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AAmong the strategies proposed to slow climate change, direct air capture (DAC) is distinctive because it removes carbon dioxide already mixed through the atmosphere. Unlike capturing emissions at a smokestack, DAC must work with air that contains only about 0.04% CO₂ by volume. Supporters say this makes the technology flexible: plants could, in theory, be built near geological storage sites or near industries that use CO₂ as an input. Critics reply that the same dilution makes the task energy-intensive, because vast quantities of air must be moved to obtain a useful amount of gas. Despite this debate, pilot and early commercial plants are operating, and DAC has become a test case for how engineering limits influence climate policy.

BMost DAC systems follow the same basic sequence. Fans draw ambient air through an intake and across a contactor, where CO₂ selectively binds to a capture medium. The medium is usually either (i) a liquid alkaline solution, such as potassium hydroxide, or (ii) a solid sorbent, typically a porous filter coated with amine groups. Once the medium is loaded, it is regenerated so the CO₂ is released as a concentrated stream. Regeneration can involve heat, vacuum, steam, or combinations of these. The CO₂ is then dried and compressed for transport, utilisation, or injection underground. Because air movement, heat supply and compression all require energy, the carbon intensity and price of that energy largely determine whether DAC delivers net climate benefits.

COne prominent design uses a liquid solvent loop adapted from established chemical engineering. Air passes through a contactor tower while an alkaline solution flows downward; CO₂ reacts in the liquid to form carbonate compounds. The CO₂-rich solution then enters a causticiser, where calcium hydroxide converts the dissolved carbonates into solid calcium carbonate pellets. These pellets are fed to a calciner, a high-temperature kiln that releases a relatively pure CO₂ stream and leaves calcium oxide behind. In a final step, a slaker adds water to convert calcium oxide back to calcium hydroxide, closing the cycle and allowing continuous operation. The main drawback is the calciner’s demand for high-grade heat, which is easiest to supply with natural gas or large quantities of low-carbon electricity.

DA second family of DAC designs relies on solid sorbents and can operate at lower temperatures. Air is blown through modular units containing filters coated with amines. During adsorption, CO₂ molecules attach to the amine sites. When the filter approaches saturation, the unit is sealed and switched to desorption: pressure is reduced and/or the filter is gently heated so CO₂ detaches. The released gas, far more concentrated than ambient air, is collected and sent to dehydration and compression. Solid-sorbent systems are often presented as easier to scale because additional modules can be installed in parallel. However, performance can be sensitive to humidity and to the long-term stability of the coating. In some trials, water vapour improves capture by enabling bicarbonate formation; in others, it reduces capacity by occupying active sites, depending on the sorbent chemistry.

EThe climate value of DAC depends on what happens after capture. If CO₂ is used to make synthetic fuels, it is usually re-released when the fuel is burned; this can still reduce net emissions if it displaces fossil carbon, but it is not permanent removal. Geological storage, by contrast, aims for long-term containment. Compressed CO₂ is transported—often by pipeline—to a suitable formation such as a deep saline aquifer or a depleted oil and gas reservoir, then injected below an impermeable caprock. Several trapping mechanisms can then act: the plume can be held beneath the caprock (structural trapping), immobilised in pore spaces (residual trapping), dissolved into brines, and in some rocks converted into stable carbonates through mineralisation. Monitoring typically combines pressure measurements, seismic surveys and chemical sampling to detect leakage. In many policy discussions, storage is treated as easier to account for than short-lived utilisation, although public acceptance of underground injection varies.

FCosts and energy intensity remain decisive barriers. Estimates vary with plant scale, local power prices and the chosen capture medium. The International Energy Agency (IEA) has noted that early commercial DAC may cost several hundred US dollars per tonne of CO₂, with potential declines if manufacturing becomes standardised. Researchers at ETH Zurich argue that modular solid-sorbent plants could fall in cost as component supply chains mature, while analysts at Carbon Engineering emphasise learning effects from deploying large liquid-solvent plants. Yet there is no agreement on how quickly costs will drop. In addition, CO₂ must be compressed to pipeline pressures, adding electricity demand, and the fans and heaters must run on low-carbon energy; otherwise, a plant could remove CO₂ while indirectly causing emissions elsewhere. For this reason, some commentators argue DAC should mainly address ‘hard-to-abate’ residual emissions rather than delay rapid emissions cuts.

GEven if DAC becomes cheaper, deployment will depend on geography and governance. Regions with abundant renewable electricity, suitable storage geology and supportive regulation—such as Iceland and parts of the United States—have moved from pilots to larger projects. Elsewhere, siting disputes may arise over industrial land use, noise from large fans and the routing of CO₂ pipelines. Standards for verifying removals are also evolving: credible accounting requires measuring captured mass, subtracting life-cycle emissions and demonstrating that storage remains secure over long timeframes. Proponents argue that transparent monitoring can build trust, while skeptics warn that verification is complex and that overpromising could distract from proven mitigation. A realistic outlook is that DAC grows as one tool among many, with its scale limited by clean energy supply and the availability of safe storage sites.