Climate change in arid and tropical humid regions
687
Hemisphere in Africa (Thomas, 1997c). The problem reaches high degrees of complexity
when interpretations are made globally and do not only refer to the African continent.
Without a doubt, a long and hard job must be done, because it is necessary to obtain new
data, reinterpret the past, and to have at one's disposal the highest number of accurate
datas. It is also essential to know the Quaternary climate evolution in the Mediterranean
areas, because this represent the transition zone between Europe and hot Africa. Therefore,
a detailed exploration in arid zones is needed.
2. Tropical regions
2.1. Introduction
Tropical climates include a wide variety of precipitation regimes and vegetation
associations, from selva to savanna and the transitions to semiarid zones. On the other
hand, the activity and intensity of the geomorphological processes change depending on the
zone. The variations in time of these morphoclimatic limits produce the modification of
geomorphologic processes and, therefore, the existing landscapes. Nevertheless, according
to Douglas (1969), humid tropical cores are stable ecosystems in which the energy is used by
vegetation and do not present much water erosion in the rainforest plains.
The most significant studies on climate change in the humid tropics have been carried
out since 1950. Brtickner (1955) in Ghana, and De Ploey (1965) in Zaire, on the basis of
the analysis of superficial deposits, found clear evidence of stages of aridity in the
Pleistocene period. In South America there are ancient dunes in the Llanos (plains) of
Venezuela, in Bahia state of Brazil and in central Amazon (Tricart, 1974a, 1975, 1985),
which show arid conditions in these humid tropical areas. Bigarella and Andrade (1965)
and Bigarella et al. (1969) developed the hypothesis of peneplanation in southeastern
Brazil. The works of Aubreville (1962) and Butzer (1978) analysed the spatial variations
of the limits of the savanna and rainforest because of climate changes. During the last 20
years there has been an important increment in the number of publications on this subject,
but there are not too many long-term chronological records (Thomas, 1994), which force
us to use temporal records of the lakes in the arid zones of Africa and Central America
(Stret-Perrott et al., 1985), because other long-term records are very rare.
For the study of climate change, there are available, apart from the record of semiarid
lakes, data from ice cores in ice sheets and from deep-sea drilling cores. There is an
important limitation, however, due to the C-14 and AMS C-14 techniques because they
can only be applied back to ---60,000 years ago.
Works carried out from cores in the equatorial Atlantic have analysed the content of
freshwater diatoms, opal phytoliths, pollen, and sapropel muds for the last 150,000 years.
From these analyses, up to seven different arid phases can be identified, depending on the
methodology used (Jausen and Van Iperen, 1991).
2.2. Biological modifications
Variations in the configuration of seas and lands, atmospheric circulation, and
physiography can produce important changes in the humidity, in such a form that the
687
Hemisphere in Africa (Thomas, 1997c). The problem reaches high degrees of complexity
when interpretations are made globally and do not only refer to the African continent.
Without a doubt, a long and hard job must be done, because it is necessary to obtain new
data, reinterpret the past, and to have at one's disposal the highest number of accurate
datas. It is also essential to know the Quaternary climate evolution in the Mediterranean
areas, because this represent the transition zone between Europe and hot Africa. Therefore,
a detailed exploration in arid zones is needed.
2. Tropical regions
2.1. Introduction
Tropical climates include a wide variety of precipitation regimes and vegetation
associations, from selva to savanna and the transitions to semiarid zones. On the other
hand, the activity and intensity of the geomorphological processes change depending on the
zone. The variations in time of these morphoclimatic limits produce the modification of
geomorphologic processes and, therefore, the existing landscapes. Nevertheless, according
to Douglas (1969), humid tropical cores are stable ecosystems in which the energy is used by
vegetation and do not present much water erosion in the rainforest plains.
The most significant studies on climate change in the humid tropics have been carried
out since 1950. Brtickner (1955) in Ghana, and De Ploey (1965) in Zaire, on the basis of
the analysis of superficial deposits, found clear evidence of stages of aridity in the
Pleistocene period. In South America there are ancient dunes in the Llanos (plains) of
Venezuela, in Bahia state of Brazil and in central Amazon (Tricart, 1974a, 1975, 1985),
which show arid conditions in these humid tropical areas. Bigarella and Andrade (1965)
and Bigarella et al. (1969) developed the hypothesis of peneplanation in southeastern
Brazil. The works of Aubreville (1962) and Butzer (1978) analysed the spatial variations
of the limits of the savanna and rainforest because of climate changes. During the last 20
years there has been an important increment in the number of publications on this subject,
but there are not too many long-term chronological records (Thomas, 1994), which force
us to use temporal records of the lakes in the arid zones of Africa and Central America
(Stret-Perrott et al., 1985), because other long-term records are very rare.
For the study of climate change, there are available, apart from the record of semiarid
lakes, data from ice cores in ice sheets and from deep-sea drilling cores. There is an
important limitation, however, due to the C-14 and AMS C-14 techniques because they
can only be applied back to ---60,000 years ago.
Works carried out from cores in the equatorial Atlantic have analysed the content of
freshwater diatoms, opal phytoliths, pollen, and sapropel muds for the last 150,000 years.
From these analyses, up to seven different arid phases can be identified, depending on the
methodology used (Jausen and Van Iperen, 1991).
2.2. Biological modifications
Variations in the configuration of seas and lands, atmospheric circulation, and
physiography can produce important changes in the humidity, in such a form that the