Impact of bioenergy crop expansion on climate-carbon cycle feedbacks in overshoot scenarios

Melnikova, Irina ; Boucher, Olivier ; Cadule, Patricia ; Tanaka, Katsumasa ; Gasser, Thomas ; Hajima, Tomohiro ; Quilcaille, Yann ; Shiogama, Hideo ; Séférian, Roland ; Tachiiri, Kaoru ; Vuichard, Nicolas ; Yokohata, Tokuta ; Ciais, Philippe

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<p align=justify>Stringent mitigation pathways frame the deployment of second-generation bioenergy crops combined with carbon capture and storage (CCS) to generate negative CO<sub>2</sub> emissions. This bioenergy with CCS (BECCS) technology facilitates the achievement of the long-term temperature goal of the Paris Agreement. Here, we use five state-of-the-art Earth system models (ESMs) to explore the consequences of large-scale BECCS deployment on the climate-carbon cycle feedbacks under the CMIP6 SSP5-3.4-OS overshoot scenario keeping in mind that all these models use generic crop vegetation to simulate BECCS. First, we evaluate the land cover representation by ESMs and highlight the inconsistencies that emerge during translation of the data from integrated assessment models (IAMs) that are used to develop the scenario. Second, we evaluate the land-use change (LUC) emissions of ESMs against bookkeeping models. Finally, we show that an extensive cropland expansion for BECCS causes ecosystem carbon loss that drives the acceleration of carbon turnover and affects the CO<sub>2</sub> fertilization effect- and climate-change-driven land carbon uptake. Over the 2000-2100 period, the LUC for BECCS leads to an offset of the CO<sub>2</sub> fertilization effect-driven carbon uptake by 12.2 % and amplifies the climate-change-driven carbon loss by 14.6 %. A human choice on land area allocation for energy crops should take into account not only the potential amount of the bioenergy yield but also the LUC emissions, and the associated loss of future potential change in the carbon uptake. The dependency of the land carbon uptake on LUC is strong in the SSP5-3.4-OS scenario, but it also affects other Shared Socioeconomic Pathway (SSP) scenarios and should be taken into account by the IAM teams. Future studies should further investigate the trade-offs between the carbon gains from the bioenergy yield and losses from the reduced CO<sub>2</sub> fertilization effect-driven carbon uptake where BECCS is applied.</p>
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