Wavelet Spectral Analysis of the Earth’s Orbital Variations and Paleoclimatic Cycles

The wavelet time-frequency spectral analysis is applied to geological records that are proxies of paleoclimatic variations: d18O in sedimentary cores, atmospheric CO2 concentration, and a loess magnetic susceptibility stratigraphy within the past million years. These spectra are compared with those for the astronomically predicted variations of the earth’s orbital eccentricity, obliquity, precession, and their resultant variations of the incoming insolation. The latter has been known to be unable to explain the characteristics of the observed 100-kyr paleoclimatic cycles. Based on similarities between the wavelet spectra of the orbital variations and paleoclimatic cycles, the authors introduce a signal–noise resonance theory to understand the dynamics of climate response to the orbital forcing. It is shown that the observed 100-kyr cycles are mainly caused by the period variation in the obliquity, which amplifies the small orbital forcing. But the observed flickers within these cycles are induced by the amplitude variation of obliquity and precession, which are two major components of the Milankovitch insolation deviations.