Introduction
In the context of seeking to reduce greenhouse gas emissions to net zero, policymakers are beginning to pay more attention to options for removing carbon dioxide from the atmosphere – a process referred to as CDR (‘carbon dioxide removal’). Without the rapid scale-up of such measures, achieving Paris Agreement temperature targets will be increasingly challenging: current emissions abatement efforts are not progressing quickly enough to prevent the world from overshooting global emissions targets.
In theory, CDR measures may permit the total costs of a climate mitigation strategy to be reduced in absolute terms (or amortized over a longer period); enable ambitious targets (such as limiting global warming to 1.5°C) to become more feasible; or delay the point when peak emissions are reached, retroactively compensating for overshooting the cumulative carbon budget.
A wide range of potential CDR measures are currently being discussed.1 Alongside afforestation and reforestation, the main option featuring in integrated assessment models (IAMs) is bioenergy with carbon capture and storage (BECCS). BECCS refers to a set of technologies and processes through which the carbon emissions from burning biomass for energy are captured before release into the atmosphere, and then stored in underground reservoirs. If this biomass energy is assumed to be carbon-neutral, BECCS should theoretically result in net negative emissions, as the accompanying carbon sequestered by biomass is permanently stored.
The prominence of BECCS in the models does not, however, represent a prescriptive judgment about its merits relative to other negative emissions options. Nor does it necessarily validate the current, highly constrained, development trajectory of BECCS technology. It is, rather, a reflection mainly of the fact that BECCS is based on well-understood biology, so it is easier to make assumptions about and model the impacts on emissions at various carbon prices. In contrast, evaluating the potential of newer and more speculative negative emissions technologies is more challenging.
In reality, there are many reasons to question the reliance on BECCS assumed in the models – including the carbon balances achievable,2 the substantial demand for land, water and other inputs that is associated with BECCS solutions, and the underlying assumption that technically and economically viable carbon capture and storage (CCS) technologies will be available ‘off the shelf’ in the near term, which is not being borne out in practice.
This is not to argue that BECCS cannot play a role in CDR, but that, as with other options, it offers no silver bullet. Rather, all CDR approaches need to be evaluated on comparable terms against a range of sustainability and socio-political criteria. Such evaluations should inform the development of a portfolio of locally appropriate (and cumulatively globally significant) CDR solutions, including those that are nature-based, such as afforestation and reforestation. Alongside the acceleration of conventional abatement action, these portfolios of solutions will need to be deployed as rapidly as possible. Delays in developing the requisite policy and regulatory frameworks will increase the risks of overshooting atmospheric carbon limits, in turn necessitating much larger and riskier negative emissions initiatives in the future.3
1 For overviews, see European Academies Science Advisory Council (EASAC) (2018), Negative Emission Technologies: What role in meeting Paris Agreement targets?, Halle, Germany: EASAC; Royal Society (2018), Greenhouse Gas Removal, London: Royal Society and Royal Academy of Engineering; Morrow, D. R., Buck, H. J., Burns, W. C. G., Nicholson, S. and Turkaly, C. (2018), Why Talk about Carbon Removal?, Washington, DC: Institute for Carbon Removal Law and Policy, American University, doi:10.17606/M6H66H; National Academies of Sciences, Engineering, and Medicine (2019), Negative Emissions Technologies and Reliable Sequestration: A Research Agenda, Washington, DC: The National Academies Press, doi:10.17226/25259; Fajardy, M., Köberle, A., Mac Dowell, N. and Fantuzzi, A. (2019), BECCS Deployment: A Reality Check, London: Imperial College, Grantham Institute Briefing Paper No. 28, https://www.imperial.ac.uk/media/imperial-college/grantham-institute/public/publications/briefing-papers/BECCS-deployment–a-reality-check.pdf; EASAC (2019), Forest Bioenergy, Carbon Capture and Storage and Carbon Dioxide Removal: An Update, https://easac.eu/publications/details/forest-bioenergy-carbon-capture-and-storage-and-carbon-dioxide-removal-an-update/.
2 These are not always negative. The actual carbon balances achievable vary significantly, depending on a wide range of factors that include, crucially, the feedstocks used.
3 The increased scale would inherently result in increased resource demand and all the downside risks that this would entail, as well as the very real possibility that, once safe atmospheric concentrations of carbon had been exceeded, irreversible earth-system processes (such as the melting of ice caps) might be triggered that no amount of CDR could retroactively reverse.