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Milutin Milankovitch was a mathematician, astronomer, climatologist, geophysicist and civil engineer. In his astronomical theory of climate (1941), Milankovitch was the first to mention the existence of orbitals parameters, governing the Earth trajectory around the sun and therefore, the climatic and environmental conditions on Earth, which is now the consensus of the scientific community.
The Earth is turning around the sun, and follows an elliptical trajectory, one year and 6 hours corresponding to the time needed to complete a full lap. This trajectory is governed by three main parameters, changing through the time and following cycles with different periodicities. These parameters are the eccentricity, the obliquity and the precession.
These orbital parameters are influencing the quantity of light and solar energy arriving to the Earth’s surface. This is the insolation, which is having a strong influence on climate, environment, atmospheric and oceanic conditions (Hays et al., 1976).
Figure 1. schematic representation of the three orbital parameters: A) the eccentricity, B) the obliquity and C) the precession (modified after Berger and Yin, 2012; Goosse, 2015; Hay et al., 1997). Proportions are not taken in account.
The eccentricity (Figure 1.A) corresponds to ‘How round is the Earth’s orbit around the sun’.
The main periodicities of this parameter are 100 ka and 404 ka (one ka is one kilo-annum, corresponding to 1000 years) (Figure 2). When the eccentricity is around zero, it means that the trajectory of the Earth around the sun is almost circular (with the sun in the centre of this circle), and when the eccentricity increases, it means that the Earth’s trajectory around the sun is more and more elliptical (the sun being then located in one of the two foci of the ellipse). When the eccentricity is low (circular orbit), the mean annual insolation (i.e. mean annual energy received at the Earth surface) is the lower, and it increases as the eccentricity increases, at all the latitudes.
Figure 2. Eccentricity over the last 1000 ka (Laskar et al., 2004).
The obliquity (Figure 1.B) is defined by ‘How tilted the Earth axis is’
The Earth rotates on itself, and its axis of rotation is not perfectly vertical. It is tilted, and the angle of this tilt changes and has changed over time from an angle of 22° to an angle of 24.5°. It is thanks to this tilt that we do have different seasons and that we do have opposite seasons in the Northern and in the southern hemisphere. This parameter is changing with the main cyclicities of 41 and 54 ka (Figure 3). When the obliquity increases (a bigger angle of the tilt), the annual mean insolation at the equator slightly decreases.
Figure 3. Obliquity over the last 1000 ka (Laskar et al., 2004).
The precession (Figure 1. C) corresponds to ‘the Earth distance to the sun at the summer solstice’
Indeed, the solstices and equinoxes are not happening at ‘the same place on the orbit’ every year. Its is slightly in advance at every revolution of the Earth around the sun. Therefore, when the Earth orbit is very elliptical (see eccentricity), sometimes the summer solstice is close to the sun, sometimes it is further away. Knowing that the closer we are to the sun, the greater is the amount of solar energy collected on earth, changes in this parameter result in summers that are hotter or colder over time. The precession is following the main periodicity of 22 ka (Figure 4). This has a strong influence on the seasonality, and the contrast between the seasons. When the precession increases (and even more when the eccentricity is high too), this is impacting the isolation at all latitudes, influencing the seasonal contrast. A higher precession means a lower insolation (Berger and Loutre, 2004; Goosse, 2015). The tropical condition and insolation are important in the global climate modulations at orbital and suborbital time scales (Beaufort et al., 1997; Berger et al., 2006).
Figure 4. Precession over the last 1000 ka (Laskar et al., 2004).
As all these three parameters are influencing the insolation, the mean annual insolation on Earth is the result of the periodicities of all these three parameters (Figure 5). Understanding the regulation of the quantity of energy arriving from the sun to the Earth is essential to understand the climate modulations. Because they are long cycles, they influence the long-term climate on Earth (Bertrand et al., 2002; Ganopolski and Calov, 2011; Past Interglacials Working Group of PAGES, 2016; Shackleton, 2000; Wara et al., 2000), but also the oceanic (Herbert, 1997) and environmental (Duque-Villegas et al., 2022; Lücke et al., 2021; Sangiorgi et al., 2008) conditions and the development of a multitude of marine organisms (e.g. Beaufort et al., 2022; Dynesius and Jansson, 2000; Girone et al., 2013).
Figure 5. Insolation over the last 1000 ka (Laskar et al., 2004).
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