Chapter 25
Climate change in arid and tropical humid regions
Rainforest, savannah, and semiarid areas, although presenting different morphogenetic
processes and being studied separately, have been affected by important climate changes
during the Cenozoic period, as is shown by the anomalous paleoforms existing in the
different environments. Thus, for example, aeolian deposits in the Congo rainforest or
laterites in the arid zones in southern Niger Republic, bear witness to these important
climate fluctuations.
The purpose of this chapter is to show the evolution of these regions during late Tertiary
and Quaternary, from the analysis of erosive and depositional landforms. Dating results
(isotopic, palaeomagnetic, thermoluminescence, and others) make it possible to elaborate
a sequence of events, adequately time-ranked, which provide the means for correlations
between different areas. From these, an approximation can be obtained of the climate
changes that occurred in the recent history of the Earth. Furthermore, it is possible to
provide data and valuable interpretations for the prediction of future climate (Petit-Maire,
1999).
1. Arid regions
The study of the landforms in arid regions that are tectonically stable and without
significant anthropic activity, reflect that many of the existing landscapes have been
generated under geomorphologic conditions that differ from the current ones. Therefore,
those landforms should be considered as paleoforms or inherited landforms, originating
under morphoclimatic conditions different from the present ones. For instance, the
extensive dune fields of the Sahelian zone, covered by grassland vegetation indicate
a higher aridity during the time of formation. Similarly, no water currently flows in the
complex network of ouads (wadis) in the central Sahara, indicating periods of higher
precipitation in the past. (Tricart, 1969, 1979). These landscapes show significant
modifications to the distribution of the arid regions over the surface of the planet.
From this evidence came the concept of the pluvial period or lacustrine phase, that can
be defined as that in which hydrological and biogeographic conditions were more humid
than present conditions. In comparison to the one before, an interpluvial or arid period
occurred under dryer conditions (Rognon, 1980). This definition has taken into account
water availability, which not only is conditioned by the precipitation, but also by
evapotranspiration. Thus, for a given precipitation, a decrease in temperature increases the
water availability of soils. Therefore, this double factor of precipitation and temperature
makes more difficult a definition of pluvial period. The alternation of pluvial and
Climate change in arid and tropical humid regions
Rainforest, savannah, and semiarid areas, although presenting different morphogenetic
processes and being studied separately, have been affected by important climate changes
during the Cenozoic period, as is shown by the anomalous paleoforms existing in the
different environments. Thus, for example, aeolian deposits in the Congo rainforest or
laterites in the arid zones in southern Niger Republic, bear witness to these important
climate fluctuations.
The purpose of this chapter is to show the evolution of these regions during late Tertiary
and Quaternary, from the analysis of erosive and depositional landforms. Dating results
(isotopic, palaeomagnetic, thermoluminescence, and others) make it possible to elaborate
a sequence of events, adequately time-ranked, which provide the means for correlations
between different areas. From these, an approximation can be obtained of the climate
changes that occurred in the recent history of the Earth. Furthermore, it is possible to
provide data and valuable interpretations for the prediction of future climate (Petit-Maire,
1999).
1. Arid regions
The study of the landforms in arid regions that are tectonically stable and without
significant anthropic activity, reflect that many of the existing landscapes have been
generated under geomorphologic conditions that differ from the current ones. Therefore,
those landforms should be considered as paleoforms or inherited landforms, originating
under morphoclimatic conditions different from the present ones. For instance, the
extensive dune fields of the Sahelian zone, covered by grassland vegetation indicate
a higher aridity during the time of formation. Similarly, no water currently flows in the
complex network of ouads (wadis) in the central Sahara, indicating periods of higher
precipitation in the past. (Tricart, 1969, 1979). These landscapes show significant
modifications to the distribution of the arid regions over the surface of the planet.
From this evidence came the concept of the pluvial period or lacustrine phase, that can
be defined as that in which hydrological and biogeographic conditions were more humid
than present conditions. In comparison to the one before, an interpluvial or arid period
occurred under dryer conditions (Rognon, 1980). This definition has taken into account
water availability, which not only is conditioned by the precipitation, but also by
evapotranspiration. Thus, for a given precipitation, a decrease in temperature increases the
water availability of soils. Therefore, this double factor of precipitation and temperature
makes more difficult a definition of pluvial period. The alternation of pluvial and