What are new CO₂ capture methods?

To limit global warming to “well below 2 degrees Celsius,” as called for by the Paris Agreement, it is necessary not only to rapidly and significantly reduce CO₂ emissions but also to actively remove carbon dioxide from the atmosphere. This is the conclusion reached by the recently published “State of Carbon Dioxide Removal” report, led by the University of Oxford. Dr. Felix Havermann, a postdoctoral researcher at the Chair of Land Use Systems at LMU and scientific coordinator of our CDRSynTra project, explains conventional and newer methods.

Removing carbon dioxide from the atmosphere—what techniques can make this happen?

First of all, every tree that grows does this naturally. Due to their photosynthetic properties, this applies to all plants—trees, grasses, but also algae and phytoplankton in the ocean. They all sequester CO₂. If, for example, forests are selectively planted, additional carbon dioxide can be removed. Another proven method is soil management. Here, the ability of fields, for instance, to store carbon dioxide is enhanced through specific farming practices. For example, fields are plowed less intensively, or plant residues are worked into the upper soil layers. This allows more carbon to be absorbed into the soil than is emitted into the atmosphere. Both afforestation and reforestation, as well as soil management, are considered conventional carbon dioxide removal (CDR) methods.

What are some newer methods?

One example is the production of biochar. This is produced from plant residues—particularly from agriculture—through combustion in the absence of oxygen, a process known as pyrolysis. It can then be reused in the same environment, for example to improve the water-holding capacity of the soil or the retention of plant nutrients.

In the so-called BECCS method (Bioenergy with Carbon Capture and Storage), for example, wood, plant residues, or biogas derived from them are burned in the conventional manner to produce bioenergy, but the CO₂ is filtered out using modern technologies immediately after the combustion process in the BECCS plant and sent to a storage facility.

In DACCS (Direct Air Capture with Carbon Storage), specialized technical systems draw in ambient air and filter it. The captured CO₂ is also stored in a suitable manner.

Another option is the accelerated weathering of rock. As a natural geological process, this method would take millions of years to remove large amounts of CO₂ from the atmosphere. To speed up the process, rock is ground into powder, thereby greatly increasing its surface area, and the powder is spread over farmland or the ocean, where it chemically binds CO₂.

Another new approach involves so-called ocean-based methods. These involve increasing the acid-binding capacity of seawater by adding substances such as limestone. This allows the ocean—already a major CO₂ sink—to absorb even more. In addition, efforts are being made to artificially bring deep-sea water to the surface. This cold, nutrient-rich water stimulates the growth of phytoplankton—which in turn stores CO₂. There are also so-called “blue carbon” methods at the interface between water and land—such as the reforestation of mangrove forests.

What methods will be available in the future?

Two methods we are researching at CDRterra are artificial photosynthesis and carbon fibers combined with stone as a building material. In artificial photosynthesis, we essentially replicate the natural process to create a material—such as oxalate or carbon flakes—that can store CO₂ for a very long time. This material could then be transformed into reusable products or buried. Another method still under research is the production of carbon fibers combined with stone for the construction industry. Instead of steel and concrete, house walls could then be made using a combination of different methods. Algal oil, byproducts from DACCS plants, rock powder for accelerated rock weathering, and biochar can all be utilized in this process.

A long list of techniques. What criteria must CDR methods—whether conventional or modern—meet?

What matters is not only a high capacity for absorption, but also consistent and safe storage capacity—on land, in the ocean, in artificial storage facilities, geological formations, or products for the construction and furniture industries. A tree—or an entire forest—stores CO₂ in its biomass, thereby reducing CO₂ in the atmosphere. When it burns, the CO₂ is released back into the atmosphere. If one were to view this solely from a long-term perspective, it might seem safer to cut down the wood and use it for products.

What, then, are the limits of traditional and newer CDR methods?

Limitations arise not only from ecological and technological aspects, but also from impacts on the local environment and society, financial viability, political feasibility, as well as energy and material requirements. Last but not least, there are competing priorities because land and ocean space are limited. Afforestation, for example, requires a great deal of land, which conflicts with food production, residential areas, or space for biodiversity.

All things considered, which of these methods are currently the most promising?

There is no single “silver bullet” among CDR methods. The question is how each individual method—with its removal potential and all its pros and cons—can be scaled up to such an extent that we can actually remove many billions of tons of CO₂ from the atmosphere. As the scientific coordinator at CDRterra, I am currently developing an evaluation framework for different methods, as well as for entire portfolios. After all, in the end, we will need to draw on a broad spectrum of options. Some of these methods will likely not be ready for use until the coming decades or toward the end of the century.

To what extent, then, can CDR contribute to the climate goals of the Paris Agreement?

So far, extremely little CO₂ has been actively removed from the atmosphere. This is also highlighted in the latest “State of Carbon Dioxide Removal” report, in which researchers from CDRterra participated. Through afforestation, reforestation, and building up soil carbon stores, 2 gigatons of CO₂ are currently removed from the atmosphere each year, compared to just 0.002 gigatons through newer methods. By comparison, it is estimated that 5–15 percent of current global emissions—40 gigatons annually—must be removed from the atmosphere. There is still a very large gap.

CDR alone will by no means be enough to achieve climate neutrality in Germany and across the EU by 2050. The top priority should be the massive reduction of greenhouse gas emissions themselves.

The interview with Felix Havermann was originally published in the LMU Newsroom.