Simulation and Projection of Global Temperature Change and Recovery of Extra‑polar Ozone using Multiple Linear Regression Models

PhD Defense: Laura McBride, UMCP

Advisor: Dr. Ross Salawitch

Abstract: Climate change and the depletion of the ozone layer are two important global environmental problems caused by the release of gases into the atmosphere. This dissertation uses multiple linear regression to separate the natural and human components that affect Earth’s global mean surface temperature (GMST) and the thickness of the ozone layer. To analyze changes in Earth’s climate, the Empirical Model of Global Climate (EM-GC) is used to simulate and project variations in GMST. Numerous scenarios of greenhouse gas and aerosol emissions are considered to analyze the probability of achieving the 1.5 and 2.0°C goals set by the Paris Agreement. There is a 53% likelihood of staying below 1.5°C if the world follows the greenhouse gas and aerosol emissions in SSP1-2.6, and a 64% probability of staying below 2.0°C if the world follows SSP4-3.4. The amount of warming attributed to humans from 1975 to 2014 based on the EM-GC is 0.157°C decade−1 (range of 0.120 to 0.195°C decade−1). Multi-model output from the Coupled Model Intercomparison Project Phase 6 (CMIP6) indicates humans contributed 0.221°C decade−1 (0.151 to 0.299°C decade−1) of warming from 1975 to 2014, which is notably faster warming than inferred from the historical climate record. The rise in GMST at 2×preindustrial concentrations of carbon dioxide is also examined. The effects of increasing methane emissions are discussed, as well as the timeline for emitting the remainder of the world’s carbon budget. Humans can emit another 150 ± 79 Gt C after 2019 to have a 66% likelihood of limiting warming to 1.5°C and another 400 ± 104 Gt C to have the same probability of limiting warming to 2.0°C. Given the estimated emission of 11.7 Gt C per year for 2019 due to human activities, carbon and methane emissions must be severely curtailed in the next 10 years to achieve the 1.5°C goal. The results from the EM-GC are compared to other reduced complexity climate models (RCMs) as part of an international collaboration, as well as multi-model output from CMIP6. All RCMs, including the EM-GC, show that the CMIP6 global climate models warm too quickly. To analyze changes in the ozone layer due to human activities, a multiple linear regression model is used that includes equivalent effective stratospheric chlorine (EESC) as a measure of the human effect on ozone. Results using the updated EESC calculation indicate VSL Cl has already caused a seven-year delay in the recovery of the ozone layer. Future simulations including VSL Cl and the updated age of air spectrum project a slower recovery of the ozone layer, up to 25 years later than previously expected.