Reply on RC1

Abstract

Stringent mitigation pathways frame the deployment of second-generation bioenergy crops combined with Carbon Capture and Storage (CCS) to generate negative CO2 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 carbon cycle and carbon-climate feedback under the CMIP6 SSP5-3.4-OS overshoot scenario, keeping in mind that all these models use generic crop vegetation to simulate BECCS crops. We show that an extensive cropland expansion for BECCS causes ecosystem carbon loss that drives the acceleration of carbon turnover and affects the estimates of the absolute values of the global carbon-concentration β and carbon-climate γ feedback parameters. Both parameters decrease so that global β becomes less positive, and γ – more negative. Over the 2000–2100 period, the land-use change (LUC) for BECCS leads to an offset of the β-driven carbon uptake by 12.2 % and amplifies the γ-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 via the β and γ feedbacks. The dependency of the estimates of β and γ on LUC is very strong after the middle of the 21st century in the SSP5-3.4-OS scenario but it also affects other SSP scenarios and should be taken into account by the integrated assessment modelling teams and accounted for in mitigation policies so as to limit the reductions of the CO2 fertilization effect where BECCS or land use expansion of short vegetation is applied.