Can we just dump more carbon in the ocean? Not so easy, argue scientists

To reach the Paris agreement and curb climate change, we must reduce carbon dioxide levels in the atmosphere. The ocean is the single largest carbon sink. It sucks up about 30 per cent of all carbon dioxide emitted. But could we get it to take in just a little more?

Scientists have looked at some new and old technology that promises to do this – and come with three warnings:

• Such technology comes with an immense knowledge gap
• Socio-ecological risks exist, but their extent is not clear
• The ocean is a difficult space to govern, and these techniques are currently mostly unregulated

In a new research article published in npj Ocean Sustainability, researchers including Robert Blasiak, discuss the results of an extensive review of literature and expert workshops.

They summarize that the restoration of mangroves, seagrass, and salt marshes – often referred to as blue carbon ecosystems – is currently the only such approach considered to be both safe and technologically ready. However, the amount of carbon stored by these systems is relatively small.

Increasing the ocean’s CO₂ uptake

Marine carbon dioxide removal – binding additional CO₂ in the ocean – can be summarized into three categories of approaches:

1. Chemical approaches – altering ocean chemistry to make it absorb and store more carbon.
2. Biological approaches – making sure that there are more ocean plants, from mangroves to phytoplankton, that convert and bind carbon.
3. Physical approaches – for example by ensuring that carbon-rich surface water is pumped deeper into the ocean.

Three challenges

An immense knowledge gap on techniques to bind extra CO₂ in the ocean is one of the foremost issues.

“It’s not just that we’re unsure about the risks of these technologies. We also have very little evidence on whether they actually are able to store carbon, and for how long,” explains Centre researcher Robert Blasiak.

In the paper, the researchers call for robust monitoring standards on marine carbon dioxide removal as a first step to see whether such approaches can be viable. Standards are of particular importance to be able to include CO₂ captures in carbon market schemes.

Environmental risks are another blind spot of marine CO₂ removal.

“Several marine carbon dioxide removal techniques can lead to pollution or harmful changes to ecosystems,” explains the article’s lead author Victor Brun from the Ocean & Climate Platform at Maison de l’Océan in France.

Whether it is about changing the chemical balance of ocean environments – e.g. through iron fertilization or alkalinisation – or just about dumping seaweeds into the deep sea; the ecological consequences are at best unknown, and sometimes outright harmful.

Negative social and economic impacts are another part of the picture. Even though the ocean might seem vast, large parts of it are in fact used by many other stakeholders, not least Indigenous peoples and local communities.

Thirdly, inadequate governance of marine carbon dioxide removal has to be addressed, the researchers argue.

Ocean governance is a patchwork of protocols, frameworks, principles and laws – some of which are legally binding, some of which are not. None of them cover marine carbon dioxide removal specifically, which opens a large grey zone for these kinds of techniques.

“Yes, it is urgent to find ways to address climate change. But this feeling of urgency must not lead us to believe that it’s sensible to use new techniques without first assessing them based on rigorous science. Marine carbon dioxide removal should be approached with extreme precaution,” argues Robert Blasiak.

“And of course, it cannot be a workaround for the urgent need to cut greenhouse gas emissions.”