologist Or Bialik of the University of Haifa in Israel and the University of Münster. . He likens the process to a soda bottle left in the back seat of a car on a hot summer day.

If you then open the bottle, all the carbon dioxide, the so-called carbonic acid, tries to escape. The gas goes off with the viscous liquid. “This is because it’s less soluble—it leaches out of the water and tries to get out of the bottle,” says Bialik.

The Mediterranean Sea releases more carbon dioxide than it absorbs

Exactly this process can happen in the Mediterranean in the summer. Bialik and his colleagues conducted studies in the eastern Mediterranean, an area that made headlines last summer because of its warming waters. The water was warm there over 30 degrees. In general, this marine area is one of the fastest warming regions in the world.

In a recent publication in the journalScientific reports“In their published study, Bialik and his colleagues found something startling. The eastern Mediterranean released more carbon dioxide than it absorbed over the course of a year.” Because we usually think that the sea absorbs carbon dioxide.”

Mysterious limestone crystals in the water

Perhaps this is partly due to the high temperatures that prevail here in the summer. The carbon dioxide is simply emitted, like the gas from a bottle of hot lemonade. On the other hand, this is probably also due to the fact that there are very few nutrients in this marine area. As a result, plants such as algae can grow less well and therefore cannot store carbon dioxide.

However, another previously unknown chemical phenomenon may play a role in the release of carbon dioxide. The researchers found limestone crystals in the water, which puzzled them at first. Usually, this form of lime, aragonite, is formed mainly by animals in the water, for example by means of shells or corals. There are no such animals in the area at all.

Previously unknown phenomenon

So Bialik began studying the chemistry of water. From this, the researchers developed an idea: The crystals could be a phenomenon not previously described in ordinary seawater. Aragonite crystals can be formed directly in the water column, that is, in an inorganic form, without the intervention of animals and plants.

There are marine areas where aragonite is heavily deposited. For example in the Bahamas, the Persian Gulf or the Red Sea. There, however, it forms huge whitish clouds. It was previously unknown that crystals could form unnoticed and then sink to the bottom of the sea.

saturated sea surface

The fact that this is happening could also have something to do with the warming temperatures in the eastern Mediterranean. Because when the water at the surface evaporates and becomes warm, this layer of water is saturated with aragonite. It can then crystallize, for example, onto tiny dust particles. It’s a bit like what sugar crystals look like on wooden sticks if you immerse them in a saturated sugar solution for a long time.

The chemical processes that take place during crystal formation can now release more carbon dioxide from the sea. Bialik calculated that about 15 percent of the carbon dioxide seeped into the eastern Mediterranean can be attributed to precipitated limestone crystals.

Locally limited or widespread?

It remains unclear whether this crystal formation is a locally limited phenomenon in the eastern Mediterranean or if it also occurs in other regions of the sea. Bialik is particularly interested in very large areas of the ocean. “It’s even more disturbing when you think about it: What other regions of the ocean are experiencing nutrient deficiencies and warming? Then you realize that’s a description of the great ocean gyres,” he says. These giant eddies are found in all oceans.

However, marine chemist Andrew Dixon of the Scripps Institution of Oceanography, who was not involved in Bialik’s research, isn’t sure. He told the American magazine that the eastern Mediterranean is a somewhat unique region Wired. “So the question is how special this environment really is, or whether the conditions are common to all oceans. And I don’t have a clear picture of that in my head.”

Whether these crystals actually occur in large ocean gyres, for example, is by no means certain and is still being researched. What is certain, however, is that the oceans will be able to store less carbon dioxide in the future due to climate change.

How does the sea store carbon dioxide?

The fact that the sea stores so much carbon dioxide at all is due to three mechanisms in particular. The first process is purely physical and results from the fact that there is more carbon dioxide in the air than in the water. Because if there is more gas in the atmosphere than water, the system seeks equilibrium. More carbon dioxide dissolves. This effect can be found in both sea and fresh water, such as rivers and lakes.

Second, the biological process is also known from the countryside. As algae and seaweeds grow in the sea, they absorb carbon dioxide. If they sink to the depths and bottom of the sea, part of the carbon dioxide also sinks. The effect can also be found in forests that store carbon dioxide for a long time.

But there is a third chemical process that is unique to the sea and is key to allowing it to absorb so much carbon dioxide. There are idiosyncrasies in ocean chemistry. Certain ions present in seawater, the carbonate and bicarbonate ions, react with the carbon dioxide dissolved in the water. As a result, the carbon is in a different form and the water can absorb more carbon dioxide.

However, this has consequences. Because these chemical reactions in the sea ensure that the pH value of the sea continues to drop. The more carbon dioxide the sea absorbs, the more acidic it is.

Climate change slows down the processes

Thus the sea ensures in physical, biological and chemical ways that climate change is mitigated because there is less man-made carbon dioxide in the atmosphere.

But due to climate change, the currents in the atmosphere and in the ocean are changing. This ensures, for example, that in certain areas water containing a lot of carbon dioxide can no longer be transported to the depths as quickly. So new surface water that could absorb more carbon dioxide is not forming so quickly.

Rising temperatures also mean that the ocean can no longer absorb as much carbon dioxide. From a purely physical point of view, less gas dissolves in warm water than in cold water.