Three absorption processes

One of the mechanisms of this carbon dioxide sink is linked to the first process of gas expulsion by the sea ice. When frozen, seawater is chemically similar to fresh water. This means that when it forms in the coldest months, the ice expels a whole series of impurities including the dissolved gases it contains. Among them, CO₂, which finishes its journey in the Southern Ocean. At the same time, when ice forms, it expels another element: salt. Here, the sea water is therefor very cold, close to its freezing point, and contains a lot of salt. Two factors that make it very dense. Denser than the lower warmer and more saline layers of water, it will sink and form deep sea currents, the main drivers of thermohaline circulation. As it sinks, this water will take with it the CO₂ it contains.

Another contributing factor to the absorption of CO₂ is primary production as soon as spring starts. Primary production is the growth of all the microalgae. There are a great many of them. "For instance", the oceanographer illustrates, "when an icebreaker breaks through the sea ice, blocks of ice overturn and reveal a layer that’s brown, not white. These are vast communities of microalgae which the krill feed off. And krill is the favourite meal of very large mammals like whales". Which gives an idea of the quantity of microalgae necessary to ensure the survival of this ecosystem. During its formation, this primary production also consumes carbon dioxide.

The phases of formation and dissolution of calcium carbonate, one of the main component salts in limescale, constitute the final process of CO₂ absorption. "In winter, calcium carbonate precipitates in the ice, in the form of crystals. This precipitation produces CO₂ which is expelled into the sea below. But in a weak saline solution it dissolves. Which is what happens in summer, when the ice crystals melt. And this dissolution consumes the CO₂ present in the ice. It contributes to carbon dioxide depletion in the ice, which will then recuperate it in the atmosphere."

Melting ice doesn’t necessarily reduce CO₂ absorption

Thanks to this research, it has been possible to reconsider the role of ice in CO₂ formation and absorption cycles. Until recently, it wasn't considered as playing a role at all. Today, sea ice includes a long list of factors and phenomena that allow us to understand how our planet’s climate works under the influence of human activity. "It’s clear that these studies have repercussions for the future, both from an environmental and scientific point of view", Bruno Delille admits. "But making forecasts for the future is rather delicate and it’s not something we’re doing at the moment."

For instance, it hasn’t been established that the melting ice will reduce CO₂ absorption in the two poles, an effect that would exacerbate the problems linked to greenhouse gases. "First of all, the ice is only melting in the Arctic.  The sea ice in Antarctica isn’t decreasing. On the contrary, it seems to be expanding. Therefore, we can ignore Antarctica for the moment. As regards the Arctic, if the ice absorbs CO₂, its decline will indeed reduce this flux. But there will also be other phenomena associated with this decline. For instance, many modellers consider that primary production will increase with the melting of the ice. If this is the case, it could compensate for the loss of absorption of CO₂ by the formation of ice. We already noticed that there was a greater quantity of carbon dioxide in the sea ice in the Arctic than in Antarctica. And, as a matter of fact, our main hypothesis is that there is more organic matter there." The Arctic Ocean, contrary to its counterpart in the south, is a closed ocean, walled off by continents whose rivers flow into it. These rivers transport organic matter that remains stuck. On the other hand, the Antarctic is far away from other continents and is in a more open environment, and is therefore more impoverished.

Be that as it may, this research has led us to rethink the models that measure the influence the disappearance of the sea ice would have on greenhouse gases. But all this takes time. After all, these results were only published in 2007. Today, the scientific community seems motivated by these discoveries, but any new paradigm needs to settle in before taking off. In the meantime, Bruno Delille and his colleagues are continuing to study the polar oceans, by examining other problems more closely, such as the iron cycle or the other greenhouse gases that are also present in the ice.