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Highlight 4: Distinct roles for precession, obliquity, and eccentricity in Pleistocene 100-kyr glacial cycles

From Barker et al. (2025): The dominant ~100-kyr period of mid- to late Pleistocene glacial cyclicity is problematic because direct orbital forcing at this period (via eccentricity) is weak and one of the oldest unresolved questions in paleoclimate. New analysis presented in this study finds that ~100-kyr glacial cycles of the mid- to late Pleistocene are largely deterministic, allowing prediction of all deglacial and interglacial periods of the past 900 kyr based on orbital phasing alone.

Specifically, analysis summarized in Fig. 1 reveals a strong correlation between deglacial duration and the phasing of precession versus obliquity during glacial terminations, with the onset of deglaciation most likely driven by peak summer intensification (i.e., precession) in combination with rising obliquity. In contrast, the timing of glacial inception depends only on a decrease in obliquity. These responses can be explained by variations in the average latitude of northern ice sheets, with inception occurring at high latitudes under the influence of obliquity, and deglaciation reflecting the dual effects of precession and obliquity across fully extended glacial ice sheets. A protracted deglaciation occurs when the responsible change in precession starts early with respect to the phase of obliquity, effectively delaying the northward retreat of ice sheets toward their interglacial state. In the absence of anthropogenic influences on climate, obliquity phasing predicts the next glacial inception would occur 7700 ± 3400 years in the future.

Finally, each of the past ten glacial terminations occurs during the first candidate precession peak (i.e., coinciding with or immediately preceding an obliquity increase) that follows a minimum in eccentricity. This is consistent with the hypothesis that eccentricity minima can enable the growth of large ice sheets by reducing the amplitude of precession.

Fig. 1. Predicted occurrence and duration of glacial terminations and interglacials. (A to C) Eccentricity, precession, and obliquity (Berger and Loutre, 1991; Crucifix, 2016). (D) Key events (Max deglac, Peak IG, and Max inception) predicted from the relationships shown in Fig. 2C, D, and figs. S3, S4 in Barker et al. (2025). (E) Same events measured directly from records of d18O. (F and G) First differential (F) of the LR04 stack (G). (H) Terminating precession peaks of the past 900 kyr (purple stars and solid purple circles) are the subset of candidate peaks (green circles), which directly follow minima in eccentricity (gray circles). Candidate peaks are the subset of precession peaks (purple crosses) that begin when obliquity is increasing (or starts to increase within 2 kyr of the turning point in precession). (I) Integrated summer energy at 65°N (Crucifix, 2016), with black symbols indicating the predicted occurrence of interglacials based on the rule of Tzedakis et al. (2017) (T17). hMIS-1 is a hypothetical future interglacial. Vertical gray boxes indicate the predicted duration of interglacial periods (from Max deglac to Max inception) based on the average of predicted events in (D). (This is Figure 6 from Barker et al., 2025.)

References:

Barker, S., Lisiecki, L.E.,  Knorr, G., Nuber, S., Tzedakis, P.C. (2025), Distinct roles for precession, obliquity, and eccentricity in Pleistocene 100-kyr glacial cycles, Science, 387, https://doi.org/10.1126/science.adp3491

Berger, A., Loutre, M.F. (1991), Insolation values for the climate of the last 10 million years. Quaternary Science Reviews, 10, 297–317, https://doi.org/10.1016/0277-3791(91)90033-Q

Crucifix, M. (2016), Palinsol: Insolation for palaeoclimate studies, R package version 0.93 (2016); https://bitbucket.org/mcrucifix/insol

Tzedakis, P.C., Crucifix, M., Mitsui, T., Wolff, E.M. (2017), A simple rule to determine which insolation cycles lead to interglacials. Nature 542, 427-432, https://doi.org/10.1038/nature21364