Status: 12/25/2022 3:23 p.m
The sea absorbs a lot of man-made CO2. It thereby mitigates climate change. But the process has its limits: in some areas, the Mediterranean Sea is already releasing more CO2 than it absorbs.
It is an interplay: the ocean swallows our CO2, and not too little. According to the research report, it is a good quarter of the total emissions this year Global Carbon Budget 2022. As a result, the sea also mitigates climate change somewhat.
However, climate change itself ensures that the sea can no longer absorb as much CO2 as it used to. It’s already four percent less. “The CO2 uptake of the oceans is not decreasing, but it would be even greater without climate change,” says Judith Hauck. 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.
Sea could emit CO2
In extreme cases, the warmer temperatures caused by climate change mean that the sea not only cannot absorb any more CO2 – it can even become a source of the greenhouse gas, explains marine geologist Or Bialik from the University of Haifa in Israel and the University of Münster. He likens the process to a bottle of soda left in the back seat of the car on a hot summer’s day.
If you then open the bottle, all the CO2, i.e. the so-called carbonic acid, tries to escape. The gas shoots out with the sticky liquid. “That’s because it’s less soluble, it leaches out of the water and tries to get out of the bottle,” says Bialik.
The Mediterranean releases more CO2 than it absorbs
Exactly this process could take place in the Mediterranean in the summer. Bialik and colleagues have conducted studies in the eastern Mediterranean, a region that made the headlines last summer because of its very high water temperatures. The water was over 30 degrees warm there. Overall, this sea area is one of the fastest warming in the world.
In a recent publication in the magazine “Scientific Reports” In their published study, Bialik and his colleagues found something startling. The eastern Mediterranean has released more CO2 than it has absorbed over the year. “This is not trivial and something we do not expect. Because we usually think that the sea absorbs CO2,” says Bialik.
Mysterious limestone crystals in the water
This is probably partly due to the high temperatures that prevail here in summer. The CO2 simply outgasses, like the gas from the hot lemonade bottle. On the other hand, it is probably also due to the fact that there are very few nutrients in this marine region. As a result, plants such as algae can grow less well and therefore cannot store the carbon dioxide.
However, another, previously unknown chemical phenomenon may also play a role in the outgassing of CO2. The researchers found limestone crystals in the water, which initially puzzled them. Normally, this form of lime, aragonite, is mainly formed by animals in the water, for example by shells or corals. Such animals are not found in the region 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 that has not previously been described in normal seawater. It could be aragonite crystals that formed directly in the water column, i.e. inorganically, without the intervention of animals and plants.
There are marine regions where aragonite precipitates heavily. For example in the Bahamas, the Persian Gulf or the Red Sea. There, however, form huge, whitish cloudy clouds. It was previously unknown that crystals can form unnoticed and then sink to the sea floor.
saturated sea surface
The fact that this is happening could also have something to do with the ever higher temperatures in the eastern Mediterranean. Because when the water on the surface evaporates and becomes warm, this layer of water is saturated with aragonite. It can then crystallize, for example, on small dust particles. A little like how sugar crystals form on wooden sticks if you immerse them in saturated sugar solution for a long time.
The chemical processes that take place during the formation of the crystals could now outgas even more CO2 from the sea. Bialik has calculated that about 15 percent of the escaping CO2 in the eastern Mediterranean can be attributed to the precipitated limestone crystals.
Locally limited or widespread?
It is still unclear whether this crystal formation is a locally limited phenomenon in the eastern Mediterranean or also takes place in other areas of the sea. Bialik is particularly concerned about very large areas of the oceans. “It’s even more disturbing when you think about it: what other areas of the ocean are nutrient-poor and warming? You then realize that this is a description of the great ocean gyres,” he says. These giant eddies are found in all oceans.
However, marine chemist Andrew Dickson of the Scripps Institute of Oceanography, who was not involved in Bialik’s research, is not sure. The eastern Mediterranean is a rather unique area, he told the American magazine Wired. “So the question is to what extent this environment is really special, or if the conditions are common to all oceans. And I don’t have a clear picture of that in my head.”
Whether such crystals actually occur in the large ocean gyres, for example, is by no means certain and is still being researched. But what is certain is that the oceans will be able to store less CO2 in the future due to climate change.
How the sea stores CO2
The fact that the sea stores so much CO2 at all is due to three mechanisms in particular. The first process is purely physical and is caused by the fact that there is simply more CO2 in the air above the water. Because if there is more gas in the atmosphere than in the water, the system strives for a balance. More CO2 dissolves. This effect can be found both in the sea and in fresh water, such as rivers and lakes.
The second, biological process is also known from the countryside. Because algae and seaweed grow in the sea, they absorb CO2. If they sink into the depths and into the sea floor, part of the carbon dioxide also sinks. The effect can also be found in forests, which store CO2 for a long time.
But there is a third chemical process that is unique to the sea and that is key to allowing it to absorb so much carbon dioxide. There are peculiarities in ocean chemistry. Certain ions contained in seawater, the carbonate and bicarbonate ions, react with the CO2 that is dissolved in the water. As a result, the carbon is in a different form and more CO2 can be absorbed by the water.
However, this has consequences. Because these chemical reactions in the sea ensure that the pH value of the sea continues to drop. The more human CO2 the sea absorbs, the more acidic it becomes.
Climate change slows down processes
The sea thus ensures in physical, biological and chemical ways that climate change is mitigated because there is less man-made CO2 in the atmosphere.
Due to climate change, however, currents in the atmosphere and in the ocean are changing. This ensures, for example, that in certain regions water with a lot of carbon dioxide can no longer be transported deep down as quickly. So new surface water that could absorb more carbon dioxide does not form so quickly.
And the higher temperatures also mean that the ocean can no longer absorb quite as much CO2. From a purely physical point of view, less gas dissolves in warm water than in cold water.