Can geoengineering be optimised?

Thesis

Geoengineering is the intentional, large-scale manipulation of Earth’s climate. It has been suggested that this could be done to counteract or ameliorate the effects of anthropogenic climate change, to reduce its negative impacts or buy time for global greenhouse gas emissions to be reduced. Stratospheric aerosol injection, where aerosols in the stratosphere are used to reflect sunlight and so cool climate, has been widely studied. Altering the altitude, latitude, or timing of aerosol injections could result in different radiative forcing patterns, which suggests there may be potential to “optimise” stratospheric aerosol injection geoengineering to achieve particular climate goals. The extent to which geoengineering could be optimised, beyond idealised studies that counteract the global mean temperature increase of greenhouse gases, has only relatively recently begun to be explored.

Chapter 1 discusses the background of geoengineering as a concept and includes a literature review and discussion of robust results that have emerged from modelling studies of geoengineering. Chapter 2 uses a combination of analytical techniques and the simple climate model, FaIR, to examine different scenarios for “peak-shaving” – temporarily using geoengineering to hold global mean temperatures below a certain threshold – and examines trade-offs between the amount of warming avoided and the implied duration of commitment to geoengineering.

Chapters 3-5 analyse simulations from the HadCM3 climate model, simulated using climateprediction.net, which uses thousands of volunteer computers to generate large ensembles of simulations with differing distributions of stratospheric aerosol optical depth counteracting an abrupt quadrupling of carbon dioxide to represent different attempts at tailoring geoengineering. Chapter 3 details initial calibration and characterisation of the response to simple patterns of radiative forcing, and establishes that the temperature response to different patterns of forcing is, to a good approximation, linear and additive. Chapter 4 expands on this and discusses various methods for optimising for temperature and precipitation, including analysis of trade-offs for attempting to optimise temperature in different regions, and analysis of whether there are a limited number of fundamental modes of response in the HadCM3 climate model to a range of imposed radiative forcing patterns. Chapter 5 examines the impact of geoengineering on climate and weather extremes, using metrics that represent heatwaves, flooding, and dry periods, and analysing any differences in the distribution of extreme events between simulations of the preindustrial and geoengineered climates. In the final chapter, results and conclusions are summarised, and possible future work is outlined.

Authors: Hornigold, T

• DOI: 10.5287/ora-dxojpxyoj
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: climate; geoengineering; climate intervention; climate science; climateprediction.net; climate modelling; climate change; HadCM3
• Subjects: Climate; Atmospheric physics; Climate change mitigation