The role of the oceans in the atmospheric CO2 absorption

Carbon sinks and acidification, the role of the oceans and its impacts

How does the ocean reduce atmospheric CO2?

Today, we are talking about a little-known phenomenon that helps to reduce the CO2 present in the atmosphere. Indeed, when we talk about CO2 absorption, we think directly of forests, trees and plants, but this absorption mechanism is also carried out by the oceans, which are literally “carbon sinks“.

That’s right, 30% of the CO2 resulting from the combustion of fossil fuels is absorbed by the oceans and then distributed in the water column, carried along by ocean currents. This absorption takes place through two distinct processes, one physical, the other biological.

In the physical process, it is simply the natural dissolution of gases present in the oceans at the surface of contact between air and water. This dissolution is clearly favored when the water temperature is low. The CO2 is then carried away when the water (cold and dense) sinks to the seabed, contributing to its vertical distribution.

In the biological process, it is the phytoplankton that participates in the reduction of CO2, by absorbing it and then transforming it into oxygen through photosynthesis. When they die, these microscopic algae will settle to the bottom of the oceans, so the CO2 will accumulate in the depths and no longer contribute to the greenhouse effect.

It is estimated that the oceans absorbed around 34 billion tons of CO2 produced by man between 1994 and 2007, which represents 30% of all CO2 emissions produced by man during this period. This amount absorbed increased in the following years (whereas it had remained stable for the previous 200 years), indicating that the more CO2 increases in the atmosphere, the more the oceans absorb. This is good news, suggesting that the oceans play an important role in this decrease in atmospheric CO2. However, this reduction also depends on water surface temperatures. As explained earlier, the dissolution process is more efficient at low temperatures, but the oceans have been warming over the past few decades, which limits the rate of CO2 absorption and is bound to decrease over the years.

Ocean acidification as a consequence of this accumulation of CO2.

Despite this immense favor that the ocean is once again doing to us, this absorption process is not without risk. As in life everything has a price, the marine environment is threatened by this accumulation of CO2. Indeed, dissolved CO2 acidifies the oceans, and this acidification extends to depths of up to 3,000 meters.

This phenomenon therefore has consequences that could be serious for many marine organisms such as mussels or corals for example, blocking their development, their population could then strongly decline or even disappear.

How does it work:

CO2 absorbed by the ocean decomposes and forms carbonic acid and hydrogen ions. These two free molecules will bind together to form bicarbonate, thus reducing the carbonic ions present in the water. Carbonic ions are necessary for marine life because they help to form calcium, which is useful for shellfish and corals.

In conclusion, even if we can rely on the oceans to reduce the impact of human actions on the planet, this is not without risks. Moreover, the oceans, forests and plants alone cannot reduce the negative effects of our actions. We are the initiators of these consequences (man-made greenhouse gases –> increase in the amount of CO2 absorbed by the oceans –> increase in ocean acidification). We have a role to play, and that role is not to diminish the negative effects of global warming, it is to stop global warming. By treating the disease and not the symptoms, we will have a chance to see the ecological pressures on earth improve, thus avoiding the extinction of species as we are currently witnessing.