Orbital forcing of climate: implications for land ice and sea ice
Creators: name: O'Neill, Grainne R.
Publisher: name: No Publisher Supplied
Year: 2024

description: The Milankovitch hypothesis posits that variations in Earth's orbital parameters—obliquity, precession, and eccentricity—drive glacial-interglacial cycles by modulating planetary insolation. Although these parameters are recognized as drivers of climate variability, their relative importance in pacing the climate cycles remains uncertain. This dissertation investigates the effects of obliquity and precession on key climate variables using idealized single-forcing simulations from Atmosphere-Ocean General Circulation Models (AOGCMs), which isolate the effects of each orbital forcing.Earth’s glacial periods are dominated by cycle lengths of approximately 100,000 years. However, records of the early Pleistocene are dominated by cycle lengths of 41,000 years. The transition from 41 ky to 100 ky cycles, known as the Mid-Pleistocene Transition (MPT), occurred without notable changes in orbital forcings. Precession has often been considered the primary driver of climate cycles since it exerts a strong influence on high-latitude summer insolation. But this is challenged by the dominance of obliquity cycles in the early Pleistocene. Obliquity plays a crucial role in influencing various aspects of insolation, such as annual mean insolation, insolation intensity integrated over the summer, and meridional insolation gradient, suggesting its potential significance in driving climate variability. In light of existing literature, the fundamental question remains: which orbital parameter—obliquity or precession—serves as the primary driver of the 100,000-year glacial cycles? This dissertation utilized model output from eight idealized single-forcing equilibrium simulations from two AOGCMs: the GFDL CM2.1 and the NCAR CESM1.2. These simulations, consisting of obliquity, precession, zero eccentricity, and preindustrial scenarios, isolated the effects of individual orbital parameters. Through linear reconstruction methods, we assessed historical climate responses to orbital forcings, yielding insights into the relative effects of obliquity and precession on key climate variables. This methodology represents sensitivity experiments relevant to glacial inception during an interglacial phase. Some aspects of the dissertation focused on Scandinavia and Baffin Island, which are associated with the inception sites of the Scandinavian and Laurentide ice sheets, respectively.Chapter 2 explores the influence of obliquity and precession on positive degree days (PDDs) and snowfall rate, metrics for ablation and accumulation, respectively. By analyzing climate model output and time series reconstructions, we found that obliquity induced greater anomalies and contributed more to the variance in annual PDDs compared to precession. While obliquity exerted a more pronounced influence on snowfall rates in Scandinavia, precession dominated annual snowfall in Baffin Island.Chapter 3 addresses the limitations of using PDDs and snowfall to determine ablation and accumulation separately by introducing equilibrium line altitude (ELA) as a comprehensive metric for ice sheet mass balance. Using a surface energy and mass balance model, ELA was computed from the climate model output. We explored the relative influence of obliquity and precession on ELA variability in the Northern Hemisphere (NH), particularly focusing on Scandinavia and Baffin Island. The findings highlighted the dominant role of precession to ELA, and emphasized the importance of accounting for multiple climate variables in understanding ablation.Chapter 4 investigates the effects of orbital forcings on sea ice dynamics in the NH and Southern Hemisphere (SH) polar regions. Climate model and time series reconstruction analysis revealed obliquity as the primary driver of annual sea ice variability. Examination of precession phasing revealed that sea ice in the warm and cold seasons was maximized when aphelion coincided with the associated solstice, except for SH sea ice thickness, which was maximized in both seasons when aphelion aligned with the SH summer solstice.This dissertation advances our understanding of the mechanisms driving Earth's glacial cycles by exploring the roles of obliquity and precession in determining key climate variables. This research contributes to the broader scientific discourse on long-term climate dynamics and our understanding of the 100 ky glacial cycles.