The temperature can be increased by the interposition of the
atmosphere, because the heat finds less obstacle to penetrate the
air, being in the state of light, than it finds it to pass through the
air when converted into dark heat.
(J. Fourier, 1824)
It is in this context that the two main physical theories are
presented, which are still relevant today, and which make it
possible to explain the existence of glacial periods: the
astronomical theory and the variations in the atmospheric
concentration of CO 2 .
From Adhémar to Milankovitch: The Role
of Insolation
Although the notion that the climate is influenced by the
stars has undoubtedly been around for a very long time, the
first astronomical scientific theory of the ice ages was formulated by Joseph Adhemar in 1842. It was based simply on
common sense: since ancient times, astronomers have
highlighted the ‘three movements of the Earth’: the diurnal
cycle, the annual cycle, and the precession of the equinoxes
(Hipparchus, about 130 before J.-C.). While it is clear that
the annual and diurnal cycles generate temperature variations, the same must be true of the third movement of the
Earth. As will be explained a little later, the precession of
equinoxes has the consequence of modifying the position of
the perihelion (point of the Earth’s orbit closest to the Sun)
in relation to the seasons: today, the Earth is closest to the
Sun around January 4 but this date changes slowly to cover
the whole of the year over about 21,000 years. In contrast to
today, 10,500 years ago, the Earth was far from the Sun in
January and close to it in July. Adhemar suggested that this
mechanism could modify the climate. More specifically, the
winters of the northern hemisphere now occur when the
Earth is close to the Sun and, conversely, those of the
southern hemisphere when the Earth is far from the Sun.
Adhemar proposed that this explains the absence of a large
ice cap in the north, due to milder and shorter winters, and
conversely, the presence of a large Antarctic cap. The situation would have been exactly the opposite 10,500 years
ago, which allowed him to explain the periods of great
glacial expansion that had just been revealed by geologists.
Adhemar’s theory was criticized for many reasons, some
largely unfounded, but it was on the very foundations of his
theory that his detractors, Charles Lyell and Alexander von
Humbolt, would find compelling arguments. In fact, the
mechanism of precession works in an anti-symmetrical way
between the poles, but also between the seasons. It can easily
be shown that although, for example, less energy is received
in winter, this is compensated for by an equivalent excess of
energy received in summer. If the seasonal contrast varies
with the precession, the full complement of energy received
does not change. How then could this have any effect on
climate? According to Adhemar, although the astronomical
forcing is effectively anti-symmetric with respect to the
seasons and zero for the annual average, the climate processes are probably not.
In 1864, James Croll clarified this concept. According to
him, the accumulation of snow occurs chiefly in winter, with
melting occurring in summer. Croll emphasized the role of the
winter accumulation which essentially supports Adhemar’s
argument: longer or colder winters favor a greater accumulation of ice allows the initiation of a glaciation. In addition,
aware of the progress that had been made in celestial
mechanics notably by Pierre Simon de Laplace and Urbain le
Verrier, Croll went on to introduce the effect of variations in
the eccentricity of the Earth’s orbit. These variations modulate
the intensity of seasonal contrasts. Indeed, in the case of a
circular orbit, the effect of the precession on climate is zero,
since there would be neither a perihelion (nearest point) nor an
aphelion (farthest point). The greater the eccentricity, the
greater the climate effects of the precession. Croll therefore
linked the great glaciations with eccentricity maxima.
Hence, he pushed back the estimate of the last glaciation to
80,000 years ago, and suggested an even more intense
glaciation 240,000 years ago. Although Croll proposed a
much more solid and evolved astronomical theory, he did not
succeed in convincing the scientific community of his time.
However, the interglacial-glacial alternations discovered in
some sediments argued in favor of a more or less periodic
mechanism. Yet the first dating of elements available from that
time, extrapolating the rates of erosion or counting lake varves,
indicated a much more recent glaciation. Croll went on to
introduce a third important astronomical parameter for the
calculation of the variations of the solar energy received in a
given place: the obliquity of the terrestrial axis, that is to say its
inclination with respect to the plane of the Earth’s orbit.
However, obliquity has little effect on winter insolation, and
therefore was of little importance in Croll’s theory.
Milankovitch (1941) formulated the astronomical theory
which still applies today. The main criticism that can be levelled at Croll’s theory is that it considered winter to be the most
pertinent season. This objection was already expressed by
Joseph Murphy during Croll’s time, as observations of the
eternal snow and mountain glaciers showed that summer
melting had much more impact on the ice mass balance than
snow accumulation. However, the precession was often
advanced to explain the current asymmetry of temperatures
between the northern hemisphere and the southern hemisphere, and the presence of Antarctica, since today, the austral
winter is longer and further from the Sun than the northern
winter. Croll’s theory was therefore probably based on this
nineteenth century misconception of the current climate.
Milankovitch resolved this problem by deducing that the
current north-south asymmetry was linked to geography and
386
D. Paillard
atmosphere, because the heat finds less obstacle to penetrate the
air, being in the state of light, than it finds it to pass through the
air when converted into dark heat.
(J. Fourier, 1824)
It is in this context that the two main physical theories are
presented, which are still relevant today, and which make it
possible to explain the existence of glacial periods: the
astronomical theory and the variations in the atmospheric
concentration of CO 2 .
From Adhémar to Milankovitch: The Role
of Insolation
Although the notion that the climate is influenced by the
stars has undoubtedly been around for a very long time, the
first astronomical scientific theory of the ice ages was formulated by Joseph Adhemar in 1842. It was based simply on
common sense: since ancient times, astronomers have
highlighted the ‘three movements of the Earth’: the diurnal
cycle, the annual cycle, and the precession of the equinoxes
(Hipparchus, about 130 before J.-C.). While it is clear that
the annual and diurnal cycles generate temperature variations, the same must be true of the third movement of the
Earth. As will be explained a little later, the precession of
equinoxes has the consequence of modifying the position of
the perihelion (point of the Earth’s orbit closest to the Sun)
in relation to the seasons: today, the Earth is closest to the
Sun around January 4 but this date changes slowly to cover
the whole of the year over about 21,000 years. In contrast to
today, 10,500 years ago, the Earth was far from the Sun in
January and close to it in July. Adhemar suggested that this
mechanism could modify the climate. More specifically, the
winters of the northern hemisphere now occur when the
Earth is close to the Sun and, conversely, those of the
southern hemisphere when the Earth is far from the Sun.
Adhemar proposed that this explains the absence of a large
ice cap in the north, due to milder and shorter winters, and
conversely, the presence of a large Antarctic cap. The situation would have been exactly the opposite 10,500 years
ago, which allowed him to explain the periods of great
glacial expansion that had just been revealed by geologists.
Adhemar’s theory was criticized for many reasons, some
largely unfounded, but it was on the very foundations of his
theory that his detractors, Charles Lyell and Alexander von
Humbolt, would find compelling arguments. In fact, the
mechanism of precession works in an anti-symmetrical way
between the poles, but also between the seasons. It can easily
be shown that although, for example, less energy is received
in winter, this is compensated for by an equivalent excess of
energy received in summer. If the seasonal contrast varies
with the precession, the full complement of energy received
does not change. How then could this have any effect on
climate? According to Adhemar, although the astronomical
forcing is effectively anti-symmetric with respect to the
seasons and zero for the annual average, the climate processes are probably not.
In 1864, James Croll clarified this concept. According to
him, the accumulation of snow occurs chiefly in winter, with
melting occurring in summer. Croll emphasized the role of the
winter accumulation which essentially supports Adhemar’s
argument: longer or colder winters favor a greater accumulation of ice allows the initiation of a glaciation. In addition,
aware of the progress that had been made in celestial
mechanics notably by Pierre Simon de Laplace and Urbain le
Verrier, Croll went on to introduce the effect of variations in
the eccentricity of the Earth’s orbit. These variations modulate
the intensity of seasonal contrasts. Indeed, in the case of a
circular orbit, the effect of the precession on climate is zero,
since there would be neither a perihelion (nearest point) nor an
aphelion (farthest point). The greater the eccentricity, the
greater the climate effects of the precession. Croll therefore
linked the great glaciations with eccentricity maxima.
Hence, he pushed back the estimate of the last glaciation to
80,000 years ago, and suggested an even more intense
glaciation 240,000 years ago. Although Croll proposed a
much more solid and evolved astronomical theory, he did not
succeed in convincing the scientific community of his time.
However, the interglacial-glacial alternations discovered in
some sediments argued in favor of a more or less periodic
mechanism. Yet the first dating of elements available from that
time, extrapolating the rates of erosion or counting lake varves,
indicated a much more recent glaciation. Croll went on to
introduce a third important astronomical parameter for the
calculation of the variations of the solar energy received in a
given place: the obliquity of the terrestrial axis, that is to say its
inclination with respect to the plane of the Earth’s orbit.
However, obliquity has little effect on winter insolation, and
therefore was of little importance in Croll’s theory.
Milankovitch (1941) formulated the astronomical theory
which still applies today. The main criticism that can be levelled at Croll’s theory is that it considered winter to be the most
pertinent season. This objection was already expressed by
Joseph Murphy during Croll’s time, as observations of the
eternal snow and mountain glaciers showed that summer
melting had much more impact on the ice mass balance than
snow accumulation. However, the precession was often
advanced to explain the current asymmetry of temperatures
between the northern hemisphere and the southern hemisphere, and the presence of Antarctica, since today, the austral
winter is longer and further from the Sun than the northern
winter. Croll’s theory was therefore probably based on this
nineteenth century misconception of the current climate.
Milankovitch resolved this problem by deducing that the
current north-south asymmetry was linked to geography and
386
D. Paillard