Environmental change
603
be relevant, namely changes in the oceanic circulation, volcanic activity, and in the gas
content of the atmosphere. On the other hand, the term "climate variability" is used for
those changes of less than 100 years, which are due basically to internal mechanisms
(Goodess et al., 1992).
In most cases, in the landscape there is a collection of landforms resulting from the past
activity of the geomorphic processes, which acted under a wide spectrum of environmental
conditions that were different from the present ones (Figure 23.4). As indicated above,
these processes, coming from tectonic, anthropic, and climatic activity can interact.
Therefore, for the study of the climate change, it is necessary to discern the contribution
of each one and try to eliminate the "noise" attached to the activity of the first two.
Undoubtedly, human action during the last millennium has been very significant in certain
areas, as in the Mediterranean countries (Goudie, 1981b, 1990a; Mannion, 1997), and
presents very complex problems for determining the real influence of climate change on
the genesis of the current landscape. Therefore, the best conditions for researching climate
change are those with a history of long crustal stability and sparse habitation. In this way,
those areas are a reflection of landform generation due exclusively to geomorphological
processes triggered by climate changes.
It is necessary to know the age of the forms related to climate change. Early researchers
used relative dating in relation to stratigraphic series, morph genetic sequences, degree of
weathering, and soil development (Birkeland, 1984). Recently, above all during the last 20
years, many dating methods have been developed, providing direct or indirect absolute
ages for buried or exposed particles. Examples of the first group include radiocarbon,
thermoluminescence, uranium series, cosmogenic isotopes, amino acid racemization, IR
luminescence, and electron spin resonance. The indirect techniques are mainly based on
paleomagnetism, and the isotopes of O and C. They are used depending on the material
available, and their precision varies according to the technique utilized. Some of these
methods are under experimental development, but in the future they promise to revolutionize our knowledge and comprehension of environmental changes (Stokes, 1997).
2. Some considerations on climate change
As indicated by Chorley et al. (1984), climate change is one of the subjects about which
many geomorphologists feel rather ignorant. According to these authors, this is due to the
following; in the course of Earth's history continuous climate changes have occurred,
change mechanisms act in complex ways, climate changes are apparent under different
patterns, and, finally, the lack of knowledge of the most significant aspects of the climate
producing erosive and sedimentary processes hinders understanding.
Another important fact comes from the knowledge of the required time for the
adaptation of landscape forms to new climate conditions. Some authors point out that
some landforms can remain during long periods of the history of the Earth. Twidale (1978)
indicated that the well-known Ayers Rock (Uluru) inselberg in the Australian desert
has remained from upper Mesozoic-lower Cenozoic to present. On the other hand, areas
with rills and active dune fields experience constant and rapid evolution. This persistence
or modification of landforms is a function, among other factors, of their spatial location
within the desert morphoclimatic region. Thus, landforms located in hyperarid
603
be relevant, namely changes in the oceanic circulation, volcanic activity, and in the gas
content of the atmosphere. On the other hand, the term "climate variability" is used for
those changes of less than 100 years, which are due basically to internal mechanisms
(Goodess et al., 1992).
In most cases, in the landscape there is a collection of landforms resulting from the past
activity of the geomorphic processes, which acted under a wide spectrum of environmental
conditions that were different from the present ones (Figure 23.4). As indicated above,
these processes, coming from tectonic, anthropic, and climatic activity can interact.
Therefore, for the study of the climate change, it is necessary to discern the contribution
of each one and try to eliminate the "noise" attached to the activity of the first two.
Undoubtedly, human action during the last millennium has been very significant in certain
areas, as in the Mediterranean countries (Goudie, 1981b, 1990a; Mannion, 1997), and
presents very complex problems for determining the real influence of climate change on
the genesis of the current landscape. Therefore, the best conditions for researching climate
change are those with a history of long crustal stability and sparse habitation. In this way,
those areas are a reflection of landform generation due exclusively to geomorphological
processes triggered by climate changes.
It is necessary to know the age of the forms related to climate change. Early researchers
used relative dating in relation to stratigraphic series, morph genetic sequences, degree of
weathering, and soil development (Birkeland, 1984). Recently, above all during the last 20
years, many dating methods have been developed, providing direct or indirect absolute
ages for buried or exposed particles. Examples of the first group include radiocarbon,
thermoluminescence, uranium series, cosmogenic isotopes, amino acid racemization, IR
luminescence, and electron spin resonance. The indirect techniques are mainly based on
paleomagnetism, and the isotopes of O and C. They are used depending on the material
available, and their precision varies according to the technique utilized. Some of these
methods are under experimental development, but in the future they promise to revolutionize our knowledge and comprehension of environmental changes (Stokes, 1997).
2. Some considerations on climate change
As indicated by Chorley et al. (1984), climate change is one of the subjects about which
many geomorphologists feel rather ignorant. According to these authors, this is due to the
following; in the course of Earth's history continuous climate changes have occurred,
change mechanisms act in complex ways, climate changes are apparent under different
patterns, and, finally, the lack of knowledge of the most significant aspects of the climate
producing erosive and sedimentary processes hinders understanding.
Another important fact comes from the knowledge of the required time for the
adaptation of landscape forms to new climate conditions. Some authors point out that
some landforms can remain during long periods of the history of the Earth. Twidale (1978)
indicated that the well-known Ayers Rock (Uluru) inselberg in the Australian desert
has remained from upper Mesozoic-lower Cenozoic to present. On the other hand, areas
with rills and active dune fields experience constant and rapid evolution. This persistence
or modification of landforms is a function, among other factors, of their spatial location
within the desert morphoclimatic region. Thus, landforms located in hyperarid