This week’s selected publications cover a wide range of issues ranging from the consideration of environmental, social and governance criteria as part of CDR assessment and the analysis of the Southern Ocean as a CO2 sink to the measurement of the efficiency of bioenergy crops, rock weathering in the UK and geological net zero. Let’s take a closer look at this week’s collection:
Assessing Carbon Removal Technologies: Integrating Technical Potential with Environmental, Social, Governance Criteria and Sequestration Permanence
Abstract
Climate modeling suggests that achieving international climate goals requires a reduction in current CO2 emissions by over 90%, with any remaining emissions to be addressed through carbon dioxide removal (CDR) solutions. Sixteen CDR strategies are evaluated by integrating technical potential, environmental, social, and governance (ESG) criteria, along with sequestration permanence. This evaluation, conducted by ENGIE’s scientific council using an interdisciplinary Delphi panel methodology, proposes a ‘quality’ measure for each technology. This measure combines ESG scores and sequestration timescales to rank and select the most promising solutions. The findings highlight the necessity for further research to understand and mitigate ESG impacts, aiming to inform both future research and current decision-making to support the effective and legitimate use of CDR strategies.
Mertens, J., Breyer, C., Belmans, R., Dimitrova, Z., Gendron, C., Geoffron, P., Fischer, C., Fornel, E., Lester, R., Nicholas, K., Miranda, P., Palhol, S., Verwee, P., Sala, O., Webber, M. & Debackere, K. (2024) Assessing Carbon Removal Technologies: Integrating Technical Potential with Environmental, Social, Governance Criteria and Sequestration Permanence. Perspective 111418.
Read the full paper here: Assessing Carbon Removal Technologies: Integrating Technical Potential with Environmental, Social, Governance Criteria and Sequestration Permanence I Perspective
Sensitivity of the Southern Ocean CO2 Sink to a Rapid Increase and Subsequent Decrease of Atmospheric CO2
Abstract
Despite the importance of the Southern Ocean carbon sink, its response to future atmospheric CO2 perturbations and warming remains highly uncertain. In this study, we use six state-of-the-art Earth system models to assess the response of Southern Ocean air-sea CO2 fluxes (FCO2) to a rapid atmospheric forcing increase and subsequent negative emissions in an idealized carbon dioxide removal reversibility experiment. We find that during positive emissions, the region north of the Polar Front only takes up atmospheric CO2 for 30-50 years before reaching equilibrium; surface stratification and reduction of CO2 solubility with warming diminishes ocean CO2 uptake in this region. In contrast, south of the Polar Front, the upper ocean continues to take up CO2 until the end of positive emissions at 140 years. Sea-ice loss and the accumulation of anthropogenic dissolved inorganic carbon in the upper ocean reduce the upwelling-driven seasonal CO2 outgassing, leading to a stronger Antarctic CO2 sink. CO2 removal triggers a CO2 uptake reduction that slowly converts the Southern Ocean into a CO2 source which persists for at least 50 years post-mitigation. Furthermore, we find that model sensitivity to atmospheric perturbation is closely linked to seasonal FCO2 dynamics. Specifically, models with a thermally dominated pCO2 seasonal cycle exhibit nearly twice the sensitivity to atmospheric perturbations compared to non-thermal models. Our findings further emphasize the necessity of accurate model representation of the seasonal CO2 dynamics for appropriately simulating the future Southern Ocean carbon sink.
Mongwe, P., Tjiputra, J., Goris, N., Falcon, Y., Noh, K., Ito, T. (2024). Sensitivity of the Southern Ocean CO2 Sink to a Rapid Increase and Subsequent Decrease of Atmospheric CO2. ESS Open Archive.
Read the full paper here: Sensitivity of the Southern Ocean CO2 Sink to a Rapid Increase and Subsequent Decrease of Atmospheric CO2 I ESS Open Archive.
How to Measure the Efficiency of Bioenergy Crops Compared to Forestation
Abstract
The climate mitigation potential of terrestrial carbon dioxide removal (tCDR) methods depends critically on the timing and magnitude of their implementation. In our study, we introduce different measures of efficiency to evaluate the carbon removal potential of afforestation and reforestation (AR) and bioenergy with carbon capture and storage (BECCS) under the low-emission scenario SSP1-2.6 and in the same area. We define efficiency as the potential to sequester carbon in the biosphere in a specific area or store carbon in geological reservoirs or woody products within a certain time. In addition to carbon capture and storage (CCS), we consider the effects of fossil fuel substitution (FFS) through the usage of bioenergy for energy production, which increases the efficiency through avoided CO2 emissions.
These efficiency measures reflect perspectives regarding climate mitigation, carbon sequestration, land availability, spatiotemporal dynamics, and the technological progress in FFS and CCS. We use the land component JSBACH3.2 of the Max Planck Institute Earth System Model (MPI-ESM) to calculate the carbon sequestration potential in the biosphere using an updated representation of second-generation bioenergy plants such as Miscanthus. Our spatially explicit modeling results reveal that, depending on FFS and CCS levels, BECCS sequesters 24–158 GtC by 2100, whereas AR methods sequester around 53 GtC on a global scale, with BECCS having an advantage in the long term. For our specific setup, BECCS has a higher potential in the South American grasslands and southeast Africa, whereas AR methods are more suitable in southeast China. Our results reveal that the efficiency of BECCS to sequester carbon compared to “nature-based solutions” like AR will depend critically on the upscaling of CCS facilities, replacing fossil fuels with bioenergy in the future, the time frame, and the location of tCDR deployment.
Egerer, S., Falk, S., Mayer, D., Nutzel, T., Obermeier, W. & Pongratz, J. (2024). How to Measure the Efficiency of Bioenergy Crops Compared to Forestation. Biogeosciences 21 (22) 5005-5025.
Read the full paper here: How to Measure the Efficiency of Bioenergy Crops Compared to Forestation I Biogeosciences
Current Rates of CO2 Removal due to Rock Weathering in the UK
Abstract
Chemical weathering of silicate and carbonate rocks via carbonic acid provides a natural sink for carbon dioxide, regulating climate over geological timescales. Although the magnitude of CO2 removal via weathering has been estimated at a global scale using the geochemistry of the world’s largest rivers, it has generally not been quantified at national level. In the United Kingdom, the variable bedrock geology and long-legacy of anthropogenic land use provide challenges to isolating carbonate and silicate mineral weathering, meaning we lack constraint on an important flux in the UK’s carbon cycle. Here we use river chemistry data collected over 40 years for 52 catchments across the UK and apply a geochemical inversion model (MEANDIR) to assess the silicate and carbonate contributions to UK river chemistry, and to estimate the CO2 consumption from natural weathering.
Silicate-derived solutes account for 0–46 % of dissolved river chemistry by mass (median 7 %) and carbonate-derived solutes for 9–82 % (median 49 %), with the remainder attributed to evaporite and cyclic sources. The maximum present-day carbon dioxide removal (CDR) through combined silicate and carbonate weathering is 2.58 MtCO2 yr−1 for the studied catchments (representing 40 % of total UK area). Extrapolated to the entire UK, the maximum CO2 consumed by silicate and carbonate weathering is 1.6 (0.8–2.8) MtCO2 yr−1 and 4.8 (4.2–5.4) MtCO2 yr−1 respectively. If sulphuric acid replaces carbonic acid in weathering, CDR may be 17 % and 28 % lower for silicate and carbonate weathering respectively. Accounting for sulphuric acid weathering, a conservative estimate suggests a net combined present-day CDR of 4.5 MtCO2 yr−1 for the UK. This is comparable to the lower-end estimates for potential additional CDR through Enhanced Weathering (EW) in the UK. If EW CDR targets are met, EW could more than double the natural weathering flux in rivers, with implications for river chemistry which warrants consideration before national implementation.
Harrington, K., Henderson, G. & Hilton, R. (2024). Current Rates of CO2 Removal due to Rock Weathering in the UK. Science of the Total Environment 957 (177458).
Read the full paper here: Current Rates of CO2 Removal Due to Rock Weathering in the UK I Science of the Total Environment
Geological Net Zero and the Need for Disaggregated Accounting for Carbon Sinks
Abstract
Achieving net zero global emissions of carbon dioxide (CO2), with declining emissions of other greenhouse gases, is widely expected to halt global warming. CO2 emissions will continue to drive warming until fully balanced by active anthropogenic CO2 removals. For practical reasons, however, many greenhouse gas accounting systems allow some “passive” CO2 uptake, such as enhanced vegetation growth due to CO2 fertilisation, to be included as removals in the definition of net anthropogenic emissions. By including passive CO2 uptake, nominal net zero emissions would not halt global warming, undermining the Paris Agreement. Here we discuss measures addressing this problem, to ensure residual fossil fuel use does not cause further global warming: land management categories should be disaggregated in emissions reporting and targets to better separate the role of passive CO2 uptake; where possible, claimed removals should be additional to passive uptake; and targets should acknowledge the need for Geological Net Zero, meaning one tonne of CO2 permanently restored to the solid Earth for every tonne still generated from fossil sources. We also argue that scientific understanding of net zero provides a basis for allocating responsibility for the protection of passive carbon sinks during and after the transition to Geological Net Zero.
Allen, M., Frame, D., Friedlingstein, P., Gillett, N., Grassi, G., Gregory, J., Hare, W., House, J., Huntingford, C., Jenkins, S., Jones, C., Knutti, R., Lowe, J., Matthews, H., Meinshausen, M., Meinshausen, N., Peters, G., Plattner, G., Raper, S., Rogelj, J., Stott, P., Solomon, S., Stocker, T., Weaver, A. & Zickfeld, K. (2024). Geological Net Zero and the Need for Disaggregated Accounting for Carbon Sinks. Nature.
Read the full paper here: Geological Net Zero and the Need for Disaggregated Accounting for Carbon Sinks I Nature