Introduction


Interest in the question of what causes the "100,000 year cycle" of glacial and interglacial periods has generated a number of possible explanations. One group of theories invokes factors external to the climate system, including
  • variations in the output of the sun
  • the amount of solar energy reaching the earth caused by changing concentrations of interstellar dust
  • the seasonal and latitudinal distribution of incoming radiation caused by changes in the earth's orbital geometry
  • the volcanic dust content of the atmosphere
  • the earth's magnetic field

Other theories are based on internal elements of the climate system including
  • growth and decay of ice sheets
  • surging of the Antarctic ice sheet
  • ice cover of the Arctic Ocean
  • distribution of carbon dioxide between atmosphere and ocean
  • deep circulation of the ocean
Additionally, it has been argued that climate could alternate between different states without the intervention of any external stimulus or internal time constant.

Among these ideas, only the orbital forcing hypothesis has been formulated so as to predict the frequencies of major glacial fluctuations and is the only explanation that can be tested using by comparing the calculated variations in solar radiation (insolation) with the paleoclimate record. This paper reports an analysis to test this hypothesis against the geological record measured in ocean sediment cores.

This study treats changes in the Earth's orbit and tilt as a forcing function of a system whose output is paleoclimate record. It does not attempt to identify the mechanisms through which climate is modified by changes in incoming solar radiation. The analysis is based on the simplifying assumption that the climate system responds linearly to orbital forcing.

Orbital forcing theory


In 1941 Milutin Milankovitch published a paper in which he outlined a mechanism by which the cycle of glacial/interglacial periods is controlled by variations in incoming solar radiation which are determined by small, predictable changes in the Earth's orbit and tilt with respect to the sun.
Milankovitch solar radiation cycles
Milankovitch solar radiation cycles

The angle of the Earth's axial tilt (in scientific terms, the obliquity of the ecliptic) varies with a period of approximately 41,000 years (41 kyr). The Earth's axis of rotation wobbles (precesses) with a period of roughly 26,000 years. The elipse ( eccentricity) of the Earth's orbit varies with a period of 413,000 years. Smaller cycles have varied between 95,000 and 125,000 years. One other small cycle, not studied by Milankovitch, has a period of 100,000 years which is due to the joint effects of Jupiter and Saturn on the Earth’s orbit.

The orbital forcing hypothesis predicts that the obliquity of the earth's axis, with a period of about 41,000 years, and the precession of the equinoxes, with a period of about 21,000 years, are the most important variables that influence climate because of how they change insolation (amount of solar radiation hitting the Earth's atmosphere). Specifically, Milankovitch argued that the distribution of summer insolation at 65°N is the critical variable which determines the growth and decay of the continental ice sheets.

Observations


The geological data comprise measurements of three variables that reflect climatic conditions in two deep-sea sediment cores.
  1. delta-oxygen-18, the oxygen isotopic composition of plankton living in the ocean surface waters
  2. Ts, an estimate of summer sea-surface temperatures calculated from relative abundance of a particular plankton species
  3. the relative abundance of a plankton species not used in the estimation of Ts
Identical samples were analyzed for the three variables at 10-cm intervals through each core. These cores were taken from an area where previous work showed that sediment is accumulating fast enough to preserve information required for the analysis.

The chronology of the sediment records was determined by "orbital tuning", tuning the sediment timescales to insolation curves calculated from orbital forcing theory.

Conclusions


Three indices of global climate have been monitored in the record of the past 450,000 years in Southern Hemisphere ocean-floor sediments. The analysis reveals that the climatic variation in the three observations is concentrated in cycles with 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.

The 42,000-year climatic component has the same period as variations in the obliquity of the earth's axis. The 23,000-year cycle displays the same period as the Earth's precession. Unexpectedly, the data reveals that the dominant, 100,000-year climatic component appears to have an average period close to the orbital eccentricity, which calculations show only weakly affects insolation.

Based on this evidence the authors conclude that changes in the earth's orbital geometry are the fundamental cause of the succession of ice ages observed over the past 800,000 years. But they also forced to conclude that orbital forcing by itself is unable to explain the 100,000 year cycle of glacial/deglacial periods that has dominated Earth's climate in the Quaternary period.

Editor's comments


The important questions that this paper attempts to answer are what triggers the origin of the ice ages, why do ice ages end, and why do they end so precipitously ? The Earth's climate system is complex. There is an unstable balance between glacial conditions in the polar regions and heating at lower latitudes. The equilibrium can be upset by small perturbations amplified by feedback mechanisms which drive the Earth's climate either toward glacial or interglacial conditions. The astronomical explanation that this seminal paper attempts to test is that long-term cyclical wobbles in the Earth's orbit are the perturbations that initiate ice ages and interglacials. Many scientists remain unconvinced. A 1992 analysis of the Devils Hole calcite climate record reported that the last four glacial cycles recorded in the calcite increased from 80,000 to 130,000 years suggesting that major climate changes are aperiodic and that orbitally controlled (Milankovitch cycles) variations in solar insolation were not a major factor in triggering deglaciations. The authors also suggested that they are consistent with the thesis that these cycles originated from internal nonlinear feedbacks within the atmosphere-ice sheet-ocean system. A recent assessment by Donald Rapp (Ice Ages and Interglacials 2009) concludes that "while there are innumerable books, reports, and articles claiming that the astronomical theory is proven, the basis for such claims remains flimsy".

Sources


Variations in the Earth's Orbit: Pacemaker of the Ice Ages, J. D. Hays, John Imbrie, N. J. Shackleton, Science 10 Dec 1976: Vol. 194, Issue 4270, pp. 1121-1132 DOI: 10.1126/science.194.4270.1121

Data

The SPECMAP paleoclimate archives at NOAA/NCDC World Data Center for Paleoclimatology consist of four archives:

ARCHIVE 1 contains 101 SPECMAP files, including planktonic counts and isotope values, age and sea surface temperature models, and SST estimates for the past 400,000 years compiled by J. Imbrie and A. Duffy at Brown University for the SPECMAP project. The O-18 chronologies of Imbrie et al. (1984) and Martinson et al. (1987) are given for reference, as well as orbital time series taken from the work of Berger (1978a,b).

Data and models archived are the basis for the paleoclimatic time series analyzed in 'Oceanic Response to Orbital Forcing in the Late Quaternary: Observational and Experimental Strategies', by J. Imbrie, A. McIntyre, and A. C. Mix. in 'Climate and Geosciences, A Challenge for Science snd Society in the 21st Century', A. Berger, S. H. Schneider, and J.-C. Duplessy, eds., 1989, D. Reidel Publishing Company.

ARCHIVE 2 contains 70 files including planktonic counts and isotope values, age and sea surface temperature models, and SST estimates. These data were compiled by J. Imbrie et al, and published as:'On the structure and origin of major glaciation cycles. 1. Linear responses to Milankovitch forcing' Paleoceanography, Dec, 1992 (Imbrie et al.,1992) .

ARCHIVE 3 contains 10 files represent analytical results and data which formed the basis for the discussion of the Devils Hole calcite record, DH-11, in the original reference: J. Imbrie, A. C. Mix, and D. G. Martinson, 1993, Milankovitch Theory Viewed from Devils Hole, NATURE, May, 1993 .

ARCHIVE 4 contains 47 files of the time series appearing in 'On the Structure and Origin of Major Glaciation Cycles: part 2', Imbrie et al., PALEOCEANOGRAPHY, Dec., 1993 .