68
H.-I. Bolle
became less favourable for a renewal of the natural resources at the pace of their
consumption. In addition the pressure due to the expanding Sahara increased. This
lead to devastation of large areas and an "exchange" of vegetation species due to the
import of "useful" plants. Technical constructions (aqueducts) helped to maintain
the living conditions for man and husbandry. The ecological-economical equilibrium
began to become fragile. The natural land-surface processes were disturbed which
had consequences for the regional climates. The decimation of the population due
to epidemics from time to time restored equilibrium for some time periods.
Since about 350 years climate variability can be measured directly but for most
locations comparable data sets are not much longer that 100 years. This period spans
from the "Little Ice Age", which seems to have made not much impact on the
Mediterranean area, to the present "global warming" era caused by the increase of
the radiatively active gases in the atmosphere. In fact, in the average the climate was
relatively balanced though large fluctuations occurred semi-periodically and
interannually. The analysis of these data also shows locally large spatial gradients.
An "antiphase" behaviour between the eastern and western Mediterranean and a
periodicity in the climate records with frequencies around roughly 0.05,0.1,0.3 and
0.4 a 1 has been found. These frequencies vary slightly from investigator to
investigator respectively from region to region. In the western Mediterranean the
influence of the North Atlantic Oscillation seems to be strong. Sometimes weak
correlations with the El Nino or Southern Oscillation are claimed. Generally it
seems to be the case that the "antiphase" coupling between the western and the
eastern Mediterranean is caused by external influences and is linked to the general
circulation system. In coastal areas the influence of the land-sea circulation is
superimposed to the variability caused by large scale influences and dominates it to
the effect that its periodicities are not very marked here.
In the analysed data series a trend towards higher average temperatures is
diagnosed. There are indications that this temperature increase mainly is due to
increasing minimum temperatures. Insofar the Mediterranean seems to follow the
general trend of global warming. Because of the large variability from year to year
and an underlying quasiperiodicity of about 25
years it is difficult to say
with final certainty that
this is a trend which will
continue in the future but
the probability is high and
supported by models.
These models do also
predict that the
Mediterranean summers
will become dryer. The
data also show this
tendency but in the case of
precipitation the variability
PRECIPITATION DEVIATION FROM MEAN
Vale Formoso 1931 - 1997
400 ~--~--~--~--~--~----~ I
1931
YEAR
1997
Fig. 16. Deviation of precipitation from the 1931 - 1997 mean
with linear trends of different length after do 6 and Roxo (200 I)
H.-I. Bolle
became less favourable for a renewal of the natural resources at the pace of their
consumption. In addition the pressure due to the expanding Sahara increased. This
lead to devastation of large areas and an "exchange" of vegetation species due to the
import of "useful" plants. Technical constructions (aqueducts) helped to maintain
the living conditions for man and husbandry. The ecological-economical equilibrium
began to become fragile. The natural land-surface processes were disturbed which
had consequences for the regional climates. The decimation of the population due
to epidemics from time to time restored equilibrium for some time periods.
Since about 350 years climate variability can be measured directly but for most
locations comparable data sets are not much longer that 100 years. This period spans
from the "Little Ice Age", which seems to have made not much impact on the
Mediterranean area, to the present "global warming" era caused by the increase of
the radiatively active gases in the atmosphere. In fact, in the average the climate was
relatively balanced though large fluctuations occurred semi-periodically and
interannually. The analysis of these data also shows locally large spatial gradients.
An "antiphase" behaviour between the eastern and western Mediterranean and a
periodicity in the climate records with frequencies around roughly 0.05,0.1,0.3 and
0.4 a 1 has been found. These frequencies vary slightly from investigator to
investigator respectively from region to region. In the western Mediterranean the
influence of the North Atlantic Oscillation seems to be strong. Sometimes weak
correlations with the El Nino or Southern Oscillation are claimed. Generally it
seems to be the case that the "antiphase" coupling between the western and the
eastern Mediterranean is caused by external influences and is linked to the general
circulation system. In coastal areas the influence of the land-sea circulation is
superimposed to the variability caused by large scale influences and dominates it to
the effect that its periodicities are not very marked here.
In the analysed data series a trend towards higher average temperatures is
diagnosed. There are indications that this temperature increase mainly is due to
increasing minimum temperatures. Insofar the Mediterranean seems to follow the
general trend of global warming. Because of the large variability from year to year
and an underlying quasiperiodicity of about 25
years it is difficult to say
with final certainty that
this is a trend which will
continue in the future but
the probability is high and
supported by models.
These models do also
predict that the
Mediterranean summers
will become dryer. The
data also show this
tendency but in the case of
precipitation the variability
PRECIPITATION DEVIATION FROM MEAN
Vale Formoso 1931 - 1997
400 ~--~--~--~--~--~----~ I
1931
YEAR
1997
Fig. 16. Deviation of precipitation from the 1931 - 1997 mean
with linear trends of different length after do 6 and Roxo (200 I)