Why the Northern Hemisphere Needs a 30-40 m Telescope and the Science at Stake: Time-domain astronomy

We outline the science case for a 30-40 m optical/infrared telescope in the Northern Hemisphere, optimised for transformative time-domain astronomy in the 2040s. Upcoming multi-wavelength and multi-messenger facilities will reveal fast, faint, rapidly evolving Northern transients whose earliest phases carry decisive diagnostics. A Northern ELT with rapid response, broad wavelength coverage, high time resolution, polarimetric capabilities, and diffraction-limited imaging is essential to capture these phases and secure deep spectroscopy and photometry as transients fade. These capabilities will enable recovery of key physical information and detailed characterisation of transient environments, while also enabling unprecedented studies of accretion phenomena at all scales. Among potential sites, La Palma uniquely combines atmospheric stability, complementary longitude to ESO’s ELT, protected dark skies, and robust infrastructure to host this facility.