Integrated summer insolation forcing and 40,000‐year glacial cycles: The perspective from an ice‐sheet/energy‐balance model

Abstract

[1] Although the origins of the 40,000-year glacial cycles during the early Pleistocene are readily attributed to changes in Earth's obliquity (also having a 40,000-year period), the lack of ice-volume variability at precession periods (20,000 years) is difficult to reconcile with most parameterizations of the insolation forcing. It was recently proposed that precession's influence on glaciation is muted because variations in the intensity of summer insolation are counterbalanced by changes in the duration of the summertime, but no climate model has yet been shown to generate obliquity period glacial cycles in response to the seasonal insolation forcing. Here we present a coupled ice-sheet/energy-balance model that reproduces the seasonal cycle and, when run over long time periods, generates glacial variability in response to changes in Earth's orbital configuration. The model is forced by the full seasonal cycle in insolation, and its response can be understood within the context of the integrated summer insolation forcing. The simple fact that obliquity's period is roughly twice as long as that of precession results in a larger amplitude glacial response to obliquity. However, for the model to generate almost exclusively obliquity period glacial variability, two other conditions must be met. First, the ice sheet's ablation zone must reside poleward of $60°N because insolation intensity is more sensitive to changes in Earth's obliquity at high latitudes. Second, the ablation season must be long enough for precession's opposing influences on summer and fall insolation intensity to counterbalance one another. These conditions are consistent with a warm climate and a thin ice sheet, where the latter is simulated as a response to subglacial sediment deformation. If a colder climate is prescribed, or in the absence of basal motion, ice sheets tend to be larger and undergo greater precession period variability, in keeping with proxy observations of late Pleistocene glaciation.Citation: Huybers, P., and E. Tziperman (2008), Integrated summer insolation forcing and 40,000-year glacial cycles: The perspective from an ice-sheet/energy-balance model, Paleoceanography, 23, PA1208,

Antarctic ice-sheet response to atmospheric CO<sub>2</sub> and insolation in the Middle Miocene

“…6). Huybers and Tziperman (2008) found a comparable relation between the ice-sheet size and the dominant frequencies for a Northern Hemisphere Pliocene ice sheet. They proposed that in the warm climate conditions under which thin (small) ice sheets occur, a long ablation season can counterbalance the opposite effect precession has on summer and fall insolation.…”

“…However, our results show a strong obliquity frequency in the ice volume without explicitly calculating a moisture flux between low and high latitudes. Our modeled obliquity could be caused by the opposing effects of insolation intensity and duration (as proposed by Huybers and Tziperman, 2008), because we included daily insolation in the ice sheet-climate model. In large ice sheets the ablation period is too short to be influenced by this effect.…”

Obliquity-paced Pliocene West Antarctic ice sheet oscillations

“…On long timescales, insolation integrated over the length of summer (summer energy) has been shown in models to control the surface melting of ice sheets at the obliquity period 8 (Supplementary Information), provided that the ablating margin is at high latitude and that the surface temperature remains above 0 uC for a significant part of the season 31 . Although the latter condition is not currently met by the Antarctic ice sheet, its surface melt threshold may have been exceeded during the early Pliocene and may be exceeded again in the next 100 years.…”

Orbital forcing of the East Antarctic ice sheet during the Pliocene and Early Pleistocene

“…The absence of a strong ∼20-kyr precession signal is confounding, because both obliquity and precession should impart a major influence on high-latitude summer insolation intensity, and therefore polar ice volume 3 . Indeed, precession is a major control on the orbital pacing of Late Pleistocene cycles 1 and, although a number of hypotheses have been proposed to explain its absence before this time [4][5][6] , reconciliation of this conundrum remains hampered by the lack of direct observational evidence of orbital-scale PlioPleistocene variance of ice sheets, in particular from Antarctica [7][8][9] . Integrated Ocean Drilling Program (IODP) core U1361 was recovered from ∼3,000 m water depth on the continental rise adjacent to the Wilkes Subglacial Basin of Antarctica, one of the largest marine-based sectors of the East Antarctic ice sheet (EAIS).…”

“…Under such a scenario, during the warmer climate state of the Early to mid-Pliocene with higher atmospheric CO 2 concentration 31,32 , the increased duration and intensity of austral summer surface warming produced a pattern approaching mean annual insolation regulated by obliquity (Fig. 2d), rather than precession that cancels out over the course of a seasonal cycle 5 . Late Pliocene cooling raised the melt threshold such that the duration of the melt season was restricted to times of austral summer insolation maxima controlled by precession (Fig.…”