Q&A: Direct air capture - Hype or hope for a cooler planet?
Please note: We have inserted sources into this Q&A in academic style to remind readers that these texts represent the scientific consensus as accurately as possible - See question 8 to find out more.
This text is part of our package about frontier climate technologies, which includes further Q&As, factsheets, and interviews on carbon capture and storage (CCS), and nuclear fusion.
Content
1. What is Direct Air Capture?
3. What does DAC cost? And what resources are required?
4. How much CO2 can be captured with DAC?
6. Why is DAC needed, for what exactly and to what extent?
7. When will Direct Air Capture be available on a larger scale?
1. What is Direct Air Capture?
The term direct air capture (abbreviated to DAC) refers to a technology for extracting carbon dioxide (CO2) directly from the ambient air. DAC is therefore one of the CO2 removal measures which sit under to the umbrella term Carbon Dioxide Removal (CDR). A distinction is made between conventional, nature-based CDR options (such as afforestation of forests or rewetting of peatlands) and innovative technologies, including direct air capture.
In its Sixth Assessment Report (AR6), the IPCC formulated three requirements for CO2 removal technologies and thus also for DAC (IPCC 2022, AR6, Volume 3: Annex I: Glossary, page 1796):
- The CO2 must be captured from the atmosphere, not from fossil emission sources (such as the combustion of coal or crude oil).
- The storage of carbon dioxide (for instance in geological formations) must be permanent.
- CO2 removal must be the result of human activity, that means it must take place in addition to natural processes.
In direct air capture, carbon dioxide is extracted directly from the air in a chemical process with the help of a binding agent (prerequisite 1 for CDR). DAC plants are not installed at a CO2 source (such as a steelworks), but can theoretically be set up anywhere in the world and can "collect" greenhouse gas that has been released into the atmosphere from many individual sources.
The captured carbon dioxide can then either be stored underground, which is then referred to as DACCS, or it can be used in long-lasting products, such as building materials, for which the abbreviation DACCU ("Direct Air Carbon Capture and Utilisation") is commonly used. Only truly permanent storage that is secured for at least several decades to centuries fulfils the "removal" criteria (requirement 2 for CDR), because only then can the carbon dioxide removed from the atmosphere by DAC reliably contribute to reducing the greenhouse effect.
The special feature of DAC is therefore that carbon dioxide emitted at an earlier point in time and/or at any location can be removed from the atmosphere – this is also referred to as "negative emissions" (IPCC 2022, AR6, Volume 3, Chapter 12.3.1.1, Lyngfelt et al. 2019). However, if the carbon dioxide captured by DAC is used for the production of short-lived products, such as e-fuels, which are quickly burned again, then this does not count as a CDR measure.
2. How exactly does DAC work?
There are several variants of DAC systems with different chemical processes. These differ in terms of energy and resource consumption and whether or not continuous operation is possible.
To "filter" the CO2 from the air in the first process step, either liquid or solid agents, known as sorbents, are used to chemically bind the carbon dioxide. In the second step, the carbon dioxide is separated from the sorbents again by changing the temperature, humidity or pressure, which requires heat and/or electricity. Water is also often needed in considerable quantities, at least in those systems that work with liquid sorbents. However, these systems can usually operate continuously, whereas DACs with solid sorbents cannot operate continuously (but the energy and water requirements are lower). In a third step, the isolated carbon dioxide is purified and processed so that it can be stored or reused.
While some conventional, nature-based methods for removing CO2 from the atmosphere (such as reforestation) have been used for decades and on a large scale for climate protection, DAC technology is still in the early stages of development. Only a few pilot projects with very small capacities are in operation.
3. What does DAC cost? And what resources are required?
For a CO2 removal technology to be used successfully and on a large scale to combat climate change, it must be "worthwhile" – in other words, its costs per tonne of carbon dioxide removed must stand up to comparison with other removal options, and with measures to avoid emissions. In a comprehensive assessment, it is also important to look at side effects, especially possible disadvantages for the environment and people, for example through high consumption of land, energy, raw materials or water.
Costs
DAC is one of the most expensive CO2 removal technologies. In its current Assessment Report, the IPCC states that capturing and injecting carbon dioxide using direct air capture (namely DACCS) is less cost-effective than capturing carbon dioxide at an industrial source (conventional CCS), simply because CO2 is contained in higher concentrations in the exhaust gases and therefore requires less effort to filter out (IPCC 2022, AR6, Volume 3, Chapter 12.3.1.1). The German Academies of Sciences and Humanities explain the reason for the high costs of DAC in clear terms:
"CO2 only makes up a very small proportion of the air (only 0.04 per cent by volume). In order to produce one cubic metre of CO2 with 1.96 kg of CO2, at least 2500 cubic metres of air must be 'filtered'. For one tonne of CO2, this corresponds to around 1.27 million cubic metres of air, even if one hundred percent filter performance is achieved." (Erlach et al. 2022)
DAC systems with solid sorbents generally have higher capital costs (especially high initial investment), while systems with liquid sorbents often have higher running costs due to their high energy requirements (IPCC 2022, AR6, Volume 3, Chapter 12.3.1.1, Fasihi et al. 2019). However, this fundamental difference becomes less important if, for example, the systems use waste heat (from industrial processes, for example) or are built at locations where wind and/or solar power can be produced cheaply.
The actual costs for future DAC plants and therefore the costs per tonne of CO2 removed in 2040 or 2050 are difficult to predict. One very optimistic study arrived at a cost of 50 US dollars per tonne in large parts of the world with strong solar radiation and the use of inexpensive photovoltaics. But this estimate only refers to the capturing plants; the cost of storing the captured carbon dioxide is not included (Breyer et al. 2019). The IPCC report cites several studies that cite costs of between 60 dollars and 1,000 dollars per tonne of CO2 removed for different types of plants and different stages of development. The IPCC itself speaks of 100 to 300 dollars per tonne (IPCC 2022, AR6, Volume 3, Chapter 12, Executive Summary).
Current data on the costs of DAC plants already in operation is provided by the regularly published report “The State of Carbon Dioxide Removal,” whose main authors work at renowned institutions and have also contributed to IPCC reports. According to the second edition of the report, anyone wishing to remove one tonne of carbon dioxide from the atmosphere using DAC in 2022 would have to pay the operating companies an average of 1,261 dollars, while the average price in 2023 was 715 dollars. In comparison: according to the report, CO2 removal using conventional methods (such as reforestation) cost between 12 and 16 dollars per tonne, while some emission reduction measures only cost around a third of this amount. Novel CO2 removal technologies are therefore currently "a factor of 100" above the costs that would have to be spent to avoid or minimise carbon dioxide emissions.
An expert survey (Shayegh et al. 2021) revealed that although the costs of direct air capture are expected to fall sharply, they will probably still be around 200 dollars per tonne of carbon dioxide in the middle of the century, which is comparatively high. According to the IPCC, the wide range and uncertainty of costs is one of the main obstacles to the use of DAC.
Energy
DAC plants require energy for all process steps, especially for the release of carbon dioxide from the liquid or solid binding agents and their regeneration, but also for fans and pumps or for compressing the CO2 for transport (Fuss et al. 2018). Some systems with liquid sorbents require high temperatures of around 700 to 900 degrees Celsius to separate the CO2 , while other DAC systems manage with much lower temperatures (Keith et al. 2018). One study put the total energy requirement of DAC technologies at around 80 percent heat and 20 percent electricity (McQueen et al. 2021).
More than a decade ago, a working group of the American Physical Society (APS) put the theoretical minimum energy requirement for the separation of carbon dioxide from the ambient air at around 0.5 gigajoules per tonne of CO2 in a foundational paper (Socolow et al. 2011). According to published estimates, however, the cost of the overall process is around eight to twenty times higher at four to ten gigajoules (Fasihi et al. 2019). Even if only a fraction of the current global greenhouse gas emissions were to be filtered out of the atmosphere at some point, this would still result in a huge energy requirement. The previously cited report “The State of Carbon Dioxide Removal” from 2024 states (page 156):
"Large-scale DACCS deployment requires [...] significant energy requirements, on the order of a quarter to a third of today’s global energy production.” (see also Yang Qiu et al., 2022).
The efficiency of a DAC system depends heavily on the chosen technology and the location of the system (Terlouw et al. 2021). If waste heat is utilised (as from other industrial plants) and the proportion of low-carbon energy in the total energy input (including the production of chemical raw materials) is high, the climate impact is high. If, on the other hand, DAC systems were operated with a CO2-intensive power supply (particularly many fossil fuels in the electricity mix), this could lead to the plant producing more CO2 during its operation than it removes from the air.
Water
Many plants that operate with liquid binding agents require large amounts of water, while those that use solid sorbents sometimes produce water themselves. The question of how much water DAC plants could require in the future ranges widely: according to estimates, the annual removal of ten billion tonnes of carbon dioxide from the atmosphere by DAC plants (the amount corresponds to around a quarter of global CO2 emissions in 2023) would require between ten and one hundred cubic kilometres (km3) of water per year (IPCC 2022, AR6, Volume 3, Chapter 12.3.1.1). For comparison: Lake Constance, Germany's largest lake, has a water volume of 48 km3.
Raw materials
The various DAC technologies require different substances or chemicals to produce the liquid or solid solvents or binders needed in the plants (such as hydroxide solutions, zeolites). According to studies, far-reaching production changes in the chemical industry (and a huge amount of energy) would be necessary to ensure that these raw materials are available in sufficient quantities for the establishment of DAC on a large scale (Realmonte et al. 2019).
Land surface area
The land consumption of the actual DAC plants is not particularly large (Madhu et al. 2021). The "Mammoth" plant in Iceland, which is run by industry pioneer Climeworks, essentially consists of just 72 containers, three of which are stacked on top of each other. Although the individual plants must be installed at some distance from each other so that they do not "breathe in" the already filtered air without CO2, even then the direct land consumption is of little concern. However, the picture changes considerably if the energy requirements of DAC systems are also taken into account, especially the areas that would be required for their own solar or wind farms, for example.
It should also be borne in mind that it must either be possible to store the CO2 on site — which places special demands on the geological conditions at the plant site — or that the carbon dioxide must be transported to suitable storage sites, for example via pipelines (Nemet et al. 2018). This incurs costs, as does the continuous maintenance and safeguarding of the storage sites.
4. How much CO2 can be captured with DAC?
In principle, the amount of carbon dioxide that can be removed from the air by the chemical processes in DAC systems is open-ended – but limiting factors are the high demand for energy and materials as well as the capacity of long-term safe CO2 storage options. In the scientific literature (such as Breyer et al. 2020), there are very optimistic models of around 150 billion tonnes of annual CO2 removal in 2050 at comparatively low costs in North Africa alone (however, these models primarily considered energy generation and costs, but not the capacity of geological formations to absorb the removed CO2 ). Other estimates, which also take into account underground storage options and other factors, are far more cautious and expect 0.5 to 5 billion tonnes of CO2 to be removed worldwide per year by 2050 (Fuss et al. 2018). By comparison, Germany's emissions alone amounted to around 0.6 billion tonnes in 2024, while global CO2 emissions amounted to more than 40 billion tonnes.
In its Sixth Assessment Report (IPCC 2022, AR6, Volume 3, Chapter 12.3.1.1), the IPCC points out that a more systematic analysis of DAC potentials is required, particularly with regard to heat, electricity, water and material requirements, the availability of geological carbon dioxide storage and the land requirements of energy sources such as solar and wind power plants (see also Section 3). Special attention is paid to underground storage, namely the "storage" part of DACCS. Theoretically, according to the IPCC, around 10,000 billion tonnes of CO2 can be stored in geological structures worldwide, but the practical potential is only likely to be around a tenth of this (IPCC 2022, AR6, WG3, Chapter 6.4.2.5) - more on this topic in our Q&A text on CCS.
5. What are the risks?
There are no particular risks associated with CO2 removal from the air itself, but the technology's energy, land, water and resource consumption (see section 3) produce significant impacts. Each or all of these could mean that DAC systems have a negative impact on the environment in certain locations (like liquid DAC systems with high water consumption in a dry environment) or, when considered as a whole (for example including the energy required to produce the chemicals used), removing significantly less CO2 than indicated for the direct operation of the system. If, on the other hand, DAC plants are set up at locations where favourable renewable energy, waste heat and good access to CO2 storage sites are available, DAC systems can operate in a more climate and environmentally friendly and cost-efficient manner.
Further potential risks may arise from the subsequent underground storage of carbon dioxide. For example, when CO2 is injected into certain geological formations, saline water could be displaced from there into the groundwater. Possible CO2 leaks are also being discussed, should the underground storage facilities leak at some point. The latter would reduce the effectiveness of DACCS measures because some of the greenhouse gas would be released back into the atmosphere. Overall, however, these risks are considered to be low for professionally operated storage facilities (more on this in our separate text on CCS technology).
Researchers are also discussing another risk that technologies such as DAC could indirectly harbour for climate policy: Governments and the public could take an overly optimistic view of their potentials despite the great uncertainties. The expectation of future CO2 removals from the air harbours the risk of delaying or even omitting other climate protection measures in the present (especially emission avoidance) that are perceived as more arduous (McLaren et al. 2021). One study warns that if policymakers do not pursue other climate protection options because they assume DAC can be deployed at large scale in the future, and it turns out to be unavailable, this could lead to additional global warming of up to 0.8 degrees Celsius (Realmonte et al., 2019).
The IPCC also has clear words on this issue:
“Carbon dioxide removal (CDR) is a necessary element of mitigation portfolios to achieve net zero CO2 and GHG emissions both globally and nationally, counterbalancing residual emissions from hard-to-transition sectors such as industry, transport and agriculture. CDR is a key element in scenarios that limit warming to 2°C […] [But] CDR cannot serve as a substitute for deep emissions reductions”. (IPCC 2022, AR6, Volume 3, Chapter 12, Cross-Chapter Box 8).
6. Why is DAC needed, for what exactly and to what extent?
There is a consensus among researchers that CO2 removal technologies should not be a substitute for or a distraction from the fastest and strongest possible emission reductions. At the same time, measures such as DAC with underground CO2 storage (CCS) are considered "a necessary element" in order to limit global warming to 2 degrees Celsius or less (IPCC 2022, AR6, Volume 3, Chapter 12, Executive Summary).
The IPCC speaks of several functions that CO2 removal technologies (CDR) can fulfil in climate protection in addition to other measures:
- Further reduce net greenhouse gas emissions from human activities in the short term (namely beyond what can be achieved through emission reductions alone);
- Offset residual emissions from sectors that are difficult to abate in the medium term, for example CO2 from industrial plants or long-distance transport (such as aircraft or container ships), as well as methane or nitrous oxide from agriculture; even with strong climate protection, some of these emissions cannot be avoided, but by simultaneously removing CO2 from the atmosphere, a net zero level of total greenhouse gas emissions can still be achieved;
- Contribute in the long term to achieving and maintaining even net negative emissions - in other words, reducing the concentration of greenhouse gases in the atmosphere. To achieve this, CDR technologies would have to be used on a scale that exceeds the residual emissions of greenhouse gases (IPCC 2022, AR6, Volume 3, Chapter 12, Cross-Chapter Box 8).
CO2 removal technologies have therefore long played an important role in climate protection scenarios in research. However, estimates of how much capacity would be needed to limit global warming to below two degrees Celsius vary widely in some cases. According to the IPCC, the various CDR technologies should be able to remove around 5.75 billion tonnes of carbon dioxide from the atmosphere worldwide in 2050; DACCS is estimated to have a maximum capacity of 1.74 billion tonnes (IPCC 2022, AR6, Volume 3, Chapter 12, Executive Summary). The estimates in the report “The State of Carbon Dioxide Removal” 2024 are even higher at seven to nine billion tonnes as the total level of CO2 removal by all CDR methods in 2050. The climate protection plans of countries around the world to date are nowhere near sufficient to reach this level – research therefore refers to a "CDR gap" (Lamb et al. 2024).
A team from the private research institute Rhodium Group has calculated how direct air capture could contribute to achieving net zero emissions by 2045 in the U.S. In two scenarios, a weaker use of DAC (annual removal capacity of around 0.6 billion tonnes) and a stronger use (1.8 billion tonnes of CO2 removal per year) were calculated. To make this possible, almost 700 or more than 2,200 large-scale DAC plants with an annual capacity of one million tonnes of CO2 removal would have to be built. To put this task into context, the publication cites a comparative figure: there are currently 613 power plants or industrial facilities in the USA, each of which emits at least one million tonnes of carbon dioxide per year or more.
7. When will Direct Air Capture be available on a larger scale?
There has already been a strong upturn in research and development into DAC technology worldwide in recent years – both through government funding programmes and private companies. However, the number of plants already in operation and, above all, their capacity is still very small. The largest projects to date are those of the start-up Climeworks in Iceland. Their pilot plant, dubbed "Orca", has an annual capacity to trap 4,000 tons of CO2, but it has never captured more than 1,000 tons in any year since its construction was completed in 2021. Their follow-up plant "Mammoth" began first operations in 2024 and is meant to reach a capacity of 36,000 tons, but completion is running slower than planned. Further, larger projects have been announced, particularly in the USA and Canada – the research team at the report “The State of Carbon Dioxide Removal” collates these regularly.
However, even the capacities of all announced DAC projects are only a fraction of what is already being removed from the atmosphere each year with traditional, natural CO2 removal projects. According to the report The State of Carbon Dioxide Removal, this amounted to a good two billion tonnes (in other words: 2,000 million tonnes) in 2023 – the DAC capacities to date account for less than 0.1 per cent of this amount.
With a previous contribution of less than 0.01 million tonnes of CO2 removal per year through DAC, the application of this technology is still in its infancy. However, announcements by various companies raise hopes of a sharp increase in CO2 removal through new DAC projects in the coming years.
Investments in start-ups in the field of carbon dioxide removal have increased significantly over the past decade, according to the authors of “The State of Carbon Dioxide Removal” 2024 – but still only account for 1.1 per cent of total investments in climate technology start-ups. The voluntary CO2 market, where companies can buy CO2 offsetting certificates, will play a major role in financing new DAC plants. However, government funds are also flowing into the sector at a considerable rate. The USA, Canada, the EU, Australia, Japan, Germany and others have pledged a total of more than 4.2 billion euros – the lion's share is accounted for by a US funding programme called "Regional Direct Air Capture Hubs" with 3.5 billion US dollars over five years. But the future of the US programme is increasingly uncertain, given president Donald Trump's ongoing funding cuts.
At the same time, the authors emphasise that the spread and associated price reductions for new CDR technologies would only be possible if there were a conducive political framework. This means that governments would have to set regulations or targets that create demand for CDR technologies such as DAC. So far, this has been lacking, as have binding regulations, for example for the monitoring and standardised accounting of CO2 reductions. DAC pioneer Climeworks also states that the production of, for example, 10,000 "Mammoth" systems with 72 containers per system is not yet possible simply because there is no corresponding supply chain for the required materials.
The scenarios considered by scientists in The State of Carbon Dioxide Removal Report 2024 assume that conventional CDR methods such as afforestation and agricultural methods to accumulate CO2 in the soil will already be used in the coming decades, while the new methods — including DACCS — will mainly become widespread in the second half of the century. In the first edition of the report, the authors used available data on the commissioning of plants to extrapolate the capacity that could be expected in the future (The State of Carbon Dioxide Removal Report 2023). They came up with a huge range of 7 to 297.5 million tonnes of CO2 by 2030.
The IPCC also expects a significant increase, concluding in its Sixth Assessment Report of 2022:
“Despite limited current deployment, moderate to large future mitigation potentials are estimated for direct air carbon capture and sequestration (DACCS) […]. The potential for DACCS (5 to 40 billion tonnes per year) is limited mainly by requirements for low-carbon energy and by cost (100 to 300 dollars per tonne of CO2). DACCS is currently at a medium technology readiness level.” (IPCC 2022, AR6, Volume 3, Chapter 12, Executive Summary).
8. Why does this text reflect the "scientific consensus"?
Scientists have accumulated a surprising amount of specific knowledge about how effective various technologies can be. Their findings can be found, for example, in the IPCC’s Sixth Assessment Report, which runs to more than 2000 pages.
To highlight that the selection of sources is central to this project, we have deviated from our usual editorial guidelines in this Q&A, and inserted sources in academic style - to remind readers that these texts represent the scientific consensus as accurately as possible.
With this aim in mind, we ranked sources in the following order, which values relevance much more than a very recent publication date:
1) Wherever possible, the texts rely on the IPCC, which provides highly reliable summaries and assessments of the state of research.
2) A second-best are thorough meta-studies (studies that evaluate many other studies), as well as synthesis reports from large research consortia or organisations, where a broad circle of participants and intensive review processes are common.
3) Only in third place, we used individual studies - limited to publications in recognised research journals that guarantee a peer review process, meaning each publication is checked by competent specialist colleagues.
This Q&A and others in this series are based on texts published by our German-language sister project Klimafakten, which were written by expert journalists, and double checked by relevant experts. Two foundations supported this editorial project, which was overseen by our colleague Toralf Staud: the Marga und Kurt Möllgaard-Stiftung and the Deutsche Bundesstiftung Umwelt.
