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cover causes a net radiative loss of about 20W
m
-2
, due to the dominance of cloud albedo
reducing shortwave radiation absorption. In lower
latitudes, this effect is much larger (up to -50 to
-100W m
-2
) whereas in high latitudes the two
factors are close to balance, or the increased
infrared absorption by clouds may lead to a small
positive value. These results are important in
terms of changing concentrations of greenhouse
gases, since the net radiative forcing by cloud
cover is four times that expected from CO
2
doubling (see Chapter 13).
The incident solar radiation averaged over the
globe is
Solar constant ×π
r
2
/ 4π
r
2
where
r
= radius of the earth and 4
r
2
is the surface
area of a sphere. This figure is approxi-
mately 342W m
-2
, or 11
π
10
9
J m
-2
yr
-1
(10
9
J =
1GJ); for convenience we will regard it as 100
units. Referring to
Figure 3.21
, incoming radiation
is absorbed in the stratosphere (3 units), by
ozone mainly, and 20 units are absorbed in the
troposphere by carbon dioxide (1), water vapour
(13), dust (3) and water droplets in clouds (3).
Twenty units are reflected back to space from
clouds, which cover about 62 percent of the earth's
surface, on average. A further 9 units are similarly
reflected from the surface and 3 units are returned
by atmospheric scattering. The total reflected
radiation is the
planetary albedo
(31 percent or
0.31). The remaining 49 units reach the earth
×
D HEAT BUDGET OF THE
EARTH
We can now summarize the net effect of the
transfers of energy in the earth-atmosphere
system averaged over the globe and over an annual
period.
SHORTWAVE
LONGWAVE AND HEAT FLUXES
TOTAL ENERGY
Gain
9
Loss
31
69
Extraterrestrial
radiation
100
Planetary
albedo
31
69
Outer edge
of atmosphere
From
Shortwave
Longwave
Latent heat
Sensible heat
To
Space
Surface
Cloud emission
9
20
102
23
7
152
57
95
152
Stratospheric
absorption
Water vapor
and CO2
emission
48
Atmospheric
window
12
Absorbed
3
Tropospheric
absorption
(water vapor:
aerosols)
Multiple tropospheric
absorption and radiation
Turbulent
heat fluxes
Back radiation
from clouds
and atmosphere
Surface
28
21
114
95
23
7
Shortwave
Longwave
49
95
144
Longwave
Latent heat
Sensible heat
114
23
7
144
Lu
L
d
LE
H
49
Absorbed at surface
Figure 3.21
The balance of the atmospheric energy budget. The transfers are explained in the text. Solid lines
indicate energy gains by the atmosphere and surface in the left-hand diagram and the troposphere in the right-hand
diagram. The exchanges are referred to 100 units of incoming solar radiation at the top of the atmosphere (equal to
342W m
-2
).
Source: After Kiehl and Trenberth (1997) Bulletin of the American Meteorological Society, by permission of the American Meteorological
Society.
