Margot Clyne

The 2022 Hunga volcanic eruption injected a significant amount of water vapor and a moderate amount of sulfur dioxide into the stratosphere, causing observable responses in the climate system. We have developed a model–observation comparison project to investigate the evolution of volcanic water and aerosols and their impacts on atmospheric dynamics, chemistry, and climate, using several state-of-the-art chemistry climate models. The project goals are (1) to evaluate the current chemistry–climate models to quantify their performance in comparison to observations and (2) to understand atmospheric responses in the Earth system after this exceptional event and investigate the potential impacts in the projected future. To achieve these goals, we designed specific experiments for direct comparisons to observations, for example from balloons and the Microwave Limb Sounder satellite instrument. Experiment 1 consists of two sets of free-running ensemble experiments from 2022 to 2031: one with fixed sea-surface temperatures and sea ice and one with coupled ocean. These experiments will help to understand the long-term evolution of water vapor and aerosols; quantify HTHH effects on stratospheric and mesospheric temperatures, dynamics, and transport; understand the impact of dynamic changes on ozone chemistry; quantify the net radiative forcings; and evaluate any surface climate impact. Experiment 2 is a nudged-run experiment from 2022 to 2023 using observed meteorology. To allow participation of more climate models with varying complexities of aerosol simulation, we include two sets of simulations in Experiment 2: Experiment 2a is designed for models with internally generated aerosol, while Experiment 2b is designed for models using prescribed aerosol surface area density. This experiment will help to analyze H₂O and aerosol evolution, quantify the net radiative forcings, understand the impacts on mid-latitude and polar O₃ chemistry, and allow close comparisons with observations.

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.

We present findings from a series of interactive stratospheric aerosol simulations of the Hunga-Tonga volcanic aerosol cloud with the UM-UKCA composition-climate model (Dhomse et al., 2020; Marshall et al., 2019; Dhomse et al., 2014).The January 2022 Hunga eruption was the most explosive eruption in the satellite era (Wright et al., 2022), an upper portion of the volcanic aerosol plume at 30-40km (Taha et al., 2022), with the main detrainment initially at ~27-30km, a highly unusual steep descent of the plume seeing the aerosol layer form at ~22-26km (e.g. Kloss et al., 2022; Legras et al., 2022; Baron et al., 2023).The eruption emitted only a modest 0.4-0.5Tg of SO2 to the stratosphere (Carn et al., 2022). but generated the strongest stratospheric aerosol optical depth for 30 years (e.g. Khaykin et al., 2022; Taha et al., 2022; Bourassa 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, the particles more readily reaching optically-active sizes.The GLOMAP aerosol module within UM-UKCA model predicts the stratospheric sulphate aerosol particles that form heterogeneously around meteoric smoke particles, alongside the sulphate aerosol that nucleate homogeneously (see Brooke et al., 2017; Dhomse et al. (2020). The model transports these two sulphate aerosol types in separate modes in the aerosol microphysics module, including with their microphysical interactions (coagulation and uptake of sulphuric acid). For the major volcanic aerosol clouds in most previous UM-UKCA stratospheric aerosol publications, the meteoric-sulphuric aerosol have only a minor role on volcanic forcing (via modulated decay timescale). Here we explore the significance of the meteoric aerosol within SO2-only simulations of the Hunga-Tonga volcanic aerosol cloud’s global dispersion.The long residence times for stratospheric aerosol particles within the tropical stratospheric reservoir means the stratospheric aerosol layer’s column burden enhancement after modest eruptions can be determined not only from the amount of volcanic SO2 emitted, but partly also reflects the residence time of the mix of stratospheric aerosol particles. We explore more generally the role of meteoric-sulphuric particles within moderate SO2 emission tropical stratosphere-injecting eruptions, exploring the transition from sheared volcanic plume to dispersed aerosol cloud, and how the additional volcanic aerosol particles combine with the two types of sulphuric acid particles in the background aerosol.The series of UM-UKCA model experiments align with protocols for the Tonga-MIP multi-model experiment (Clyne et al., 2022), and explore how moderate volcanic enhancements to the stratospheric aerosol layer evolve in the transitional post-plume phase across months 2 to 4 after the eruption.We analyse how the predicted size distributions of the two sulphate aerosol types progress after moderate volcanic eruptions, exploring simulated co-variations of particle size with multi-wavelength aerosol extinction and sulphate mass. We compare also to satellite measurements after Hunga-Tonga and for other recent moderate stratosphere-injecting eruptions.

The 2022 Hunga volcanic eruption injected a significant amount of water vapor and a moderate amount of sulfur dioxide into the stratosphere causing observable responses in the climate system. We have developed a model-observation comparison project to investigate the evolution of volcanic water and aerosols, and their impacts on atmospheric dynamics, chemistry, and climate, using several state-of-the-art chemistry climate models. The project goals are: 1. Evaluate the current chemistry-climate models to quantify their performance in comparison to observations; and 2. Understand atmospheric responses in the Earth system after this exceptional event and investigate the potential impacts in the projected future. To achieve these goals, we designed specific experiments for direct comparisons to observations, for example from balloons and the Microwave Limb Sounder satellite instrument. Experiment 1 is a free-running ensemble experiment from 2022 to 2031. Experiment 2 is a nudged-run experiment from 2022 to 2023 using observed meteorology. To allow participation of more climate models with varying complexities of aerosol simulation, we include two sets of simulations in Experiment 2: Experiment 2a is designed for models with internally-generated aerosol while Experiment 2b is designed for models using prescribed aerosol surface area density. We take model results from the previously developed Tonga-MIP to fulfill Experiment 3, which focuses on the initial dispersion and microphysical evolution of aerosol and water plumes. Experiment 4 is designed to understand the climate impact on the mesosphere from 2022–2027, for which the experiment design is the same as Experiment 1 but for models that resolve the upper stratosphere and mesosphere.

For the first time, we use a modern climate model with interactive chemistry including the effects of aerosols on photolysis rates to simulate the consequences of regional and global scale nuclear wars (injecting 5 and 150 Tg of soot respectively) for the ozone layer and surface ultraviolet (UV) light. For a global nuclear war, heating in the stratosphere, reduced photolysis, and an increase in catalytic loss from the HO_x cycle cause a 15 year‐long reduction in the ozone column, with a peak loss of 75% globally and 65% in the tropics. This is larger than predictions from the 1980s, which assumed large injections of nitrogen oxides (NO_x), but did not include the effects of smoke. NO_x from the fireball and the fires provide a small (5%) increase to the global average ozone loss for the first few years. Initially, soot would shield the surface from UV‐B, but UV Index values would become extreme: greater than 35 in the tropics for 4 years, and greater than 45 during the summer in the southern polar regions for 3 years. For a regional war, global column ozone would be reduced by 25% with recovery taking 12 years. This is similar to previous simulations, but with a faster recovery time due to a shorter lifetime for soot in our simulations. In‐line photolysis provides process specific action spectra enabling future integration with biogeochemistry models and allows output that quantifies the potential health impacts from changes in surface UV for this and other larger aerosol injections.

As part of the Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP), several climate modeling centers performed a coordinated pre-study experiment with interactive stratospheric aerosol models simulating the volcanic aerosol cloud from an eruption resembling the 1815 Mt. Tambora eruption (VolMIP-Tambora ISA ensemble). The pre-study provided the ancillary ability to assess intermodel diversity in the radiative forcing for a large stratospheric-injecting equatorial eruption when the volcanic aerosol cloud is simulated interactively. An initial analysis of the VolMIP-Tambora ISA ensemble showed large disparities between models in the stratospheric global mean aerosol optical depth (AOD). In this study, we now show that stratospheric global mean AOD differences among the participating models are primarily due to differences in aerosol size, which we track here by effective radius. We identify specific physical and chemical processes that are missing in some models and/or parameterized differently between models, which are together causing the differences in effective radius. In particular, our analysis indicates that interactively tracking hydroxyl radical (OH) chemistry following a large volcanic injection of sulfur dioxide (SO₂) is an important factor in allowing for the timescale for sulfate formation to be properly simulated. In addition, depending on the timescale of sulfate formation, there can be a large difference in effective radius and subsequently AOD that results from whether the SO₂ is injected in a single model grid cell near the location of the volcanic eruption, or whether it is injected as a longitudinally averaged band around the Earth.

As part of the Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP), several climate modeling centers performed a coordinated pre-study experiment with interactive stratospheric aerosol models simulating the volcanic aerosol cloud from an eruption resembling the 1815 Mt Tambora eruption (VolMIP-Tambora ISA ensemble). The pre-study provided the ancillary ability to assess intermodel diversity in the radiative forcing for a large stratospheric-injecting equatorial eruption when the volcanic aerosol cloud is simulated interactively. An initial analysis of the VolMIP-Tambora ISA ensemble showed large disparities between models in the stratospheric global mean aerosol optical depth (AOD). In this study, we now show that stratospheric global mean AOD differences among the participating models are primarily due to differences in aerosol size, which we track here by effective radius. We identify specific physical and chemical processes that are missing in some models and/or parameterized differently between models, which are together causing the differences in effective radius. In particular, our analysis indicates that interactively tracking hydroxyl radical (OH) chemistry following a large volcanic injection of sulfur dioxide (SO₂) is an important factor in allowing for the timescale for sulfate formation to be properly simulated. In addition, depending on the timescale of sulfate formation, there can be a large difference in effective radius and subsequently AOD that results from whether the SO₂ is injected in a single model gridcell near the location of the volcanic eruption, or whether it is injected as a longitudinally averaged band around the Earth.

Volcanic eruptions have a significant impact on climate when they inject sulfur gases into the stratosphere. The dynamics of eruption plumes is also affected by climate itself, as atmospheric stratification impacts plumes' height. We use an integral plume model to assess changes in volcanic plume maximum rise heights as a consequence of global warming, with atmospheric conditions from an ensemble of global climate models, using three representative concentration pathways (RCP) scenarios. Predicted changes in atmospheric temperature profiles decrease the heights of tropospheric and lowermost stratospheric volcanic plumes and increase the tropopause height, for the RCP4.5 and RCP8.5 scenarios in the coming three centuries. Consequently, the critical mass eruption rate required to cross the tropopause increases by up to a factor of 3 for tropical regions and up to 2 for high‐latitude regions. A number of recent lower stratospheric plumes, mostly in the tropics (e.g., Merapi, 2010), would be expected to not cross the tropopause starting from the late 21st century, under RCP4.5 and RCP8.5 scenarios. This effect could result in a ≃5–25% decrease in the average SO₂ flux into the stratosphere carried by small plumes, the frequency of which is larger than the rate of decay of volcanic stratospheric aerosol, and a ≃2–12% decrease of the total flux. Our results suggest the existence of a positive feedback between climate and volcanic aerosol forcing. Such feedback may have minor implications for global warming rate but can prove to be important to understand the long‐term evolution of volcanic atmospheric inputs.