Variations in the Earth’s Orbit: Pacemaker of the Ice Ages
J. D. Hays, John Imbrie, N. J. Shackleton. 1976. Variations in the Earth’s Orbit: Pacemaker of the Ice Ages. Science, New Series, Vol. 194, No. 4270, (Dec. 10, 1976), pp. 1121-1132.
Note: this classic, seminal paper changed the science of paleoclimatology. It is one of the most important papers ever written in any scientific discipline.
Full text [here]
Selected excerpts:
Introduction
For more than a century the cause of fluctuations in the Pleistocene ice sheets has remained an intriguing and unsolved scientific mystery. Interest in this problem has generated a number of possible explanations (1, 2). One group of theories invokes factors external to the climate system, including variations in the output of the sun, or the amount of solar energy reaching the earth caused by changing concentrations of interstellar dust (3); the seasonal and latitudinal distribution of incoming radiation caused by changes in the earth’s orbital geometry (4); the volcanic dust content of the atmosphere (5); and the earth’s magnetic field (6). Other theories are based on internal elements of the system believed to have response times sufficiently long to yield fluctuations in the range 104 to 106 years. Such features include the growth and decay of ice sheets (7), the surging of the Antarctic ice sheet (8); the ice cover of the Arctic Ocean (9); the distribution of carbon dioxide between atmosphere and ocean (10); and the deep circulation of the ocean (11). Additionally, it has been argued that as an almost intransitive system, climate could alternate between different states on an appropriate time scale without the intervention of any external stimulus or internal time constant (12).
Among these ideas, only the orbital hypothesis has been formulated so as to predict the frequencies of major Pleistocene glacial fluctuations. Thus it is the only explanation that can be tested geologically by determining what these frequencies are. Our main purpose here is to make such a test. Previous work has provided strong suggestive evidence that orbital changes induced climatic change (13-20). However, two primary obstacles have led to continuing controversy. The first is the uncertainty in identifying which aspects of the radiation budget are critical to climatic change. Depending on the latitude and season considered most significant, grossly different climatic records can be predicted from the same astronomical data. Milankovitch (4) followed Koppen and Wegener’s (21) view that the distribution of summer insolation (solar radiation received at the top of the atmosphere) at 65N should be critical to the growth and decay of ice sheets. Hence the curve of summer insolation at this latitude has been taken by many as a prediction of the world climate curve. Kukla (19) has pointed out weaknesses in Koppen and Wegener’s proposal and has suggested that the critical time may be September and October in both hemispheres. However, several other curves have been supported by plausible arguments. As a result, dates estimated for the last interglacial on the basis of these curves have ranged from 80,000 to 180,000 years ago (22).
The second and more critical problem in testing the orbital theory has been the uncertainty of geological chronology. Until recently the inaccuracy of dating methods limited the interval over which a meaningful test could be made to the last 150,000 years. Hence the most convincing arguments advanced in support of the orbital theory to date have been based on the ages of 80,000, 105,000, and 125,000 years obtained for coral terraces first on Barbados (15) and later on New Guinea (23) and Hawaii (24). These structures record episodes of high sea level (and therefore low ice volume) at times predicted by the Milankovitch theory. Unfortunately, dates for older terraces are too uncertain to yield a definitive test (25).
More climatic information is provided by the continuous records from deep-sea cores, especially the oxygen isotope record obtained by Emiliani (26). However, the quasi-periodic nature of both the isotopic and insolation curves, and the uncertain chronology of the older geologic records, have combined to render plausible different astronomical interpretations of the same geologic data (13, 14, 17, 27).
Strategy
All versions of the orbital hypothesis of climatic change predict that the obliquity of the earth’s axis (with a period of about 41,000 years) and the precession of the equinoxes (period of about 21,000 years) are the underlying, controlling variables that influence climate through their impact on planetary insolation. Most of these hypotheses single out mechanisms of climatic change which are presumed to respond to particular elements in the insolation regime (28). In our more generalized version of the hypothesis we treat secular changes in the orbit as a forcing function of a system whose output is the geological record of climate — without identifying or evaluating the mechanisms through which climate is modified by changes in the global pattern of incoming radiation (29). Most of our climatic analysis is based on the simplifying assumption that the climate system responds linearly to orbital forcing. The consequences of a more realistic, nonlinear response are examined in a final section here.
Our geological data comprise measurements of three climatically sensitive parameters in two deep-sea sediment cores. These cores were taken from an area where previous work shows that sediment is accumulating fast enough to preserve information at the frequencies of interest. Measurements of one variable, the per mil enrichment of oxygen 18 (6180), make it possible to correlate these records with others throughout the world, and to establish that the sediment studied accumulated without significant hiatuses and at rates which show no major fluctuations. To be used in tests of the orbital hypothesis, these data are transformed into geological time series. In our first test we make the simplest geochronological asumption, that sediment accumulated in each core at a constant rate throughout the period of study. Later we relax this assumption and allow slight changes in accumulation rate, as indicated by additional geochronological data.
Our frequency-domain tests use the numerical techniques of spectral analysis and are designed to seek evidence for a concentration of spectral energy at the frequencies of variation in obliquity and precession. We consider that support for the hypothesis can be decisive if both frequencies are detected and, to allow for geochronological uncertainties, if it can be clearly demonstrated that the ratio of the two frequencies detected does not differ significantly from the predicted ratio (about 1.8).
Finally, our time-domain tests are designed to examine the phase coherence between the three climatic records and between each record and the postulated forcing function. To this end we use the numerical techniques of bandpass filter analysis. Such an approach makes it possible to examine separately the variance components of the geological records that correspond in frequency to the variations of obliquity and precession. …
Summary
1) Three indices of global climate have been monitored in the record of the past 450,000 years in Southern Hemisphere ocean-floor sediments.
2) Over the frequency range 10^-4 to 10^-5 cycle per year, climatic variance of these records is concentrated in three discrete spectral peaks at periods of 23,000, 42,000, and approximately 100,000 years. These peaks correspond to the dominant periods of the earth’s solar orbit, and contain respectively about 10, 25, and 50 percent of the climatic variance.
3) The 42,000-year climatic component has the same period as variations in the obliquity of the earth’s axis and retains a constant phase relationship with it.
4) The 23,000-year portion of the variance displays the same periods (about 23,000 and 19,000 years) as the quasi-periodic precession index.
5) The dominant, 100,000-year climatic component has an average period close to, and is in phase with, orbital eccentricity. Unlike the correlations between climate and the higher-frequency orbital variations (which can be explained on the assumption that the climate system responds linearly to orbital forcing), an explanation of the correlation between climate and eccentricity probably requires an assumption of non-linearity.
6) It is concluded that changes in the earth’s orbital geometry are the fundamental cause of the succession of Quaternary ice ages.
7) A model of future climate based on the observed orbital-climate relationships, but ignoring anthropogenic effects, predicts that the long-term trend over the next several thousand years is toward extensive Northern Hemisphere glaciation.
