Empirical support for the orbital forcing hypothesis from marine sediments

For more than a century the cause of fluctuations in the Earth’s climate responsible for the growth and ebb of the great ice sheets has remained an unsolved scientific mystery. Only the hypothesis that relates these changes to small variations in the Earth’s tilt and orbit changing the amount of solar radiation hitting the Earth has been formulated so as to predict the frequencies of major glacial/deglacial cycles. This seminal 1976 paper tested this hypothesis by comparing the 450,000 year geological record of the climate as recorded in marine sediments with the cycle of variations in Earth’s tilt and orbit. The analysis provides strong support for orbital forcing affecting Earth’s climate, but the authors conclude that orbital forcing by itself is not able to explain the dominant 100,000 year glacial/deglacial cycle.

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.

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.

The authors conclude that clear evidence for orbital forcing is found in the marine sediment record.  But they also 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 over the past 800,000 years.

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