After finishing his Master’s thesis of an Life Cycle Assessment (LCA) of Direct Air Capture, Stefan reflects on the usefulness of the technology.
When talking about climate change, Direct Air Capture (DAC) has become a polarizing buzzword – while some put a lot of hope into it, others decline it resolutely. DAC is a technology that aims at capturing CO2 directly from the ambient air and is distinguished from point-source carbon capture where CO2 is captured from exhaust gas streams of plants (e.g coal-fired power plants). Even though this sounds like a minor difference, it has significant implications. Particularly, there are two crucial differences. Firstly, the concentration in the ambient air is with about 420 ppm significantly lower than the concentration in exhaust gas streams with up to 100,000 ppm, leading to a significantly higher energy demand for DAC. Secondly, while DAC aims at removing CO2 from the ambient air to eventually reduce the atmospheric CO2 concentration, point-source carbon capture is an abatement technology avoiding CO2 from getting into the atmosphere in the first place. Thus, point-source carbon capture is more efficient in capturing CO2, however, it cannot create so-called negative emissions.
The sense of DAC
In the 2018 special report Global Warming of 1.5 °C, the Intergovernmental Panel on Climate Change (IPCC) outlined a multitude of emission pathways on how to reach the 1.5°C goal. Their patterns are very similar and noteworthy, there is no scenario forgoing without net negative global emissions at one point. That means, that more CO2 is removed from the atmosphere than added to it. But why is this? This can be explained by the carbon budget which is a defined amount of CO2 that is allowed to be added to the atmosphere while ensuring a temperature increase not higher than 1.5°C. All scenarios of the IPCC assume though that we will overspend this budget during the first half of the century and thus have to balance it out with negative emissions in the second half. Thus, following the IPCC, negative emissions to some extent are inevitable if we want to reach 1.5°C.
However, DAC is not the only way for the creation of negative emissions. Instead, the probably most obvious way is the planting of trees. Not only are trees capable of capturing and binding CO2, they also offer other services to ecosystems (e.g. regulating the water household) and society (e.g. being a place for recreation). Yet, they have some drawbacks, as there is land competition with food production which is particularly relevant with a growing global population. Moreover, the change in albedo especially in boreal and temperate regions is expected to cause net-warming. This does not mean that afforestation is not a mean for creating negative emissions but it supports the position that a portfolio of negative emission solutions is required; with DAC being a part of it.
The nonsense of DAC
When discussing DAC, a multitude of pitfalls occurs. Due to the low concentration of CO2 in the atmosphere, DAC requires high amounts of energy for the capturing process. To actually achieve overall negative emissions, this energy has to be low-carbon since otherwise the emissions associated with the energy demand counteract the carbon capturing. Powering DAC with today’s German electricity mix for example would lead to more emissions than are removed. At the same time, decarbonizing the industry – be it the chemical, cement or steel – or mobility are dependent on electrification and low-carbon electricity as well. Thus, DAC plant compete for low-carbon electricity with technologies that otherwise would keep emitting CO2.
Beyond, DAC comes with ethical and political concerns. Particularly the risk of perceiving DAC as the “silver bullet” against climate change and a potential slowing down of ambitious climate action is discussed. Moreover, locking in a fossil-based economy due to the trust in DAC may counteract crucial transitions. This is dangerous as it might lead to an “all-eggs-in-one-basket”-strategy which, if DAC would fail at scale, could result in a disaster (a global temperature overshoot of up to 0.8°C tied to this is estimated).
The (non)sense of DAC
So, is the use of DAC sense or nonsense? Well, it depends. Based on the climate action claims of countries and corporates, we are currently far off-track for the 1.5°C target; in fact, we are rather running towards 2.7°C. Thus, if we want to get our act together, we need to reverse our course and the later we do so, the more negative emissions are required. DAC is one core element of a negative emission technology portfolio which would be able to achieve these negative emissions but it comes with disadvantages such as its high energy demand. Therefore, building upon the claim “do our best, remove the rest” of the reinsurance company Swiss Re, “do our very best, remove the rest” is what is needed. It is highly probable that we will need DAC to some extent, however, it needs to be embedded in a decarbonization strategy, focusing on the elimination of emissions. However, if DAC has to play a role in such a strategy, R&D is crucial to increase its performance (i.e. reduce the energy demand) and reduce its costs (with costs of about 500$/t CO2 which is 10 times the current carbon price, DAC is among the most expensive ways of fighting climate change). So, when its time has come, DAC is able to do its bit.
Written by: Stefan Lübke
B.A. Environmental Sciences; MSc. Industrial Ecology
Bui, M., C.S. Adjiman, A. Bardow, E.J. Anthony, A. Boston, S. Brown, P.S. Fennell, et al. 2018. Carbon capture and storage (CCS): The way forward. Energy and Environmental Science 11(5): 1062–1176.
Chestney, N. 2021. U.N. warns world set for 2.7C rise on today’s emissions pledges. https://www.reuters.com/business/cop/un-warns-world-set-27c-rise-todays-emissions-pledges-2021-10-26/.
Deutz, S. and A. Bardow. 2021. Life-cycle assessment of an industrial direct air capture process based on temperature–vacuum swing adsorption. Nature Energy.
Goldman Sachs. 2020. Carbonomics – Innovation, Deflation and Affordable De-carbonization. https://www.goldmansachs.com/insights/pages/gs-research/carbonomics-innovation-deflation-and-affordable-de-carbonization/report.pdf.
IPCC. 2018. Global warming of 1.5°C. https://www.ipcc.ch/sr15/.
Kreidenweis, U., F. Humpenöder, M. Stevanović, B.L. Bodirsky, E. Kriegler, H. Lotze-Campen, and A. Popp. 2016. Afforestation to mitigate climate change: impacts on food prices under consideration of albedo effects.
Realmonte, G., L. Drouet, A. Gambhir, J. Glynn, A. Hawkes, A.C. Köberle, and M. Tavoni. 2019. An inter-model assessment of the role of direct air capture in deep mitigation pathways. Nature Communications 10(1): 1–12.
Rueda, O., J.M. Mogollón, A. Tukker, and L. Scherer. 2021. Negative-emissions technology portfolios to meet the 1.5 °C target. Global Environmental Change 67(May 2020).
SwissRe. 2021. Net-zero emissions: do our best, remove the rest. https://www.swissre.com/sustainability/stories/net-zero-emissions.html.