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AMost climate plans prioritise cutting emissions, yet many researchers argue this alone will not meet mid-century temperature goals. CO₂ persists in the atmosphere for decades, and some activities—such as aviation and cement—are hard to decarbonise rapidly. Direct air capture (DAC) aims to remove CO₂ after it has mixed into ambient air. Unlike “point-source” capture, which treats a concentrated exhaust stream, DAC must process huge volumes because atmospheric CO₂ is extremely dilute (hundreds of parts per million). Supporters say engineering can overcome this; critics reply that dilution makes the process unavoidably energy-hungry. Even so, DAC has progressed from lab trials to small commercial plants and is now discussed alongside reforestation and soil carbon as a form of “negative emissions.”

BCurrent DAC designs mainly use either liquid solvents or solid sorbents. In solvent systems, fans push air through a contactor where CO₂ reacts with an alkaline solution. The “carbonated” liquid is sent to a regeneration unit, where heat and chemical steps release a concentrated CO₂ stream and restore the solvent for reuse. Solid-sorbent systems instead pass air over porous materials coated with amines that selectively bind CO₂. When the material nears saturation, the unit switches to desorption: heating, vacuum, or both cause CO₂ to detach. Solid sorbents are often favoured for modular plants, while solvents can suit larger facilities. In both cases, the output is a CO₂ stream that must be stored or used, and performance depends on controlling airflow, temperature and humidity.

CA solid-sorbent DAC module typically runs as a repeating cycle. First, a fan draws ambient air through a filter to remove dust that could block the sorbent. The air then passes through a capture bed containing a structured material (for example, a honeycomb) coated with amine groups. During adsorption, CO₂ binds to these sites while most nitrogen and oxygen exit as treated air. When sensors show the bed is close to saturation, valves isolate it and the system shifts to regeneration. A vacuum pump may lower pressure to ease release, and heaters raise the bed temperature to speed desorption. The CO₂-rich gas then goes to a condenser to remove water vapour, after which it is compressed into a pipeline-ready stream. Finally, the bed is cooled and returned to adsorption. Plants use multiple beds so that some capture while others regenerate, enabling continuous operation.

DEnergy demand is the constraint most frequently highlighted. DAC needs electricity for fans, pumps and compression, and it also requires heat for regeneration, particularly in solvent systems. If this energy is fossil-based, net removal can fall sharply. For that reason, life-cycle studies usually assume low-carbon electricity and low-grade heat, such as industrial waste heat or geothermal sources. Water and humidity also matter. Many sorbents work best at moderate humidity, but excessive moisture can compete with CO₂ for binding sites and increase the load on condensation and compression. In dry regions, operators may add humidity to maintain capture efficiency; in humid regions, they may have to remove more water downstream. These factors help explain why identical units can perform differently across locations.

EThe climate value of DAC depends on what happens to the captured CO₂. One option is geological storage: CO₂ is compressed and injected into deep saline aquifers or depleted oil and gas reservoirs, where impermeable layers can trap it. Another is mineralisation, where CO₂ reacts with magnesium- or calcium-rich rocks to form stable carbonates, either underground or in engineered reactors. A third route is utilisation—turning CO₂ into fuels, plastics or building materials—but this does not automatically deliver “negative emissions.” The key distinction is storage duration: synthetic fuels may return CO₂ to the air within months, whereas mineralised carbon can remain locked away for geological timescales. In policy debates, these outcomes are sometimes grouped together, obscuring what “removal” really means.

FDAC cost estimates vary because they depend on energy prices, plant scale and how quickly equipment improves with mass production. Early projects reported costs in the hundreds of dollars per tonne of CO₂, though developers expect reductions through better sorbents and manufacturing. The Swiss firm Climeworks has emphasised modular solid-sorbent collectors, while Canada’s Carbon Engineering has promoted large solvent-based plants aimed at economies of scale. Analyses from the Massachusetts Institute of Technology (MIT) suggest that the cheapest designs are not always those that maximise capture per pass, because pushing capture toward 100% can increase pressure drop and fan energy. A UK-led consortium, the Greenhouse Gas Removal Hub, has also argued that monitoring must be transparent: reporting capture rates without accounting for emissions from energy use can make comparisons unreliable.

GDAC is unlikely to be a universal solution. Advocates note that it offers measurable, engineerable removal without the large land footprint of some biological approaches. Skeptics counter that even a modest contribution would require many plants and substantial clean energy, potentially competing with electrification elsewhere. A practical view is that DAC is most defensible when powered by additional low-carbon energy and paired with verifiable long-term storage. In the near term, small plants can function as learning platforms, improving materials, reducing maintenance and refining monitoring. Whether DAC scales to gigatonnes will depend less on a single breakthrough than on infrastructure, regulation and the price society is willing to pay for durable carbon removal.