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.