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The underwater Hunga Tonga-Hunga Ha-apai (HT) volcano violently erupted on January 15th, 2022, injecting volcanic gases and aerosols at over 50 km altitude. Here we show the stratospheric aerosol and water vapour perturbations due to the HT eruption, the plume evolution during the first month dispersion and we estimate its short-term radiative impact. The HT eruption produced the largest perturbation of stratospheric aerosols and water vapour since the eruption of Pinatubo volcano in 1991. During the first three weeks following the eruption, water vapour radiative cooling dominates the plume’s heating/cooling rates, reaching values as large as -10 K/d and produces a fast plume descent of several km. At the top-of-the-atmosphere (TOA) and surface, volcanic aerosol cooling dominates the radiative forcing (RF) for the fresh plume. After two weeks, due to dispersion/dilution, water vapour heating starts to dominate the TOA RF, leading to a net warming of the climate system, which was never reported before for a volcanic plume. The surface RF, on the contrary, is dominated by the aerosol effect and reaches values of near -2 Wm-2, exceeding the hemispheric-averaged surface impact of stratospheric events of the last 30 years.