1st PART




The causes of glaciations and warm periods lie both in astronomy and geophysics :

One thinks first and foremost to the Earth's environment, i.e. its relationship with the Sun. Indeed, our star is evolving and its activity makes vary its brightness, which directly affects the temperature of our planet. 400 million years ago, the solar constant was lower than its current value of 4%. However, we don't know the relative importance of solar activity compared to a variation of CO2. Some models could overestimate the solar constant. For others, it would be the reverse.

The response of the climate to astronomical variations is complex : one should also take into account the multiple nonlinear interactions that occur between the atmosphere, ice, ocean, land and even the biosphere (all living organisms). The dynamics of huge ice caps are at work in several ways. Once there, ice caps tend to spread and to intensify the cooling of the Earth's surface. A glacier also distorts the bedrock on which it rests. By its slowness, this process of isostatic readjustment creates an asymmetry between the phases of accumulation and melting of the ice caps. It promotes the accumulation of as long as the sinking of the base isn't significant, but, instead, accelerates the melting as soon as a significant decrease in altitude of the ice occurs.

According Milutin Milankovitch, who developed this theory in the 1920s, when sunshine received during the summer at the high latitudes of the northern hemisphere decreases then the snow which fell in the winter does not melt completely during the summer and begins to accumulate. In addition, the snow strongly reflects sunlight, which tends to increase cooling and allows a glaciation.

Besides this climate blur, Earth also has its say.


The movement of the Earth also has many influences on climate. The serbian astronomer Milutin Milankovitch demonstrated between 1920 and 1941 that the Earth goes through three phases of variations which are the causes of glacial and warm periods :

  • The Precession of the Equinoxes and Perihelion

Two orbital parameters intervene at the same time to explain the variations in the seasons.

  1. The Precession of the Equinoxes

    The Earth's rotation axis which is tilted 23,27° justifies the existence of the different seasons the different seasons. But its axis varies in the course over time, it describes a cone in 25.868 years : the seasons move on the Earth's orbit. This movement, called axial precession is due to the combined attraction of the Sun and Moon on the equatorial bulge. The moon our satellite that supports life on Earth slows the period of variation of its axis of rotation. So without the Moon, the Earth's axis would describe a cone in only 10.000 years which would have effects on the climate.

    Currently the rotation axis of Earth is directed to Pole Star (Polaris) in constellation of Ursa Minor but in 12.934 years she will be directed to the star Vega in the constellation of Lyra and 12.934 years later the Earth's axis will be again pointed towards Polaris. In the northern hemisphere, the minimum distance from Sun is in winter and the maximum in summer, and opposite in the southern hemisphere. We are in a situation that softens the winters and cools the summers in the northern hemisphere, while it increases the seasonal contrasts in the southern hemisphere. On the contrary, there are approximately 10.000 years,  the Earth passed through the point closest to the Sun at the time of the boreal summer solstice and not the winter solstice as today. The northern hemisphere received more solar energy in summer and less in winter.

    the evolution of the inclination of the polar axis

    A video of the precession of the equinoxes

    The axial precession

    Ripples overlap to the precession underlined in 1748 by James Bradley, called nutation. The latter is a slight oscillating movement of the axis of the Earth caused by the evolution of the orbit of the moon around the Earth disturbed by the Sun . This movement traverses a path in a loop on the path of precession which each oscillations is a period of 18.6 years. This represents a low tipping of the polar axis of a maximum of 0.0025 ° (9.21 ") in a 18.6-year time interval.

    In addition to this, the axis of rotation of the Earth also varies in a coordinate system associated with the planet itself and not for the Sun and the moon. The main components of the variations are :

    - an annual oscillation, forced by seasonal variations in oceanic and atmospheric pressure.

    - an oscillation of a complex trajectory that the North Pole describes to the Earth's surface called : Polhodie or Chandler movement. The polhodie is a movement due to the ellipsoidal shape of the globe of 430 days and an amplitude of the order of 10 m, whose excitement is related to internal movements of magma and not to the action of the Moon and the Sun as to the nutation and precession.

    - A variation due to drift of the Canadian mainland.

    The different forces changing the rotation of the Earth

    This graph represents the "movement of the pole" (here represented by the curve
    in black dashed) contained in a square of 20 meters wide and in full line seen
    mean development year by year since 1890. Here you have the coordinates of the
    mean axis of Rotation in the Earth Reference Systems of the IERS.

    Filtering the X coordinate North Pole raw values
    (top) and Y South (bottom) since 1890 with the
    seasonal component, Chandler and irregular variations.

  2. The Precession of the perihelion
    Relative to the stars, the elliptical Earth orbit makes a rotation (the precession of the perihelion). This changes the months of the year when the Earth is at perihelion ie when the Earth is closest to the Sun. This is also due to the attraction of the planets, the Sun and the Moon.

    Evolution of the rotation of the Earth orbit viewed from above

    These two combined movements reduce to 22.000 years the periodicity of the precession of the equinoxes. The resulting movement, combined with the variation of the eccentricity of the orbit of the Earth varies the Earth-Sun distance in two periods, 19.000 and 23.000 years.



    Seasonal variations of insolation associated with the
    precession cycle often exceed 10%. This is the main
    climate forcing on the scale of thousands of years.

    • The obliquity
    The axis of rotation of the Earth is currently tilted by 23°27'. But it varies between 21°59' and 24°50' over a period 41.000 years. This fluctuation affects the distribution of the energy received at different latitudes following the seasons in particular the duration of the polar night to the highest latitudes. When the obliquity reaches 24°50' this causes harsh winters at midlatitudes. But when the obliquity is less important that favours the glaciations and reverse when it is most important.

    The obliquity of the Earth


    Evolution of the obliquity of the ecliptic during historic time

    Click here to see the obliquity of the Earth during the last 5 million years

    • The variation in the Earth's orbit

    The mass of the Sun controls the movement of the Earth in space, but the presence of other planets of the solar system (especially Jupiter) disrupts this movement and may cause long-term changes in orbital parameters of the Earth. The eccentricity which is 0,02 measures the difference between the Earth's orbit and a perfect circle. It varies between 0 and 0,07. Its period varies with a period of 400.000 years and another of 100.000 ans. Therefore the global flow of the radiation that it receives from the Sun varies according to its distribution in space and time.

    Eccentricity : the elliptical nature of the orbit has been exaggerated

    The eccentricity during these last 1 million years

    The eccentricity during these last 5 million years

    The Serbian Astronomer Milutin Milankovitch demonstrated between 1920 and 1941 that all these changes are the cause of the ice ages the Earth has sustained. The last major ice age reached its maximum 22.000 years ago.

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