The cumulative concentration of carbon dioxide ($\text{CO}_2$) in the atmosphere, primarily from human activities since the Industrial Revolution, necessitates a dual approach to managing climate change. The first effort is mitigation, which focuses on reducing the flow of new greenhouse gas emissions by transitioning to clean energy and improving efficiency. However, even with aggressive mitigation, the existing stock of $\text{CO}_2$ in the atmosphere remains a long-term problem that will continue to drive global warming.
Carbon Dioxide Removal (CDR) is the second, complementary strategy, defined as extracting $\text{CO}_2$ directly from the ambient air and locking it away in durable reservoirs. CDR does not replace emissions reduction; instead, it addresses past emissions and any remaining emissions from hard-to-decarbonize sectors like agriculture, aviation, and certain heavy industries. Achieving net-zero emissions and limiting global temperature rise to 1.5°C will require the deliberate, large-scale deployment of CDR techniques, which span from leveraging natural processes to developing complex mechanical technologies.
Enhancing Natural Carbon Sinks
Nature-based solutions accelerate the Earth’s existing ability to absorb and store $\text{CO}_2$, often providing additional environmental advantages. Reforestation and afforestation involve planting trees on lands where forests previously existed or on lands that have historically been non-forested. As trees grow, they absorb $\text{CO}_2$ through photosynthesis, storing the carbon in their biomass. These terrestrial sinks face limitations from land availability and the permanence risk posed by wildfires and disease, which can rapidly return stored carbon to the atmosphere.
Agricultural lands offer a vast opportunity to enhance carbon storage through soil carbon sequestration, particularly via regenerative farming practices. Methods like no-till farming minimize soil disturbance, preventing the oxidation of organic matter that releases carbon back into the air. Cover crops and diverse crop rotations ensure living roots are in the ground year-round, feeding the soil microbiome with carbon drawn from the atmosphere. This process increases soil organic carbon, improving soil health, water retention, and crop resilience.
Coastal ecosystems, known as “blue carbon” habitats, offer some of the most durable natural storage options. Mangroves, salt marshes, and seagrass meadows draw down atmospheric $\text{CO}_2$ and store it primarily in their waterlogged, anaerobic soils. The lack of oxygen dramatically slows decomposition, allowing carbon to accumulate and remain locked away for millennia. These habitats can sequester carbon at rates far greater per unit area than most terrestrial forests.
A more engineered approach is Ocean Alkalinity Enhancement (OAE). This method involves adding finely ground alkaline minerals, such as olivine or magnesium hydroxide, to seawater. The added alkalinity shifts the ocean’s carbonate chemistry, converting dissolved $\text{CO}_2$ into stable bicarbonate ions, which are stored long-term. The resulting deficit of $\text{CO}_2$ in the surface water prompts the ocean to draw more $\text{CO}_2$ from the atmosphere to restore chemical equilibrium, accelerating a natural geological weathering process.
Direct Air Capture Technology
Direct Air Capture (DAC) represents the most technologically advanced form of CDR, using specialized chemical processes to filter $\text{CO}_2$ directly from the ambient air. The core challenge for DAC systems is the low concentration of $\text{CO}_2$ in the atmosphere, currently around 420 parts per million (ppm). This low concentration means enormous volumes of air must be processed to capture a single ton of $\text{CO}_2$, contributing significantly to the high energy demand.
DAC systems generally fall into two main categories based on the chemical agent, or sorbent, used. Liquid solvent systems pass air through a chemical solution that selectively binds with the $\text{CO}_2$. To release the concentrated gas for storage, this solution must be heated to extremely high temperatures, sometimes up to 900°C, demanding a substantial energy input.
Solid sorbent systems use porous, solid materials coated with amines that chemically adsorb $\text{CO}_2$ from the air. These systems typically operate at much lower temperatures, often between 80°C and 120°C, making them more compatible with low-grade waste heat or renewable heat sources. While the theoretical minimum energy required is low, commercial systems currently require near 2,000 kWh per ton of $\text{CO}_2$ captured and compressed.
The deployment of DAC is proceeding along two distinct scaling pathways. Modular systems, often employing solid sorbents, consist of standardized units allowing for flexible siting and easier technology upgrades. Conversely, large-scale, centralized facilities, often using liquid solvents, are designed to operate like massive chemical plants, leveraging economies of scale to achieve multi-megaton-per-year removal capacity.
Durable Carbon Storage Methods
The effectiveness of any CDR method hinges on the long-term permanence of the storage solution, ensuring the captured $\text{CO}_2$ does not return to the atmosphere. Geological sequestration is the most mature method, involving the injection of compressed $\text{CO}_2$ deep underground into porous rock formations sealed by an impermeable caprock layer. The two primary geological reservoirs are deep saline aquifers and depleted oil and gas reservoirs.
Deep saline aquifers offer the largest theoretical storage capacity globally, trapping $\text{CO}_2$ structurally beneath the caprock and through dissolution into the brine. Storage efficiency here is relatively low, typically ranging from 2% to 20% of the pore space. Depleted hydrocarbon reservoirs benefit from extensive pre-existing geological data and proven containment history. Due to lower pressure, these exhausted fields can accommodate a higher volume of injected $\text{CO}_2$, with storage efficiencies potentially reaching up to 80%.
Another highly permanent storage method is carbon mineralization, which accelerates the natural weathering of rock. This involves reacting $\text{CO}_2$ with calcium- or magnesium-bearing silicate minerals to form stable, solid carbonate materials. The $\text{CO}_2$ is chemically locked into an inert rock form, ensuring permanence for thousands of years. This process can be done in situ by injecting $\text{CO}_2$ into reactive basalt formations or ex situ using industrial waste materials like steel slag.
Carbon Utilization (CCU) is a third pathway where captured $\text{CO}_2$ is converted into commercial products, but it only counts as durable storage if the carbon is permanently locked away. Injecting $\text{CO}_2$ into concrete during the curing process forms stable compounds that sequester the carbon for the life of the building. Converting captured $\text{CO}_2$ into synthetic fuels is not considered permanent storage because the carbon is released back into the atmosphere when the fuel is burned.
Current Global Removal Capacity
The current scale of deliberate carbon removal, both nature-based and engineered, is vastly insufficient to meet projected climate goals. Natural carbon sinks already absorb a significant amount of human emissions, but this unmanaged process is increasingly threatened by climate change itself. The Intergovernmental Panel on Climate Change (IPCC) estimates that limiting warming to 1.5°C will require the annual removal of 6 to 10 gigatons of $\text{CO}_2$ by 2050.
The technological removal capacity deployed today remains extremely limited. The total operational capacity of all DAC facilities worldwide is currently only in the thousands of tons of $\text{CO}_2$ per year. While large-scale, one-megaton-per-year DAC projects are under development, the overall technological capacity is far smaller than the gigaton scale required. This substantial gap highlights the need for accelerated deployment across the entire portfolio of CDR methods.
Governments and industry are attempting to bridge this gap by establishing a clear policy landscape and financial incentives. In the United States, the 45Q tax credit provides financial support for each ton of $\text{CO}_2$ permanently stored or utilized. These policies are designed to reduce the cost barrier of early-stage technologies and stimulate the private investment needed to build the necessary infrastructure. Scaling these technologies requires continued research, development, and robust long-term demand signals to mobilize supply chains.