Is BECCS carbon-negative?
In reality, the net impact on the atmosphere is a combination of all these variables, along with the emission/capture volumes at the point of combustion and counterfactual factors – i.e. what would have happened to the biomass if it had not been burnt for energy?
The vast majority of papers discussing BECCS take as their starting point the assumption that biomass energy is by definition carbon-neutral: i.e., that the emissions from consuming the biomass are part of a natural cycle in which, over time, tree or plant growth balances the carbon emitted on combustion (as long as the trees or crops are regrown after harvesting). Hence, the reasoning goes, if even a proportion of those combustive emissions are captured, BECCS will result in net negative emissions. The assumption of carbon neutrality is very widely held, underlying, for example, all the models of the potential for BECCS reviewed by the IPCC in its Fifth Assessment Report. Although recent studies,36 including some reviewed in SR1.5,37 have increasingly recognized that this central assumption is invalid,38 understanding of the potential limitations of BECCS is not yet widespread. For example, although ways to account for the impact of bioenergy production on soil degradation are actively being developed in IAMs, such impact was not comprehensively accounted for in SR1.5.39
A full carbon lifecycle analysis must take into account a wide range of factors that affect the balance between carbon in biomass and in the atmosphere. These factors include: the impacts of any initial land clearance to grow trees or crops; any indirect land-use effects (e.g. from the clearance of forests to grow agricultural crops displaced by energy crops); any losses of soil carbon during harvesting (which are generally significant); supply-chain emissions from the energy consumed in harvesting, processing and transporting biomass; and – particularly for trees – the time delay until replacement trees are large enough to absorb carbon at the same rate as the harvested trees. In reality, the net impact on the atmosphere is a combination of all these variables, along with the emission/capture volumes at the point of combustion and counterfactual factors – i.e. what would have happened to the biomass if it had not been burnt for energy?
Left to themselves, trees continue to grow and sequester carbon. If trees or energy crops are harvested specifically for energy, not only is the stored biomass converted into carbon dioxide but the future carbon sequestration potential of that vegetation is lost.
Left to themselves, trees continue to grow and sequester carbon. If trees or energy crops are harvested specifically for energy, not only is the stored biomass converted into carbon dioxide (which may or may not be captured) but the future carbon sequestration potential of that vegetation – i.e. the carbon that would have been absorbed during the remainder of the tree or plant’s lifetime – is lost. This foregone sequestration can be replaced if replanting occurs after harvesting, but the initial rate of absorption may be slower; this is particularly true for trees, since although young trees grow faster than mature specimens, their much lower leaf cover means they absorb much less carbon from the atmosphere.40 As a 2014 study concluded:
For most species mass growth rate increases continuously with tree size. Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree.41
There is a difference between the carbon sequestration rates of individual trees and entire forests, however; older forests tend to contain fewer trees, as an increasing number succumb to pests or disease. Nevertheless, a 2008 study concluded that ‘in forests between 15 and 800 years of age, net ecosystem productivity (the net carbon balance of the forest including soils) is usually positive’.42 Older trees and forests also store higher volumes of carbon than their younger counterparts, though there is a risk that – unlike geological carbon repositories that are more or less permanent – vegetative stores of carbon can rapidly be transformed from carbon sinks into carbon sources if unsustainably felled or lost to wildfires; they are therefore vulnerable to social, political, economic and environmental changes, and need safeguarding against dangerous positive-feedback cycles.
The balance is different for faster-growing coppices, where the difference in carbon sequestration rates between young and mature plants is much smaller. Even in these cases, however, it is important to measure the climate impacts of regular planting and harvesting. Tree plantations are much poorer at storing carbon than are natural forests, which develop with little or no disturbance from humans. The regular harvesting and clearing of plantations – which would be necessary to guarantee a regular supply of BECCS feedstock (if wood were the main feedstock) – releases stored carbon dioxide back into the atmosphere every 10–20 years. In practice, as a 2019 study concluded, ‘plantations hold little more carbon, on average, than the land cleared to plant them’.43 By contrast, natural forests continue to sequester carbon for many decades or even centuries, implying that stopping deforestation and promoting natural forest restoration, if and where possible, are likely to be better CDR options than using forests as BECCS feedstock sources. These options will not provide energy co-benefits, but the energy balance of BECCS is in any case contestable (see next section).
Although the potential deployment rates for carbon-negative biomass are frequently lower than suggested by IAMs, the possibilities that its use presents are nonetheless substantial and could be exploited in appropriate circumstances.
Dedicated energy crops grow much more rapidly than trees, and harvesting therefore has a smaller impact on carbon sequestration rates; the main impact on the carbon break-even time – i.e. the time needed until the net carbon balance is zero – derives from the nature of the land-use conversion to establish the crop in the first place. If energy crops are planted on lands with a high carbon stock (e.g. as is the case when established forests are converted to such crops), then it can take anything from decades to over a century to compensate for the carbon losses from the initial land-use change.44 Conversely, establishing energy crops on marginal lands can result in much faster carbon neutrality and much deeper carbon dioxide removals over time (see Figure 2). There is also the potential for positive, synergistic outcomes – such as management of dryland salinity, enhancement of biodiversity and reduced eutrophication – if perennial bioenergy crops are strategically and appropriately integrated with conventional crops.45 Thus, although the potential deployment rates for carbon-negative biomass are frequently lower than suggested by IAMs, the possibilities that its use presents are nonetheless substantial and could be exploited in appropriate circumstances.
Figure 2: Impacts of land-use change on BECCS carbon break-even times
— Source: Fajardy et al. (2019), BECCS Deployment: A Reality Check.
Figure 2: Impacts of land-use change on BECCS carbon break-even times
If agricultural or forest wastes or residues are used, clearly the impacts on forest or soil carbon stocks are much lower, since these do not involve harvesting (and consequent loss of future sequestration). Calculating the overall carbon impact is particularly complicated, however, due to the varying consequences of counterfactual uses. Sawmill residues can be used for engineered wood products, locking the carbon in the built environment, as well as for energy. If forest and agricultural residues that would otherwise have been left to rot and fertilize soils in situ are removed, this may have significant negative impacts in terms of soil degradation46 and associated declines in levels of soil carbon and rates of tree growth.
A further important component of carbon lifecycle balances in BECCS systems concerns the question of how much carbon is captured and stored during the combustion process. In SR1.5, the IPCC reported assumed capture rates of about 90 per cent of the carbon dioxide emitted from electricity and hydrogen production, and rates of about 40–50 per cent from liquid fuel production.47 More complex models including supply-chain emissions suggested lower capture rates, of about 50 per cent and 25 per cent respectively.48
The net effect on carbon balances between living biomass, soil carbon, the atmosphere and carbon captured are not the only impacts of BECCS processes on climate change.49 Biomass and biofuel production and combustion can lead to emissions of largely uncapturable ‘black carbon’, a short-lived climate forcer; of nitrous oxide from fertilizer use; and of methane from combustion and biomass decomposition in anaerobic conditions. Land-cover changes or land-use disturbances, including forest harvesting or conversion of natural lands to energy crops, can also lead to changes in albedo, surface roughness and evapotranspiration, with negative impacts on the climate system.
Thus, there are many circumstances under which BECCS may not result in net negative emissions – or could even increase carbon dioxide concentrations in the atmosphere by eroding forest carbon sinks. The carbon removal potential and time required for BECCS to become carbon-negative vary greatly depending on the feedstocks, supply-chain emissions, direct and indirect land-use changes, and whether the system is optimized for negative emissions or for power or heat production. Fully accounting for all these net changes is difficult, as some of the factors are highly species- and location-specific, but doing so is crucial in calculating the net effect of BECCS on the atmosphere and in stress-testing the efficiencies assumed by some IAMs.
While some of these factors – supply-chain and direct (though not usually indirect) land-use change emissions in particular – may be taken into account in models and policy frameworks, losses of soil carbon and changes in forest carbon stock over time are generally ignored in both models and policy frameworks, even though these are crucial to the net impact on the atmosphere. A 2019 review of 100 widely cited lifecycle assessments of bioenergy found that 71 of them assumed that ‘biogenic carbon dioxide does not impact climate change’.50 This casts serious doubts over the potential for BECCS assumed in many of the models. If BECCS involves replacing high-carbon-content ecosystems with energy crops, it is likely to be ineffectual in reducing atmospheric carbon dioxide for a sustained period; leaving the ecosystem intact is thus likely to be a better option.51
36 Fuss et al. (2016), ‘Research priorities for negative emissions’.
37 IPCC (2018), Global Warming of 1.5°C, Chapter 2, ‘Supplementary Material’.
38 For a longer discussion of this topic, see Brack, D. (2017), Woody Biomass for Power and Heat: Impacts on the Global Climate, London: Royal Institute of International Affairs, https://www.chathamhouse.org/publication/woody-biomass-power-and-heat-impacts-global-climate; and EASAC (2017), Multi-functionality and sustainability in the European Union’s forests, Halle, Germany: EASAC, https://easac.eu/fileadmin/PDF_s/reports_statements/Forests/EASAC_Forests_web_complete.pdf.
39 IPCC (2018), Global Warming of 1.5°C, Chapter 2.
40 See, for example, Luyssaert, S., Schulze, E.-D., Börner, A., Knohl, A., Hessenmöller, D., Law, B. E., Ciais, P. and Grace, J. (2008), ‘Old-growth forests as global carbon sinks’, Nature, 455, doi: 10.1038/nature07276; Lewis, S., Lopez-Gonzalez, G., Sonké, B., Affum-Baffoe, K., Baker, T. R., Ojo, L. O., Phillips, O. L., Reitsma, J. M., White, L., Comiskey, J. A., Djuikouo K, M.-N., Ewango, C. E. N., Feldpausch, T. R., Hamilton, A. C., Gloor, M., Hart, T., Hladik, A., Lloyd, J., Lovett, J. C., Makana, J.-R., Malhi, Y., Mbago, F. M., Ndangalasi, H. J., Peacock, J., Peh, K. S.-H., Sheil, D., Sunderland, T., Swaine, M. D., Taplin, J., Taylor, D., Thomas, S. C., Votere, R. and Wöll, H. (2009), ‘Increasing carbon storage in intact African tropical forests’, Nature, 457, https://www.nature.com/articles/nature07771; Bellassen, V. and Luyssaert, S. (2014), ‘Carbon sequestration: Managing forests in uncertain times’, Nature, 506, https://www.nature.com/news/carbon-sequestration-managing-forests-in-uncertain-times-1.14687; Stephenson, N. L., Das, A. J., Condit, R., Russo, S. E., Baker, P. J., Beckman, N. G., Coomes, D. A., Lines, E. R., Morris, W. K., Rüger, N., Álvarez, E., Blundo, C., Bunyavejchewin, S., Chuyong, G., Davies, S. J., Duque, Á., Ewango, C. N., Flores. O., Franklin, J. F., Grau, H. R., Hao, Z., Harmon, M. E., Hubbell, S. P., Kenfack, D., Lin, Y., Makana, J.-R., Malizia, A., Malizia, L. R., Pabst, R. J., Pongpattananurak, N., Su, S.-H., Sun, I.-F., Tan, S., Thomas, D., van Mantgem, P. J., Wang, X., Wiser, S. K. and Zavala, M. A. (2014), ‘Rate of tree carbon accumulation increases continuously with tree size’, Nature, 507, doi: 10.1038/nature12914; and Craggs, G. (2016), The Role of Old-Growth Forests in Carbon Sequestration, Future Directions International. Over 60 studies showing the same phenomenon are summarized in CO2 Science (2014), ‘Forests (Growth Rates of Old vs. Young Trees) – Summary’.
41 Stephenson et al. (2014), ‘Rate of tree carbon accumulation increases continuously with tree size’.
42 Luyssaert et al. (2008), ‘Old-growth forests as global carbon sinks’, pp. 213–15.
43 Lewis, S. L., Wheeler, C., Mitchard, E. T. A. and Koch, A. (2019), ‘Regenerate natural forests to store carbon’, Nature, 568, pp: 25–28, doi: 10.1038/d41586-019-01026-8.
44 IPCC (2019), Climate Change and Land, Chapter 6, cross-chapter box 7.
45 Ibid.
46 Ibid.
47 IPCC (2018), Global Warming of 1.5°C, p. 124.
48 Fajardy et al. (2019), BECCS Deployment: A Reality Check.
49 National Academies of Sciences, Engineering, and Medicine (2019), Negative Emissions Technologies and Reliable Sequestration: A Research Agenda.
50 Agostini, A., Giuntoli, J., Marelli, L. and Amaducci, S. (2019), ‘Flaws in the interpretation phase of bioenergy LCA fuel the debate and mislead policymakers’, The International Journal of Life Cycle Assessment, doi:10.1007/s11367-019-01654-2.
51 See, for example, IPCC (2019), Climate Change and Land; and Harper, A., Powell, T., Cox, P., House, J., Huntingford, C., Lenton, T. M., Sitch, S., Burke, E., Chadburn, S. E., Collins, W. J., Comyn-Platt, E., Daioglou, V., Doelman, J. C., Hayman, G., Robertson, E., van Vuuren, D., Wiltshire, A., Webber, C. P., Bastos, A., Boysen, L., Ciais, P., Devaraju, N., Jain, A. K., Krause, A., Poulter, B. and Shu, S. (2018), ‘Land-use emissions play a critical role in land-based mitigation for Paris climate targets’, Nature Communications, 9(1), https://www.nature.com/articles/s41467-018-05340-z.