Techno-economics and value judgments of modeling biochar production for carbon removal in climate change mitigation scenarios

Integrated assessment models (IAMs) create policy-relevant expectations about emerging technologies. Their modeling requires a stylized representation of many possible technological futures, which involves value judgments. Acknowledging calls for reflection on value-laden assumptions, we jointly present the techno-economic considerations and value judgments in modeling biochar production and its use for carbon removal in the IAM REMIND. Our framework distinguishes between the selection of technology configurations and their parameterization. For biochar, we first introduce a classification of production configurations into artisanal, mobile, established, and advanced industrial categories. In selecting biochar configurations, value judgments are embedded in assumptions of equivalence and fungibility with other mitigation options, given the socio-political and carbon permanence differences between them. Modelers also have to decide whether to include established or more advanced configurations that deliver greater systemic value but are associated with higher uncertainty. For parameterization, our data collection shows a wide range of quotable assumptions, providing scope for different levels of optimism. We then test the impact of cautious versus optimistic biochar production assumptions on biochar deployment in scenarios limiting climate change to low overshoot above 1.5 °C. The primary determinant of deployment is not the parameterization of biochar production, but rather assumptions about geologic CO 2 storage, which constrain the deployment of competing BECCS technologies. Nonetheless, biochar parameterization and the inclusion of an advanced, currently non-existent, configuration also influence deployment. The analysis highlights how technological framing and assumptions shape modeled technology potentials, underscoring the need for transparent and reflexive modeling practices.