The carbon footprint of proposed $e^+e^-$ Higgs factories

The energy consumption of any of the ${\mathrm{e}}^+{\mathrm{e}}^-$ Higgs factory projects that can credibly operate immediately after the end of LHC, namely three linear colliders (CLIC, operating at $\sqrt{s}\,=\,380$ GeV; and ILC and ${\mathrm{C}}^3$, operating at $\sqrt{s}\,=\,250$ GeV) and two circular colliders (CEPC and FCC-ee, operating at $\sqrt{s}\,=\,240$ GeV), will be everything but negligible. Future Higgs boson studies may therefore have a significant environmental impact. This note proposes to include the carbon footprint for a given physics performance as a top-level gauge for the design optimisation and, eventually, the choice of the future facility. The projected footprints per Higgs boson produced, evaluated using the 2021 carbon emission of available electricity, are found to vary by a factor 100 depending on the considered Higgs factory project.