On the Dominance of Obliquity in the Early Pleistocene Glacial Cycles: Insights from an Energy-Balance Climate Model

In Milanković theory, changes in the Earth’s obliquity and climatic precession combine to pace the growth and decay of Northern Hemisphere ice sheets by controlling summer insolation at the high northern latitudes. However, in the glacial-interglacial cycles of the Early Pleistocene, the strength of the obliquity cycle consistently outweighs precession, despite the fact that precession strongly controls the intensity of summer insolation. Consequently, the dominance of the obliquity cycle during the ‘41-kyr world’ of the Early Pleistocene has been referred to as Milanković’s other unsolved mystery. In this study, we present simulations of a zonally-averaged energy balance model (ZEMBA) in response to these orbital cycles. Transient simulations spanning the Early Pleistocene (from 2.4 to 1.2 Ma) show a pronounced 41-kyr obliquity cyclicity in polar temperature similar to the paleoclimate records. Further sensitivity experiments underscore the importance of obliquity over precession, and of sea ice over snow cover, in driving this polar temperature variability. We present new results that attribute the prevalence of the 41-kyr obliquity cycles in ZEMBA to the influence of the orbital parameter on winter sea ice, which regulates the release of large stores of ocean heat to the atmosphere. In contrast, the muted effect of precession on surface air temperature arises from the counterbalancing relationship between insolation intensity and summertime duration, limiting its influence on winter sea ice extent and thereby temperature variability.