A gigantic ecosystem that’s difficult to access

But the main interest of Bruno Delille remains this flux of CO₂ between the sea ice and the atmosphere. His team quickly sought to determine the quantity of gas that the whole of the sea ice can absorb. But they were faced an enormous problem. "The surface area of the world’s sea ice varies between 18 and 28 million kilometres square", the researcher points out. "To give an idea of its size, it’s larger than all the world’s farmland put together. Antarctic campaigns are difficult and expensive. We can’t just study small regions. However, we understood that the temperature of the ice controlled its permeability to gas, but also the consumption of CO₂ by microalgae and the phases of formation and dissolution of calcium carbonate, consumer phenomena of carbon dioxide.  It was consequently possible to establish good correlation coefficients between the fluxes of CO₂ and the temperature of the ice in the regions we studied. A team from Louvain-la-Neuve, which modelled the sea ice, then helped us to simulate the temperature across the entire ice cover. With the temperature and gas fluxes, we therefore had two elements of the equation. We could extrapolate our calculations to the whole of the sea ice." After several verifications, the result was that all the estimations converged towards the same figure. The Antarctic sea ice absorbed approximately 30 megatons of CO₂ between spring and summer, i.e. 50 to 60 % of the amount the Southern Ocean absorbs every year.

The ocean, an immense CO₂ sink

Why does sea ice make such a feast of CO₂ in spring and summer? Because the rest of the time, it consumes it (see "three absorbtion processes" below) or expels it to the ocean depths. Ice and air, like all the major groupings that live side by side in an ecosystem, act like communicating vessels, seeking a balance. When it has expelled CO₂ into the sea, the sea ice is literally left ‘starving’, and will draw it from the atmosphere until a new balance is established.

What allows this flux of CO₂ during the warmer seasons is temperature variation, which makes the ice more permeable. "The sea ice", explain Bruno Delille, "is composed of pure ice crystals, which are made from fresh water and are impermeable. But these crystals are surrounded by pockets, brine channels rich in salts, where biological activity develops (microalgae, etc.). In a nutshell, it’s concentrated sea water. And the atmosphere’s gases are absorbed by these channels." When it’s very cold, the majority of these channels freeze. The brine doesn’t circulate anymore, everything is frozen, nothing is connected anymore, the ice is impermeable. On the other hand, when the temperature rises, the water from the ice crystals melts and increases the amount of brine. The ice is covered in long channels allowing exchanges of gas, matter and nutrients between the underlying water, the sea ice and the atmosphere.

But the CO₂ that has been consumed has to go somewhere. A little detour via the open sea is necessary before coming to the three processes combining this expulsion. "The seas absorb a huge amount of carbon dioxide. It’s estimated that they have absorbed approximately 60 % of what has been discharged by human activities since the beginning of the industrial revolution. And every year, a third of our CO₂ production disappears into the oceans. Which causes an increase in their acidity, but that’s another problem." The more CO₂ we produce, the more the oceans absorb, always in the name of balance. Initially, the CO₂ absorbed by the sea is found in the surface layer, which is rapidly saturated. Although the balance has been achieved, the sea still hasn’t absorbed a great deal of gas. It’s only once the gas has migrated towards the deep layers that the sea can draw more. One of the stakes of oceanography is therefore to understand how this CO₂ descends to the sea bed.