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Science 23 July 1999:
Vol. 285. no. 5427, pp. 564 - 568
DOI: 10.1126/science.285.5427.564
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Reports

Pacemaking the Ice Ages by Frequency Modulation of Earth's Orbital Eccentricity

J. A. Rial

Evidence from power spectra of deep-sea oxygen isotope time series suggests that the climate system of Earth responds nonlinearly to astronomical forcing by frequency modulating eccentricity-related variations in insolation. With the help of a simple model, it is shown that frequency modulation of the approximate 100,000-year eccentricity cycles by the 413,000-year component accounts for the variable duration of the ice ages, the multiple-peak character of the time series spectra, and the notorious absence of significant spectral amplitude at the 413,000-year period. The observed spectra are consistent with the classic Milankovitch theories of insolation, so that climate forcing by 100,000-year variations in orbital inclination that cause periodic dust accretion appear unnecessary.

Department of Geological Sciences, and Wave Propagation Laboratory, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3315, USA. E-mail: jar{at}email.unc.edu

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