Early Pleistocene Glacial Cycles and the Integrated Summer Insolation Forcing The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Huybers, Peter J. 2006. Early Pleistocene glacial cycles and the integrated summer insolation forcing. Science 313(5786): 508-511. Published Version http://dx.doi.org/10.1126/science.1125249 Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:3382981 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Other Posted Material, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#LAA Early Pleistocene Glacial Cycles and the Integrated Summer Insolation Forcing Peter Huybers, et al. Science 313, 508 (2006); DOI: 10.1126/science.1125249 The following resources related to this article are available online at www.sciencemag.org (this information is current as of January 5, 2007 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/cgi/content/full/313/5786/508 Supporting Online Material can be found at: http://www.sciencemag.org/cgi/content/full/1125249/DC1 A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/cgi/content/full/313/5786/508#related-content This article cites 12 articles, 5 of which can be accessed for free: http://www.sciencemag.org/cgi/content/full/313/5786/508#otherarticles on January 5, 2007 This article has been cited by 1 article(s) on the ISI Web of Science. 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REPORTS showed predominantly precession-period glacial Early Pleistocene Glacial Cycles variability during the early Pleistocene, whereas another, more sophisticated, coupled climate–ice and the Integrated Summer sheet model (18) showed primarily obliquity period variability (although the latter model is for Antarctica near È34 My ago), and neither Insolation Forcing study identified mechanisms for the differing sensitivities to orbital variations. The origins of Peter Huybers strong obliquity over precession-period glacial variability during the early Pleistocene remain Long-term variations in Northern Hemisphere summer insolation are generally thought to control unresolved. glaciation. But the intensity of summer insolation is primarily controlled by 20,000-year cycles in the Tying insolation at the top of the atmo- precession of the equinoxes, whereas early Pleistocene glacial cycles occur at 40,000-year intervals, sphere to climate on the ground poses a serious matching the period of changes in Earth’s obliquity. The resolution of this 40,000-year problem is challenge. It is useful to consider empirical that glaciers are sensitive to insolation integrated over the duration of the summer. The integrated relationships between insolation (19) and mod- summer insolation is primarily controlled by obliquity and not precession because, by Kepler’s second ern temperature (20). Insolation lagged by 30 law, the duration of the summer is inversely proportional to Earth’s distance from the Sun. days shows an excellent correlation with zonally and diurnally averaged land tempera- link between changes in glacial extent tion (11). However, models used to explore the ture, T, for latitude bands north of 30-N(r2 9 and Earth_s orbital configuration was effects of changes in the insolation gradient 0.99) (Fig. 1C). Insolation is apparently a good A apparently first proposed by Adh2mar have found that local insolation is the more predictor of T. (1, 2), who postulated that the Antarctic ice important control on glacial mass balance (12). A more complicated relationship might have sheet exists because the Southern Hemisphere Simple models that used summer insolation as been expected between insolation and T when winter is 8 days longer than the Northern one. the forcing (13–15) exhibited more precession- one considers processes such as reflection of In this case, winter is the period between the period variability than is observed in the early radiation by snow, ice, and clouds; changes in equinoxes. This difference in duration follows Pleistocene climate record. Another possibility heat storage; and the redistribution of heat by from Kepler_s second law and from the fact is that glaciation is controlled by the annual the ocean and the atmosphere. The linear on January 5, 2007 that Earth_s closest approach to the Sun, that is, average insolation, which is independent of relationship between insolation and average perihelion, currently occurs during Northern precession, but this hypothesis requires glacial temperature does not exclude the importance Hemisphere winter. Croll modified this hypoth- mass balance to be equally sensitive to winter of these processes but does suggest that their esis, arguing that the decrease in insolation and summer insolation (16). One climate model aggregate influence is also correlated with the associated with being further from the Sun (17) that is forced by the complete seasonal cycle insolation. Furthermore, the combined heat leads to glaciation (2, 3). Milankovitch, in turn, argued that summer insolation determines glaci- ation (4). More recently, once proxies of past Fig. 1. Relationships glaciation showed that late Pleistocene glacial between insolation and www.sciencemag.org cycles occurred at È100,000-year (100-ky) temperatures. (A)Tem- - intervals (5), the amplitude envelope of the perature in Ccontoured precession (i.e., the eccentricity) was identified as a function of latitude as accounting for the 100-ky glacial cycles (5–7). and month. Temper- This thread of glacial hypotheses thus atures, T, are diurnal averages from WMO sta- implies that precession of the equinoxes controls tions and are averaged the occurrence of glacial cycles. Indeed, var- according to latitude af- Downloaded from iations in the intensity of summer insolation ter adjusting for eleva- are primarily controlled by precession. For ex- tion using a lapse rate ample, average insolation on the 21st day of of 6.5-C/km. (B)Insola- - June at 65 N has 80% of its variance at the pre- tion at the top of the cession periods (1/21 ky T 1/100 ky). The caloric atmosphere. (C) T plotted summer half-year at 65-N, defined as the energy against insolation for dif- received during the half of the year with the ferent latitudes (r2 9 greatest insolation intensity (4), also has more 0.99). Latitude bins are than half its variance in the precession bands. 10-, and insolation bins But a major problem exists for the standard are 10 W/m2 where inso- orbital hypothesis of glaciation: Late Plio- lation has been lagged cene and early Pleistocene glacial cycles oc- by 1 month. (D) Posi- cur at intervals of 40 ky (8–11), matching the tive degree days plotted obliquity period, but have negligible 20-ky against summer energy 2 0 variability. (r 0.98). (E)Positive One possibility is that the latitudinal gradient degree days plotted in insolation, which enhances obliquity over against the intensity of precession, is more important than local insola- diurnally averaged in- solation on June 21st (r2 0 0.04). Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA. E-mail: phuybers@ fas.harvard.edu 508 28 JULY 2006 VOL 313 SCIENCE www.sciencemag.org REPORTS transport of the ocean and the atmosphere to low the sum of theP insolation on days exceeding however, anticorrelated. This is the Achilles_ - 0 latitudes above 30 N amounts to 5 PW (peta- this threshold, J ibi(Wi  86,400), where J heel of precession control of glaciation: just Watts) (21) and, when spatially averaged, is termed the summer energy and is measured in when Earth is closest to the sun during summer, 2 2 corresponds to 40 W/m , or less than 10% of joules. Wi is mean insolation in W/m on day i, summertime is shortest. When the intensity is Q the summer insolation at the top of the and b equals one when Wi t and zero integrated over the summertime, precession- atmosphere at any latitude. In this light, it is otherwise. Note that ablation responds to both related changes in duration and intensity nearly reasonable for insolation to primarily control radiative transfer and heat flux from the atmo- balance one another (25), and the obliquity local temperature, particularly during the sum- sphere into the ice, but this distinction is not component is dominant. When t 0 275 W/m2, mer months. made because insolation and temperature are 80% of the summer energy variance is in the If one accepts the empirical relationship strongly correlated. obliquity band (1/41 ky T 1/100 ky) (Fig. 2, C between insolation and temperature, then what S, computed by using T, monotonically and D). is the best measure of insolation_s influence on decreases from 6000 at 30-Nto400at70-N. As an example, Earth_s orbital configuration ablation? It is not mean annual insolation: The The summer energy also steadily decreases when perihelion occurs variously at the equi- ablation season is not more than 6 months in toward high latitudes and is highly correlated noxes and at the solstices is shown (Fig.