Multi-model analysis of the impact of water vapor on the radiative forcing of volcanic aerosols after the 2022 Hunga Eruption

On 15 January 2022, the Hunga volcano eruption released unprecedented amounts of water vapor into the atmosphere alongside a modest amount of SO₂. In this work we analyse results from multiple Earth system models as part of the Hunga Tonga-Hunga Ha'apai Volcano Impact Model Observation Comparison Project. Our results show a good model agreement over the climatic outcomes of the eruption, overall indicating a significant negative radiative forcing from the Hunga eruption. The multi-model mean of global instantaneous radiative forcing averaged over 2022–2023 is estimated at -0.19 ± 0.06 Wm^-2 at the top-of-atmosphere (TOA), and -0.16 ± 0.06 Wm^-2 at the surface. Simulations with free-running meteorology and climatological sea surface temperatures and sea ice yield a global mean TOA forcing of -0.14 ± 0.10 Wm^-2 across two models for the first 2 years, decreasing to -0.09 ± 0.10 Wm^-2 on average between 2022 and 2027. However, these global values may be underestimated by about 50 %, considering that recent SO₂ injection retrievals suggest nearly twice the amount than the 0.5 Tg-SO₂ used in the protocol. We also find that the contribution from added stratospheric water vapor is minimal and that the injected SO₂ and the resulting formation of stratospheric sulfate dominate the radiative forcing. However, water vapor played a key role in the initial aerosol growth, leading to a stronger negative radiative forcing during the first 6 months after the eruption compared to simulations without water vapor co-injection.