Do Danish oceans release or absorb CO2?

By Birgitte Svennevig, birs@sdu.dk, 3/30/2022

CO₂ is one of the biggest climate sinners, and everywhere in the world, efforts to reduce emissions are being made.

Also in Denmark, we keep a close eye on our national CO₂ balance, and we now know that many types of nature can contribute to the task and thus be worth preserving: forests, salt marshes, bogs, eelgrass beds, etc.

But then there is the seawater, of which we have almost infinite amounts in Denmark: It both absorbs and releases CO₂, but we have no idea whether it absorbs the most or releases the most.

What's going on in the sea?

– We know that the world's oceans are responsible for removing up to 40 per cent of man-made CO₂, but can they continue to do so? We cannot be sure, because when seawater begins to absorb a lot of CO₂, acidification of the seawater kicks in – and that may have consequences for the sea's ability to absorb CO₂, Christian Furbo Reeder explains.

Acidification occurs when CO₂ reacts with water: In the process, carbonic acid, which dissolves lime, is made, posing a threat to calcareous algae, mussels, crustaceans, corals, etc.

– If acidification reduces the populations of lime-based organisms, there will be fewer organisms to absorb CO₂ in the ocean, says Reeder, adding that acidification can destroy entire ecological cycles.

Is there a difference between hot and cold oceans?

– Seawater is not the same everywhere in the world. Warmer water may actually be less effective at absorbing CO₂, while cold water may absorb and store more CO₂. One can therefore well expect that Danish waters contain more CO₂ than warmer waters, says Christian Furbo Reeder.

However, temperature is not the only thing that comes into play: ocean currents also play a big role. South of Greenland, by the Labrador Sea, for example, large amounts of CO₂ are absorbed and then transported down to the deep sea via ocean currents.

Factors like the salt content of the water also has an effect, so you cannot "just" take a few water samples in a few places in the world and then think that you get the full picture of the ocean's ability to absorb and release CO₂.

To understand the capability of Danish waters - or reduced capability - to absorb CO₂, Principal Investigator Carolin Löscher and her group plans to map the CO₂ content in Danish waters over a year.

– We will need a lot of water samples, and there is no way, we can collect them all ourselves. That is why we want to involve students from upper secondary schools all over the country to help us take water samples, says Jakob Bang Rønning.

The team expects to start in September 2022 and to collect water samples four times over a year, thus representing all four seasons.

How will the answer help us?

– If it turns out that Danish waters release more CO₂ than they absorb, you can actually do something to improve the water's ability to absorb CO₂>. We know that adding basic minerals can counteract acidification and increase the uptake of CO₂ from the atmosphere, says Jakob Bang Rønning.>

Such an alkaline mineral may, for example, be the common rock type olivine.

– We are currently doing some experiments with this. We crush the stone to powder and pour it into containers with seawater, and we also test it in an indoor, artificial river system. Our hope is that one day you can remedy acidification in vulnerable and waters, he says.