Text

theses

Geoengineering is the large-scale intentional manipulation of climate processes designed to reduce global temperature. Absent the implementation of an effective global mitigation strategy, it may be difficult to avoid experiencing an amount of anthropogenic global warming that would adversely impact both civilization and natural ecosystems without solar radiation management (SRM) geoengineering. One of the major issues with global warming is a possible increase in tropical precipitation over already wet tropical land regions. A major risk associated with stratospheric SRM, an intervention designed to reduce the impacts of global warming, is a reduction of tropical precipitation over land. In this thesis, I will explore the climate response to SRM with a special focus on impacts in tropical regions, particularly with regard to changes in precipitation patterns. Changes in the El Niño Southern Oscillation (ENSO) are either not present in Geoengineering Model Intercomparison Project (GeoMIP) G1-G4 output, or not detectable, due to the short simulation duration and to the large inherent variability of ENSO. The changes in precipitation distribution and monsoon strength relative to anthropogenic global warming or the historical climate under G1-G4 cannot be attributed to changes in ENSO. After determining that no possible marine cloud brightening scheme could be an effective method of cooling Earth without reducing tropical precipitation, in heavily populated, highly cultivated tropical regions, I developed the G4Foam experiment, which achieved significant global cooling and a large redistribution of precipitation from ocean to land. No direct forcing was applied to tropical latitudes, but G4Foam cooled the tropics by 0.6 K, while increasing precipitation in most areas, including areas that typically get less precipitation with global warming. However, the Southern Hemisphere (SH) regional forcing was, as expected, not effective in cooling Northern Hemisphere (NH) continents. In an attempt to cool the entire planet while maintaining tropical precipitation at present day levels, I combined stratospheric solar radiation management and regional ocean albedo enhancement in designing the G4SSAFoam experiment. In this experiment, 1.5 K of global mean cooling was achieved and tropical precipitation remained at or near RCP6.0 levels and slightly above G4SSA levels. However, the spatial distribution of positive and negative precipitation and precipitation minus evaporation (P-E) anomalies in the tropics was heterogeneous, with some heavily populated areas experiencing large increases, while others experience large decreases. The severe cooling of about 2 K in the SH extratropics causes a precipitation reduction of almost 6% in G4SSAFoam when compared to RCP6.0. While P-E anomalies over land in the SH are only negative in certain regions, future research would be needed to determine the full societal and ecological impact of the SH extratropical temperature and precipitation perturbations. The deployment of microbubbles in the ocean is currently not possible and significant innovation would need to occur if a need to conduct geoengineering in the manner of G4Foam arose, for example, in the event of a stratospheric solar radiation management deployment that reduced tropical precipitation to the extent that it or reduced tropical temperature.

ETD_8000

Ph.D.

Includes bibliographical references

by Corey J. Gabriel

doi:10.7282/T3CF9SZG

ETD doctoral

The author owns the copyright to this work.