Volume 2 the state of our knowledge about the Milankovitch
theory, or the ‘astronomical theory’ of paleoclimates.
The Sun’s Evolution
Since the formation of the solar system, the Sun, like all stars
of the same type, slowly consumes its hydrogen to produce
helium, and the amount of heat it emits varies very slowly
over long time scales. The standard stellar evolution models
estimate that four billion years ago the luminosity of the Sun
was 25–30% lower than it is today, and that it has increased
more or less linearly over time. This model seems in
accordance with observations made by astronomers of
young stars. With the same Earth’s atmosphere as today four
billion years ago, the average temperature of the Earth would
be below 0 °C, oceans would be frozen, and life would be
impossible. Geological observations, however, indicate the
presence of water in the liquid state and the first traces of life
3.5 billion years ago. This is the ‘Pale Young Sun paradox’
which is solved by assuming that the atmosphere had a very
different chemical composition from that of today. Indeed,
the elimination of carbon dioxide by the young Earth, and
the low rates of weathering given the absence of continental
crust, meant that the atmosphere during these ancient periods
acquired an exceptionally high level of CO 2 and hence was
responsible for a strong greenhouse effect, further enhanced
by the presence of methane produced by bacteria. This will
be discussed in detail in the chapter on the Precambrian
(Chap. 26, Volume 2).
Reconstruction of the History of Atmospheric
Composition
Although the Sun is the source of energy for the Earth, the
energy made available depends essentially on the composition of the atmosphere: greenhouse gases and particles.
Reconstructing the past history of the composition of the
atmosphere is therefore an important element in understanding climate dynamics.
Again, the last hundreds of thousands of years constitute
the best documented period because of the fossil air bubbles
contained in the polar ice caps. The snow falling on the polar
caps forms a porous firn, within which air circulates freely.
Under the weight of accumulated snow, the pores gradually
compact and the firn turns into ice that traps tiny air bubbles
within it. This air keeps its original chemical composition,
which allows the reconstitution of variations in the composition of the atmosphere over time, as long as we can find
well-preserved ancient ice. The oldest ice is found in
Antarctica, where a continuous recording of the greenhouse
gas content (CO 2 , CH 4 ) over the last 800,000 years has been
established. These records show that the levels of carbon
dioxide have not remained constant; they were high in warm
periods, around 280 ppmv (280 cm
3 of CO 2 per m
3 of air)
and only 200 ppmv during cold periods. Similarly, methane
ranged from *700 ppbv (mm
3 per m
3 of air) in warm
periods to less than 400 ppbv during cold ones, with a high
temporal variability (Fig. 1.4).
Polar ice is the only direct recording of the composition
of the atmosphere. As ice in the ice caps flows very slowly
and is continuously renewed throughout geological time, it is
impossible to reconstruct a record of carbon dioxide levels
before a million years ago. For earlier periods, it is therefore
necessary to use indirect empirical methods which have a
much lower level of precision. These indirect “proxies” to
reconstruct atmospheric CO 2 may be derived from stomata,
boron isotope or alkenone (Chap. 27, Volume 2). For
example, botanists observed that the stomata—pores through
which leaves absorb carbon dioxide from the air—are
smaller and fewer when carbon dioxide is high. This plant
characteristic has been used as a means of establishing CO 2
levels for the past. However, the results were not clear-cut.
For one, the fossil species being studied must be the same as
the current species on which the empirical relationship
between the levels of carbon dioxide and the number or
diameter of the stomata is being established. Moreover, the
relationship, which can only be determined in the current
conditions, also depends on the availability of water to the
plant, so it is not clear whether changes observed in fossil
stomata are due to variations in CO 2 or in moisture.
As the CO 2 content of the air is governed by the partial
pressure of this gas in the surface waters of the ocean,
geochemists have tried to use carbon-13, a tracer of the
oceanic phase of the carbon cycle, as a tool to reconstruct
changes in atmospheric CO 2 . One of the proposed markers is
the
13 C/
12 C ratio in foraminifera, microscopic animals in the
form of plankton living in the surface waters of the oceans.
These animals secrete a calcareous shell whose size is a few
tenths of a millimeter and which are found in abundance in
marine sediments. The
13
C/
12
C ratio of planktonic foraminifera therefore depends on the isotopic composition of
dissolved CO 2 in the surface waters, and indirectly on that of
the atmosphere.
This isotopic approach can be compared against the
independent records provided by the polar ice cores, so that
the method can be evaluated over the last few hundred
thousand years. The correlation is only proximate due to the
complexity of the oceanic carbon cycle which depends in
particular on the temperature of the sea water, on the primary
production of the ocean, on the decomposition of organic
matter and on the circulation of the bodies of water. Biologists came up with another method when they noticed that
the fractionation of carbon isotopes during the absorption of
carbon dioxide by seaweed depends on the dissolved carbon
dioxide content and therefore the partial pressure of CO 2 in
the seawater. This led them to the hypothesis that variations
1 The Climate System: Its Functioning and History
7
theory, or the ‘astronomical theory’ of paleoclimates.
The Sun’s Evolution
Since the formation of the solar system, the Sun, like all stars
of the same type, slowly consumes its hydrogen to produce
helium, and the amount of heat it emits varies very slowly
over long time scales. The standard stellar evolution models
estimate that four billion years ago the luminosity of the Sun
was 25–30% lower than it is today, and that it has increased
more or less linearly over time. This model seems in
accordance with observations made by astronomers of
young stars. With the same Earth’s atmosphere as today four
billion years ago, the average temperature of the Earth would
be below 0 °C, oceans would be frozen, and life would be
impossible. Geological observations, however, indicate the
presence of water in the liquid state and the first traces of life
3.5 billion years ago. This is the ‘Pale Young Sun paradox’
which is solved by assuming that the atmosphere had a very
different chemical composition from that of today. Indeed,
the elimination of carbon dioxide by the young Earth, and
the low rates of weathering given the absence of continental
crust, meant that the atmosphere during these ancient periods
acquired an exceptionally high level of CO 2 and hence was
responsible for a strong greenhouse effect, further enhanced
by the presence of methane produced by bacteria. This will
be discussed in detail in the chapter on the Precambrian
(Chap. 26, Volume 2).
Reconstruction of the History of Atmospheric
Composition
Although the Sun is the source of energy for the Earth, the
energy made available depends essentially on the composition of the atmosphere: greenhouse gases and particles.
Reconstructing the past history of the composition of the
atmosphere is therefore an important element in understanding climate dynamics.
Again, the last hundreds of thousands of years constitute
the best documented period because of the fossil air bubbles
contained in the polar ice caps. The snow falling on the polar
caps forms a porous firn, within which air circulates freely.
Under the weight of accumulated snow, the pores gradually
compact and the firn turns into ice that traps tiny air bubbles
within it. This air keeps its original chemical composition,
which allows the reconstitution of variations in the composition of the atmosphere over time, as long as we can find
well-preserved ancient ice. The oldest ice is found in
Antarctica, where a continuous recording of the greenhouse
gas content (CO 2 , CH 4 ) over the last 800,000 years has been
established. These records show that the levels of carbon
dioxide have not remained constant; they were high in warm
periods, around 280 ppmv (280 cm
3 of CO 2 per m
3 of air)
and only 200 ppmv during cold periods. Similarly, methane
ranged from *700 ppbv (mm
3 per m
3 of air) in warm
periods to less than 400 ppbv during cold ones, with a high
temporal variability (Fig. 1.4).
Polar ice is the only direct recording of the composition
of the atmosphere. As ice in the ice caps flows very slowly
and is continuously renewed throughout geological time, it is
impossible to reconstruct a record of carbon dioxide levels
before a million years ago. For earlier periods, it is therefore
necessary to use indirect empirical methods which have a
much lower level of precision. These indirect “proxies” to
reconstruct atmospheric CO 2 may be derived from stomata,
boron isotope or alkenone (Chap. 27, Volume 2). For
example, botanists observed that the stomata—pores through
which leaves absorb carbon dioxide from the air—are
smaller and fewer when carbon dioxide is high. This plant
characteristic has been used as a means of establishing CO 2
levels for the past. However, the results were not clear-cut.
For one, the fossil species being studied must be the same as
the current species on which the empirical relationship
between the levels of carbon dioxide and the number or
diameter of the stomata is being established. Moreover, the
relationship, which can only be determined in the current
conditions, also depends on the availability of water to the
plant, so it is not clear whether changes observed in fossil
stomata are due to variations in CO 2 or in moisture.
As the CO 2 content of the air is governed by the partial
pressure of this gas in the surface waters of the ocean,
geochemists have tried to use carbon-13, a tracer of the
oceanic phase of the carbon cycle, as a tool to reconstruct
changes in atmospheric CO 2 . One of the proposed markers is
the
13 C/
12 C ratio in foraminifera, microscopic animals in the
form of plankton living in the surface waters of the oceans.
These animals secrete a calcareous shell whose size is a few
tenths of a millimeter and which are found in abundance in
marine sediments. The
13
C/
12
C ratio of planktonic foraminifera therefore depends on the isotopic composition of
dissolved CO 2 in the surface waters, and indirectly on that of
the atmosphere.
This isotopic approach can be compared against the
independent records provided by the polar ice cores, so that
the method can be evaluated over the last few hundred
thousand years. The correlation is only proximate due to the
complexity of the oceanic carbon cycle which depends in
particular on the temperature of the sea water, on the primary
production of the ocean, on the decomposition of organic
matter and on the circulation of the bodies of water. Biologists came up with another method when they noticed that
the fractionation of carbon isotopes during the absorption of
carbon dioxide by seaweed depends on the dissolved carbon
dioxide content and therefore the partial pressure of CO 2 in
the seawater. This led them to the hypothesis that variations
1 The Climate System: Its Functioning and History
7