Carbon accounting for negative emissions technologies

ABSTRACT

Negative emissions technologies (NETs) are an essential part of most scenarios for achieving the Paris Agreement goal of limiting warming to below 2°C and for all scenarios that limit warming to 1.5 °C. The deployment of these technologies requires carbon accounting methods for a range of different purposes, such as determining the effectiveness of specific technologies or incentivising NETs. Although the need for carbon accounting methods is discussed within the literature on NETs, there does not appear to be a clear understanding of the range of different accounting challenges. Based on a systematic literature review this study identifies five distinct accounting issues related to NETs: 1. estimating total system-wide change in emissions/removals; 2. non-permanence; 3. non-equivalence of ‘no overshoot’ and ‘overshoot and removal’; 4. accounting for incentives for NETs; and 5. the temporal distribution of emissions/removals. Solutions to these accounting challenges are proposed, or alternatively, areas for further research and the development of solutions are highlighted. One key recommendation is that carbon accounting methods should follow a ‘reality principle’ to report emissions and removals when and where they actually occur, and an important overall conclusion is that it is essential to use the correct accounting method for its appropriate purpose. For example, consequential methods that take account of total system-wide changes in emissions/removals should be used if the purpose is to inform decisions on the deployment or incentivisation of NETs. Attributional methods, however, should be used if the purpose is to construct static descriptions of possible net zero worlds.

Key policy insights

• Negative emissions technologies (NETs) raise a number of distinct carbon accounting challenges, the importance of which varies across different NETs.

• Attributional life cycle assessment is not an appropriate method for estimating the system-wide changes caused by the deployment of NETs.

• Consequential greenhouse gas accounting methods should be used to estimate system-wide changes, and should be used as much as possible for guiding incentives for NETs.

• Greenhouse gas accounting methods should follow a ‘reality principle’ to report emissions and removals when and where they actually occur.

KEYWORDS:

• NETS
• negative emissions technologies
• carbon accounting
• consequential LCA
• greenhouse gas removal
• BECCS

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

1 A related issue is the efficiency (or cost-effectiveness) of different NETs, i.e. $/net tCO₂ removed (with net change in emissions/removals providing the denominator in the metric), which has also been a central topic within the literature (McLaren, 2012a; Smith et al., 2016; Bhave et al., 2017; Alcalde, Smith, et al., 2018). For example, McLaren (2012a) reports cost effectiveness figures of between $8/tCO₂ and $600/tCO₂ for different NETs, while Smith et al. (2016) report figures equivalent to between $24tCO₂ and $567tCO₂.

2 A further accounting issue related to non-CO₂ GHGs that is not prominent within the literature identified, but nevertheless appears to be a highly important for some types of NET, is how to compare emissions and removals of different GHGs. Neubauer and Megonigal (2015) suggest that the use of GWPs is not appropriate for ecosystem-based NETs as the emissions and removals are sustained fluxes rather than single pulses, and pulse-based GWPs may underestimate the radiative forcing of CH₄ and N₂O by as much as 40%. Cain et al. (2019) also consider how the calculation of CO₂ equivalence for CH₄ might be improved with a method equating a change in the emission rate of a short-lived climate pollutant as equivalent to a single emissions pulse of a long-lived pollutant.

3 The two illustrative charts are not completely equivalent as, for simplicity, the chart for BECCS shows a single instance of harvesting, combustion, capture, storage and regrowth (rather than overlaying on-going instances of these activities), while the chart for DACCS shows on-going removals and operational emissions over the lifetime of a DACCS facility. However, the illustrative comparison of carbon payback periods remains valid. If ongoing instances of the BECCS activities were included, the payback period would be longer (i.e. extend further to the right).

Additional information

Funding

This work was supported by Natural Environment Research Council [Grant Number NE/P019749/1].