Modeling the global potential and limitations of biomass pyrolysis as a negative emission technology using a dynamic vegetation model

The ongoing rise in anthropogenic greenhouse gas emissions is significantly exacerbating climate change, which poses an increasing threat to the integrity of the biosphere and societies worldwide. Negative emission technologies (NETs) extracting CO${}_2$ from the atmosphere are being widely discussed in science and policy as climate change mitigation measures in addition to decarbonization. Among the proposed NETs, Pyrogenic carbon capture and storage (PyCCS) has emerged as a promising candidate for early deployment entailing a range of agronomic co-benefits. It is based on biomass pyrolysis and the subsequent storage of the produced biochar, bio-oil or permanent-pyrogas. Since a systematic assessment of global negative emission (NE) potentials of PyCCS from purpose-grown feedstocks is currently lacking, this dissertation comprises three studies that apply the Dynamic Global Vegetation Model LPJmL to estimate global biogeochemical potentials of PyCCS under different deployment scenarios and evaluate the associated land use dynamics, which are among the most critical potential trade-offs. The first study is a demand-driven analysis aiming to achieve NEs projected to be required in climate economic scenarios limiting global warming of this century to 1.5°C by PyCCS deployment. It finds that that biochar application has the potential to deliver these NEs - yet only under significant land use expansion, ranging from 300 to 1500 Mha, if natural land was converted to biomass plantations. Furthermore, the study finds that carbon balancing for failed mitigation actions would require an additional 400 1200 Mha for an extra of +200 Gt C (634 Gt CO${}_2$) NE, posing a significant threat to areas identified as particularly relevant for conservation. Subsequently, a novel approach to PyCCS deployment was assessed that reduces land pressure by releasing cropland to PyCCS feedstock production while maintaining calorie supply through biochar-mediated yield increases on remaining cropland. Based on this allocation scheme and LPJmL-computed biomass yields, a sequestration potential of 0.44-2.62 Gt CO${}_2$ yr${}^{-1}$ was quantified for a literature-based range of +15-30% yield increase. Assessing the potential of replacing NE from BECCS (bioenergy with carbon capture and storage - a prominent NET in stabilization scenarios of climate economics) with this approach and dedicating the plantation area for BECCS to alternative uses, the study finds that global calorie production could be increased by 2-16% or 14-96 Mha of conservationally relevant areas could be allocated to nature protection. The understanding of the potential for LCN-PyCCS as a strategy for climate stabilization was further expanded by the representation of the emerging practice of biochar-based fertilization (i.e., biochar applied as mixtures with fertilizer at lower rates than the previously evaluated soil ii amendment) and sensitivity analyses of assumed pyrolysis parameters and management intensities in the third study. Results showed that assuming +10% yield increase achieved through biochar-based fertilization would lead to a global NE potential of LCN-PyCCS of 0.20-1.04 Gt CO${}_2$ yr${}^{-1}$. Overall, the three studies have contributed significant quantitative knowledge to the ongoing discussion about the deployment of NETs. The first study demonstrates the significant potential of PyCCS to contribute to international climate targets and emphasizes the risk of increasing environmental pressures that result from insufficient emission reductions. The subsequent assessments further advocate for supply-driven approaches to analyze potential NET deployment, accounting for environmental limitations. However, the findings reveal discrepancies between the large NE potentials in demand-driven scenarios following economic optimization and the constrained potentials in supply-driven approaches like LCN-PyCCS, highlighting the need for transparent discussions of these in science and policy in order to develop responsible NET deployment strategies.