effect. It is not clear if global warming will be accompanied
by more low-level clouds with a moderating effect, or
high-level clouds with an amplifying effect. The answer is
complicated further by the fact that it depends on changes in
the general circulation in the atmosphere.
Through coalescence, micro-droplets of water grow and
turn into rain, snow or ice, depending on the temperature.
How precipitations are distributed reflects the main features
of the general circulation of the atmosphere: ascendance and
heavy rains at the equator, subsidence associated with a lack
of rainfall in the sub-tropics, rainfall in the mid-latitudes
linked to the passage of depressions. On average, excess in
the evaporation rate over the oceans is offset by a surplus of
rain over land. This transfer of water from oceans to land is
particularly apparent during the seasonal phenomenon of the
‘monsoon’, well known in South East Asia but also in
Africa. In summer, when the land warms up, low-pressure
heat expands and causes a convergence of ocean winds
towards the mainland. Charged with humidity, the winds rise
and discharge a large amount of rain over land.
Reconstructing Changes in Precipitation
There are many signs to be found on the surface of our
planet of how precipitation has changed over a given area.
The accumulation of ice in the polar ice caps is directly
dependent on the supply of snow. Glaciologists have shown
that snowfall was half as abundant during glacial periods as
it is today. At lower latitudes, fluctuations in rainfall are
recorded in lake sediments. Deposits left above the current
water level are evidence of phases of intense rain, particularly in the beginning of the interglacial periods. The levels
of the lakes drop when rains abate. For example, an aerial
view of Lake Chad shows tracks of the various shorelines
that record the regression of the lake over past millennia.
About 6000–8000 years ago, it occupied an area of
340,000 km
2 (equivalent to more than 2/3 of the area of
France). By 2000, it was reduced to 1500 km
2 , or less than
1% of its maximum size.
Lakes are not the only records of rainfall fluctuations on
land. In limestone terrain, variations in the growth of concretions in caves are another indicator of fluctuations in the
supply of groundwater by rain. Dating using geochronological methods (see Chap. 14) can detect slowdowns or
arrested growth during dry periods, followed by recovery
when the groundwater supply resumes in wet periods.
Monsoons are a prime example of intense rainfall
affecting large areas, both in Africa and Asia. Rain falls
during the summer months when the overheated land masses
are the source of low pressures towards which the
humidity-charged oceanic air masses converge. The intensity
of the monsoons has fluctuated considerably during the
Quaternary. This has resulted in enormous variations in the
volume of water flowing in the major rivers which are fed by
the rains, such as the Niger and the Nile in Africa, or the
great rivers that drain the Himalayas and discharge into the
Bay of Bengal. Variations in freshwater inputs to the ocean
have been so significant that they have resulted in large
fluctuations in salinity in areas close to the mouths of these
rivers. Paleoceanographers are able to reconstruct these
changes through isotopic and micropaleontological analysis
of marine sediments deposited near the river mouth
(Fig. 1.6, Chap. 21, Volume 1).
Land surfaces contain many other traces of major changes
in the hydrological cycle in the past. During periods of great
aridity, dry winds have facilitated the creation of dunes which
became established when the rains returned. The vegetation
growing in different regions is as much determined by air
temperature as it is by precipitation and water availability.
This is what causes the variation in the thickness and density
of the annual rings of trees. Pollens found in lake sediments,
peat bogs and in marine sediments close to the coast are used
by geologists to reconstruct the major vegetation types that
developed throughout the various geological periods allowing them to infer the temperature and humidity conditions
that then prevailed (Chap. 12, Volume 1).
Modes of Variability of the Atmosphere
Atmospheric circulation is very variable. Over short timescales, variability is dominated by the duration of depressions, usually a few days. Over longer time scales, the
circulation shows variability patterns over periods of up to
several years. In Europe, the variability is dominated by
fluctuations in the system caused by low-pressure from
Iceland and high pressure from the Azores. This dipole
oscillates between a ‘positive’ phase which is marked by a
strengthening of the low and high pressures, stronger westerly winds bringing rain, and high temperatures in northern
Europe, and the ‘negative’ phase where pressures and
westerly winds subside, moving the rainy zone to the south
of Europe (Fig. 1.7). The ‘North Atlantic Oscillation’ (often
designated by the acronym NAO) occurs at all time scales
and explains about a third of the variability in weather
conditions in Western Europe, especially in winter. Positive
and negative phases tend to predominate for ten years or
more, which makes the NAO particularly interesting in the
study of the climate of Europe. This mode seems to be
caused by the atmosphere alone, and yet it has an influence
over ocean circulation. The mechanisms that allow atmospheric circulation to present an oscillation over such a long
period are still not fully understood.
Did the North Atlantic Oscillation exist in the past?
Measurements of atmospheric pressure, in particular ones
1 The Climate System: Its Functioning and History
11
by more low-level clouds with a moderating effect, or
high-level clouds with an amplifying effect. The answer is
complicated further by the fact that it depends on changes in
the general circulation in the atmosphere.
Through coalescence, micro-droplets of water grow and
turn into rain, snow or ice, depending on the temperature.
How precipitations are distributed reflects the main features
of the general circulation of the atmosphere: ascendance and
heavy rains at the equator, subsidence associated with a lack
of rainfall in the sub-tropics, rainfall in the mid-latitudes
linked to the passage of depressions. On average, excess in
the evaporation rate over the oceans is offset by a surplus of
rain over land. This transfer of water from oceans to land is
particularly apparent during the seasonal phenomenon of the
‘monsoon’, well known in South East Asia but also in
Africa. In summer, when the land warms up, low-pressure
heat expands and causes a convergence of ocean winds
towards the mainland. Charged with humidity, the winds rise
and discharge a large amount of rain over land.
Reconstructing Changes in Precipitation
There are many signs to be found on the surface of our
planet of how precipitation has changed over a given area.
The accumulation of ice in the polar ice caps is directly
dependent on the supply of snow. Glaciologists have shown
that snowfall was half as abundant during glacial periods as
it is today. At lower latitudes, fluctuations in rainfall are
recorded in lake sediments. Deposits left above the current
water level are evidence of phases of intense rain, particularly in the beginning of the interglacial periods. The levels
of the lakes drop when rains abate. For example, an aerial
view of Lake Chad shows tracks of the various shorelines
that record the regression of the lake over past millennia.
About 6000–8000 years ago, it occupied an area of
340,000 km
2 (equivalent to more than 2/3 of the area of
France). By 2000, it was reduced to 1500 km
2 , or less than
1% of its maximum size.
Lakes are not the only records of rainfall fluctuations on
land. In limestone terrain, variations in the growth of concretions in caves are another indicator of fluctuations in the
supply of groundwater by rain. Dating using geochronological methods (see Chap. 14) can detect slowdowns or
arrested growth during dry periods, followed by recovery
when the groundwater supply resumes in wet periods.
Monsoons are a prime example of intense rainfall
affecting large areas, both in Africa and Asia. Rain falls
during the summer months when the overheated land masses
are the source of low pressures towards which the
humidity-charged oceanic air masses converge. The intensity
of the monsoons has fluctuated considerably during the
Quaternary. This has resulted in enormous variations in the
volume of water flowing in the major rivers which are fed by
the rains, such as the Niger and the Nile in Africa, or the
great rivers that drain the Himalayas and discharge into the
Bay of Bengal. Variations in freshwater inputs to the ocean
have been so significant that they have resulted in large
fluctuations in salinity in areas close to the mouths of these
rivers. Paleoceanographers are able to reconstruct these
changes through isotopic and micropaleontological analysis
of marine sediments deposited near the river mouth
(Fig. 1.6, Chap. 21, Volume 1).
Land surfaces contain many other traces of major changes
in the hydrological cycle in the past. During periods of great
aridity, dry winds have facilitated the creation of dunes which
became established when the rains returned. The vegetation
growing in different regions is as much determined by air
temperature as it is by precipitation and water availability.
This is what causes the variation in the thickness and density
of the annual rings of trees. Pollens found in lake sediments,
peat bogs and in marine sediments close to the coast are used
by geologists to reconstruct the major vegetation types that
developed throughout the various geological periods allowing them to infer the temperature and humidity conditions
that then prevailed (Chap. 12, Volume 1).
Modes of Variability of the Atmosphere
Atmospheric circulation is very variable. Over short timescales, variability is dominated by the duration of depressions, usually a few days. Over longer time scales, the
circulation shows variability patterns over periods of up to
several years. In Europe, the variability is dominated by
fluctuations in the system caused by low-pressure from
Iceland and high pressure from the Azores. This dipole
oscillates between a ‘positive’ phase which is marked by a
strengthening of the low and high pressures, stronger westerly winds bringing rain, and high temperatures in northern
Europe, and the ‘negative’ phase where pressures and
westerly winds subside, moving the rainy zone to the south
of Europe (Fig. 1.7). The ‘North Atlantic Oscillation’ (often
designated by the acronym NAO) occurs at all time scales
and explains about a third of the variability in weather
conditions in Western Europe, especially in winter. Positive
and negative phases tend to predominate for ten years or
more, which makes the NAO particularly interesting in the
study of the climate of Europe. This mode seems to be
caused by the atmosphere alone, and yet it has an influence
over ocean circulation. The mechanisms that allow atmospheric circulation to present an oscillation over such a long
period are still not fully understood.
Did the North Atlantic Oscillation exist in the past?
Measurements of atmospheric pressure, in particular ones
1 The Climate System: Its Functioning and History
11