4. Process for Carbon Dioxide Removal
There are many pathways outlined for CDR in literature, involving systems that can scrub CO₂ from air, land, and ocean waters. The CDR taxonomy is ever evolving, and novel pathways for CDR can appear as science advances.
Set forth below is a description of current CDR pathways as published by CDR.fyi (Durable CDR Methods Update – June 2025):
Biomass Carbon Removal and Storage (BiCRS)
Bioenergy with Carbon Capture and Sequestration (BECCS) Combines biomass energy production with carbon capture and storage technologies. CO₂ released during biomass combustion or fermentation is captured and permanently stored, creating net-negative emissions.
Biochar Carbon Removal (BCR) Involves the pyrolysis of organic material to produce biochar, which stores carbon in a stable form when added to soil. This method can also improve soil health.
Biomass Geological Sequestration (BGS) Involves converting biomass into forms such as bio-oil or bioslurry and storing it in geological formations for long-term carbon storage.
Biomass Direct Storage Directly stores terrestrial biomass in stable environments, such as controlled terrestrial storage sites or sinking to the deep sea, where decomposition is minimized, and carbon is sequestered over the long term.
Direct Air Carbon Capture and Sequestration (DACCS)
The capture of CO₂ directly from the atmosphere using chemical processes combined with its permanent underground storage.
Marine CDR (mCDR)
Alkalinity Enhancement This combines ocean, river, and the emergent coastal and wastewater alkalinity enhancement methods under one category. This streamlined approach focuses on adding alkaline materials to aquatic systems to neutralize acidity and store carbon as bicarbonates or carbonates.
Direct Ocean Removal Captures CO₂ directly from ocean water, reducing atmospheric concentrations as the ocean reabsorbs CO₂ to maintain equilibrium.
Marine Biomass Carbon Capture and Sequestration (MBCCS) This combines marine biomass sinking, which involves cultivating marine biomass, such as seaweed, and sinking it into deep ocean layers, and microalgal capture and storage, which harnesses the carbon-capturing capacity of microalgae that can be processed into products or stored in stable environments.
It can also include hybrid approaches where the biomass that captures the carbon is marine, rather than terrestrial.
Enhanced Weathering (EW)
Accelerates the natural rock weathering process to remove CO₂ from the atmosphere. Crushed silicate rocks are spread across landscapes or oceans, where they react with CO₂ and lock it into mineral form.
Mineralization
A broad category involving the conversion of CO₂ into stable mineral forms.
Ex-situ Mineralization: A carbon storage method in which captured CO₂ is reacted with alkaline minerals outside their natural setting—typically involving mined and ground rocks or industrial by-products—within engineered systems such as reactors or treatment facilities.
In-situ Mineralization: A carbon storage method that involves injecting CO₂ into underground rock formations, where it reacts with naturally occurring minerals to form stable carbonates. While the process itself is storage-focused, the overall classification depends on the source of the CO₂; for example, CO₂ captured from biomass combustion qualifies the process as BECCS.
Microbial Mineralization: Leverages microbes to accelerate the natural process of converting CO₂ into stable mineral forms, such as carbonates. These microbes often enhance weathering reactions by producing acids or enzymes that facilitate the breakdown of silicate or carbonate rocks, enabling long-term geological storage of carbon. The microbial nature of the method will be retained as an attribute.
Surficial Mineralization: Applies alkaline minerals to surface environments where they passively react with atmospheric CO₂ to form carbonate minerals, with the aim of enhancing the mineralization capacity of alkaline feedstock. The surficial nature of the method will be retained as an attribute.
Additional Pathways (Nature-Based Solutions)
Afforestation / Reforestation Planting trees in a site where there previously was not a forest (afforestation) or where there previously was a forest (reforestation). They will grow and store carbon biogenically through photosynthesis.
Soil Carbon Sequestration Increase the carbon content of soil through agricultural practices such as no-till and compost.