When carbon counts differ: the complexities of biogenic carbon accounting in BECCUS and why accounting choices matter

Bioenergy with Carbon Capture, Utilization and Storage (BECCUS) is an important pathway towards net-zero and net-negative emissions. Its mitigation potential depends on net atmospheric CO2 removal into durable storage after accounting for system-wide emissions and removals. However, lifecycle assessment, national greenhouse gas inventories and carbon-market frameworks apply differing accounting approaches, producing divergent mitigation estimates. This study identifies key sources of variability and examines their influence on reported outcomes through a systematic literature review and illustrative calculations for a hypothetical BECCUS system. Depending on the accounting approach, the net system outcome ranges from +422 kt CO2 (net-emitting) to −2,084 kt CO2 (net-reducing), while reported carbon removal ranges from 0 to −1,688 kt CO2. These deviations arise from four interconnected sources: (1) goals and framing, including definitions of concepts, terms and carbon flows; (2) methodological and analytical choices; (3) scope and system boundaries; and (4) baselines and counterfactuals. Governance, monitoring, reporting and verification, and crediting frameworks further shape these sources. Reported outcomes therefore vary primarily because of accounting choices rather than technical performance. The study calls for better alignment of existing methodologies and frameworks so that BECCUS accounting reflects atmospheric outcomes and supports credible climate mitigation.