Climate impacts of stratospheric particle injection

Geoengineering has attracted large attention over recent years as to being a possible way to ameliorate some of the effects of climate change. One of the proposals, involving injecting sulphate aerosols into the stratosphere in order to cool Earth's temperature back to pre-industrial levels, has been assessed as one of the leading geoengineering proposals. Despite this, large uncertainties remain in both the physical and social sciences. Small scale trials of sulphate aerosol injection are not seen as ways to provide large amounts of useful data to inform on the climate response to stratospheric sulphate aerosol loading (whilst also facing many social and ethical barriers). Large scale trials involving injecting amounts of aerosol more comparable to what would be required to cool the Earth's temperature back to pre-industrial levels are viewed as too risky.
Assessments of the climate effects of sulphate aerosol geoengineering by the scientific community therefore have largely relied on climate modelling studies. The thesis begins by reviewing sulphate aerosol geoengineering and the modelling that have been conducted to date. In light of the need to verify modelling results with observations the thesis seeks to understand the effects of nature's analogue to sulphate aerosol geoengineering: large volcanic eruptions. When a volcano erupts it can inject large amounts of SO2 gas into the stratosphere, which then undergo conversion to form sulphate aerosol, cooling the Earth in a way analogous to sulphate aerosol engineering.
The ability of the climate models submitted to the Coupled Model Intercomparison Project 5 (CMIP5) database is assessed, with a particular focus on dynamical changes in the Northern Hemisphere winter period. These models fail to capture the observed NH dynamical response following eruptions, which is of concern for the accuracy of geoengineering modelling studies that assess the atmospheric response to sulphate aerosol geoengineering.
Simulations of volcanic eruptions are then performed with high-top and low-top configurations of the HadGEM2-CC climate model. The high-top version of HadGEM2-CC, with enhanced vertical resolution and model height, gives a markedly improved and statistically significant post-volcanic winter dynamical simulation to its low-top counterpart. The post-winter dynamical simulation in the high-top model agrees with the observed response following volcanic eruptions. Accordingly, mechanisms involved in the dynamical changes are analysed and it is concluded that the HadGEM2-CC high-top model would give more confident simulations of sulphate aerosol geoengineering over its low-top counterpart.
Given the identification of a more suitable model for geoengineering simulations following extensive investigation, the final chapter analyses simulations of the HadGEM2-CC high-top model for asymmetries between the climate response to an immediate onset of geoengineering and a rapid cessation of geoengineering - known as a 'termination' of geoengineering. The project is summarised and discussed, and future work is proposed, involving a large host of projects.

Advisor: Grainger, D