“There is no silver bullet to remove CO2”

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To keep global warming to “well below two degrees Celsius”, as the Paris Agreement demands, it is necessary not only to swiftly and significantly reduce CO₂ emissions, but also to actively remove carbon dioxide from the atmosphere. This is the conclusion reached by the newly published “State of Carbon Dioxide Removal” report led by the University of Oxford. Dr. Felix Havermann, postdoctoral researcher at LMU’s Chair of Land-Use Systems, explains both conventional and more modern methods.

Removing carbon dioxide from the atmosphere – Which technologies can we use to make that happen?

Let us start by noting that every growing tree does this, of course. This is true of all green plant life – trees and grasses, but also algae and phytoplankton in the sea – due to their photosynthetic properties. All of them bind CO₂. Selective reforestation, for example, can remove additional carbon dioxide. Specific soil management is another proven method, where special forms of cultivation enhance the ability of farmland to store carbon dioxide. The ground can, for example, be plowed less intensively or plant residues can be returned into the soil. In this way, more carbon is absorbed into the soil than is given off into the atmosphere. Afforestation, reforestation and soil management rank among the conventional methods of carbon dioxide removal (CDR).

What are the more modern methods?

One is the production of biochar. Biochar is made by burning plant residue – especially farm waste – without oxygen in a process known as pyrolysis. It can then be reused in an agricultural setting, for example to improve the soil’s ability to store water and plant nutrients.

Using a method referred to as bioenergy with carbon capture and storage, or BECCS, wood, plant residues and/or biogas from these substances can be burned in conventional fashion in order to produce bioenergy. In this case, however, modern technologies are used immediately after the burning process in the BECCS plant to filter out the CO₂ and feed it into a storage system.

Where the direct air capture with carbon storage (DACCS) method is deployed, special technical equipment draws in and filters ambient air. Here again, a suitable method is used to store the captured CO₂.

Yet another way is to accelerate the weathering of rocks. As a natural geological process, this method would take millions of years to remove large volumes of CO₂ from the atmosphere. To speed things up, grinding the rock hugely increases its surface area, and the resultant powder is scattered across farmland or the sea, where a chemical process captures CO₂.

Ocean methods, as they are called, are also new. The acid binding capacity of seawater is increased by adding substances such as limestone. The ocean, which is already a major store of CO₂, can thus absorb even more. At the same time, attempts are made to artificially upwell deep water. Cold water from the depths is nutrient-rich and thus stimulates the growth of plant-based plankton, which in turn stores CO₂. And we can also talk about “blue carbon” methods at the point where water meets land – such as the afforestation of mangrove woodlands.

What methods will we see in the future?

Two methods we are exploring as part of the CDRterra research program, which is funded by the Federal Ministry of Education and Research (BMBF) and spearheaded by Professor Julia Pongratz, are artificial photosynthesis and the use of carbon fibers with stone as a construction material. Artificial photosynthesis effectively emulates natural photosynthesis and produces a material – such as oxalate or carbon flakes – that can store CO₂ for the very long term. It could, for instance, be transformed into reusable products or simply buried. Another method we are currently researching is the production of carbon fibers together with stone for use in the construction industry. In place of steel and concrete, the walls of buildings could then be made by combining various methods (algae oil, deposits from DACCS plants, rock flour to accelerate the weathering of rock and biochar) to be a carbon sink product in the end.

That is a long list of technologies. What criteria must CDR methods meet, be they conventional or modern ones?

High removal potential is important, but so too is the ability to store CO₂ constantly and securely – on land, in the oceans, in artificial storage systems, in geological formations and/or in products for the construction and furniture industries. A tree – or a whole forest – stores CO2 in its biomass, thereby reducing the amount of CO₂ in the atmosphere. If the tree is burned down, the CO₂ is released into the atmosphere again. If we only saw the issue from this long-term perspective, we might conclude that the safer option is to cut the trees down and use the wood for products.

What are the limits to conventional and more modern CDR methods?

Limits are imposed not only by ecological and technological considerations, but also by the influence of these methods on the environment and local society, whether they are affordable, their political feasibility, and also the requirement for energy and materials.

By no means the least issue is the conflict of interests that can arise because land and the oceans are themselves inherently limited spaces. Afforestation, for example, takes up a very large amount of space, and that can come into conflict with issues such as food production, living space and space for biodiversity.

Having weighed up all these factors, which of the methods are the most promising at the present time?

None of the CDR methods is a silver bullet. The question is how each one, with its removal potential and all its benefits and drawbacks, can be scaled up to such an extent that we really can remove many billions of tons of CO₂ from the atmosphere. As scientific coordinator of the BMBF-funded CDRterra research program, I am currently developing a framework for assessing different methods, but also for whole portfolios. Ultimately, we will need recourse to a broad spectrum of options. Some of the methods will probably only be ready to go into service in the decades ahead or toward the end of the century.

To what extent can CDR contribute to the climate targets defined in the Paris Agreement, then?

As things stand, only a tiny amount of CO₂ is actively being removed from the atmosphere. This is pointed out in the current “State of Carbon Dioxide Removal” report, in which researchers from CDRterra were involved. Today we are removing two gigatons of CO₂ a year from the atmosphere through afforestation, reforestation and building up the store of carbon in the soil. But we are only removing 0.002 gigatons using newer methods. To put that in perspective: The assumption is that 5 to 15% of current global emissions of 40 gigatons a year must be removed from the atmosphere by 2050. So, the gap is still very wide. To be climate neutral in Germany and the whole of the EU by 2050, CDR alone will certainly not be enough. Absolute top priority should be given to massively reducing greenhouse gases per se.

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Dr. Felix Havermann is a postdoctoral researcher at LMU’s Physical Geography and Land-Use Systems teaching and research unit. He is also scientific coordinator of the CDRSynTra project, which brings together the findings of the CDRterra research program funded by the Federal Ministry of Education and Research (BMBF). CDRterra investigates the feasibility of implementing carbon dioxide removal methods on a large scale in Germany and Europe.

For further reading: The State of Carbon Dioxide Removal