Katmai (1912), Agung (1963), El Chichon (1982) and
Pinatubo (1991) have provided material to better understand
the effects of aerosols that have recently been incorporated
into climate models. The indirect effects of aerosols by
modifying clouds were discovered more recently. The
presence of aerosols can modify cloud characteristics by
making them more reflective, or by extending their lifetime
before precipitation, for example.
These chemical elements and aerosols are closely linked
to climate and climate changes. Their concentration in the
atmosphere impacts the Earth’s energy budget, either
directly or indirectly, while changes in climate modify the
exchanges between reservoirs of these compounds and, in
fine, their concentration in the atmosphere. Some atmospheric gases have the ability to modify the energy budget of
the Earth (Volume 1, Chap. 1). The Earth receives shortwave radiation (ultraviolet, visible, and near-infrared) from
the sun. Part of this radiation is reflected back by the surface,
clouds and the atmosphere, part of it is absorbed by the
atmosphere and clouds, and the last part is absorbed by the
surface of the Earth. The Earth’s surface emits longwave
radiation (infrared) because it is colder than the sun, and also
transfers energy to the atmosphere by latent and sensible
heat. The longwave radiation from the Earth’s surface is
partly absorbed by the atmospheric greenhouse gases, which
then re-emit radiation in all directions, including towards the
surface of the Earth. The latter is then heated, resulting in a
higher temperature than on an Earth without greenhouse
gases. The radiation absorbed by the gases depends on their
properties and in which zones they absorb radiation. Among
the gases present in the atmosphere, the main greenhouse
gases are, in decreasing order (excluding water vapor) carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O).
Aerosols have two effects on the energy budget in the
atmosphere: direct and indirect. Aerosols are tiny particles—
such as sea salt, dust from deserts and fires—in suspension
in the atmosphere, either in liquid or solid form. They can
absorb and disperse solar radiation, as well as absorb and
emit thermal radiation. This is the direct effect. Aerosols also
form cloud condensation nuclei and ice nuclei: raindrops and
ice develop around these nuclei. This is the indirect effect
because it modifies the energy budget through the modification of the microphysics of clouds. Depending on the size
of the drops, which is dependent on the type and size of
nuclei, the clouds will reflect or absorb radiation. In addition,
aerosols deposited back to the surface will alter the amount
of solar radiation reflected back to space, and will disperse
chemical elements that can influence various biogeochemical cycles.
Some of the changes impacting on the biogeochemical
cycles and aerosols are recorded and preserved for thousands
of years. The main natural archives that have been used to
track such changes are sediment cores extracted from oceans
or lakes, and ice cores, drilled from polar ice sheets
(Fig. 23.1).
Marine sediment cores, collected from the bottom of the
ocean, include various organic and inorganic elements,
which can be used to get direct information. For example,
examining the type of plankton that lived in a region at a
given time can tell us how cold it was. A succession of
species that thrive in warm or cold environments will indicate a succession of warm and cold periods. An analysis of
the pollen present in sediment will yield information on the
proportion of the major plants that lived on the nearby
continent. In addition, the material is measured to obtain the
ratio of chemical elements such as Pa/Th, and the ratio of
isotopes such as oxygen and carbon isotopes (d
18 O, d
13 C,
D
14 C), which can be used as indicators of specific processes
such as ocean circulation changes, temperature changes,
terrestrial biosphere changes, etc. (see Volume 1, Chap. 21).
The idea that ice from ice sheets could be used to provide
information on past changes originated in the 1950s with the
work of Willi Dansgaard and others, who hypothesized that
the link between temperature and the number of heavy
oxygen isotopes in precipitation could be applied to old ice
to reconstruct past temperature changes. Ice core drilling
began in Antarctica, Alaska and Greenland in the 1950s, but
these cores were around 100 m deep and the recovery
quality was low. Drilling to extract ice cores was spurred by
the International Geophysical Year (1957–1958) and longer
ice cores were drilled in Greenland at Site 2 (1956–1957)
and in Antarctica at Byrd station (1957–1958) and at Little
America V (1958–1959). Many more ice cores have been
drilled since then, mainly in Greenland and Antarctica. Past
climate and environmental changes are recorded both in the
ice and in air bubbles trapped within the ice of the ice cores.
For instance, the proportion of hydrogen and oxygen isotopes in the ice provide information on past surface temperatures, while the concentration of greenhouse gases can
be directly measured in the air bubbles.
To understand the changes recorded in climate archives
and to test various hypotheses on feedback mechanisms,
more and more climate models now include biogeochemical
cycles, and sometimes isotopes (see Volume 2, Chaps. 25
and 29). Additional mechanisms and elements are added
progressively so that simulations and measured data can be
compared directly. This continuous comparison helps to
increase our knowledge of the climate system resulting in
improved models that can be used to evaluate possible future
changes. These coupled carbon-climate models are valuable
tools to help understand past changes and increase our
confidence in future climate projections.
In this chapter we describe the main biogeochemical
cycles interacting with the climate: carbon (CO 2 and CH 4 ),
272
N. Bouttes et al.
Pinatubo (1991) have provided material to better understand
the effects of aerosols that have recently been incorporated
into climate models. The indirect effects of aerosols by
modifying clouds were discovered more recently. The
presence of aerosols can modify cloud characteristics by
making them more reflective, or by extending their lifetime
before precipitation, for example.
These chemical elements and aerosols are closely linked
to climate and climate changes. Their concentration in the
atmosphere impacts the Earth’s energy budget, either
directly or indirectly, while changes in climate modify the
exchanges between reservoirs of these compounds and, in
fine, their concentration in the atmosphere. Some atmospheric gases have the ability to modify the energy budget of
the Earth (Volume 1, Chap. 1). The Earth receives shortwave radiation (ultraviolet, visible, and near-infrared) from
the sun. Part of this radiation is reflected back by the surface,
clouds and the atmosphere, part of it is absorbed by the
atmosphere and clouds, and the last part is absorbed by the
surface of the Earth. The Earth’s surface emits longwave
radiation (infrared) because it is colder than the sun, and also
transfers energy to the atmosphere by latent and sensible
heat. The longwave radiation from the Earth’s surface is
partly absorbed by the atmospheric greenhouse gases, which
then re-emit radiation in all directions, including towards the
surface of the Earth. The latter is then heated, resulting in a
higher temperature than on an Earth without greenhouse
gases. The radiation absorbed by the gases depends on their
properties and in which zones they absorb radiation. Among
the gases present in the atmosphere, the main greenhouse
gases are, in decreasing order (excluding water vapor) carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O).
Aerosols have two effects on the energy budget in the
atmosphere: direct and indirect. Aerosols are tiny particles—
such as sea salt, dust from deserts and fires—in suspension
in the atmosphere, either in liquid or solid form. They can
absorb and disperse solar radiation, as well as absorb and
emit thermal radiation. This is the direct effect. Aerosols also
form cloud condensation nuclei and ice nuclei: raindrops and
ice develop around these nuclei. This is the indirect effect
because it modifies the energy budget through the modification of the microphysics of clouds. Depending on the size
of the drops, which is dependent on the type and size of
nuclei, the clouds will reflect or absorb radiation. In addition,
aerosols deposited back to the surface will alter the amount
of solar radiation reflected back to space, and will disperse
chemical elements that can influence various biogeochemical cycles.
Some of the changes impacting on the biogeochemical
cycles and aerosols are recorded and preserved for thousands
of years. The main natural archives that have been used to
track such changes are sediment cores extracted from oceans
or lakes, and ice cores, drilled from polar ice sheets
(Fig. 23.1).
Marine sediment cores, collected from the bottom of the
ocean, include various organic and inorganic elements,
which can be used to get direct information. For example,
examining the type of plankton that lived in a region at a
given time can tell us how cold it was. A succession of
species that thrive in warm or cold environments will indicate a succession of warm and cold periods. An analysis of
the pollen present in sediment will yield information on the
proportion of the major plants that lived on the nearby
continent. In addition, the material is measured to obtain the
ratio of chemical elements such as Pa/Th, and the ratio of
isotopes such as oxygen and carbon isotopes (d
18 O, d
13 C,
D
14 C), which can be used as indicators of specific processes
such as ocean circulation changes, temperature changes,
terrestrial biosphere changes, etc. (see Volume 1, Chap. 21).
The idea that ice from ice sheets could be used to provide
information on past changes originated in the 1950s with the
work of Willi Dansgaard and others, who hypothesized that
the link between temperature and the number of heavy
oxygen isotopes in precipitation could be applied to old ice
to reconstruct past temperature changes. Ice core drilling
began in Antarctica, Alaska and Greenland in the 1950s, but
these cores were around 100 m deep and the recovery
quality was low. Drilling to extract ice cores was spurred by
the International Geophysical Year (1957–1958) and longer
ice cores were drilled in Greenland at Site 2 (1956–1957)
and in Antarctica at Byrd station (1957–1958) and at Little
America V (1958–1959). Many more ice cores have been
drilled since then, mainly in Greenland and Antarctica. Past
climate and environmental changes are recorded both in the
ice and in air bubbles trapped within the ice of the ice cores.
For instance, the proportion of hydrogen and oxygen isotopes in the ice provide information on past surface temperatures, while the concentration of greenhouse gases can
be directly measured in the air bubbles.
To understand the changes recorded in climate archives
and to test various hypotheses on feedback mechanisms,
more and more climate models now include biogeochemical
cycles, and sometimes isotopes (see Volume 2, Chaps. 25
and 29). Additional mechanisms and elements are added
progressively so that simulations and measured data can be
compared directly. This continuous comparison helps to
increase our knowledge of the climate system resulting in
improved models that can be used to evaluate possible future
changes. These coupled carbon-climate models are valuable
tools to help understand past changes and increase our
confidence in future climate projections.
In this chapter we describe the main biogeochemical
cycles interacting with the climate: carbon (CO 2 and CH 4 ),
272
N. Bouttes et al.