Amplitude and frequency modulations of the Earth's obliquity for the last million years

The variability of the earth's obliquity is a result of the deviation of the obliquity from its main component at 41 kyr and is generated by amplitude and frequency modulation. For the last million years, the amplitude modulation of the obliquity explains 8% of the obliquity's total variance and the frequency modulation explains 10.3%. The spectra of both the amplitude and frequency modulations display significant power at 171 and 97 kyr. The contribution of these two modulations to the variance of the insolation is evaluated. At 65 degreesN in June, the amplitude modulation of the obliquity explains 1.3% of the variance of the insolation and the frequency modulation explains 1.6%. It is shown that the obliquity's frequency modulation is physically questionable and could only be meaningful for its components at 41, 54, and 29 kyr taken separately.