Direct Air Capture Explained: How We Remove CO2 from the Atmosphere

February 10, 2025 Earthmover Science Team

Of all the carbon removal technologies on the horizon, direct air capture (DAC) is perhaps the most radical in its ambition. While most carbon removal approaches rely on biological processes — trees, soil microbes, kelp, or agricultural practices — to draw CO2 out of the atmosphere, DAC uses industrial chemistry and engineering to accomplish the same goal mechanically. In theory, this means DAC can be sited almost anywhere, scaled proportionally to the energy supply available, and run with a level of precision and predictability that biological systems cannot match. In practice, it remains expensive, energy-intensive, and early in its development curve. Understanding both its promise and its constraints is essential for anyone working in carbon markets or climate policy.

The basic concept of direct air capture is simple. Air is drawn through a contactor system that contains a chemical sorbent — either a liquid solvent or a solid material — that preferentially binds CO2 molecules while allowing nitrogen, oxygen, and water vapor to pass through. Once the sorbent is saturated with CO2, it is heated (or subjected to a pressure swing) to release the captured gas in a concentrated stream. That concentrated CO2 can then be compressed and injected into geological formations for permanent storage, or used as a feedstock for synthetic fuels, materials, or other products. The challenge is doing all of this efficiently enough, and cheaply enough, to make it a viable climate intervention at scale.

The Chemistry: Liquid Solvent vs. Solid Sorbent Systems

There are two main chemical approaches to direct air capture that have reached commercial or near-commercial status. The first, pioneered by Carbon Engineering (now acquired by Occidental Petroleum), uses a liquid solvent system. Air is passed over a potassium hydroxide (KOH) solution in an open-air contactor tower. The CO2 in the air reacts with the KOH to form potassium carbonate (K2CO3). This carbonate solution is then mixed with calcium hydroxide (Ca(OH)2) in a pellet reactor, causing calcium carbonate (CaCO3) to precipitate out. The calcium carbonate is heated in a calciner to around 900°C, releasing pure CO2 and regenerating calcium oxide (CaO), which is slaked back to Ca(OH)2 and returned to the cycle. The system is thermally intensive but well-understood, drawing on a century of industrial chemistry.

The second approach, commercialized by Climeworks and several other companies, uses solid sorbent materials — typically amine-functionalized porous structures — to capture CO2 at ambient temperatures. These materials bind CO2 at room temperature and release it when heated to a moderate temperature, typically 80–120°C. Because the release temperature is much lower than in liquid solvent systems, solid sorbent DAC can use waste heat or low-temperature geothermal energy as its heat source, potentially reducing the overall energy requirement. Solid sorbent systems are more modular and easier to scale incrementally, but the sorbents degrade over time and must be replaced, adding to operating costs.

Energy Requirements and the Carbon Math

The energy requirements for DAC are substantial and must be carefully accounted for in any honest assessment of its climate value. Current commercial solid sorbent systems require approximately 1,500–2,000 kilowatt-hours of energy per tonne of CO2 captured, split roughly 80/20 between heat and electricity. Liquid solvent systems require roughly 1,750–2,500 kWh per tonne. For context, an average American household consumes about 10,500 kWh of electricity per year — meaning that capturing one tonne of CO2 via DAC requires the equivalent of several weeks of household electricity consumption.

The critical implication is that DAC must be powered by low-carbon or zero-carbon energy sources to deliver a genuine net carbon removal. If a DAC plant is powered by coal or natural gas, the emissions from generating that energy could equal or exceed the CO2 captured. The most favorable deployment scenarios pair DAC with dedicated renewable energy — solar, wind, or geothermal — or with excess electricity during grid curtailment events. Iceland's Hellisheidi geothermal field, where Climeworks' Orca and Mammoth plants are located, provides one of the most favorable energy profiles in the world for this reason. Scaling DAC globally will require securing large quantities of low-cost, low-carbon energy — a challenge that intersects directly with the broader energy transition.

Cost Trajectory and Deployment Milestones

The current cost of DAC is the most frequently cited barrier to its deployment at scale. Early commercial plants from Climeworks charge $400–$1,000 per tonne of CO2 removed, reflecting the costs of a first-of-kind technology with limited economies of scale. Carbon Engineering's plant design, which uses liquid solvents in a larger, more energy-efficient configuration, has been projected at $300–$400 per tonne at commercial scale. These costs stand in stark contrast to nature-based carbon credits, which often trade at $10–$50 per tonne, and even to other engineered removal approaches like biochar ($100–$300 per tonne) or enhanced weathering ($50–$200 per tonne).

However, DAC's cost trajectory is attracting serious attention because of its learning rate — the rate at which costs decline per doubling of cumulative capacity. Early estimates from researchers at MIT and the Rocky Mountain Institute suggest a learning rate of 15–20% for DAC, comparable to solar photovoltaics and battery storage. If this holds, costs could potentially fall to $100–$150 per tonne by 2035 and below $100 per tonne by 2040 as deployment scales. The U.S. Department of Energy's "Carbon Negative Shot" initiative has set a target of $100 per tonne by 2032, and has committed $3.5 billion in funding toward large-scale DAC hubs that could demonstrate and accelerate this cost reduction.

Permanence and Verification Advantages

One of DAC's most important advantages over biological carbon removal approaches is the permanence and verifiability of its carbon storage. When CO2 captured by a DAC plant is injected into geological formations — deep saline aquifers or depleted oil and gas reservoirs — it is chemically and physically isolated from the atmosphere for timescales of thousands to millions of years. This is as close to permanent removal as currently achievable, and it represents a fundamentally different risk profile from forest carbon credits (vulnerable to fire, disease, and land use change) or soil carbon sequestration (vulnerable to tillage, drought, and land management changes).

From a measurement and verification standpoint, DAC with geological storage is also the most straightforward carbon removal pathway to quantify. The amount of CO2 captured is directly measurable via industrial flow meters and sensors with accuracy of better than 1%. The amount stored underground can be monitored via seismic surveys, pressure monitoring, and tracer gas analysis. This high verifiability means that DAC credits can command premium prices in voluntary carbon markets and are increasingly favored by corporate buyers who face scrutiny over the quality of their carbon claims. Microsoft, Shopify, Swiss Re, and others have made large advance purchases of DAC credits precisely because of this verifiability advantage.

Integration with Carbon Markets and Corporate Net-Zero

The market for DAC credits is nascent but growing rapidly. Stripe's Frontier initiative, which has committed $925 million in advance purchases of DAC and other engineered carbon removal credits through 2030, has been instrumental in providing demand signals that give project developers confidence to build. The Science Based Targets initiative (SBTi) Net-Zero Standard, published in 2021, requires companies reaching their 2050 net-zero targets to neutralize any residual emissions with permanent carbon removal — a requirement that many analysts expect to drive significant DAC demand as other decarbonization options are exhausted.

For Earthmover, DAC represents a critical part of the measurement and verification landscape we serve. While DAC's own carbon accounting is relatively straightforward, the broader question of where DAC fits within a corporate carbon portfolio — how it compares to other removal methods, how it should be weighted against emissions reductions, and how it contributes to a credible net-zero claim — requires exactly the kind of nuanced, science-based analysis we provide. As the DAC sector scales and prices fall, we expect measurement and integrity services to become an increasingly important part of ensuring that the market delivers genuine climate outcomes.

Key Takeaways

Direct air capture uses industrial chemistry — liquid solvents or solid sorbents — to extract CO2 directly from ambient air.
Current costs range from $300–$1,000 per tonne, but learning rates suggest costs could fall below $100/tonne by the late 2030s.
DAC requires 1,500–2,500 kWh of energy per tonne of CO2 captured and must use low-carbon energy to deliver genuine net removal.
Geological storage of captured CO2 provides near-permanent removal with high verifiability — key advantages over biological approaches.
Corporate buyers increasingly favor DAC credits because of their permanence and ease of independent verification.
The U.S. DOE Carbon Negative Shot and Frontier initiative are driving demand and cost reduction through advance purchases and R&D funding.

Conclusion

Direct air capture is not a silver bullet, and it is not cheap. But it is one of the very few carbon removal technologies that can be deployed at arbitrary scale, independent of land availability, ecosystem constraints, or agricultural practice adoption rates. For that reason, virtually every credible net-zero scenario for 2050 includes hundreds of millions to billions of tonnes of annual DAC capacity. The question is not whether DAC will play a major role in the climate solution — it is how quickly costs can fall, how the necessary clean energy can be sourced, and how the measurement and verification infrastructure can be built to ensure that every tonne claimed is genuinely delivered. Those are the questions that drive our work at Earthmover every day.