Atmospheric carbon capture involves technologies designed to remove carbon dioxide (CO2) directly from the air. The main goal of these technologies is to reduce the concentration of greenhouse gases in the atmosphere. This process plays a role in efforts to mitigate climate change by actively drawing down existing CO2.
Why Atmospheric Carbon Capture is Important
The concentration of carbon dioxide in the atmosphere has significantly increased since the Industrial Revolution, rising from approximately 280 parts per million (PPM) to around 415 PPM currently, with an annual increase of about 2.5 PPM. This elevated CO2 traps heat from the sun, leading to global warming and climate change, which can disrupt ecosystems and cause sea levels to rise.
While reducing new emissions from sources like burning fossil fuels is a primary focus, removing existing CO2 from the atmosphere is also recognized as a necessary step to meet global climate targets. The Paris Agreement, signed by 195 countries in 2015, aims to limit global warming to well below 2°C above pre-industrial levels, striving for 1.5°C. To achieve these goals, both emissions reduction and active carbon dioxide removal are needed.
Carbon capture technologies address emissions in hard-to-abate industries, such as cement and steel production, that will continue to release CO2 even with a shift to renewable energy sources. This dual approach of reducing new emissions and removing existing atmospheric carbon accelerates the transition to a low-carbon economy.
How Atmospheric Carbon Capture Works
Direct Air Capture (DAC) technology removes CO2 directly from ambient air using engineered systems. These systems draw in large volumes of air, which then passes through specialized filters or chemical solutions designed to selectively bind with CO2 molecules. The captured CO2 is then released as a concentrated stream, while the CO2-depleted air is returned to the atmosphere.
Two main approaches are used in DAC: liquid DAC and solid DAC. In liquid DAC systems, ambient air moves through chemical solutions, which absorb the CO2. After the CO2 is captured, the chemical solutions are regenerated using high-temperature heat, allowing them to be reused in a continuous, closed-loop process. This regeneration step concentrates the captured CO2 for further processing.
Solid DAC technology employs solid sorbent filters that chemically bond with CO2 from the air. Once these filters become saturated with CO2, they are isolated from the air flow, and the captured CO2 is released by applying heat and creating a vacuum. The concentrated CO2 is collected, and the sorbent filters are regenerated. Companies like Climeworks pursue this solid sorbent method, while Carbon Engineering utilizes a liquid solvent approach.
Bioenergy with Carbon Capture and Storage (BECCS) is another approach. BECCS integrates the process of generating bioenergy from biomass with carbon capture technology. When this biomass is used to produce energy, such as through combustion in a power plant, the CO2 emissions are captured before they are released back into the atmosphere.
The captured CO2 from BECCS processes is then stored, resulting in “negative emissions” because it removes CO2 that was initially drawn from the atmosphere by the growing biomass. This technology is scalable and applies to various biogenic processes, including ethanol production or industrial systems that burn biomass for fuel. However, the effectiveness of BECCS depends on sustainable biomass sourcing to ensure that the carbon removed from the atmosphere is not outweighed by emissions from biomass cultivation, harvesting, and transport.
Storing and Utilizing Captured Carbon
Once carbon dioxide has been captured from the atmosphere, it can be managed in two main ways: long-term storage or utilization. Geological sequestration is a primary method for long-term storage, involving the injection of captured CO2 deep underground into suitable geological formations. These formations include deep saline aquifers, which are vast underground layers of porous rock filled with saltwater, and depleted oil and gas reservoirs. These sites are chosen for their capacity to securely contain the CO2 over extended periods, often for thousands of years, due to impermeable rock layers that act as seals.
For effective geological storage, CO2 is compressed into a supercritical fluid, a state where it exhibits properties of both a gas and a liquid, allowing for higher density and more efficient injection. This compressed CO2 is then transported, often via pipelines, to the injection sites, which must be at depths greater than 800 meters (approximately 2,600 feet) to maintain the CO2 in its supercritical state. Depleted oil and gas reservoirs are attractive because their ability to store fluids has already been proven over geological timescales, and they often have existing infrastructure that can be repurposed.
Beyond storage, captured CO2 can also be utilized as a resource in various applications, known as Carbon Capture, Utilization, and Storage (CCUS). One use is creating synthetic fuels, where CO2 can be combined with hydrogen to produce hydrocarbons that serve as alternatives to traditional fossil fuels for sectors like shipping and aviation. This transformation helps reduce greenhouse gas emissions by displacing fossil resources.
Captured CO2 can also be incorporated into building materials, such as concrete, where the carbon becomes chemically bound within the material, preventing its release back into the atmosphere. CO2 also applies in industrial processes, such as the fertilizer industry for urea manufacturing and in enhanced oil recovery (EOR), where it is injected into oil reservoirs to extract more oil. While some utilization pathways offer climate benefits, permanent geological storage remains the primary focus for large-scale CO2 reduction due to the limited market size for many CO2-derived products.