The ebb and flow of Pleistocene glacial cycles is not random; it follows a predictable pattern dictated by the distinct and deterministic influence of Earth’s orbital geometry, according to a new study. The findings highlight the roles of precession, obliquity, and eccentricity – factors influencing the tilt and movement of Earth's axis, and the shape of Earth's orbit around the Sun – in glacial transitions. They also establish a predictive model for past and future glacial cycles based solely on orbital forcing. The Quaternary ice ages are thought to be driven by variations in Earth’s orbital configuration. However, due to imprecise age constraints concerning the timing of glacial cycles, researchers have struggled to disentangle the specific influences of precession, obliquity, and eccentricity on the rhythmic advance and retreat of ice sheets. A major challenge in determining the precise influence of orbital variations on glacial cycles lies in the striking similarity between the periodicities of precession (~21,000 years) and the second harmonic of obliquity (~20,500 years). Additionally, the tendency for glacial terminations to occur at ~100,000-year intervals – corresponding to a key eccentricity cycle – remains unresolved, a problem commonly known as the "100-thousand-year (kyr) problem." Rather than relying on the given precision of paleoproxy age models, Stephen Barker and colleagues took a novel approach by analyzing the morphology of the beginning and end of glacial periods over the past 800 kyr, a period characterized by ~100-kyr glacial cycles. Drawing on three independent benthic oxygen isotope records, Barker et al. quantified the timing of critical phases within each glacial-interglacial transition and found that those steps strongly align with the relative phasing of orbital precession versus obliquity. Although both parameters are crucial, precession plays a dominant role in initiating deglaciation, say the authors, while obliquity is more critical for achieving peak interglacial conditions and triggering glacial inception. The findings suggest that glacial terminations occur at specific precession minima that align with increasing obliquity following an eccentricity minimum. Given this, Barker et al. estimate that – in the absence of rising anthropogenic greenhouse gasses – the next glacial period would likely begin within the next 11,000 years as Earth’s obliquity declines toward its next minimum.