seventeenth century. They led to the discovery of a cycle of
80–90 years, called the Gleisberg cycle, which modulates
the Schwabe cycle.
Solar wind plays only a minor role in the flow of charged
particles received by the Earth. Most comes from galactic
cosmic rays, which consists of electrons, protons, a particles
(ionized helium nuclei) and heavier ions in very small
quantities. It is isotropic and comes from everywhere in
space. In periods of high solar activity, intense solar wind,
through the magnetic field it creates, acts as a shield repelling the galactic cosmic radiation falling to Earth. This
phenomenon inspired a geochemical method for determining
variations in solar activity. Indeed, galactic cosmic rays,
through spallation reaction on the atoms in the upper
atmosphere, are responsible for the production of several
cosmonucleides, the most well-known of which is
Carbon-14. Less Carbon-14 is produced during intense solar
activity. Measurements by geochemists on well-dated tree
rings showed pseudo-periodic variations in the production of
this cosmonucleide. This is attributed to fluctuations in solar
activity, with periods of about 150–300 years (Suess cycles)
and 2300 years (Hallstattzeit cycles). The existence of these
cycles has been confirmed by the measurement of other
cosmonucleides, such as beryllium-10, in polar ice. These
are trapped in ice in Greenland, whose location in time can
be determined simply by visually counting the annual layers
or, for earlier periods, through more complex methods
described in Chap. 9. The paleoclimatologists are now
investigating if these periodicities can be reflected in geological records.
Long-Term Variations in the Movement
of the Earth Around the Sun
The movement of the Earth around the Sun varies over time
under the influence of the gravitational attraction of other
planets (see Chap. 28, Volume 2). The orbit traveled by the
Earth over a full year is almost exactly an ellipse with an
eccentricity (the parameter which defines the degree of
flattening of the ellipse with respect to a circle) that can vary
over time. With periodicities close to 100,000 and to
400,000 years, the orbit goes from a circle with an eccentricity of zero to a slightly flattened ellipse with a maximum
eccentricity of 6%.
The tilt in the axis of the Earth relative to the ecliptic
plane is known as its obliquity and it influences the amount
of sunshine received at different latitudes in different seasons. It is the reason for phenomena such as the polar night
in winter and the midnight sun in summer at the highest
latitudes. For this reason, the climate at high latitudes is
especially sensitive to variations in the obliquity. With a
periodicity of around 41,000 years, the obliquity angle
oscillates between 22° and 25°, the current value being close
to 23° 26′.
Because of the elliptical nature of the Earth’s orbit, the
distance between the Earth and the Sun varies at different
times of the year. Currently, in the northern hemisphere, this
distance is at its minimum in winter and at its maximum in
summer, and the opposite is true for the southern hemisphere. In fact, the amount of solar radiation intercepted by
the Earth decreases as the distance increases. This causes
milder winters and cooler summers in the northern hemisphere, while the seasonal contrasts are accentuated in the
southern hemisphere (although this impact is minor compared with the seasonal variations in high latitudes caused by
obliquity).
Over the millennia, the position of the solstices and
equinoxes slowly moves along the ellipse resulting in a
variation in the solar energy received during each season.
This movement of precession of the equinoxes is caused by a
combination of two rotational movements. The first is the
rotation of the Earth around an axis running through the
poles which is perpendicular to the elliptic plane. A gradual
shift in the orientation of the axis of rotation is caused by the
attraction of the Sun and the Moon and traces out a circle
over the North Pole in a cycle of approximately
26,000 years. The second is the elliptical orbit of the Earth
around the sun which is superimposed on the first. The
combination of these two movements results in a periodicity
of the precession of the equinoxes of about 22,000 years.
More specifically, the Earth’s distance from the Sun fluctuates, not only due to the precession movement of the equinoxes, but also due to variations in the eccentricity of its
orbit which varies according to a set of cyclical changes
occurring over two proximate periods, one of 19,000 years
and the other 23,000 years. Thus, approximately
10,000 years ago, the Earth reached its closest point to the
Sun at the time of the summer solstice and not at the boreal
winter solstice as it does today. At that time, the northern
hemisphere received more solar energy in summer than it
does today and obviously less in winter.
All of these modifications in the orbital parameters affect
sunshine levels (still referred to as the insolation) at the different bands of latitude on Earth, and particularly the intensity
of the seasonal cycle. Already, in 1924, the Serbian mathematician, Milutin Milankovitch, proposed that these slow
variations of the movement of the Earth around the Sun could
explain the glacial cycles. Indeed, as these slow variations
induce a decrease in solar energy received in the summer at 60°
N, snow, which has fallen in the winter, does not melt completely. Furthermore, it strongly reflects solar radiation,
facilitating the snow to persist. Gradually, the snow accumulates and turns into an ice cap. This hypothesis has been
debated for many years and was strongly opposed until the
1970s, when the cycles predicted by this theory were clearly
confirmed by paleoclimate records in marine sediments and
later in polar ice. We will see more precisely in Chap. 28 of
6
S. Joussaume and J.-C. Duplessy
80–90 years, called the Gleisberg cycle, which modulates
the Schwabe cycle.
Solar wind plays only a minor role in the flow of charged
particles received by the Earth. Most comes from galactic
cosmic rays, which consists of electrons, protons, a particles
(ionized helium nuclei) and heavier ions in very small
quantities. It is isotropic and comes from everywhere in
space. In periods of high solar activity, intense solar wind,
through the magnetic field it creates, acts as a shield repelling the galactic cosmic radiation falling to Earth. This
phenomenon inspired a geochemical method for determining
variations in solar activity. Indeed, galactic cosmic rays,
through spallation reaction on the atoms in the upper
atmosphere, are responsible for the production of several
cosmonucleides, the most well-known of which is
Carbon-14. Less Carbon-14 is produced during intense solar
activity. Measurements by geochemists on well-dated tree
rings showed pseudo-periodic variations in the production of
this cosmonucleide. This is attributed to fluctuations in solar
activity, with periods of about 150–300 years (Suess cycles)
and 2300 years (Hallstattzeit cycles). The existence of these
cycles has been confirmed by the measurement of other
cosmonucleides, such as beryllium-10, in polar ice. These
are trapped in ice in Greenland, whose location in time can
be determined simply by visually counting the annual layers
or, for earlier periods, through more complex methods
described in Chap. 9. The paleoclimatologists are now
investigating if these periodicities can be reflected in geological records.
Long-Term Variations in the Movement
of the Earth Around the Sun
The movement of the Earth around the Sun varies over time
under the influence of the gravitational attraction of other
planets (see Chap. 28, Volume 2). The orbit traveled by the
Earth over a full year is almost exactly an ellipse with an
eccentricity (the parameter which defines the degree of
flattening of the ellipse with respect to a circle) that can vary
over time. With periodicities close to 100,000 and to
400,000 years, the orbit goes from a circle with an eccentricity of zero to a slightly flattened ellipse with a maximum
eccentricity of 6%.
The tilt in the axis of the Earth relative to the ecliptic
plane is known as its obliquity and it influences the amount
of sunshine received at different latitudes in different seasons. It is the reason for phenomena such as the polar night
in winter and the midnight sun in summer at the highest
latitudes. For this reason, the climate at high latitudes is
especially sensitive to variations in the obliquity. With a
periodicity of around 41,000 years, the obliquity angle
oscillates between 22° and 25°, the current value being close
to 23° 26′.
Because of the elliptical nature of the Earth’s orbit, the
distance between the Earth and the Sun varies at different
times of the year. Currently, in the northern hemisphere, this
distance is at its minimum in winter and at its maximum in
summer, and the opposite is true for the southern hemisphere. In fact, the amount of solar radiation intercepted by
the Earth decreases as the distance increases. This causes
milder winters and cooler summers in the northern hemisphere, while the seasonal contrasts are accentuated in the
southern hemisphere (although this impact is minor compared with the seasonal variations in high latitudes caused by
obliquity).
Over the millennia, the position of the solstices and
equinoxes slowly moves along the ellipse resulting in a
variation in the solar energy received during each season.
This movement of precession of the equinoxes is caused by a
combination of two rotational movements. The first is the
rotation of the Earth around an axis running through the
poles which is perpendicular to the elliptic plane. A gradual
shift in the orientation of the axis of rotation is caused by the
attraction of the Sun and the Moon and traces out a circle
over the North Pole in a cycle of approximately
26,000 years. The second is the elliptical orbit of the Earth
around the sun which is superimposed on the first. The
combination of these two movements results in a periodicity
of the precession of the equinoxes of about 22,000 years.
More specifically, the Earth’s distance from the Sun fluctuates, not only due to the precession movement of the equinoxes, but also due to variations in the eccentricity of its
orbit which varies according to a set of cyclical changes
occurring over two proximate periods, one of 19,000 years
and the other 23,000 years. Thus, approximately
10,000 years ago, the Earth reached its closest point to the
Sun at the time of the summer solstice and not at the boreal
winter solstice as it does today. At that time, the northern
hemisphere received more solar energy in summer than it
does today and obviously less in winter.
All of these modifications in the orbital parameters affect
sunshine levels (still referred to as the insolation) at the different bands of latitude on Earth, and particularly the intensity
of the seasonal cycle. Already, in 1924, the Serbian mathematician, Milutin Milankovitch, proposed that these slow
variations of the movement of the Earth around the Sun could
explain the glacial cycles. Indeed, as these slow variations
induce a decrease in solar energy received in the summer at 60°
N, snow, which has fallen in the winter, does not melt completely. Furthermore, it strongly reflects solar radiation,
facilitating the snow to persist. Gradually, the snow accumulates and turns into an ice cap. This hypothesis has been
debated for many years and was strongly opposed until the
1970s, when the cycles predicted by this theory were clearly
confirmed by paleoclimate records in marine sediments and
later in polar ice. We will see more precisely in Chap. 28 of
6
S. Joussaume and J.-C. Duplessy