The recognition that most IPCC scenarios for to avoid exceeding the 2°C “guardrail” require large-scale deployment of negative emissions technologies (NETs) has led to extensive recent discussion of the potential effectiveness and risks associated with a range of option. However, as the authors of a new study published in the journal Global Change Biology conclude, most studies to date have focused on bioenergy with carbon capture and sequestration (BECCS), direct air capture, enhanced weathering of minerals, and afforestation and reforestation. This study, by Pete Smith at the University of Aberdeen, expands the scope of inquiry to two other NETs options: 1. soil carbon sequestration (SCS), through methods such as alternation of agricultural practices, including no-till or low-till with residue management, organic amendment and fire management; and 2. Biochar, which is production of charcoal as soil amendment via the process of pyrolysis which can, inter alia, sequester carbon. Biochar, at least, is often included under the rubric of “climate geoengineering” options, in the subcategory of carbon dioxide removal (CDR) approaches.

Among the study’s findings:

1. SCS at global scale could sequester from 0.4-0.7GtCeq. yr^-1, with technical potential of 1.37GtCeq. yr^-1, at a cost of ~$70-370 per ton of Ceq. Biochar could effectuate sequestration of ~1 GtCeq yr^-1, with a maximum potential of 1.8 GtCeq yr^-1
2. By contrast, BECCS might be able to sequester 3.3 GtCeq yr^-1 by 2100, and direct air capture a comparable amount. However, the potential of SCS and biochar are higher than either enhanced weathering and comparable to afforestation and deforestation;
3. About 20% of the mitigation to be derived from SCS could occur at negative cost, and 80% between $0-40 tCeq. Biochar costs range from -$581-1560 billion;
4. In terms of water requirements, SCS and biochar are virtually zero, while direct air capture has medium to high water demands, and BECCS creating “a very large water footprint;”
5. In terms of energy requirements, SCS has a negligible energy impact, and biochar can actually produce energy during the pyrolysis process; by contrast, both direct air capture and enhancing mineral weathering have significant energy requirements;
6. One significant issue in terms of both SCS and biochar is “sink saturation,” i.e. decreased carbon sequestration potential as soils approach a new, higher equilibrium level. This can occur after 10-100 years for SCS, and is also an issue for biochar. This has implications for deployment of these technologies, as most scenarios for use of NETs envision primary importance in the second half of this century, meaning that deployment of some approaches in the next few years might have little impact later this century.

Overall, the author of the study concludes that SCS and biochar should be given serious consideration in integrated assessment models given their advantages over some other NET approaches.

Among the classroom questions that this study might generate:

1. How do we determine the optimal mix of R&D funding for NETs?
2. What should be the most important criteria for determining if we proceed with research on individual NETs options?
3. What kind of governance architecture should be established for NETs research and development and/or deployment?