LMU researchers show that combining afforestation and oceanic carbon dioxide removal does not lead to efficiency losses – this could reduce pressure on land areas.

To reach the goals of the Paris Agreement, we not only have to reduce CO₂ emissions, but also remove CO₂ from the atmosphere (carbon dioxide removal, CDR) on a large scale. This can involve both land- and ocean-based methods. As LMU geographers Yiannis Moustakis, Tobias Nützel, and Julia Pongratz show in a new study, these methods can be combined without compromising each other. This is important for finding flexible solutions and alleviating potential land use conflicts.

On land, CO₂ removal mainly takes the form of afforestation and reforestation. In the ocean, by contrast, there is a lot of discussion around chemical methods. One such method is ocean alkalinity enhancement (OAE), whereby carbonates or silicates are added to the ocean so that it can sequester more carbon dioxide. It was previously uncertain whether a combination of these two approaches (land and sea) might impair their effectiveness. After all, CO₂ is part of a global carbon cycle, and if it is taken from the atmosphere in one place, it could theoretically lead to lower absorption at another place.

The study by LMU scientists offers initial evidence dispelling these fears. Using two Earth System Models, the researchers simulated scenarios with afforestation/reforestation and ocean alkalinity enhancement – individually and combined. In total, the researchers performed 42 model runs under a scenario with high emissions. These simulations show that global CO₂ flows increase linearly when the two methods are used in combination – in other words, efficiency losses do not occur.

“Our results are very encouraging,” says Moustakis. “They suggest that diverse CDR portfolios are possible without losing effectiveness. This reduces the pressure on land areas, which offers social and ecological advantages. However, more research is urgently required, including into marine CDR methods and their environmental impacts. And it should be clearly stated that CDR cannot replace the rapid reduction in emissions we urgently require, but only complement it.”