When will the stratospheric water vapour return to pre-Hunga level?

The January 2022 eruption of the Hunga volcano (20°S), the most explosive in the past 30 years, injected 150 Tg of water vapour into the middle atmosphere, leading to an increase in the stratospheric water burden of 10%, unprecedented in the observational record. This excess moisture altered stratospheric chemistry, aerosol microphysics and radiative balance, offsetting the expected surface cooling from volcanic aerosols. Yet the residence time of volcanic-injected water vapour, a key control on its climate impact, has remained uncertain. In the first two years post eruption the stratospheric burden hardly changed, except for a small decay due to Antarctic polar stratospheric cloud dehydration in 2023. Here, using satellite observations, we show a substantial decline from 2024 to early 2025, the largest drop since the eruption. Comparison with 3-D model simulations shows that the long-term removal of the Hunga water has now entered a new phase, with stratosphere-troposphere exchange playing an increasingly important role, exceeding Antarctic dehydration in 2024. Based on the observations and model, we estimate the additional stratospheric water vapour is now decaying steadily with an e-folding time of 3 years and will reach the observed pre-Hunga range of variability around 2030.