Flow, deformation, and transport - Physical Processes in Earth and Environmental Sciences - page 196

Box 4.4

Box 4.4(a) Lambert-Bouguer absorption law.
d = exp(- bx )
The fraction of energy, d , transmitted through the
atmosphere depends on the path length, x , and an
absorption coefficient, b , whose value at sea level is
about 0.1 km -1 .
In 10 km of travel, only 1/ e (37%) of energy remains.

Box 4.4(b) Other relevant aspects regarding Solar radiation
1 The solar radiation “constant” has probably decreased
over geological time since Earth nucleated as a planet. This
has severe implications for estimates of geological palaeo climates.
2 Sunspots cause variations in the incoming solar energy.
3 The number of sunspots seem to vary over about an
11-year cycle. There is increasing evidence that a longer term
variability has severe effects on the global climate system
for example, the 80-year long Maunder Minimum in sunspots
coincides with the “Little Ice Age” of northern Europe.

are not fixed and though cooler than surrounding areas
the sun's irradiance is increased due to unusually high
bursts of electromagnetic activity from them, with solar
flaring generating intense geomagnetic storms. The dark
patches were well known to ancient Chinese, Korean, and
Japanese astronomers and to European telescopic
observers from the late-Medieval epoch onward: nowadays
they are termed sunspots . We owe this long historical
record to the dread with which the ancient civilizations
regarded sunspots, as omens of doom. Systematic visual
observations over a c .2 ky time period reveals distinct
waxing and waning of the area covered by sunspots.
An approximately 11-year waxing and waning sunspot
cycle is well established, with a longer multidecadal
Gleissberg cycle of about 90 years also evident. Because the
electromagnetic effects of sunspot activity reach all the way
into Earth's ionosphere, where they interfere with (reduce)
the “normal” incoming flux of cosmic rays, longer-term
proxies gained from measuring the abundance of cosmo-
genically produced nucleides (like 14 C preserved in tree-
rings) accurately push back the radiation record to 11 ka.
What emerges is a fascinating record of solar misbehavior,
culminating in the record-breaking solar activity of the last
50 or so years, which is the strongest on record, ever. This
increased irradiance is thought to contribute about one-
third to the recent global warming trend. But this estimate
is model driven: what if the models are wrong? A chilling
thought is the fact that the global “Little Ice Age” of
1645-1715 correlates exactly with the sunspot minimum
named the Maunder minimum .

incident radiation. Thus solids like ice, rocks, and sand are
opaque and the short wavelength solar radiation is either
reflected or absorbed. Water, on the other hand, is translu-
cent to solar radiation in its surface waters, although when
the angle of incidence is large in the late afternoon or early
morning, or over a season, the amount of reflected radia-
tion increases. It is the radiation that penetrates into the
shallow depths of the oceans that is responsible for the
energy made available to primary producers like algae. It is
useful to have a measure of the reflectivity of natural
surfaces to incoming shortwave solar radiation. This is the
albedo , the ratio of the reflected to incident shortwave
radiation. Snow and icefields have very high albedos,
reflecting up to 80 percent of incident rays, while the
equatorial forests have low albedos due to a multiplicity of
internal reflections and absorptions from leaf surfaces,
water vapor, and the low albedo of water. The high albedo
of snow is thought to play a very important feedback role
in the expansion of snowfields during periods of global cli-
mate deterioration.

4.19.4 Earth's reradiation and the “greenhouse”

concept

Incoming shortwave solar radiation in the visible
wavelength range has little direct effect upon Earth's
atmosphere, but heats up the surface in proportion to the
magnitude of the incoming energy flux, the surface albedo,
and the thermal properties of the surface materials. It is
the reradiated infrared radiation (Fig. 4.150) that is
responsible for the elevation of atmospheric temperatures
above those appropriate to a gray body of zero absolute
temperature. It was the savant , Fourier, who first postu-
lated this loss of what he called at the time, chaleur obscure ,
in 1827. We now know that the reradiated infrared energy

4.19.3 Reflection and absorption of radiated energy

The Sun's radiation falls upon a bewildering array of natu-
ral surfaces; each has a different behavior with respect to