The impact of volcanic halogens, climate change, and sulfate aerosol geo-engineering on the atmospheric effects of volcanic eruptions

Abstract

The evolution of volcanic sulfur, stratospheric composition and radiative forcing following explosive volcanic eruptions injecting sulfur directly into the stratosphere is well understood. However, how the co-emission of volcanic halogens, climate change, and concurrent sulfate aerosol geo-engineering would alter the atmospheric effects of sulfur-only explosive volcanic eruptions has, previously, been under-researched. In this thesis, aerosol-chemistry-climate model simulations are utilised to show that the co-emission of volcanic halogens alongside sulfur (halogen co-emission) and climate change amplify the volcanic forcing of volcanic eruptions, mainly caused by a reduction in the aerosol lifetime and aerosol size. Stratospheric ozone is shown to be less vulnerable to sulfur-only eruptions, but more sensitive to halogen co-emission eruption scenarios in the future compared to the present-day, due to the projected decline in stratospheric halogen abundance and ozone recovery. An explosive volcanic eruption during stratospheric sulfate aerosol geo-engineering is found to lead to additional negative radiative forcing and total column ozone depletion, but the response is complex and non-additive. This thesis emphasises the need to include volcanic halogen emissions when simulating the climate effects of past or future eruptions as well as the necessity to maintain space-borne observations of stratospheric compounds to better constrain the stratospheric injection estimates of volcanic eruptions. It identifies a novel climate-volcano feedback and highlights the need to re-evaluate the use of constant volcanic forcing typically used in future climate projections. Furthermore, this thesis demonstrates the difficulties associated with maintaining a stable level of radiative forcing in the event of an explosive volcanic eruption during sulfate aerosol geo-engineering.