Swiss air

Giant fans are sucking in fresh air from the Swiss Alps and Iceland's frozen interior, capturing the carbon dioxide and turning it into fizzy drinks or burying it deep underground.

Tom Heap gets up close to the extraordinary Climeworks device at the Science Museum in London and talks to the team that's developed it to ask if they're the solution to climate change or a symbol of our failure to cut carbon emissions. Dr Tamsin Edwards of King's College London joins Tom to crunch the numbers.

Listen now on BBC Radio 4

What our experts say

We asked Society Fellows Dr Samuel Krevor and Professor Nilay Shah from Imperial College London, and Professor Jon Gluyas from the Durham Energy Institute, to offer some observations on the potential of direct air capture facilities in reducing carbon emissions. Their points take some of the themes of the programme a step further.

What are the limiting factors?

Dr Samuel Krevor

It is not clear that incentives can be strong enough to drive growth in direct air carbon capture and sequestration (DACCS) to gigaton per year scales (gt/year) by mid-century. Integrated assessment models used to identify technology pathways to climate change mitigation permit extraordinarily high rates of annual growth or scaleup in technologies for long periods of time. If we consider more modest, but still historically ambitious, scenarios of an initial 50 megatonnes per year (Mt yr-1) injection achieved by 2030 (Note the IEA SDS only assumes 10 Mt yr-1 achieved by 2030), followed by 10% annual growth, then by 2050 DACCS is achieving around 350 Mt yr-1.

Using the first large saline aquifer storage project as a benchmark, the mineralisation process is around 20 years behind the scale of deployment accomplished in saline aquifers. If all goes well in the development of the mineralisation technologies then a similar development trajectory would suggest around 50 Mt/yr could be achieved by 2050.

Deploying DACCS at scales of 0.5 Gt/yr will place demands on chemicals for the sorbent material that are equal to or larger than the total global demand today; e.g., sodium and potassium hydroxide production will need to be scaled up. However, given that global production at this scale is already demonstrated, then it seems plausible that this could continue or be expanded to accommodate demand from DACCS. Manufacture of tens of thousands of air capture plants is commensurate with large scale manufacture, e.g., of automobiles, airplanes, powerplants (see Realmonte et al. 2019 for an efficient discussion of this).

Saline aquifer storage resources are estimated at >10,000 gigatons of carbon dioxide (Gt CO2) globally, but in highly uncertain assessments. A low-end benchmark may be to assume that the carbon dioxide (CO2) is only stored in those reservoirs where oil has been produced historically (around a trillion barrels), which would amount to around 350 Gt CO2. Even this low figure is more than enough to accommodate the above estimated rates of DACCS. Thus, DACCS should not be limited by the availability of geological storage sites.  

There are at times concerns expressed about the potential for CO2 to leak back to the atmosphere once stored. Carbonate minerals are the thermodynamically favoured end-state of carbon at near-surface conditions. Buoyant fluids (oil, gas, CO2) have been trapped underground over geologic timescales, and storage engineered over decadal-centurial timescales. Issues of using more marginal reservoir sites, and issues of induced seismicity may emerge as Gt scales of CO2 storage are approached. But we cannot currently predict how much of a limiting factor this may be. Once we are at the scale of injecting 100s of Mt/yr we should have a better idea of potential geophysical limitations.

Professor Nilay Shah

Technology development and deployment needs to start early enough to avoid bottlenecks and leaving too much to do in the 2040s. Renewable power generation would also have to scale quickly to match the demand. Finally, we need permanent carbon dioxide storage.

What are the co-benefits?

Dr Samuel Krevor

• DACCS avoids CO2 transport infrastructure: CO2 storage of over 1 Gt yr-1 is fluids handling, similar in scale to the current hydrocarbon industry; minimising transport infrastructure (pipes, tanker ships, etc.) can mitigate the industrial footprint of the activity.

• DACCS allows for a reduction in atmospheric CO2 concentration, mitigating past emissions and overshoot. With the carbon budget to maintain warming <1.5ºC, down in the low hundreds of Gigatonnes of CO2, DAC may be necessary to achieve this. 

• DACCS directly mitigates the leading cause of climate change, i.e., CO2 emissions, rather than treating some of the impacts, as with other geoengineering technologies.

• The cost of DACCS places a ceiling on the cost of mitigating climate change; Bill Gates does this calculation in his recent book “How to avoid a climate disaster”: $100 per ton of CO2 x 51 gigatonnes of CO2 per year = $5.1 trillion per year ($100/t CO2 x 51 GtCO2/yr = $5.1 trillion/yr), or 6% of the global economy. Of course, we would not use it in this way, but it provides a simple way to bound potential costs of mitigating climate change.

• DACCS may be useful as an approach to offsets for some hard to decarbonise sectors, e.g., airline transport.

Professor Nilay Shah

• Direct air carbon capture and storage systems can be located wherever the energy system and carbon storage opportunity is optimal and does not need to be near the emissions source.

• They can provide economic development opportunities.

• They can be used on non-productive land and the land use per total carbon dioxide (tCO2) is low

• Some versions can also co-produce water which could be useful in arid regions. It also has a high capacity.

Professor Jon Gluyas

• Such systems could be set up anywhere on the planet, irrespective of where or how the CO2 is emitted.

• If renewable sources of energy are used to maintain operation of the units then that is very positive.

Potential negative impacts of this idea?

Dr Samuel Krevor

Expensive mitigation: It is far more efficient and less expensive to avoid emitting CO2 or capture CO2 at high flux and high concentration point sources like power plants than to extract it from the air.

The issue of perverse incentives: Some environmental NGOs express concern that technologies like DACCS are cover for the fossil fuel industry to avoid action, or that they will use these technologies to permit their continued exploitation of fossil fuels. Over 90% of CO2 storage (through carbon capture) is associated with enhanced oil recovery, and the first 1 megatonne per year DACCS plant is being developed for combined use with enhanced oil recovery (EOR). In Michael Mann’s recent book The New Climate War, the positive potential of the technologies were acknowledged. However bad-faith advocates of DACCS were memorably described as in-activists for reasoning along these lines.

Professor Jon Gluyas

If renewable sources of energy are not used to power the system then the purpose is compromised. Such direct capture systems can only answer a modest part of the CO2 problem, given the volume emitted globally.

Making fuels from CO2 is possible but it requires more energy to make the fuels than you get from burning them. This means that you still need more energy from somewhere and it must be renewable energy. This is true of almost all applications/uses of CO2. Currently, the few possible uses of CO2 that can liberate energy have not yet been commercialised and it is questionable as to whether they could be sufficient scale to make an impact on emissions. For example, the reinfection of CO2 into basalt in Iceland, while great in concept and application, is currently modest in volume terms. Direct air capture is not an end in itself.

Further reading

• Global CCS Institute. CCS Facilities Database, Global CCS Institute

• Gluyas, J.G. and Mathias, S.A. (2013), Geological storage of carbon dioxide (CO2), Geoscience, technologies, environmental aspects and legal frameworks, Woodhead Publishers

• Hanna, R., Abdulla, A., Xu, Y. and Victor, D. (2021), Emergency deployment of direct air capture as a response to the climate crisis, Nature Communications 12, 368

• Iyer, G., Hultman, N., Eom, J., McJeon, H., Patel, P., & Clarke, L. (2015), Diffusion of low-carbon technologies and the feasibility of long-term climate targets, Technological Forecasting and Social Change, 90, 103-118

• Oil and Gas Climate Initiative, CO2 Storage Resource Catalogue

• Realmonte, G., Drouet, L., Gambhir, A. et al. (2010), An inter-model assessment of the role of direct air capture in deep mitigation pathways, Natural Communications 10, 3277

• Snæbjörnsdóttir, S. Ó., Sigfússon, B., Marieni, C., Goldberg, D., Gislason, S. R., & Oelkers, E. H. (2020), Carbon dioxide storage through mineral carbonation, Nature Reviews Earth & Environment, 1(2), 90-102.

• Staff, R. (2020), Occidental-backed company will build new U.S. CO2 removal plant, Reuters

• The Royal Society, Greenhouse Gas Removal

About the series

39 Ways to Save the Planet is a new radio series by BBC Radio 4 developed in partnership with the Society and broadcast in 2021. It showcases 39 ideas to relieve the stress that climate change is placing on the Earth. In each 15 minute episode Tom Heap and Dr Tamsin Edwards meet the people behind a fresh and fascinating idea to cut the carbon.

Programme website

Over the course of 2021, the Society will be producing events and digital content to accompany the series.