Net Zero: How Does That Actually Work?

Biochar carbon removal, enhanced weathering, and direct air capture are the three most prominent and promising methods of enduringly extracting CO₂ from the atmosphere. Below, we explain how those technologies work.

Photosynthesis is the most efficient way to absorb and sequester CO₂ from the atmosphere. However, the photosynthesis process works only for a limited time. When plants decompose or are burned, the carbon stored in them is released back into the atmosphere as CO2 or, in specific circumstances, even as methane, which exerts an even stronger greenhouse gas effect. This is exactly where biochar carbon removal goes to work. Carbon dioxide accounts for around half the weight of different types of dried plant-based biomass. When that biomass is heated to high temperatures without the presence of oxygen, it produces vegetable carbon, or biochar. Biochar, in essence, transforms carbon that was absorbed by plants into a very stable form. Biochar resembles charcoal and remains stable for several centuries, particularly when it is used in a targeted way, for instance as a soil conditioner in agriculture. That way, carbon is lastingly sequestered in the ground and no longer ends up in the atmosphere.

Another benefit is that biochar ameliorates soil quality. In arid areas in particular, biochar acts as a very efficient reservoir for water and minerals. And that’s not all: the production of biochar generates gases that in turn can be converted into energy. Part of that is used to operate biochar manufacturing plants, and the residual gases can be used to produce renewable energy.

In nature, rocks get deteriorated by weather influences like rain, heat, or frost. During the weathering process, certain minerals in rock chemically react with the CO₂ in the air or in rainwater. This reaction binds CO₂ in the form of carbonates, which deposit as sediment in the ground or in bodies of water over time. However, this natural process is very slow and can take thousands to millions of years. But it can be sped up. To do that, machines are employed to pulverize rocks, thus enlarging their surface. The resulting powder is then dispersed in soils, grassland, or bodies of water. Its contact with rainwater and CO₂ in the air causes a chemical reaction that permanently converts CO₂ into carbonates that remain sequestered in the ground or in water. Enhanced weathering makes this entire process around 50,000 times faster than natural rock weathering. Enhanced weathering removes CO₂ from the atmosphere and, just like biochar, can improve soil quality and replace costly fertilizers. But there are also challenges involved. The wide variability of weathering processes, which depend greatly on the rock material used and on the soil and climate, and the difficulty of reliably quantifying how much CO₂ is sequestered in a given period are the biggest factors. Sampling and modeling particularly still pose tough challenges.

Direct air capture, as the name implies, directly extracts CO2 from the atmosphere by using giant fans to suck in ambient air. Since outdoor air contains only a trace concentration of CO₂, the flow of air must be as large as possible. The air subsequently passes through chemical filters that capture CO₂ with the aid of special materials. Afterwards, the CO₂ is compressed and purified before it can be transported for storage. The compression and purification process produces a colorless and odorless liquid. Since the CO₂ extracted by direct air capture has a very high degree of purity, it can also be reused to manufacture synthetic fuels, building materials, or chemicals, for example. Alternatively, the captured CO₂ can be permanently sequestered in geological formations such as underground rock layers, for instance. However, direct air capture currently is very cost-intensive mainly due to the high energy requirements of the fans and the infant state of the technology.