Environmental Engineering Reference
In-Depth Information
As a result, nearly 40% of all solar energy is carried by
visible wavelengths, only about 8% by UV (
<
400 nm),
and 53% by IR frequencies (
>
700 nm). Longer UV
(UVA, 320-400 nm) is not very harmful to organisms,
but shorter wavelengths (UVB, 290-320 nm) burn skin,
and the highest frequencies are lethal to most organisms,
particularly marine phytoplankton (de Mora, Demers,
and Vernet 2000). Visible wavelengths, from 400 nm
(the deepest violet) to 700 nm (the darkest red), ener-
gize photosynthesis and are sensed by heterotrophs
(fig. 2.2). The peak sensitivity of human eyes is in green
(491-575 nm) and yellow (576-585 nm) light, and
maximum visibility is at 556 nm (3
:
58
10
19
J/
photon). Most of the heat supplied to the biosphere
comes from wavelengths shorter than 2 mm (less than
1
10
20
J/photon).
There is hardly any attenuation of the Sun's radiation
as it travels through interplanetary space, and the irradi-
ance at the top of the Earth's atmosphere is simply the
quotient of the Sun's total energy flux (3
:
89
10
26
W)
and the area of the sphere with the radius equal to the
Earth's mean orbital distance from the star (149.6 Gm),
or about 1370 W/m
2
. This rate is known as the solar
constant, and atmospheric interference makes it impossi-
ble to detect its subtle changes by ground-based moni-
toring. During the 1960s and the early 1970s, NASA
used a solar constant of 1353 W/m
2
as the design
value for its space vehicles, although unsystematic pre-
1980 measurements from high-altitude balloons, planes,
rockets, and spacecraft indicated an increase of 0.029%/
a after 1967 (Fr¨hlich 1987).
Systematic extraterrestrial studies of solar irradiance
began with the Earth Radiation Budget (ERB) radiome-
ter on Nimbus 7 in 1978, and they were followed by the
Active Cavity Radiometer Irradiance Monitor (ACRIM
I) in 1980, the Earth Radiation Budget Satellite (ERBS)
in 1984, ACRIM II in 1991, Variability of Solar Irradi-
ance and Gravity Oscillations (VIRGO) monitoring on
the Solar and Heliospheric Observatory (SOHO) in
1996, and ACRIM 3 in 2000 (Foukal 1990; de Toma
et al. 2004). A composite record of 24 years of solar irra-
diance reveals the average of 1366 W/m
2
. Short-term
dips of up to 0.2-0.3 W/m
2
correspond to the passage
of large sunspots across the Sun, and long-term undula-
tions are due to the 11-year solar cycle with peaks about
1.3 W/m
2
above the minimum of 1365.6 W/m
2
(de
Toma et al. 2004). Spectral irradiance during the 11-
year cycle varies at least 100% for wavelengths shorter
than 100 nm, but the total declines only by about 0.1%
between the cycle's peak and trough (NRC 1994). Vari-
ations in solar luminosity have not had a significant influ-
ence on global warming since the seventeenth century,
but additional climate forcing due to enhanced solar UV
output cannot be ruled out (Foukal et al. 2006).
A reconstruction of the sunspot frequency during the
past 11,400 years (based on dendrochronologically dated
radiocarbon) shows that the level of solar activity has
been exceptionally high since about 1940 but that this
episode (with more than 70 sunspots a year compared
to an average of about 30 during the past millennium) is
unlikely to continue for more than two or three decades
(Solanki et al. 2004). Satellite observations show that the
Sun's shape and temperature vary with latitude in a com-
plex way, and indicate that variations of radius and lumi-
nosity do not originate in the star's inner depths but
rather in its outer layers (Emilio et al. 2000). None
of this changes the mean value of about 1366 W/m
2
for the solar constant, the base for tracing the conver-
sions of solar radiation within the planet's interacting
spheres.
