Earthʹs orbital eccentricity and the rhythm of the Pleistocene ice ages: the concealed pacemaker

Most paleoclimate researchers would probably agree that variations in Earthʹs axial tilt and precession parameters have influenced past climate change. However, claims of connections between orbital eccentricity and ice age climate are more difficult to demonstrate or accept, especially since the amplitude of the strongest component of eccentricity-induced insolation, the 413-ky signal, is conspicuously small or absent from the power spectra of the last million years of paleoclimate data, and climate models without external forcing can easily reproduce the main 100-ky cycles of the late Pleistocene and Holocene. Here I show that it is possible to tease out the 413-ky component of eccentricity directly from orbitally untuned deep-sea δ18O time series, and that the signal is strong, albeit buried deep in the δ18O time series, concealed by frequency modulation (FM). To extract the 413-ky signal, the data is frequency and phase demodulated numerically, while synthetic surrogate time series with properties believed similar to the actual data are used to test the nature of the modulator and the accuracy of each step in the inversion.