Abstract
Human activities have already transgressed several planetary boundaries, yet energy system models remain largely focused on greenhouse gas mitigation, reflecting their original purpose of addressing climate change. Recent integrations of Planetary boundary-based Life Cycle Assessment into Energy System Optimisation Models show that even cost-optimal low-carbon pathways systematically violate multiple planetary boundaries, indicating that supply-side decarbonisation alone is insufficient for absolute environmental sustainability. At a 2050 horizon, where energy supply is largely decarbonised and technologies are assumed mature, further impact reductions through techno-economic optimisation become limited, positioning final energy demand as a key remaining lever for restoring feasibility under planetary constraints. To address this gap, we ex-tend an Energy System Optimisation framework coupled with a Planetary Boundary framework by explicitly treating final energy demand as a decision variable and exploring energy sufficiency configurations within a multi-objective formulation minimising system cost and environmental pressures. The national-scale EnergyScopeTD model is coupled with the RHEIA uncertainty analysis framework to propagate uncertainty in final energy demands and to identify demand-side drivers through global sensitivity analysis. Applied to Belgium, the results show that individual mobility demand is the dominant driver of environmental pressures across multiple planetary boundaries, with consistently higher influence than other end-use demands. While large-scale vehicle fleet electrification substantially reduces climate impacts, it simultaneously shifts pressures toward other environmental dimensions, indicating that technological substitution alone, even combined with moderate sufficiency, is insufficient to achieve absolute sustainability. Overall, the results confirm that energy sufficiency, particularly in mobility, is necessary for operating energy systems within planetary limits, and that anchoring system design in absolute sustainability thresholds provides a robust basis for prioritising demand-side mitigation strategies.