The progression and global dispersion of the Hunga aerosol cloud, and influence from co-emitted water vapour, aligned to the APARC Hunga impacts report

The January 2022 Hunga eruption generated the strongest stratospheric aerosol optical depth for 30 years (e.g. Khaykin et al., 2022; Taha et al., 2022; Bourassa et al., 2023), but the eruption emitted only a modest 0.4-0.5Tg of SO2 to the stratosphere (Carn et al., 2022).

The most explosive eruption in the satellite era (Wright et al., 2022), an upper portion of the Hunga plume was initially at ~35-40km altitude (Taha et al., 2022), but the main detrainment occurred lower at ~27-30km, with a highly unusual initial steep descent of the plume seeing the layer of Hunga-enhanced aerosol form at ~22-26km (e.g. Kloss et al., 2022; Legras et al., 2022; Baron et al., 2023).

The shallow underwater explosion also detrained ~150Tg of water vapour deep into the stratosphere (e.g. Millan et al., 2022), shown by Zhu et al. (2022) and Asher et al. (2023) to have accelerated SO2 oxidation and enhanced the growth of volcanic sulphate aerosol to optically-active sizes. The total water vapour present within the Hunga plume was greater, with also an estimated 23 Tg of SO2 present (Colombier et al., 2023), the vast majority of emitted sulphur removed via ice sedimentation in the initial days.

The potential for such an explosive eruption to influence climate and the ozone layer, and the effects from the strong enhancement to stratospheric water vapour, motivated APARC to begin a special “Hunga impacts” cross-activity project. The activity’s main role is to co-ordinate community activity to write a special “Hunga impacts report”, and author teams were convened in early 2024, each chapter 1st draft peer-reviewed in autumn 2024, ahead of publication in summer 2025.

This presentation will focus on the Hunga aerosol, and the initial months after the eruption, aligned to the 2025 report. We will present findings from the interactive stratospheric aerosol HTHH-MOC experiment 3, a co-ordinate multi-model analysis of the Hunga aerosol progression, the protocols to identify how the model predictions of the co-emitted water vapour effects vary.

The Hunga aerosol progression has commonalities with the 1883 Krakatau eruption, both eruptions injecting very large amounts of vaporised seawater deep into the stratosphere. Krakatau is estimated to have emitted 500Tg water vapour (Joshi and Jones, 2009), i.e. 4 times greater than Hunga. Krakatau’s highest plume-altitude explosions are thought to have occurred after caldera collapse, from pyroclastic density currents entering the sea (see Self and Rampino, 1981; Self 1992),

Purple twilight duration observations in the Royal Society Krakatoa committee report  (Russell & Archibald, 1888) show the Krakatau cloud descended between August 1883 and January 1884 (see Nature Feb 1888 summary of the report). The observations presented in Pernter (1889) indicate an initial descent from 32km to 24km in the first few weeks.  a similar altitude for the subsequent 2-3 months (September to November 1883), then a descent resuming to 19km in December 1883 and 17km in January 1884.