Designing sustainable processes is gaining momentum in the chemical engineering community. Conventional life cycle assessment approaches often employed to assess the sustainability level of chemical processes can be used to compare alternatives. However, because they lack clear quantitative thresholds above which a process should be deemed unsustainable, the insight provided into whether a technology is truly sustainable in absolute terms is limited. This work covers this gap by incorporating absolute sustainability criteria in process design using the planetary boundaries concept, which defines ecological limits on critical Earth systems. Our method, integrating process simulation, surrogate modeling, and a recent characterization method to compute the impact on the planetary boundaries, is applied to methanol production from hydrogen and CO2. Our results show that the sustainability level of the fossil-based chemical can be improved substantially by adequately selecting the hydrogen source. The new approach unfolds new avenues for including absolute sustainability criteria in process design.