Volcanic Impacts

For over a decade, our research group has specialised in quantifying the complex impacts of volcanic eruptions on the global climate system. To achieve this, we utilize a dual-modelling approach featuring the Unified Model – United Kingdom Chemistry and Aerosol (UM-UKCA), which serves as the composition-climate configuration of UKESM, and the TOMCAT/SLIMCAT three-dimensional chemical transport model (CTM). While our historical work has focused on SO2-rich eruptions such as Mt. Pinatubo, El Chichón, and Mt. Agung, our recent efforts have shifted toward the unique atmospheric challenges posed by the January 2022 eruption of Hunga Tonga-Hunga Ha'apai.

The Hunga eruption was unprecedented in the satellite era, injecting approximately 150 Tg of water vapour directly into the sub-tropical mid-stratosphere. This massive injection increased the global stratospheric water burden by roughly 10%. Unlike major 20th-century eruptions that primarily influenced climate through sulphate aerosol cooling, Hunga’s water-rich plume has altered stratospheric chemistry, aerosol microphysics, and the radiative balance in ways that are still being unravelled. We are currently employing the TOMCAT CTM to investigate how this persistent moisture impacts stratospheric composition, specifically regarding the formation of polar stratospheric clouds (PSCs) and the resulting ozone depletion.

Our recent simulations with TOMCAT reveal that the enhanced water vapour has raised the threshold temperatures for PSC formation. This shift leads to the earlier onset of heterogeneous chemistry and the activation of chlorine, which are the primary drivers of the seasonal Antarctic ozone hole. However, our CTM analysis shows that these effects are not uniform across the polar vortex. In the Antarctic "vortex core," widespread mid-winter dehydration—caused by the sedimentation of ice-containing PSCs—limits the chemical impact by removing much of the excess water vapor from the stratosphere.

In contrast, the vortex edge region and the Arctic stratosphere present different chemical risks. Because the Arctic is generally warmer and less prone to the extreme dehydration seen in the Antarctic, the Hunga-injected moisture may have a more pronounced impact on ozone depletion during cold Arctic winters. For instance, TOMCAT experiments simulating a Hunga-like injection followed by a cold Arctic winter indicated a maximum additional ozone depletion of approximately 16 Dobson Units (DU) at the vortex edge (Chipperfield et al., 2025). This highlights the necessity of using high-resolution CTMs to track the plume’s movement and its interaction with varying meteorological conditions.

Looking toward the future, our latest research indicates that the removal of Hunga water vapour has entered a new phase (Zhou et al, 2026). While Antarctic dehydration was a primary removal pathway in 2023, stratosphere-troposphere exchange became the dominant sink by the end of 2024. Based on a combination of satellite observations and TOMCAT simulations, we estimate the excess stratospheric water vapour is now decaying with an e-folding time of approximately 3 years. We project that stratospheric water vapour levels will return to their pre-Hunga range of variability by around 2030, and we will continue to use our modelling suite to monitor its ongoing impact on the stratospheric composition.

References

Chipperfield, M.P., S.G. Heddell, S. Dhomse, W. Feng, S. Chang, G. Mann, X. Zhou and H.C. Pumphrey, Ongoing large ozone depletion in the polar lower stratospheres: The role of increased water vapour,
Faraday Discussions, 258, 216-233, doi:10.1039/D4FD00163J, 2025.

Dhomse, S., K.M. Emmerson, G.W. Mann, N. Bellouin, K.S. Carslaw, M.P. Chipperfield, R. Hommel, N.L. Abraham, P. Telford, P. Braesicke, M. Dalvi, C.E. Johnson, F. O'Connor, O. Morgenstern, J.A. Pyle, T. Deshler, J.M. Zawodny and L.W. Thomason, Aerosol microphysics simulations of the Mt Pinatubo eruption with the UKCA composition-climate model, Atmos. Chem. Phys., 14, 11221-11246, doi:10.5194/acp-14-11221-2014, 2014.

Dhomse, S., M.P. Chipperfield, W. Feng, R. Hossaini, G.W. Mann and M.L. Santee, Revisiting the hemispheric asymmetry in mid-latitude ozone changes following the Mount Pinatubo eruption: A 3-D model study,
Geophys. Res. Lett., 42, 3038-3047, doi:10.1002/2015GL063052, 2015.

Dhomse, S., G.W. Mann, J.C. Antuna Marrero, S.E. Shallcross, M.P. Chipperfield, K.S. Carslaw, L. Marshall, N.L. Abraham and C.E. Johnson,
Evaluating the simulated radiative forcings, aerosol properties, and stratospheric warmings from the 1963 Mt Agung, 1982 El Chichon and 1991 Mt Pinatubo volcanic aerosol clouds,
Atmos. Chem. Phys., 20, 13627-12408, doi:10.5194/acp-20-13627-2020, 2020.

Zhou, X., S.S. Dhomse, W. Feng, G. Mann, S. Heddell, H. Pumphrey, B.J. Kerridge, B. Latter, R. Siddans, L. Ventress, R. Querel, P. Smale, E. Asher, E.G. Hall, S. Bekki and M.P. Chipperfield, Antarctic vortex dehydration in 2023 as a substantial removal pathway for Hunga Tonga-Hunga Ha'apai water vapour,
Geophys. Res. Lett., 51, e2023GL107630, doi:10.1029/2023GL107630, 2024.

Zhou, X., Q. Chen, W. Feng, S. Heddell, S.S. Dhomse, G. Mann, H.C. Pumphrey, L. Millan, M.L. Santee, M.P. Chipperfield, When will the stratospheric water vapour return to pre-Hunga level?
Communications Earth and Environment, xx, xx, doi:10.1038/sxxx, 2026.