Precession, Eccentricity, Obliquity, Insolation and Paleoclimates

Abstract

Different parameters related to the solar radiation received at the top of the atmosphere are derived and their long-term variations analysed in the frequency domain. The annual mean energy received by the whole Earth is varying in time according to the variations of the mean distance from the Earth to the Sun, i.e. as a function of (1 − e 2)−1/2. The so-called 100 ka period of the eccentricity (and the other periods as well) are originating from a combination of the fundamental periods of the climatic precession parameters. The insolation at a given latitude and for a fixed longitude of the Earth on its orbit is a function of obliquity through the factor related to the zenith angle of the Sun and of precession through the distance factor. Moreover a deeper analysis of the spectrum of the distance factor shows that it contains also, with much less power, half precession periods, eccentricity periods and combination tones between eccentricity and precession. Over the Quaternary, the latitudes of the polar and the tropical circles, varying with obliquity, are situated respectively between 65.5° and 68° and between 22° and 24.5°. Their present-day motion towards north is estimated to be 14.4 m per year. Finally, it is shown that in most insolation parameters, the precessional signal dominates the obliquity one, except in high polar latitudes mainly of the winter hemisphere, although the power of the obliquity signal increases from low to high latitudes.