Conservation and Sustainable Use of the Biosphere - WBGU

The biosphere between forest and desert: a simulation F 2.2
219
Figure F 2.2-1
a) Global hydrological cycle
in 10 12 m 3 year -1
b) energy levels over land as
a proportion of solar
irradiation (100 per cent =
341.3W m -2 ) for climate
simulations with and
without vegetation control
of the processes on the land
surface: ‘Global Desert’
scenario (upper figure),
‘Global Forest’ scenario
(middle figure) and the
difference between the two
(in per cent, lower figure).
At maximum biospheric
control, evapotranspiration
is three times as high,
precipitation on land is twice
as high and temperatures
close to the surface are
reduced over land by 1.2°C.
Source: after Fraedrich et al,
1999
a)
b)
100.0
100.0
0%
421
410
-3%
38.9
36.0
-8%
26
27
+4%
Ocean
464
443
-5%
68.2
66.3
-3%
Atmosphere
Water vapour content in kg m -3
37
28
-24%
Space
71
137
+93%
16
21
+31%
Land
31
108
-248%
Atmosphere
55.6
46.6
-16%
18.9
8.6
-55%
111.1
106.9
-4%
89.3
91.6
+3%
36.7
37.9
+3%
21.9
15.3
-30%
Land
6.6
2.2
-67%
5.2
18.0
+248%
Short-wave
radiation
Long-wave
radiation
Sensible
heat flux
Latent
heat flux
desert comes about as a result of the increased ability
of the soils to store humidity and release it in drier
times. Furthermore, there is the increased radiation
balance at the surface that is available for evapotranspiration
(Milly and Dunne, 1994; Eltahir, 1998).
The differences between the two extremes can be
understood through the mechanisms described
above as a direct consequence of the change in evapotranspiration.
They are particularly striking in
regions and in times of maximum solar radiation
(summer) (Fig. F 2.2-2c).
Atmospheric circulation is influenced considerably
by increased levels in the hydrological cycle:
Increased evapotranspiration increases the Hadley
circulation in the Tropics (tropical cell in which warm
air masses rise at the Equator and lower in the Subtropics)
and leads to a general warming of the mid
troposphere in the sub-tropics (by around 5°C). The
increase in the North-South temperature gradient
associated with that phenomenon increases the
intensity of the mid-latitude West wind drift. Consequently,
the East-West asymmetry is reduced in the
quasi-permanent high and low-pressure areas. These
global changes in atmospheric circulation cause
regional and seasonal weakening of the Aleutian
Depression, warming in Eastern Asia and cooling in
Alaska (Fig. F 2.2-3). Furthermore, the influence of
the forest on the snow albedo leads to large-scale
warming in the boreal regions during the Spring that
is in part compensated by Summer cooling. Overall, a
warming is noted in certain extra-tropical regions in
the middle of the year.
Maximum influence of vegetation on the
CO 2
-induced greenhouse effect
The atmospheric greenhouse effect is weaker in the
Global Forest scenario than in the Global Desert
scenario because the layers of near surface air cool
off more and the troposphere is more stable. The
greenhouse effect can be quantified by calculating
the difference in terrestrial net radiation between the
upper and lower borders of the atmosphere (land