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surfaces are just 0.04 for the 90
2.2 Energy Balance of the Earth: Radiation Fluxes
Although the disk with the Earth's diameter (12.74 Mm)
catches only a tiny fraction of total solar output
(4 : 5 10 10 ), the rate of this intercept, 174.26 PW,
and its annual aggregate, 5.495 YJ, are enormous: at the
beginning of the twenty-first century the global con-
sumption of all fossil fuels was just above 10 TW, or
about 0.006% of the solar irradiance. Total resources
of fossil fuels are perhaps as large as 200 ZJ, but even
this generous estimate would be no more energy than
in the solar radiation intercept by the Earth in only
about 13 days. Average insolation (radiation received
per m 2 of the planet's surface) is considerably lower:
even without any atmospheric interference it would
be only a quarter of the solar constant, or about
342 W/m 2 (a sphere's area is four times larger than that
of a circle of the same radius). And because the Earth's
clouds and surfaces reflect about 30% of the incoming
shortwave (SW) radiation, the total actually absorbed
by the atmosphere and by the ground is 240 W/m 2
(122 PW, 3.85 YJ/a) and the mean flux reaching the
surface is about 173 W/m 2 .
Albedo, a, the share of SW radiation that is reflected
and scattered back to space without any change of wave-
length, can be commonly as high as 0.8-0.9 for thick
cumulonimbus clouds and only 0.02-0.03 for wispy cir-
rus. Fresh snow is as efficient a reflector as thick clouds,
and hence the snow-covered parts of the Northern
Hemisphere greatly affect the planetary energy balance.
Satellite observations indicate average winter albedos of
0.6 in Eurasia and 0.56 in North America, with a North-
ern Hemisphere mean of 0.59 (Robinson and Kukla
1984). Albedo of older snow falls below 0.50, light
sandy deserts rate 0.3-0.4, green meadows 0.1-0.2, and
coniferous
sun angle but 0.6 for
the 3
angle (and the Moon averages just 0.07, Mars
0.16).
The Earth's albedo is thus variable on a seasonal basis,
and it changes faster on regional and local scales with
cloud cover, aerosols in the atmosphere, and the extent
of snow and ice. Its presatellite estimates ranged between
0.28-0.42. Remote sensing by satellites constrained the
range, but because the planetary albedo cannot be mea-
sured directly, theoretical models are needed to derive it
from the monitored parameters. Six standard models
used in 2005 had annual averages between 0.29-0.31,
most had the maximum interseasonal amplitude of less
than 0.02, and all of them shared the expected minimum
global albedo in September, when the Northern Hemi-
sphere's radiation-absorbing vegetation cover is at its
peak (Charlson, Valero, and Seinfeld 2005). The global
albedo of 0.3 is largely due to cloud reflection (returning
20% of the incoming SW), with back scattering and sur-
face reflection sending outward, respectively 6% and 4%
of all SW input.
Deforestation has been the single largest anthropo-
genic cause of about 0.01 albedo increase during the his-
toric era. With average irradiance at 342 W/m 2 , albedo
change of just 0.01 means a global energy balance shift
of 3.4 W/m 2 , the rate similar to that caused by the dou-
bling of current atmospheric CO 2 level. Unfortunately,
the evidence regarding the recent changes in Earth's
albedo is inconsistent. Pall ยด et al. (2004) used earthshine
(the sunlight reflected from the Earth's bright side to the
Moon and then back to an observer on the Earth's dark
side) to conclude that the albedo decreased steadily from
1984 to 2000 (by an equivalent of@10 W/m 2 ) and then
rose by 1.7% (@6 W/m 2 ) between 2000 and 2004. Sat-
ellite observations indicate a small decrease of 0.6%
forests
just 0.05-0.15. Albedos of water
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