YES-27-A-15: Climate impacts longevity from high-altitude major volcanic eruptions

The different regime relates to the first direct measurement dataset of a volcanic solar dimming, the solar radiation measurement record from Montpellier after Krakatoa (Kimball, 1918; Wexler, 1951).  A recent undergraduate dissertation project at the Univ. Leeds (Marshall, 2026) has digitized the Montpellier solar radiation anomaly dataset (Figure 1a), deriving a stratospheric aerosol optical depth (Figure 1b), each dataset providing alternative constraints for climate model predictions.

This PhD project will involve model experiments with the UK Earth System Model, to explore the climate influences from high-altitude major eruption case studies.  The PhD is a co-operation with Met Office scientists, within the Leeds-Met Office academic partnership.

Figure 1: a), b) Montpellier solar radiation measurement dataset for 1883-1901 has two-year post-Krakatoa peak solar dimming, consistent with deep-branch transport pathway from high-altitude volcanic sulfur emission. c) The Hunga excess water vapour decay rate has e-folding timescale of 2.89-3.65 years, a longevity of ~9 years. d) schematic of stratospheric meridional transport pathways in deep branch (from Boenisch et al., 2009).

For the historical integrations in CMIP climate model intercomparison activities (e.g. Eyring et al., 2016), volcanic impacts are represented from “volcanic forcing datasets”, to enact the solar dimming effect from best-estimate time-varying stratospheric aerosol optical properties (e.g. Sato et al., 1993; Joerimann et al., 2025). For the current CMIP7 activity (Van Vuuren et al., 2026), a new approach (Aubry et al., 2026) provides also a self-consistent historical volcanic forcing dataset based on eruption source parameters for each individual eruption, including the altitude of sulfur emission.

The January 2022 Hunga eruption was the most explosive volcanic event in the satellite era (Wright et al., 2022) and its shallow underwater setting caused emitted water vapour to remain in the stratosphere for 5-10 years (Millan et al., 2022; Zhuo et al., 2025).  The potential for substantial impacts on polar ozone (Zhu et al., 2022; Wang et al., 2023), and the effects from the strong cooling of the stratosphere (e.g. Randel et al., 2024), motivated the 2025 Hunga impacts report (e.g. APARC, 2025), aligned to the 2026 WMO/UNEP Scientific Assessment of Ozone Depletion report.

Major volcanic eruptions such as Pinatubo cause prolonged increases to the stratospheric  aerosol layer and subsequent heterogenous chemistry, on the surfaces of volcanic aerosol, and at high latitudes also on polar stratospheric clouds (PSCs).  Both effects cause substantial changes to the ozone layer (e.g., Solomon, 1999; WMO, 2022). Recent new understandings include a volcaniic “vortex strengthening effect” (Kodera, 1994; Toohey et al., 2014), seen for both Hunga (Bednarz et al., 2025; 2026) and Pinatubo (Dalla Santa et al., 2019); and how emitted water vapour can increase PSCs and ozone depletion when volcanogenic water vapour reaches high latitudes (Heddell et al., 2026).

This PhD project will focus on high sulfur major eruption case studies, aligned to analysis of Hunga transport pathways (Toohey et al., 2025; Schoeberl et al., 2025; Zhou et al., 2026) that provides new constraints for the longer-duration timescales of deep-branch water vapour and aerosol transport, relevant for reliably predicting impacts high-altitude high-sulphur volcanic eruptions.

The 1883 eruption is the most recent high-sulphur water co-emission case, with large amounts of water vapour thought to have been injected into the stratospheric from co-ignimbrite plume explosions from the ocean pyroclastic flow, alongside also high sulphur emission (Rampino and Self, 1982; Colombier et al., 2025).

In addition to the Montpellier solar dimming measurement constraints, the project could explore initial similarities with the Hunga eruption, with twilight duration measurements indicating an initial descent of the Krakatoa aerosol, strikingly similar to that observed after Hunga (see Figure 2).

Figure 2: Observations from the optical phemonena synthesis (Russell and Archibald, 1888) of the Royal Society report on Krakatoa (Symons et al., 1888) show the Krakatoa aerosol descended from ~32km to ~25km (left-hand panels), strikingly similar to the initial water vapour forced descent after Hunga (right-hand panels).

The PhD is a co-operation with Met Office scientists, within the Leeds-Met Office academic partnership, and could also align with other impacts from the long-lived stratospheric water vapour, for example polar stratospheric clouds. An alternative very high-altitude high-sulfur eruption case study (e.g. Taupo or Santorini) could also be another alternative for exploring within the PhD.

Similarly, the student will have the opportunity to make use of NCAR’s Community Earth System Model (CESM) which has been developed in Leeds University (e.g. Feng et al., 2013; Kovacs et al., 2016; Li et al., 2026) to investigate the impacts of volcanic eruptions on changes in stratospheric ozone, aerosol, radiations and dynamics etc) using a self-consistent whole atmospheric model.

For very large eruptions, impacts on food security, and another alternative route the PhD project could be to explore how the longevity compounds into more severe economic and hence societal impacts, for example when two successive growing seasons are affected.