Why does the pH value increase with decreasing CO2

What does CO2 do in the ocean?

That we humans blow more carbon dioxide into the air is nothing new. It is now also known that almost half of this carbon dioxide is absorbed by the ocean. But what exactly this carbon dioxide triggers in the ocean can only be guessed at in many places. One thing is certain, however: it acidifies the ocean by lowering the pH, which is a measure of the acidity of a solution.

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In any case, all organisms that build a skeleton out of lime are affected. Their list is quite long, starting with mussels and snails over corals, single-celled and large algae, sea urchins, starfish, crabs and fish. If they were to be permanently damaged, this would have enormous effects on the food chains in the sea, fisheries, coastal protection and the climate.

The following happens chemically: If carbon dioxide (CO2) dissolves in water, it reacts with it and produces carbonic acid (CO2 + H2O results in H2CO3). This breaks down into different ions, namely first into bicarbonate (HCO3-) and then into carbonate (CO32-), whereby one or two hydrogen ions (H +) are released. These ions determine the pH value, the higher their concentration, the lower the pH, the more acidic the solution. If the CO2 concentration in the water increases, as more carbon dioxide is absorbed from the atmosphere, more bicarbonate and thus more H + ions are produced.

Carbonate Starting material for calcareous skeletons

In order to buffer this, part of the carbonate reacts with the H + ions and turns back into bicarbonate. Thus, the carbonate concentration ultimately falls through the uptake of CO2, as the equilibrium shifts towards bicarbonate. But carbonate is the starting material for the calcareous skeletons and shells of animals and algae. If his concentration drops, it is more difficult for them to separate lime. Lime that has already formed is also at risk because it simply dissolves again when the pH drops.

Due to the increasing content of CO2 in the sea, the area in which lime is stable in the water is becoming increasingly shallow. Even today, lime is unstable below a certain depth, the so-called calcite compensation depth. This depth depends on various parameters. In the North Atlantic it is between 3,500 and 5,000 meters, in the Pacific and at high latitudes it is z. Sometimes only a few hundred meters deep.

Below this depth, lime dissolves due to the low temperature and higher pressure. Because the deeper and colder, the lower the pH value. Therefore, researchers like Prof. Ulf Riebesell from the Leibniz Institute for Marine Sciences, IFM-GEOMAR, in Kiel, who are dealing with the phenomenon of acidification, fear that cold-water corals will be the first victims. Because they live in the deep sea and at high latitudes near the surface. They also consist of aragonite, the more soluble of the two lime minerals that are formed by organisms.

pH drops

The big problem here is not necessarily the fact that the pH value changes, but - as with climate change overall - the speed at which it does it. Because in the last 200 years the pH value in the surface water of the oceans has decreased by 0.1. That doesn't sound like much, but it corresponds to a change in the hydrogen ion concentration of 30%, since the pH scale is a logarithmic scale. By 2100, according to Riebesell and other researchers, the pH could drop by a total of 0.5 units to an average of 7.7. A rate of change that is around 100 times faster than anything that has taken place in the last millennia. The pH value of seawater has changed several times over the course of geological times.

But the changes over time were so small that calcareous marine sediments could dissolve to provide additional carbonate. However, this is not possible at today's speed. In any case, the seas at the poles would in all probability be undersaturated with regard to aragonite at the pH value forecast for 2100, so that there would probably no longer be any living beings that produce these calcium minerals.

(Kirsten Achenbach / MARUM_Research Center Ocean Margins, 25.10.2007 - DLO)

October 25, 2007