While it was initially believed that the atmosphere and sea ice were impermeable to each other, it has since been discovered that sea ice absorbs millions of tons of the CO₂ present in the air. Discoveries made by Belgian researchers, from the University of Liège in particular, that are increasingly attracting the attention of the scientific community. They are forcing modellers to rethink the role of sea ice in environmental issues linked to greenhouse gases. That said, this research has just begun and there are still many questions that remain unanswered. For instance, it still hasn't been ascertained what influence the melting ice would have on atmospheric CO₂.   

Until recently, the scientific community was sure that sea ice was impermeable to the atmosphere. In other words, there were no exchanges of gas fluxes between them. Unlike the air and sea, which are permeable. Their chemical components influence and balance each other like two communicating vessels. Over the past decade, campaigns led in particular by Bruno Delille, an FNRS research associate at the University of Liège’s Chemical Oceanography Unit, have proved the contrary. The sea ice at both poles is indeed permeable. In spring and summer, it absorbs millions of tons of carbon dioxide (CO₂) and releases other greenhouse gases, such as methane and nitrous oxide. These observations, which are becoming widespread on an international level, call into question a paradigm. A recent publication offers particularly innovative ideas on these dynamics of CO₂(1). Ideas that will have to be integrated into the models studying the evolution of greenhouse gases present in the atmosphere.

Bruno Delille may very well have bypassed these properties. "During my thesis on the Southern Ocean’s absorption of CO₂, I spoke with a researcher who told me that, in principle, there could be an exchange of gases between the ice and the atmosphere. Everyone thought the contrary but I remained sceptical. In 1999, I led an initial somewhat improvised campaign, which proved unfruitful, and I didn’t take the research any further".

A lucky encounter

In 2003, the young oceanographer, based at ULg, met Jean-Louis Tison, a glaciologist from ULB who submitted an ambitious project aimed at studying the gases present in sea ice, as well as the iron cycle, and the study of other biogeochemical components. "The project was highly innovative", Bruno Delille remembers. "No-one was working on the gases dissolved in the ice. No-one was interested in them. As for iron, you have to take a lot of precautions when working on it because contamination can occur very easily, but studying it is interesting because it controls the oceans’ primary production (phytoplankton)." Nevertheless, Jean-Louis Tison wasn't easily put off. He was interested in the experiences of Bruno Delille, who had worked on CO₂ in the Southern Ocean. The collaboration was promising, and the first campaigns more encouraging than the attempt four years earlier. Thanks to the chamber technique, they observed the ice absorbing atmospheric CO₂ in spring and summer. "The chamber technique consists of a large cloche that is placed on the ice. We then measure the variation in the CO₂ present in the air trapped inside it. If the concentration of CO₂ increases, this means that gas is released from the sea ice. On the other hand, a decrease means that it absorbs it. In this case, the concentration fell."

In 2007, they published their first paper, and haven’t stopped since. Over the years, they have created a whole network of researchers concerning the biogeochemistry of sea ice and its effect on the climate. They increased the campaigns, publications and conferences. Gradually, other scientists became involved. Among them, modellers from UCL specialised in the physics of sea ice.

There have been a great variety of studies. Because it isn’t just CO₂ that’s absorbed by the ice. Other greenhouse gases have been detected. First of all, methane, and nitrous oxide, which is found in small quantities but whose impact in terms of warming is 300 times greater than CO₂. "As regards the study of methane", the oceanographer explains, "it’s particularly motivated by recent research carried out on the melting of the underwater permafrost in Siberia, which releases an enormous quantity of this gas." Understanding the dynamics of these air-ice interactions with regard to methane is becoming necessary in order to better measure the impact of its release into the atmosphere. Finally, another climatic gas, also present in the air-ice fluxes, is currently drawing the researchers’ attention: dimethyl sulfide (DMS). DMS isn’t a greenhouse gas, on the contrary. It encourages the appearance of clouds and increases terrestrial albedo. "It allows a greater quantity of light to be reflected into space thus helping to cool the planet".  

[1] Bruno Delille, Martin Vancoppenolle, Nicolas-Xavier Geilfus, Bronte Tilbrook, Delphine Lannuzel, Véronique Schoemann, Sylvie Becquevort, Gauthier Carnat, Daniel Delille, Christiane Lancelot, Lei Chou, Gerhard S. Dieckmann, Jean-Louis Tison, Southern Ocean CO₂ sink: The contribution of the sea ice, Journal of Geophysical Research, 2014