The ocean absorbs carbon from the air, but what if the temperature increases?

‘Fortunately, seawater absorbs carbon dioxide (CO₂). If it didn’t, things would have been over and done with already,’ according to climate and ocean researchers Richard Bintanja and Rob Middag. ‘The ocean is a massive carbon reservoir.’ But what actually happens to carbon absorption as the climate changes? For example, when glaciers and sea ice melt due to higher temperatures?

FSE Science Newsroom | Text Charlotte Vlek | Images Leoni von Ristok

This article is the second in a series about the carbon cycle: the cycle in which carbon is exchanged between the atmosphere and the ocean, and between plants and rocks. But this carbon cycle is being disrupted by the CO₂ added to the atmosphere through human activity. In this article, you can read about the carbon exchange between the atmosphere and seawater.

The amount of carbon dioxide absorbed by seawater depends on how much CO₂ concentrations in the air and in the water differ. Compare it to a warm room and a cold one: heat flows faster into the cold room when the temperature difference is greater. This is how the Earth regulates its own balance: more CO₂ in the atmosphere leads to greater carbon absorption by seawater.

However, there is a limit to the ocean’s carbon intake: once the top layer becomes saturated with CO₂, it no longer absorbs more from the atmosphere. The ocean can absorb more carbon from the air only after the existing carbon in surface waters has been transported to the depths. Carbon is transported to deeper waters by currents or by phytoplankton: single-celled plants that take in CO₂ and release oxygen, much like plants on land.

‘When we think about the oxygen we breathe, we instantly think of the rainforest. But in fact, a large share comes from phytoplankton,’ laughs Richard Bintanja, Professor of Climate and Environmental Change at the University of Groningen and the Royal Netherlands Meteorological Institute (KNMI). As dead phytoplankton sink to the bottom, they bring the carbon they absorbed with them. This helps remove carbon from surface waters efficiently.

Iron deficiency around Antarctica

‘But to convert CO₂, phytoplankton need iron as a nutrient,’ says Rob Middag, Professor by special appointment at the University of Groningen and the Royal Netherlands Institute for Sea Research (NIOZ). You can notice it instantly when looking at iron-poor seawater in a bottle. Add iron to the water in its correct chemical form, you can’t just add paper clips. Then you can see the water turn green from the phytoplankton.’

But iron dissolves poorly in seawater. This means that it precipitates quickly and then easily sinks to the bottom of the ocean. Along the coast, this is less of a problem because enough iron-rich dust enters the sea from the land, and rivers also carry iron. However, in the remote Southern Ocean around Antarctica, iron is scarce. ‘Researchers once thought it would be a good idea to add iron to the ocean there, encouraging more phytoplankton to grow and absorb more carbon. But that proved not viable,’ says Middag (see text box).

Middag himself travelled to Antarctica with his research team on icebreakers, but for a different purpose: ‘We thought that melting glaciers might carry iron into the Southern Ocean, which would in turn support more phytoplankton growth.’ Several PhD students explored this hypothesis and arrived at both positive and less positive conclusions.

Middag: ‘One PhD student studied in the lab how phytoplankton is coping with these changing conditions: more iron, but also higher temperatures due to the melt water. How does that affect growth?’ The good news is that phytoplankton thrives in higher temperatures, provided there is enough iron. The bad news, discovered during the icebreaker expedition, is that the iron from the glacier is in the wrong chemical form and not easily absorbed. So phytoplankton hardly benefits from it. Middag: ‘It’s a bit like someone with an iron deficiency chewing on paper clips: it’s pointless.’

Melting Arctic sea ice

Bintanja now focuses on the Arctic, though his polar expeditions are largely a thing of the past. Bintanja: ‘You would think that more open water would mean more phytoplankton, and with it, more CO₂ absorption.’ That’s because phytoplankton needs sunlight for photosynthesis and therefore cannot grow under sea ice.

But does it actually work like that? ‘What you picture in your mind is often oversimplified,’ Bintanja says. That is why he is using KNMI’s climate models to investigate exactly how this scenario of melting sea ice and phytoplankton could play out. ‘Our climate models consistently show that everything is connected, and if you change one thing, everything changes.’

The current conclusion is that there is no clear conclusion. Different models produce different results. Bintanja explains: ‘Phytoplankton also needs nutrients, such as nitrogen and iron. The plankton itself stays near the surface, but nutrients need to be supplied from the depths by ocean currents. It’s very difficult to model the precise movement of these vertical currents.’

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