66
H.-J. Bolle
acceleration of the hydrological cycle by in the average 15% which is equivalent to
an average increase of precipitation of 0.42 mm/day (Randall et aI., 1992). In the
coupled atmosphere-ocean model ECHAM of the Max-Planck-Institut in Hamburg
all components of the hydrological cycle are increased by about 5% if the CO 2 -
concentration is increased from 400 ppm to 1200 ppm within 100 years (IPCC
scenario "A": rate of increase 1.3% as present). The already effective enhancement
of the hydrological cycle manifests itself in the increase of the global recycling rate
of water from 2.95 to 3.03 within the short period from 1988 to 1994 (Chahine,
Hoskins, and Fetzer, 1997).
Over a warmer sea the air could take up more water vapour and if it is
transported by the land-sea circulation systems over land, condensates and rains out,
the land would stay cooler due to either enhanced cloudiness and consequently less
net radiation flux or due to enhanced evaporation. One presupposition of the
hypothesis that the hydrological cycle may be enhanced would be that the
Mediterranean receives enough solar radiation to warm up the sea which is only the
case if cloudiness remains low. Metaxas et aI. (1991) investigated the variability of
the sea surface temperature in the western, central and eastern Mediterranean during
the last 120 years and showed that it reached a minimum in 1909 (l9.18
c
C) and
1980 (l9.3'C) with maxima 1940-1950 and 1965 (around 19.78'C). Since 1980 an
steady increase was observed which did not reach the maximum of 1965 by the end
of the study, 1990.
During the last few years in autumn the precipitation intensity seems to increase
after hot and dry summers due to both thunderstorms and frontal activities. This
resulted in disastrous floods in northern Italy, southern France, Spain and Algeria.
Though the synoptic scale disturbances are an integral part ofthe general circulation
they may have intensified by taking up, like local thunderstorms, additional latent
heat from the Mediterranean Sea. One hypothesis for the generation of enhanced
storm activities in autumn could be that the still warm sea leads to higher
evaporation rates into the quicker cooling air which is labilized. The land surfaces
are cooling not only because it is a later stage of the year but also because, with the
first rains, the vegetation recovers and increases evaporation.
Worrying is that a combination of man's activities and an enhanced climate
variability and trend may foster the process of desertification. This term does not
imply a quick conversion from agricultural land into deserts but stands for the whole
complex of possible land degradation. The increasing tendency of irrigation,
overgrazing and land-surface conversion as well as the in parallel developing
urbanization, tourism, and industrialization specifically in coastal areas will result
in a continuation of the overexploitation of water resources. Wildfires will further
denude large areas in which primarily low garrique-like vegetation will recover.
This type of vegetation as well as dry agricultural crops generally will not have the
cooling potential as forests and will reduce storage of water in the soil. Therefore
summer temperatures may regionally increase and latent heat fluxes will only be
strong for short times after rains by which most of the precipitated water returns to
the atmosphere. During prolonged periods the flux of sensible heat will nearly equal
the absorbed radiative energy and enhance dry convection over these areas. This
H.-J. Bolle
acceleration of the hydrological cycle by in the average 15% which is equivalent to
an average increase of precipitation of 0.42 mm/day (Randall et aI., 1992). In the
coupled atmosphere-ocean model ECHAM of the Max-Planck-Institut in Hamburg
all components of the hydrological cycle are increased by about 5% if the CO 2 -
concentration is increased from 400 ppm to 1200 ppm within 100 years (IPCC
scenario "A": rate of increase 1.3% as present). The already effective enhancement
of the hydrological cycle manifests itself in the increase of the global recycling rate
of water from 2.95 to 3.03 within the short period from 1988 to 1994 (Chahine,
Hoskins, and Fetzer, 1997).
Over a warmer sea the air could take up more water vapour and if it is
transported by the land-sea circulation systems over land, condensates and rains out,
the land would stay cooler due to either enhanced cloudiness and consequently less
net radiation flux or due to enhanced evaporation. One presupposition of the
hypothesis that the hydrological cycle may be enhanced would be that the
Mediterranean receives enough solar radiation to warm up the sea which is only the
case if cloudiness remains low. Metaxas et aI. (1991) investigated the variability of
the sea surface temperature in the western, central and eastern Mediterranean during
the last 120 years and showed that it reached a minimum in 1909 (l9.18
c
C) and
1980 (l9.3'C) with maxima 1940-1950 and 1965 (around 19.78'C). Since 1980 an
steady increase was observed which did not reach the maximum of 1965 by the end
of the study, 1990.
During the last few years in autumn the precipitation intensity seems to increase
after hot and dry summers due to both thunderstorms and frontal activities. This
resulted in disastrous floods in northern Italy, southern France, Spain and Algeria.
Though the synoptic scale disturbances are an integral part ofthe general circulation
they may have intensified by taking up, like local thunderstorms, additional latent
heat from the Mediterranean Sea. One hypothesis for the generation of enhanced
storm activities in autumn could be that the still warm sea leads to higher
evaporation rates into the quicker cooling air which is labilized. The land surfaces
are cooling not only because it is a later stage of the year but also because, with the
first rains, the vegetation recovers and increases evaporation.
Worrying is that a combination of man's activities and an enhanced climate
variability and trend may foster the process of desertification. This term does not
imply a quick conversion from agricultural land into deserts but stands for the whole
complex of possible land degradation. The increasing tendency of irrigation,
overgrazing and land-surface conversion as well as the in parallel developing
urbanization, tourism, and industrialization specifically in coastal areas will result
in a continuation of the overexploitation of water resources. Wildfires will further
denude large areas in which primarily low garrique-like vegetation will recover.
This type of vegetation as well as dry agricultural crops generally will not have the
cooling potential as forests and will reduce storage of water in the soil. Therefore
summer temperatures may regionally increase and latent heat fluxes will only be
strong for short times after rains by which most of the precipitated water returns to
the atmosphere. During prolonged periods the flux of sensible heat will nearly equal
the absorbed radiative energy and enhance dry convection over these areas. This