not to astronomical forcing. The critical season for the
evolution of ice caps is therefore summer, which completely
reverses the reasoning of Adhemar and Croll. The obliquity
of the Earth’s axis, that is to say its inclination with respect
to the plane of Earth’s orbit, becomes the most important
parameter for glacial-interglacial evolution. The foundations
of modern astronomical theory were thus laid down.
From Tyndall to Arrhénius: The Role of Carbon
Dioxide
The importance of the role of the greenhouse effect was well
understood by the nineteenth century, notably through the
work of Joseph Fourier. As early as 1845, Jacques Joseph
Ebelmen, a French chemist, first suggested that changes in
the atmospheric concentration of CO 2 might have consequences for the climate (Bard 2004). Indeed, by focusing on
the chemistry of minerals, he established the bases of carbon
geochemistry: a source primarily of volcanic origin and
sinks related to the erosion of silicates and the burial of the
organic material. Since all these processes appear to be
disconnected, it seems unlikely that the atmospheric concentration of CO 2 would be constant over geological time. In
1861, John Tyndall further authenticated the theory of the
greenhouse effect. By measuring the absorption and infrared
emission of the various gases present in the air, he demonstrated that nitrogen or oxygen are essentially transparent to
infrared rays and that the greenhouse effect of our planet is
caused primarily by gases in very small quantities, in large
part by water vapor, but also carbon dioxide, methane,
nitrous oxide and ozone. Tyndall then suggested that all of
the climate changes discovered by geologists, including ice
ages, could be explained by changes in the levels of atmospheric greenhouse gases.
But it was the Swedish chemist Svante Arrhenius (1896)
who managed to calculate the effect of carbon dioxide on the
climate, in an attempt to explain the ice ages. Based on
geological data on moraine positions during glacial periods,
he estimated a cooling of 4 or 5 °C and calculated that this
could be explained by a reduction of about 40% in the
atmospheric concentration of CO 2 . The measurement of
pCO 2 from this glacial period, carried out on air bubbles
from ice cores taken from Antarctica in the years 1980–
1990, confirmed his calculations: the atmospheric concentration of CO 2 was indeed 30% lower during the ice age.
This scientific prediction, nearly a century before it could be
confirmed through observation, is a good illustration of the
essential role of greenhouse gases in the deployment of the
Quaternary cycles. Arrhenius also considered that future
global warming would be linked to anthropogenic CO 2
emissions. He calculated an increase in global temperatures
of about 5 °C for a doubling of CO 2 , a figure surprisingly
close to the most recent estimates of about 3.5 °C, another
prediction likely to become true in the not too distant future.
All these arguments were underscored by the American
geologist Chamberlin, who highlighted the succession of at
least five glacial stages in the United States. According to
Chamberlin, there was a sort of oscillation between the climate and the geochemistry of the Earth: a decrease in CO 2
leading to cooling, with the effect of reducing carbon sinks
on Earth by reducing the burial of organic matter as well as
the erosion of silicates. This would then lead to a gradual
increase in atmospheric CO 2 until it switches over to the
opposite situation. Chamberlin therefore attempted to formulate an ‘internal oscillation’ to explain the succession of
ice ages, without calling on a ‘Deus ex machina’ such as the
astronomical forcing.
It is interesting to note that these two opposing theories of
the glacial periods have existed since the middle of the
nineteenth century and are still largely valid today: it
remains to be understood how they relate and complement
each other.
Astronomical Parameters and Insolation
Before proceeding further, it is useful to review the various
astronomical parameters that influence the energy received at
the top of the Earth’s atmosphere, which we call ‘insolation’.
Eccentricity
According to Kepler’s first law, the Earth’s orbit is an
ellipse. This is characterized by a major parameter, the
semi-major axis, often denoted a, by a shape or flattening
parameter, the eccentricity, often denoted e, and also by
three parameters defining the position of this ellipse in space,
two of which define the orbital plane (the inclination i with
respect to a reference plane and the longitude of the
ascending node X defined by the intersection of these two
planes), and another to define the absolute position of the
perihelion (the longitude p). In fact, as soon as the system is
made up of three material bodies (the Sun with two planets)
or more, the movement is no longer strictly an ellipse, and
there is no analytical solution to the problem of celestial
mechanics at N bodies, for N > 2. It is therefore appropriate
to calculate the perturbations or the approximate numerical
resolutions. The notion of terrestrial orbit nevertheless still
makes sense because the perturbations are secondary. It is
therefore useful to reason in terms of elliptical orbit, which
deforms and moves over time.
The perturbations induced by the other planets do not
modify the semi-major axis of the ellipse a, only the terrestrial trajectory, i.e. the eccentricity e and the orientation
28 Climate and Astronomical Cycles
387
evolution of ice caps is therefore summer, which completely
reverses the reasoning of Adhemar and Croll. The obliquity
of the Earth’s axis, that is to say its inclination with respect
to the plane of Earth’s orbit, becomes the most important
parameter for glacial-interglacial evolution. The foundations
of modern astronomical theory were thus laid down.
From Tyndall to Arrhénius: The Role of Carbon
Dioxide
The importance of the role of the greenhouse effect was well
understood by the nineteenth century, notably through the
work of Joseph Fourier. As early as 1845, Jacques Joseph
Ebelmen, a French chemist, first suggested that changes in
the atmospheric concentration of CO 2 might have consequences for the climate (Bard 2004). Indeed, by focusing on
the chemistry of minerals, he established the bases of carbon
geochemistry: a source primarily of volcanic origin and
sinks related to the erosion of silicates and the burial of the
organic material. Since all these processes appear to be
disconnected, it seems unlikely that the atmospheric concentration of CO 2 would be constant over geological time. In
1861, John Tyndall further authenticated the theory of the
greenhouse effect. By measuring the absorption and infrared
emission of the various gases present in the air, he demonstrated that nitrogen or oxygen are essentially transparent to
infrared rays and that the greenhouse effect of our planet is
caused primarily by gases in very small quantities, in large
part by water vapor, but also carbon dioxide, methane,
nitrous oxide and ozone. Tyndall then suggested that all of
the climate changes discovered by geologists, including ice
ages, could be explained by changes in the levels of atmospheric greenhouse gases.
But it was the Swedish chemist Svante Arrhenius (1896)
who managed to calculate the effect of carbon dioxide on the
climate, in an attempt to explain the ice ages. Based on
geological data on moraine positions during glacial periods,
he estimated a cooling of 4 or 5 °C and calculated that this
could be explained by a reduction of about 40% in the
atmospheric concentration of CO 2 . The measurement of
pCO 2 from this glacial period, carried out on air bubbles
from ice cores taken from Antarctica in the years 1980–
1990, confirmed his calculations: the atmospheric concentration of CO 2 was indeed 30% lower during the ice age.
This scientific prediction, nearly a century before it could be
confirmed through observation, is a good illustration of the
essential role of greenhouse gases in the deployment of the
Quaternary cycles. Arrhenius also considered that future
global warming would be linked to anthropogenic CO 2
emissions. He calculated an increase in global temperatures
of about 5 °C for a doubling of CO 2 , a figure surprisingly
close to the most recent estimates of about 3.5 °C, another
prediction likely to become true in the not too distant future.
All these arguments were underscored by the American
geologist Chamberlin, who highlighted the succession of at
least five glacial stages in the United States. According to
Chamberlin, there was a sort of oscillation between the climate and the geochemistry of the Earth: a decrease in CO 2
leading to cooling, with the effect of reducing carbon sinks
on Earth by reducing the burial of organic matter as well as
the erosion of silicates. This would then lead to a gradual
increase in atmospheric CO 2 until it switches over to the
opposite situation. Chamberlin therefore attempted to formulate an ‘internal oscillation’ to explain the succession of
ice ages, without calling on a ‘Deus ex machina’ such as the
astronomical forcing.
It is interesting to note that these two opposing theories of
the glacial periods have existed since the middle of the
nineteenth century and are still largely valid today: it
remains to be understood how they relate and complement
each other.
Astronomical Parameters and Insolation
Before proceeding further, it is useful to review the various
astronomical parameters that influence the energy received at
the top of the Earth’s atmosphere, which we call ‘insolation’.
Eccentricity
According to Kepler’s first law, the Earth’s orbit is an
ellipse. This is characterized by a major parameter, the
semi-major axis, often denoted a, by a shape or flattening
parameter, the eccentricity, often denoted e, and also by
three parameters defining the position of this ellipse in space,
two of which define the orbital plane (the inclination i with
respect to a reference plane and the longitude of the
ascending node X defined by the intersection of these two
planes), and another to define the absolute position of the
perihelion (the longitude p). In fact, as soon as the system is
made up of three material bodies (the Sun with two planets)
or more, the movement is no longer strictly an ellipse, and
there is no analytical solution to the problem of celestial
mechanics at N bodies, for N > 2. It is therefore appropriate
to calculate the perturbations or the approximate numerical
resolutions. The notion of terrestrial orbit nevertheless still
makes sense because the perturbations are secondary. It is
therefore useful to reason in terms of elliptical orbit, which
deforms and moves over time.
The perturbations induced by the other planets do not
modify the semi-major axis of the ellipse a, only the terrestrial trajectory, i.e. the eccentricity e and the orientation
28 Climate and Astronomical Cycles
387