in the salinity of the water at high latitudes, preventing
downwelling of dense water in winter, thus stopping the
thermohaline circulation and the transport of warm water by
the Gulf Stream and North Atlantic Drift. The result was a
freezing of Europe and a disruption of the climate over
almost the entire planet. The behavior of the
ocean-atmosphere-cryosphere interactions during glacial
climate periods is complex. In addition to the Heinrich
events that occurred during the last glaciation at intervals of
eight to ten thousand years, the paleoclimate record inferred
from the isotopic analysis of Greenland ice indicates a
sudden warming (>10 °C) between Heinrich events, over
periods of less than a few centuries that end with a slower
cooling leading to a return to glacial conditions. These are
the Dansgaard-Oeschger events which occur every two to
three thousand years and which are not explained by any one
unanimously accepted theory (instability of the European ice
sheets, internal oscillation in ocean circulation, amplification
of a weak solar forcing). The climate of the ice ages appears
to have been much more variable than the climate we have
known for the past 10,000 years, but the conditions for
stability of the climate system remain a research topic that is
far from fully understood (Chap. 29, Volume 2).
The Lithosphere: Over Large Timescales
The surface of the Earth, which makes up the lithosphere,
also intervenes over long time scales of the order of a million
years or more. Plate tectonics determine the position of the
continents, the relief, the shape of the ocean basins, as well
as the level of carbon dioxide emitted by volcanoes (Chap. 2,
Volume 1 and Chap. 22, Volume 2). For example, the
glaciation of the Sahara during the Ordovician is linked with
the movement of the African plate, which had a polar
location at that time. Pioneer studies suggested that the
Antarctic ice cap was triggered 34 million years ago, when
the American and Australian continents had sufficiently
drifted from the Antarctic to allow the establishment of the
circumpolar current that isolated Antarctica from the
mid-latitudes. More recently, it has been shown that the long
trend decrease of atmospheric CO 2 was pivotal in the triggering of Antarctica ice sheet. The uplift of the Himalayas,
through its impact on the weathering and erosion of surface
rocks, has likely contributed to increased consumption of
atmospheric carbon dioxide and to the global cooling
observed since the Eocene. The uplift of Tibetan Plateau as
well as the Paratethys shrinkage amplified the Asian monsoon. The major tectonic phenomena therefore have a major
impact on global climate on a geological scale (Chap. 2
Volume 1 and Chaps. 22, 26 and 27, Volume 2).
The lithosphere also interacts with the atmosphere
through volcanism. During violent volcanic eruptions, large
amounts of dust and gas are ejected into the atmosphere, up
to several tens of kilometers in altitude. The emitted sulfur
dioxide combines with the water vapor present in the
stratosphere to form micro droplets of sulfuric acid, which is
very effective in reflecting solar rays and causes a significant
cooling of the surface of the Earth, of around 0.5–1 °C on
average. In addition, these very small droplets can remain in
the stratosphere for several years before falling into the
troposphere where they are eliminated by rain. For example,
the most intense volcanic eruption of the last two centuries,
that of the Indonesian volcano Tambora in 1815, projected
about 150 km
3 of debris and gases into the atmosphere and
was followed by two exceptionally cold years. The year
1816 was even deemed to Canada and New England to have
been ‘the year with no summer’. Recently, after the eruption
of the Philippine volcano Pinatubo in June 1991, one of the
largest of this century, the weather stations recorded a significant lowering of temperature for several months.
Although the influence of volcanic eruptions on climate is
limited mostly to a cooling for a relatively short time, rarely
exceeding a few years, geologists think that unusually
intense eruptions, called trapps, could have contributed to
large-scale disruptions in climate and mass extinctions
through their emissions. For example, it is estimated that the
fissure eruptions that continued for several hundred thousand
years to form the gigantic Deccan plateau, could have been
of sufficient magnitude severally increased the carbon
dioxide content of the air and to warm it by 3–4 °C until the
weathering of rocks consumed the excess. Further in the
past, 250 million years ago, huge fissure eruptions in Siberia
are often cited as one of the factors responsible for global
warming that marked the end of the Permian (Chap. 27,
Volume 2).
The Climate System
Atmosphere, oceans, cryosphere, biosphere, and the lithosphere, all contribute to climate changes through a complex
set of actions and reactions, both physicochemical and biological (Fig. 1.10). The atmosphere and the ocean, the two
fluids that transport the excess energy received in the tropics
to the poles, are central in climate dynamics, and this is true
for all periods of time ranging from a few years to millennia.
Strongly coupled with the mechanical effect of wind on the
surface of the sea and through the exchange of heat and
water, together they can cause fluctuations in natural climate
phenomena, the scale of which can be glimpsed in El Niño
for example. Their interactions can be strongly modified by
changes in the land surface (vegetation, snow, ice). By their
impact on the albedo and the water cycle, these changes
modulate the exchange of heat and water between the two
hemispheres, and between the oceans and land. Finally, past
20
S. Joussaume and J.-C. Duplessy
downwelling of dense water in winter, thus stopping the
thermohaline circulation and the transport of warm water by
the Gulf Stream and North Atlantic Drift. The result was a
freezing of Europe and a disruption of the climate over
almost the entire planet. The behavior of the
ocean-atmosphere-cryosphere interactions during glacial
climate periods is complex. In addition to the Heinrich
events that occurred during the last glaciation at intervals of
eight to ten thousand years, the paleoclimate record inferred
from the isotopic analysis of Greenland ice indicates a
sudden warming (>10 °C) between Heinrich events, over
periods of less than a few centuries that end with a slower
cooling leading to a return to glacial conditions. These are
the Dansgaard-Oeschger events which occur every two to
three thousand years and which are not explained by any one
unanimously accepted theory (instability of the European ice
sheets, internal oscillation in ocean circulation, amplification
of a weak solar forcing). The climate of the ice ages appears
to have been much more variable than the climate we have
known for the past 10,000 years, but the conditions for
stability of the climate system remain a research topic that is
far from fully understood (Chap. 29, Volume 2).
The Lithosphere: Over Large Timescales
The surface of the Earth, which makes up the lithosphere,
also intervenes over long time scales of the order of a million
years or more. Plate tectonics determine the position of the
continents, the relief, the shape of the ocean basins, as well
as the level of carbon dioxide emitted by volcanoes (Chap. 2,
Volume 1 and Chap. 22, Volume 2). For example, the
glaciation of the Sahara during the Ordovician is linked with
the movement of the African plate, which had a polar
location at that time. Pioneer studies suggested that the
Antarctic ice cap was triggered 34 million years ago, when
the American and Australian continents had sufficiently
drifted from the Antarctic to allow the establishment of the
circumpolar current that isolated Antarctica from the
mid-latitudes. More recently, it has been shown that the long
trend decrease of atmospheric CO 2 was pivotal in the triggering of Antarctica ice sheet. The uplift of the Himalayas,
through its impact on the weathering and erosion of surface
rocks, has likely contributed to increased consumption of
atmospheric carbon dioxide and to the global cooling
observed since the Eocene. The uplift of Tibetan Plateau as
well as the Paratethys shrinkage amplified the Asian monsoon. The major tectonic phenomena therefore have a major
impact on global climate on a geological scale (Chap. 2
Volume 1 and Chaps. 22, 26 and 27, Volume 2).
The lithosphere also interacts with the atmosphere
through volcanism. During violent volcanic eruptions, large
amounts of dust and gas are ejected into the atmosphere, up
to several tens of kilometers in altitude. The emitted sulfur
dioxide combines with the water vapor present in the
stratosphere to form micro droplets of sulfuric acid, which is
very effective in reflecting solar rays and causes a significant
cooling of the surface of the Earth, of around 0.5–1 °C on
average. In addition, these very small droplets can remain in
the stratosphere for several years before falling into the
troposphere where they are eliminated by rain. For example,
the most intense volcanic eruption of the last two centuries,
that of the Indonesian volcano Tambora in 1815, projected
about 150 km
3 of debris and gases into the atmosphere and
was followed by two exceptionally cold years. The year
1816 was even deemed to Canada and New England to have
been ‘the year with no summer’. Recently, after the eruption
of the Philippine volcano Pinatubo in June 1991, one of the
largest of this century, the weather stations recorded a significant lowering of temperature for several months.
Although the influence of volcanic eruptions on climate is
limited mostly to a cooling for a relatively short time, rarely
exceeding a few years, geologists think that unusually
intense eruptions, called trapps, could have contributed to
large-scale disruptions in climate and mass extinctions
through their emissions. For example, it is estimated that the
fissure eruptions that continued for several hundred thousand
years to form the gigantic Deccan plateau, could have been
of sufficient magnitude severally increased the carbon
dioxide content of the air and to warm it by 3–4 °C until the
weathering of rocks consumed the excess. Further in the
past, 250 million years ago, huge fissure eruptions in Siberia
are often cited as one of the factors responsible for global
warming that marked the end of the Permian (Chap. 27,
Volume 2).
The Climate System
Atmosphere, oceans, cryosphere, biosphere, and the lithosphere, all contribute to climate changes through a complex
set of actions and reactions, both physicochemical and biological (Fig. 1.10). The atmosphere and the ocean, the two
fluids that transport the excess energy received in the tropics
to the poles, are central in climate dynamics, and this is true
for all periods of time ranging from a few years to millennia.
Strongly coupled with the mechanical effect of wind on the
surface of the sea and through the exchange of heat and
water, together they can cause fluctuations in natural climate
phenomena, the scale of which can be glimpsed in El Niño
for example. Their interactions can be strongly modified by
changes in the land surface (vegetation, snow, ice). By their
impact on the albedo and the water cycle, these changes
modulate the exchange of heat and water between the two
hemispheres, and between the oceans and land. Finally, past
20
S. Joussaume and J.-C. Duplessy