the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
How to measure the efficiency of terrestrial carbon dioxide removal methods
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/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, spatio-temporal dynamics, and the technological progress in FFS and CCS. We use the land component JSBACH3.2 of the 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 until 2100, whereas AR sequesters around 53 GtC on a global scale with BECCS having an advantage in the long term. For our specific setup, we find that BECCS has a higher potential in the South American grasslands and Southeast Africa, whereas AR is more suitable in Southeast China. Hence, 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.
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RC1: 'Comment on egusphere-2024-1451', Wei Li, 23 Jun 2024
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This manuscript estimated the CDR potentials of afforestation/reforestation (AR) and bioenergy with carbon capture and storage (BECCS, Miscanthus) in the low-warming scenario using JSBACH3.2. The analyses of different factors that impact CDR such as locations, plantation area, and levels of FFS and CCS are important for evaluating the feasibility and climate benefits of AR and BECCS. More importantly, both AR and BECCS simulations were done within the same model, which makes the comparison more consistent. The model has also been well calibrated and validated against field observations. In general, the manuscript is clearly written, and the results are scientifically important and technically sound. I would recommend the publication of this manuscript after some minor revisions.
My main concern is about the treatment of AR in the model. The authors used the current preferences of existing forests as AR, but these forest PFTs are more parameterized by and considered as natural forests, so AR should refer to natural recovery if I understood correctly. In reality, AR may adopt some fast-growing species such as eucalypt and poplar, especially in Southern China, so they may have much higher yields than natural recovery. It may partly explain the lower potential of AR compared to BECCS (e.g., L280-282, L284).
The lifetime of harvested wood from AR is also very important. The authors mentioned the wood harvest in the methods but didn’t report much in the results. Some simple calculations may be done by assuming a decay rate/turnover time in the wood product pool as assumed in the bookkeeping models.
Table 2, may also compare with CDR in Wang et al. (2023, https://doi.org/10.1021/acs.est.2c05253)
L410, L428 Although bioenergy crops may not need fertilization to improve yields, but herbaceous bioenergy crops are harvested every year, so the nutrients in the aboveground biomass will be lost. Please see the nutrient demands by bioenergy crops in Li et al. (2021, https://doi.org/10.1021/acs.est.1c02238)
Citation: https://doi.org/10.5194/egusphere-2024-1451-RC1 -
CC1: 'Comment on egusphere-2024-1451', Andreas Krause, 08 Jul 2024
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This is an interesting paper contributing to the debate on the role of CDR in climate mitigation. As the authors compare their results to one of my own papers (https://bg.copernicus.org/articles/14/4829/2017/) I want to note a few points:
- While this paper investigated LUC impacts on a range of ES indicators, we focused on carbon uptake in a subsequent paper (https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.14144) which might therefore be more suited for comparison.
- We used a CCS carbon capture efficiency of 80% instead of 100%.
- In our 2018 paper we computed the forest area / carbon uptake relative to the baseline scenario by year 2099. Of course you can also use the present-day values as a baseline, just make sure to be consistent when comparing different studies.
Andreas Krause
Citation: https://doi.org/10.5194/egusphere-2024-1451-CC1
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