The relevance of Negative Emission Technologies (NETs) in the scientific and public discourse on climate change stems from the fact that, according to IPCC [1], all the assessed pathways to limit global warming to 1.5 °C include carbon dioxide removal and, in most cases, net negative emissions in the second half of this century. Even allowing for a 2 °C limit, only a minority of scenarios do not include negative emissions. Annual removal up to ~20 Gt of carbon dioxide by the end of the century is implied by Integrated Assessment Modelling (IAM) and it is considered technically achievable. Together with afforestation, which has limited sequestration potential, the most explored technology in IAM has so far been Bioenergy with Carbon Capture and Storage (BECCS). However, this solution demands huge land areas, and scaling it up to the required extent would raise concerns about carbon neutrality, biodiversity conservation, competition with food production and with other land uses [2]. Competing NETs with lower land requirement exist, but IAM structure tends to favor BECCS due to its versatility in energy production [3].
Though technically possible, the deployment of NETs at the scale assumed in IAM may turn out to be unfeasible [4]. IAM is indeed based on strong assumptions about the future that may prove wrong. For example, the trade-off between strong mitigation in the near term and later carbon removal is ultimately set by the level of discounting of future costs. Besides, IAM unrealistically assumes a single, perfectly informed policy maker for all the world. More in general, massive development of NETs would require societal acceptance and adequate governance, which shouldn’t be taken for granted in view of the huge costs implied and the already mentioned environmental concerns. We can adequately apply here the term “Deep Uncertainty”, proposed in the context of prediction-based decision analysis to describe circumstances where the key assumptions on which predictions are based are highly uncertain [5].
Excessive reliance on NETs may weaken the near-term mitigation efforts, creating the risk of rapid temperature surge if those technologies fail at scale [2]. Even so, since NETs are needed to achieve the temperature target(s), we must deal with them: research in carbon dioxide removal is bound to expand, and we can suppose that large-scale initiatives will be at least attempted. Hence, the question arises how NETs are and will be framed in the scientific and public discourse, how they will enter the imagery of society and, consequently, how they will affect people’s feeling of uncertainty related to climate change. To this regard, I see two major ways of interpreting NETs. The first one follows their gradual integration among the measures to combat climate change, as envisioned by IAM. In this situation, the world embarks on a climate safe pathway and NETs are thought of as a price that must necessarily be paid. However, in view of the current divergence between the global emission pledges and the efforts implied by the Paris Agreement [6], a future in which the world has made little progress in mitigating climate change is possible, if not plausible. In this second scenario, rapid and massive deployment of NETs would be invoked as our last chance to avoid the disaster.
I deem that societies may be dangerously inclined to the latter line of thought, if only for the appealing fantasy of having a quick, last-minute solution to an issue that would otherwise require immediate and drastic measures. If we merely consider that carbon removal is technically doable, failing the mitigation commitments for 2050 would not represent an irreversible problem. In fact, it would be possible to transgress the carbon budget for the 1.5 °C or 2 °C thresholds, and later come back thanks to net negative emissions. People might even evolve the misleading sensation that, from a certain point in time on, Earth’s temperature could be controlled through climate engineering. Conceptually, this could be represented as a narrowing confidence interval for global temperature projections, starting somewhere after 2050. From a psychological point of view this is a reassuring perspective, perhaps able to ease the distress related to climate change uncertainties. It is nevertheless clearly unsafe, because it neglects the obstacles that NETs may encounter during the scaling process – namely, the deeply uncertain side of the story.
If the detrimental perception of NETs as “ultimate weapon” became dominant, the risk of failing climate mitigation and leaving the world worse off to a hardly predictable extent would be concrete. For this reason, I think that a careful narrative of carbon dioxide removal will be needed in the years to come.
References:
[1] Rogelj, J., D. Shindell, K. Jiang, S. Fifita, P. Forster, V. Ginzburg, C. Handa, H. Kheshgi, S. Kobayashi, E. Kriegler, L. Mundaca, R. Séférian, and M.V. Vilariño, 2018: Mitigation Pathways Compatible with 1.5°C in the Context of Sustainable Development. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)]. In Press.
[2] Anderson, K., Peters, G., The trouble with negative emissions, Science, 14 Oct 2016 : 182-183
[3] Köberle, A.C. The Value of BECCS in IAMs: a Review. Curr Sustainable Renewable Energy Rep 6, 107–115 (2019). https://doi.org/10.1007/s40518-019-00142-3
[4] Naomi E Vaughan and Clair Gough 2016 Environ. Res. Lett. 11 095003
[5] Mark Workman, Kate Dooley, Guy Lomax, James Maltby, Geoff Darch, Decision making in contexts of deep uncertainty – An alternative approach for long-term climate policy, Environmental Science & Policy, Volume 103, 2020, Pages 77-84, ISSN 1462-9011, https://doi.org/10.1016/j.envsci.2019.10.002.
[6] Rogelj J, den Elzen M, Höhne N, Fransen T, Fekete H, Winkler H, Schaeffer R, Sha F, Riahi K, Meinshausen M. Paris Agreement climate proposals need a boost to keep warming well below 2 °C. Nature. 2016 Jun 30;534(7609):631-9. doi: 10.1038/nature18307. PMID: 27357792.
Hello Anne, thanks for your comment! Yes of course there is research on this, since all scenarios assessed through integrated modelling are evaluated based on the cost of their implementation. But apparently there is no option to stay in the temperature target avoiding NETs (except mitigating a lot while reducing the pro-capita GDP…?). Anyway, in my opinion research should not directed towards demostrating something in which you believe, in this case mitigation vs. carbon engeneering. Instead of comparing the presumed costs of the two options, a perspective that may help is stressing the fact that the costs of near-term mitigation are better known than the costs of carbon dioxide removal in the second half of the century (due to the problem of discounting and to the assumptions on how the technology will evolve). Then, it’s in principle better to incur costs that you can well estimate rather than costs which are poorly known. Hope this was an interesting point!
I agree, Giovanni, NETs can become quite dangerous if we factor them into our emission paths, leading us to be less stringent with mitigation now. One way I could imagine to solve this dilemma would be to find reliable projections of mitigation costs now vs. NET costs in the future (taking into account different discounting rates). If we could show that heavy mitigation now is by far the cheapest (and also least risky) option that might tip the scales a bit, hopefully. Do you know if there is research on this?