Recent research findings and escalating natural disasters confirm a dramatic – and probably irreversible –damage to the environment, posing an existential threat to humanity. The urgent need to mitigate climate change has intensified efforts to swiftly decelerate global warming, translating to strict measures for reducing greenhouse gas emissions in every human activity. The civil infrastructure, and among them, the tunnelling sector, is a major contributor, responsible for more than 13% of annual global environmental impacts from energy consumption and more than 20% from construction material production. While extensive research and governance actions have been vested with defining and reducing the climate impacts of manufacturing materials and fossil fuel consumption, fundamental design decisions can significantly impact a tunnel’s environmental footprint. A recently coined concept, the so-called Climate Limit State, is presented herein to reconcile environmental responsibility with structural safety and underpin rationalised sustainability evaluations of engineering structures. Its implementation in conventional tunnel design exercises is demonstrated herein, exploring key optimisation factors such as cross-section profile, reduction of material related emissions, lining load-sharing system, material non-linearity, and service life. By challenging traditional concepts, this study offers a fresh perspective on driving sustainability in tunnelling and provides a crucial step towards a more environmentally responsible future.