The accumulation of carbon dioxide ($\text{CO}_2$) in the atmosphere is the most significant challenge to stabilizing the global climate. Simply reducing new emissions is insufficient to limit warming to international targets, necessitating the removal of legacy emissions already dispersed in the air. This need has prompted the development of “negative emissions” technologies. Atmospheric carbon capture, formally known as Direct Air Capture (DAC), is an engineered solution designed to mechanically scrub $\text{CO}_2$ from the ambient air, offering a pathway to reverse the historical buildup of emissions.
Defining Atmospheric Carbon Capture
Atmospheric Carbon Capture chemically extracts $\text{CO}_2$ directly from the surrounding air using engineered systems. This technology differs from traditional Carbon Capture and Storage (CCS), which captures $\text{CO}_2$ from concentrated “point sources” like power plants. Point source streams contain $4\%$ to $15\%$ $\text{CO}_2$, while ambient air contains only about $0.04\%$ (420 parts per million). This extreme dilution means DAC systems must process approximately 250 times more air volume than point-source capture to yield the same amount of $\text{CO}_2$. Consequently, the low concentration significantly increases the energy required for separation, making DAC inherently more challenging and energy-intensive than traditional CCS.
Primary Methods of Direct Air Capture
Direct Air Capture systems rely on a reversible chemical reaction cycle that first binds $\text{CO}_2$ and then releases it in a concentrated stream for collection. The two main technological approaches use either liquid solvents or solid sorbents. Both methods require air to be drawn into specialized equipment called air contactors, where the $\text{CO}_2$ interacts with the capture medium.
Liquid Solvent Approach
This approach typically uses a highly alkaline solution, such as potassium hydroxide, sprayed as a fine mist through the air contactor. The $\text{CO}_2$ reacts chemically with the solvent to form a carbonate compound, dissolving the gas out of the air stream. To regenerate the solvent and release the captured $\text{CO}_2$, the carbonate is subjected to chemical reactions, often involving a high-temperature kiln reaching up to $900^\circ \text{C}$. This process breaks down the compound and yields a pure stream of $\text{CO}_2$ gas.
Solid Sorbent Method
The solid sorbent method uses porous materials, often structured as filters or pellets, chemically treated to bind selectively with $\text{CO}_2$. This binding process, known as adsorption, allows the $\text{CO}_2$ molecules to adhere to the surface of the sorbent. Once the material is saturated, the captured $\text{CO}_2$ is released through Temperature Swing Adsorption (TSA) or Pressure Swing Adsorption (PSA). TSA involves heating the sorbent to a lower temperature, typically between $80^\circ \text{C}$ and $120^\circ \text{C}$, which breaks the weak bond. The release of the concentrated gas regenerates the solid material, allowing the cycle to repeat.
Utilizing and Storing Captured Carbon
Once the $\text{CO}_2$ is separated and compressed, it follows one of two main pathways: utilization or permanent storage. Carbon Capture and Utilization (CCU) uses the gas as a feedstock for industrial applications, including synthesizing transportation fuels or creating building materials like concrete. While CCU recycles $\text{CO}_2$ and displaces fossil fuels, it is generally considered carbon-neutral rather than carbon-negative, as the $\text{CO}_2$ is often released back into the atmosphere later (e.g., when synthetic fuel is burned).
True atmospheric carbon removal is achieved through Carbon Capture and Storage (CCS), which involves deep geological sequestration. This process injects the compressed $\text{CO}_2$ into deep underground porous rock formations, such as saline aquifers, where it is trapped permanently. Another permanent storage method is mineralization, where $\text{CO}_2$ reacts with minerals to form stable, solid carbonate rock, locking the carbon away for geologic timescales.
Scale and Energy Requirements
Scaling Atmospheric Carbon Capture to meaningfully impact climate goals presents immense logistical and energy challenges. Climate models suggest gigatons of $\text{CO}_2$ must be removed annually by mid-century to meet the $1.5^\circ \text{C}$ warming target. Achieving this scale requires building tens of thousands of large-scale DAC facilities globally, an effort comparable to the current size of the global fossil fuel industry.
The greatest hurdle is the immense energy demand required for the capture and regeneration cycles. A DAC system typically requires between $1500$ and $3000$ kilowatt-hours of energy to capture one ton of $\text{CO}_2$. This energy is consumed as electricity for the fans and high-grade heat to regenerate the solvents or sorbents. To ensure a net environmental benefit, all energy must come from zero-carbon sources, otherwise the DAC process would generate more $\text{CO}_2$ than it removes. This requirement translates into high operational costs and necessitates a huge expansion of renewable energy capacity.