U5 is comparable in investment scale to Stuttgart 21, running almost entirely underground, which means significant construction-phase greenhouse gas emissions. Hamburger Hochbahn AG’s own life cycle assessment puts emissions at 2.7 million tCO₂e using conventional construction methods.

In his MSc thesis, our student Georg Benjamin Fiedler extended this analysis with economic input-output data to capture upstream emissions outside the LCA’s system boundary. Our upper-bound estimate for conventional construction: 3.6 million tCO₂e.

Furthermore, the transport authorities project up to 70% emission savings during construction, largely contingent on future climate-friendly cement production. Using stochastic sensitivity analysis, we showed that small uncertainties in cement production emissions swing the project’s overall climate balance substantially. Our recommendation: a methodologically robust forecast of cement decarbonisation pathways is needed to substantiate the 70% reduction claim.

Great work by Georg and Sven Lundie, who co-supervised this thesis together with Johannes.