Climate, Climate Variability and Impacts in the Mediterranean Area: An Overview
65
0.5 mm/day throughout the year while measured amounts are twice as high. But also
the measurement of local rainfall as well as the spatial integration of these punctual
measurements can be in error by 15-30%. Climate model computations of the
Hadley Centre confirm the trend towards a dryer climate in many areas for the past
decennia and some models predict an even more dramatic decrease of (summer)
precipitation from 2040 on. The temperature trend underlying these computations,
however, is severely diminished if an increase of atmospheric sulphate aerosols
would be taken into account. It also is not very clear to what degree regional effects
of the sea are included in the models.
The question of the net water budget of the Mediterranean basin and its future
development under global warming to date cannot be answered conclusively. The
crucial question is to determine the net import of water into the area. Based upon the
work of Grennon and Batisse (1988), leftic et al. (1990), Margat (1992), and Huber
(1993), Wagner (2001) designed a map of the water influx to the Mediterranean sea.
The net water influx from the North Atlantic Ocean and the Black Sea is 1.4 ~. 10 12
m 3 /a and the input from all rivers around the Mediterranean sea sums up to 0.439
* 10 12 m 3 /a. If in a rough estimate an average of 2.5 mm/day precipitation is
assumed (see Table 1), the total annual rainfall over the sea would be of the order
of 2.3 * 10 12 m 3 /a. The total water input of the order of 4.1 10 12 mO/a (equivalent
to 4.5 mm/day) would have to evaporate to keep the sea level constant. This amount
of water would be available for precipitation over land and export from the region.
The air over the Mediterranean basin transports also water from the Atlantic Ocean.
This water partly is included in the number of precipitation over the sea but a
fraction may also be exported with the general circulation to the East. The moisture
recycling in the Mediterranean basin is geographically most variable. Trenberth
(1977) has considered the annual turnover of water at a scale of 1,000 km. Over
North Africa only 2% ofthe local precipitation comes from the domain of 1,000 km,
the rest is imported from more distant regions. For the Iberian Peninsula the number
is about 18% while in central and eastern Mediterranean regions the values are more
around 26% with "hot spots" of 34%.
To solve the question of the balance, a larger cage-type experiment, a GEWEXlike effort, would be necessary. The denser hydrological network that will include
a number of new radiosonde stations, which the World Meteorological Organization
plans to establish around the Mediterranean sea (MED-HYCOS project), will greatly
contribute to answer this question (Abrate, 1999). On the basis of this knowledge
then it would become possible to turn to the second part of the problem, how this
budget would be changed if the world is warming and what impact this would make
on its vegetational productivity.
Related to this problem is the question of the turnover of water within the
Mediterranean Basin. Global temperatures seem to increase during the last ten years
towards the highest values reached during this interglacial. If the Mediterranean
Sea, which to a certain degree is decoupled from what happens in the world ocean,
would warm at a considerable rate, then its hydrological cycle would be enhanced
and more precipitation should also be expected over land. Global climate models in
the average respond to a doubling of the CO 2 amount in the atmosphere with an
65
0.5 mm/day throughout the year while measured amounts are twice as high. But also
the measurement of local rainfall as well as the spatial integration of these punctual
measurements can be in error by 15-30%. Climate model computations of the
Hadley Centre confirm the trend towards a dryer climate in many areas for the past
decennia and some models predict an even more dramatic decrease of (summer)
precipitation from 2040 on. The temperature trend underlying these computations,
however, is severely diminished if an increase of atmospheric sulphate aerosols
would be taken into account. It also is not very clear to what degree regional effects
of the sea are included in the models.
The question of the net water budget of the Mediterranean basin and its future
development under global warming to date cannot be answered conclusively. The
crucial question is to determine the net import of water into the area. Based upon the
work of Grennon and Batisse (1988), leftic et al. (1990), Margat (1992), and Huber
(1993), Wagner (2001) designed a map of the water influx to the Mediterranean sea.
The net water influx from the North Atlantic Ocean and the Black Sea is 1.4 ~. 10 12
m 3 /a and the input from all rivers around the Mediterranean sea sums up to 0.439
* 10 12 m 3 /a. If in a rough estimate an average of 2.5 mm/day precipitation is
assumed (see Table 1), the total annual rainfall over the sea would be of the order
of 2.3 * 10 12 m 3 /a. The total water input of the order of 4.1 10 12 mO/a (equivalent
to 4.5 mm/day) would have to evaporate to keep the sea level constant. This amount
of water would be available for precipitation over land and export from the region.
The air over the Mediterranean basin transports also water from the Atlantic Ocean.
This water partly is included in the number of precipitation over the sea but a
fraction may also be exported with the general circulation to the East. The moisture
recycling in the Mediterranean basin is geographically most variable. Trenberth
(1977) has considered the annual turnover of water at a scale of 1,000 km. Over
North Africa only 2% ofthe local precipitation comes from the domain of 1,000 km,
the rest is imported from more distant regions. For the Iberian Peninsula the number
is about 18% while in central and eastern Mediterranean regions the values are more
around 26% with "hot spots" of 34%.
To solve the question of the balance, a larger cage-type experiment, a GEWEXlike effort, would be necessary. The denser hydrological network that will include
a number of new radiosonde stations, which the World Meteorological Organization
plans to establish around the Mediterranean sea (MED-HYCOS project), will greatly
contribute to answer this question (Abrate, 1999). On the basis of this knowledge
then it would become possible to turn to the second part of the problem, how this
budget would be changed if the world is warming and what impact this would make
on its vegetational productivity.
Related to this problem is the question of the turnover of water within the
Mediterranean Basin. Global temperatures seem to increase during the last ten years
towards the highest values reached during this interglacial. If the Mediterranean
Sea, which to a certain degree is decoupled from what happens in the world ocean,
would warm at a considerable rate, then its hydrological cycle would be enhanced
and more precipitation should also be expected over land. Global climate models in
the average respond to a doubling of the CO 2 amount in the atmosphere with an