Chap. 2, Volume 1 as well was Chaps. 22, 26 and 27,
Volume 2). Since the beginning of the industrial era (about
1850), human activities have significantly increased the
concentration of greenhouse gases already naturally present
in the air and have introduced new ones, such as
chlorofluorocarbons (CFCs) which are active agents of the
greenhouse effect.
The Water Cycle
As solar radiation passes through the different layers of the
atmosphere, part of it is absorbed by ozone in the stratosphere and by water vapor in the troposphere. About half of
the incident energy reaches the surface of Earth, where it is
partially compensated for by the loss of infrared radiation to
the atmosphere. An energy surplus of 104 W/m
2 (Fig. 1.2)
remains available at the surface. This energy warms up the
surrounding air and causes evaporation of water from the
surface of oceans and land, feeding into the water cycle on
our planet. The water vapor is then transported by winds
until it condenses as precipitation, releasing into the atmosphere the energy acquired at the surface during evaporation.
Thus, the cycle of evaporation and precipitation of water
takes energy from the surface of the oceans and land and
redistributes it in the atmosphere. This transfer of latent heat
cools the surface and warms up the atmosphere, thus lessening the differences in temperature between the upper and
lower layers of the atmosphere, as well as between the
equator and the poles. The water cycle thus plays a fundamental role in the redistribution of energy between the surface and the atmosphere.
Evaporation and condensation continuously renew the
store of water vapor in the atmosphere. However, the
amount of water vapor in the air at any given moment
remains quite low. If it were completely condensed, the
liquid layer thus formed would cover the Earth’s surface in a
layer 2.5 cm thick. Yet, on average, the water cycle involves
the evaporation and the precipitation of water which would
correspond to a layer of about 80 cm per year. The recycling
time of water in the atmosphere is therefore very fast and the
water vapor is completely renewed in ten days. The water,
most of which evaporates from the oceans (86%), returns
there either by precipitation or through the flow of rivers and
streams after runoff from land. Globally, on average, evaporation and rainfall balance each other exactly, thereby
maintaining a constant concentration of water vapor in air, as
long as the average temperature of the air remains constant.
Sun-Related Variability
Variations in energy emitted by the Sun and the variations in
the solar energy received by the Earth will affect the climate.
In the first case, the solar activity cycles and the evolution of
the Sun since the formation of the solar system modify the
amount of energy it emits. In the second case, the slow
variations of the movement of the Earth around the Sun
influence the seasonal and geographical distribution of
energy received in a given place on our planet.
Solar Cycles
In the mid-nineteenth century, the German astronomer,
H. Schwabe, discovered spots on the Sun’s surface that
appear and disappear over an eleven-year cycle. When solar
activity is more intense, marked by a greater number of
spots, the Sun emits more energy. Since the 1980s, satellite
measurements allow the estimation of variations in intensity
of solar energy. These are around 0.1%, which corresponds
to a very small perturbation (0.24 W/m
2 ) in the radiation
balance of the Earth. Solar activity directly reflects changes
in the Sun’s magnetic field. The spots reappear in larger
numbers when the magnetic field intensifies. Solar flares
then become stronger; they eject a larger number of particles
toward outer space and thus reinforce the solar wind. These
electrically charged particles, mainly electrons and protons,
reach the Earth’s atmosphere where they cause magnetic
storms—strong disturbances in the magnetic field—as well
as magnificent auroras in the polar regions.
The influence of solar activity on climate has been
debated for many years. In the second half of the seventeenth
century, documented observations indicate an almost total
disappearance of spots for a period of several decades,
during the Little Ice Age (Fig. 1.3). At the end of the
nineteenth century, the German astronomer H. Spörer and
his English colleague W. Maunder linked these two phenomena, thus starting a controversy that persists today. The
nature of the connection between the minimum solar activity
(called the Maunder minimum) and a decrease in the
intensity of solar radiation sufficient to induce a marked
cooling that coincided with that time, still needs to be
explained.
As the direct disruption in the solar radiation balance is
too small to explain the phenomenon, it is believed that solar
activity may affect climate through circulation in the upper
atmosphere. Nevertheless, the link between variations in
solar activity and the Earth’s climate remains a subject of
research and a source of controversy given the absence of a
recognized physical mechanism. The relative role of external
forcing (solar radiation) and internal/geological forcing
(volcanism) in explaining the Little Ice Age still needs to be
assessed.
The Sun exhibits variations over longer periods. These
can be seen not only in the number of sunspots, but also in
variations in solar diameter. This varies with a periodicity of
900 days, but this oscillation is influenced by solar activity.
It is minimal when the activity is at its maximum. Like
sunspots, solar diameter measurements started in the
4
S. Joussaume and J.-C. Duplessy
Volume 2). Since the beginning of the industrial era (about
1850), human activities have significantly increased the
concentration of greenhouse gases already naturally present
in the air and have introduced new ones, such as
chlorofluorocarbons (CFCs) which are active agents of the
greenhouse effect.
The Water Cycle
As solar radiation passes through the different layers of the
atmosphere, part of it is absorbed by ozone in the stratosphere and by water vapor in the troposphere. About half of
the incident energy reaches the surface of Earth, where it is
partially compensated for by the loss of infrared radiation to
the atmosphere. An energy surplus of 104 W/m
2 (Fig. 1.2)
remains available at the surface. This energy warms up the
surrounding air and causes evaporation of water from the
surface of oceans and land, feeding into the water cycle on
our planet. The water vapor is then transported by winds
until it condenses as precipitation, releasing into the atmosphere the energy acquired at the surface during evaporation.
Thus, the cycle of evaporation and precipitation of water
takes energy from the surface of the oceans and land and
redistributes it in the atmosphere. This transfer of latent heat
cools the surface and warms up the atmosphere, thus lessening the differences in temperature between the upper and
lower layers of the atmosphere, as well as between the
equator and the poles. The water cycle thus plays a fundamental role in the redistribution of energy between the surface and the atmosphere.
Evaporation and condensation continuously renew the
store of water vapor in the atmosphere. However, the
amount of water vapor in the air at any given moment
remains quite low. If it were completely condensed, the
liquid layer thus formed would cover the Earth’s surface in a
layer 2.5 cm thick. Yet, on average, the water cycle involves
the evaporation and the precipitation of water which would
correspond to a layer of about 80 cm per year. The recycling
time of water in the atmosphere is therefore very fast and the
water vapor is completely renewed in ten days. The water,
most of which evaporates from the oceans (86%), returns
there either by precipitation or through the flow of rivers and
streams after runoff from land. Globally, on average, evaporation and rainfall balance each other exactly, thereby
maintaining a constant concentration of water vapor in air, as
long as the average temperature of the air remains constant.
Sun-Related Variability
Variations in energy emitted by the Sun and the variations in
the solar energy received by the Earth will affect the climate.
In the first case, the solar activity cycles and the evolution of
the Sun since the formation of the solar system modify the
amount of energy it emits. In the second case, the slow
variations of the movement of the Earth around the Sun
influence the seasonal and geographical distribution of
energy received in a given place on our planet.
Solar Cycles
In the mid-nineteenth century, the German astronomer,
H. Schwabe, discovered spots on the Sun’s surface that
appear and disappear over an eleven-year cycle. When solar
activity is more intense, marked by a greater number of
spots, the Sun emits more energy. Since the 1980s, satellite
measurements allow the estimation of variations in intensity
of solar energy. These are around 0.1%, which corresponds
to a very small perturbation (0.24 W/m
2 ) in the radiation
balance of the Earth. Solar activity directly reflects changes
in the Sun’s magnetic field. The spots reappear in larger
numbers when the magnetic field intensifies. Solar flares
then become stronger; they eject a larger number of particles
toward outer space and thus reinforce the solar wind. These
electrically charged particles, mainly electrons and protons,
reach the Earth’s atmosphere where they cause magnetic
storms—strong disturbances in the magnetic field—as well
as magnificent auroras in the polar regions.
The influence of solar activity on climate has been
debated for many years. In the second half of the seventeenth
century, documented observations indicate an almost total
disappearance of spots for a period of several decades,
during the Little Ice Age (Fig. 1.3). At the end of the
nineteenth century, the German astronomer H. Spörer and
his English colleague W. Maunder linked these two phenomena, thus starting a controversy that persists today. The
nature of the connection between the minimum solar activity
(called the Maunder minimum) and a decrease in the
intensity of solar radiation sufficient to induce a marked
cooling that coincided with that time, still needs to be
explained.
As the direct disruption in the solar radiation balance is
too small to explain the phenomenon, it is believed that solar
activity may affect climate through circulation in the upper
atmosphere. Nevertheless, the link between variations in
solar activity and the Earth’s climate remains a subject of
research and a source of controversy given the absence of a
recognized physical mechanism. The relative role of external
forcing (solar radiation) and internal/geological forcing
(volcanism) in explaining the Little Ice Age still needs to be
assessed.
The Sun exhibits variations over longer periods. These
can be seen not only in the number of sunspots, but also in
variations in solar diameter. This varies with a periodicity of
900 days, but this oscillation is influenced by solar activity.
It is minimal when the activity is at its maximum. Like
sunspots, solar diameter measurements started in the
4
S. Joussaume and J.-C. Duplessy