Hunga Tonga-Hunga Ha'apai: a paradigm-changing eruption

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The 2022 eruption of Hunga Tonga-Hunga Ha'apai (HT-HH) is the most intense volcanic eruption in 30 years. The eruption challenges many preconceptions about the impact of volcanic plumes on the atmosphere and climate, and calls for new, more comprehensive methods for studying volcanic plumes. These would improve future monitoring capability and enhance understanding of plume dynamics and impacts. Monitoring volcanic plumes is important for a number of reasons including their climatic impact. This climate impact is usually driven by the emission of sulphur dioxide (SO2). This gas can be converted to sulphate aerosol which deflects incoming solar radiation and causes planetary cooling. Despite the intensity of the eruption, HT-HH emitted only small quantities of SO2: not sufficient to cause cooling. However, due to it being a submarine volcano, the eruption injected significant quantities of water vapour into the atmosphere. Water vapour is a greenhouse gas and in contrast to 'normal' eruption-climate impacts, the eruption is expected to have a warming effect in the troposphere: amplifying anthropogenically driven climate change. Water vapour has been largely neglected in previous studies of volcano-atmosphere-climate interactions and requires significant further study. It has an additional effect of causing the rapid emergence of sulphate aerosol: possibly through accelerating the conversion of SO2 to sulphate, or more directly from sulphate in seawater. Early formation of sulphate has also been identified during other eruptions. Regardless of the formation mechanism, early emergence of sulphate aerosol means that measurements of SO2 alone (the conventional approach), are not sufficient for quantifying the emission of sulphur to the atmosphere. Another motive for studying volcanic plumes is their hazardous nature, in particular, to aviation. Volcanic ash can cause significant damage to aircraft, in some cases so severe that it causes engine failure and potentially life-threatening circumstances. Volcanic gases can also damage aircraft. Monitoring these plumes is therefore essential to minimize the hazard they present. Satellite data plays an important part of monitoring and studying volcanic plumes. The HT-HH eruption has identified a number of areas for improvement which can be adressed with new instrumentation making frequent global observations (unavailable for past eruptions). This project will develop the next generation of satellite retrievals for the quantification of volcanic plume properties. These will be developed for the Infrared Atmospheric Sounding Interferometer (IASI): a meteorological satellite instrument with sensitivity to multiple types of volcanic plumes. The new retrievals will be more comprehensive: simultaneously obtaining information about water vapour, SO2, sulphate and volcanic ash, rather than treating them separately. This will make them valuable tools for both hazard detection and providing data to rapidly assess climate impacts. Working closely with the UK Met Office will ensure the newly developed tools can be used operationally for future eruptive events, so ensuring a lasting impact from the project. Following the development of these retrievals, they will be applied, along with other datasets, to study the HT-HH in detail. Climate impact models and seasonal forecasts, initialized with results from the new satellite data, will be used to study the future impact of this eruption on climate and atmospheric dynamics. This will build a better understanding of this unusual event. The impact goes beyond this eruption, as applying the retrievals to study other eruptive events will help to improve understanding of volcanic plume dynamics. The HT-HH eruption has raised numerous questions about volcanic plumes. It is likely that this will have a lasting impact on the direction of research over the next few decades, with this project playing a key role in this.