Major volcanic eruptions significantly impact the atmospheric radiative balance and climate system by emitting gases and aerosols into the stratosphere. The Hunga Tonga-Hunga Ha'apai (HT) volcano's eruption on January 15, 2022, was exceptionally violent, with plumes reaching altitudes over 50 km. While the estimated sulfur dioxide (SO2) emission (around 0.4 Tg) was initially thought to suggest a negligible climate impact, this study challenges that assumption. The research questions center around understanding the magnitude and nature of the stratospheric perturbations caused by the HT eruption, and subsequently the impact this eruption had on the atmospheric radiation budget and the Earth's climate. The study aimed to quantify the radiative forcing resulting from both aerosol and water vapor injections, and to examine the plume's evolution and its implications for the broader climate system. This is important because accurate assessment of volcanic impacts on climate requires detailed understanding of both aerosol and water vapor contributions, particularly in unprecedented events like the HT eruption. The significance of this research lies in improving our ability to predict and model the climate effects of volcanic eruptions, especially those with unique characteristics, thus contributing to a more robust understanding of the Earth’s climate system.

Previous research has extensively studied the impact of volcanic eruptions on climate, particularly focusing on the role of stratospheric aerosols in radiative forcing. Studies have shown a correlation between the amount of SO2 injected into the stratosphere and the magnitude of the resulting climate cooling. The Pinatubo eruption of 1991, for instance, is frequently cited as a prime example of a large-scale volcanic event causing significant global cooling. However, the understanding of water vapor's role in volcanic radiative forcing remains less clear, although research has highlighted its potential contribution. This study builds upon these existing investigations by examining a unique eruption—the HT eruption—with a significant, unprecedented water vapor injection, forcing a reevaluation of the relative impact of aerosols and water vapor on the atmospheric radiation balance and climate system. The existing literature provides a baseline for understanding the typical impact of volcanic eruptions, but the HT event presented an opportunity to expand upon this understanding by examining the consequences of an exceptionally large water vapor release in tandem with a relatively smaller aerosol release.

This research employed a multi-faceted approach combining various data sources and modeling techniques. High-resolution geostationary satellite instruments (Himawari-8 Ash RGB product) tracked the volcanic plume's initial dispersion and evolution. The Himawari data provided insights into the plume's composition and movement. The Infrared Atmospheric Sounding Interferometer (IASI) and the Advanced Microwave Sounding Unit (AMSU), alongside the Microwave Humidity Sounder (MHS) on board the MetOp spacecraft series, enabled the quantification of volcanic sulfur dioxide (SO2) and sulfate aerosol through the use of the RAL Infra-red/Microwave Sounder (IMS) retrieval core scheme, using an optimal estimation spectral fitting procedure and RTTOV 12 (Radiative Transfer for TOVS) as the forward radiative transfer model. The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument onboard the CALIPSO satellite provided high-vertical-resolution aerosol observations, offering crucial information on the plume's vertical structure and optical properties. The Ozone Mapping and Profiler Suite Limb Profiler (OMPS-LP) onboard the Suomi-NPP satellite provided long-term observations for comparison with recent stratospheric events. Ground-based aerosol sun-photometer observations from the AERONET network helped confirm plume evolution across Australia and La Réunion. In situ observations from the Light Optical Aerosol Counter (LOAC) during a balloon campaign at La Réunion provided microphysical characterization of the dispersed plume. Radio-sounding observations in Australia and Saint Helena Island were used to capture water vapor concentrations. Offline radiative transfer modeling (using UVSPEC within the libRadtran suite) calculated the localized stratospheric radiative heating/cooling and top-of-atmosphere (TOA) and surface radiative forcing, considering both aerosol and water vapor perturbations, using CALIOP-derived aerosol extinction and radio-sounding water vapor data. This integrated approach using multiple data sources and modeling allowed for a robust assessment of the eruption's radiative impact.

The Hunga Tonga eruption produced exceptionally large perturbations in both stratospheric aerosols and water vapor. The study found that the eruption generated the largest global perturbation of stratospheric aerosols since the 1991 Pinatubo eruption and the largest perturbation of stratospheric water vapor observed during the satellite era. Satellite imagery revealed a rapid conversion of SO2 to sulfate aerosols, potentially due to the abundant water vapor. CALIOP observations showed an initial dominance of aspherical particles (ash and/or ice) which transitioned to spherical sulfate aerosols. AERONET data confirmed the movement of the plume and the increase in small, reflective sulfate aerosols. Analysis of IASI/IMS data estimated a sulfate aerosol total mass burden of 1.0 to 3.0 Tg by early February 2022. The eruption injected an estimated >100 Tg of water vapor into the stratosphere—unprecedented in the satellite era. Radio-sounding data showed high water vapor concentrations within the plume. Radiative transfer modeling showed that in the initial weeks following the eruption, water vapor radiative cooling dominated local stratospheric heating/cooling rates, while at the top-of-the-atmosphere and surface, volcanic aerosol cooling dominated the radiative forcing. After two weeks, however, water vapor heating began to dominate top-of-atmosphere radiative forcing, leading to a net warming of the climate system. The modeling also highlighted a rapid radiatively-driven plume descent caused by the large water vapor concentrations, unlike typical volcanic plumes which ascend.

The findings challenge the initial assumption that the HT eruption’s climate impact would be negligible due to the relatively low SO2 emission. The exceptionally high water vapor content played a crucial role, leading to rapid sulfate aerosol formation and a unique radiative response. The initial cooling effect, dominated by water vapor, transitioned to a net warming effect as the plume dispersed. The extremely fast conversion rate of SO2 to sulfate aerosols and the unexpectedly large contribution of water vapor to radiative forcing highlight the complexity of assessing volcanic impacts on climate. The magnitude of the stratospheric aerosol and water vapor perturbations observed during and after the HT eruption were larger than those observed after other recent stratospheric events such as the Raikoke eruption and the Australian fires, emphasizing the HT eruption's significance. This research significantly improves our understanding of the relative contributions of water vapor and aerosol to volcanic radiative forcing. The study's findings highlight the need to refine climate models to adequately capture the diverse impacts of different volcanic eruption types.

The Hunga Tonga eruption of January 15, 2022, produced unprecedented perturbations in stratospheric aerosols and water vapor, leading to a complex radiative response. The findings emphasize the significant role of water vapor in volcanic radiative forcing and highlight the limitations of relying solely on SO2 emissions to predict climate impacts. Future research should focus on refining climate models to accurately represent the combined effects of aerosols and water vapor in various volcanic eruption scenarios, particularly for phreatomagmatic eruptions. Further investigation into the long-term effects of the HT eruption on the stratospheric ozone layer is also warranted.

The study's radiative forcing calculations were made for specific stages of plume dispersion, potentially limiting the generalizability of the results to the entire lifespan of the plume. While representative of the plume’s initial state and subsequent evolution, the fact that the aerosol and water vapor profiles in the radiative modeling were not sampled at precisely the same location and time represents a potential limitation. Furthermore, the long-term effects on the stratospheric ozone layer were not explicitly addressed due to the time frame considered. Finally, the uncertainties associated with radio-sounding water vapor measurements at high altitudes, particularly the issue of sonde outgassing, should be considered when evaluating the results.