Risks to global water resources from geoengineering the climate with solar radiation management

Climate change, including global temperature rise, is one of the greatest threats facing humanity and the planet. The response of the collective human population to reduce greenhouse gas emissions, the driver of this change, has been far from adequate. To this end, methods of geoengineering the climate have been proposed, ranging from removal of carbon dioxide presently in the atmosphere (including Carbon Capture and Storage and Direct Air Capture) to methods of adjusting the amount of incoming solar radiation reaching the troposphere. Solar geoengineering (SG), also known as solar radiation management (SRM), is a proposed method of geoengineering which reflects a portion of incoming solar radiation back into space to slow or reverse global temperature rise. However, whilst simulations suggest that SG may be able to halt global mean temperature changes, it will not be able to counteract all aspects of climate change which threaten both society and ecosystems. One particularly well-known effect of current SG modelling experiments is that, when designed to offset a specific temperature rise under greenhouse gas forcing, precipitation values would be excessively offset and decrease below the baseline, reducing the overall intensity of the global hydrological cycle. A global-scale assessment is urgently needed to quantify the risks and opportunities SG poses to global water resources. This project has examined how water resources (including drought and water scarcity) could be affected across the globe through deployment of SG. Throughout this work, the Geoengineering Large Ensemble Project (GLENS) modelling has been used as the scenario of SG; which aimed to reduce some of the negative outcomes of previous SG modelling results, including the negative changes in precipitation. Runoff directly output from the GLENS modelling was compared to simulated runoff generated by the Global Hydrology Model (GHM) Mac-PDM.17 using climate forcing from GLENS, in order to gain some understanding of intra-model variability. Aspects of both the hazard and risk solar geoengineering may pose, contrasted against those seen under unmitigated climate change, were examined through consideration of drought and water scarcity projections across the globe. The findings of this research help answer policy-relevant research questions such as: which parts of the globe may benefit/suffer from SG; which regions may be better off with global warming than with a cooler climate achieved by SG; and how long may it take for such changes to occur; all within the context of global water resources. To help answer the questions set out in this thesis a number of mathematical techniques have been employed. Extreme value theory was used to establish minimum monthly drought runoff return periods, whilst permutation tests were used to determine statistical significance between the solar geoengineering and climate change modelling ensembles. Further investigation of the permutation test revealed the importance of sample size on the power of the test. Summary statistics to summarise averages, variability and percentage change were used to interpret trends in the data. Mathematical methods were critical to model outputs, with the feedback algorithms governing the amount of stratospheric aerosol injection (and thus climate outcomes) and the probability distribution moisture method in Mac-PDM determining soil moisture across a gridcell (and thus saturation excess runoff). The importance of mathematical equations in controlling outputs, e.g. by the method used to calculate potential evapotranspiration, was also highlighted. Under the specific scenarios of climate change and solar geoengineering used in this research, it has been seen that some areas may benefit from the amelioration of impacts from climate change through the implementation of solar geoengineering, namely many parts of the Americas, Europe, Australia and the Middle East. Other regions are projected to face greater water resource challenges under solar geoengineering, including parts of Southern Asia, Southeast Asia and many regions of Africa (Western, Middle and Eastern). This raises important questions around global equability of outcomes and equality for all nations, as well as the implications for future governance of any solar geoengineering deployment. A further important finding in the context of water scarcity was that the largest degree of future change often arose from future projected changes in population, with the climate scenario having a smaller overall impact. In addition to this, in some instances where solar geoengineering was projected to worsen conditions in comparison to climate change, this resulted from solar geoengineering maintaining conditions closer to the present-day climate, with climate change reducing future water scarcity when calculated at the annual scale. This demonstrates the complex interconnectedness of humans and the environment and advocates for more extensive research into possible impacts of solar geoengineering on global water resources. Future work into the affect of solar geoengineering on global water resources should particularly focus on using a wider range of modelling projections (including more realistic scenarios, i.e. not offsetting all warming) and greater consideration of variation in water availability throughout the year