Residence time of Hunga stratospheric water vapour perturbation quantified at 9 years

The January 2022 eruption of the Hunga volcano (20 ∘ S) 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. In the first two years post eruption the stratospheric burden hardly changed, leaving the residence time of volcanically injected water vapour, a key control on its climate impact, uncertain. Here, using satellite observations, we show a substantial decline from 2024 to early 2025, the largest drop since the eruption. Comparison with 3-D numerical 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. We estimate that 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.