Environmental Engineering Reference
In-Depth Information
ENDOGENETIC ENERGY
In addition to the exogenetic energy, a minute proportion of the total energy comes from
within Earth and is thus
endogenetic
(Greek
endon
, 'within', +
genos
).
The most obvious source of endogenetic energy is the hot interior of Earth. The outer
core of the globe consists of molten materials at immense pressures, and at temperatures
up to 2600° C. There is an almost immeasurably small and continuous conduction of this
heat to the ground surface that adds to the energy inputs acting on the landscape. It can be
detected in deep mines and caves. Locally the decay of radioactive minerals can provide
energy to the surface. More dramatic leakages of this endogenetic energy are seen in the
form of volcanoes, hot springs and various other tectonic activities. Taken together, all
sources of endogenetic energy appear to contribute no more than 0·0001 per cent of the
total energy supply averaged over Earth's entire surface.
ENERGY OUTPUTS OF THE GLOBE
THE NATURE OF EARTH'S ENERGY OUTPUT
The output of energy from Earth is in radiant form, but it is not identical to the input of
radiant energy from the sun. Earth has modified the input by a variety of processes. Some
of the original solar energy input is reflected by clouds or the ground surface and returned
to space with little change in its radiative properties; it is still short-wave radiation. As
insolation passes through the atmosphere it is scattered, much of it towards Earth, but a
small proportion goes back to space as an output of short-wave energy.
Of greatest importance is the emission of radiant energy from Earth itself. As a result
of the absorption of solar energy in the atmosphere and at the surface, Earth will have
gained energy that will be converted into heat. In turn, Earth and its atmosphere emit
radiation following Wien's law. The average temperature of Earth is about 290 K, while
that of the atmosphere is a chilly 250 K. Consequently the energy emission will reach a
maximum at a wavelength of 2898/290 K or 2898/250 K, which is 9·99 µm or 11·59 µm;
and overall emission is entirely within the infra-red range (Figure 2.3).
In this form the energy is susceptible to absorption by the atmosphere, so very little
escapes directly to space; most is repeatedly absorbed and emitted before it is able to
leave the system. The ability of the globe to modify energy flows in this way helps to
keep the temperature of Earth and its atmosphere higher than it would otherwise be. In
other words, it promotes energy storage within the system.
At a global scale these processes lead to energy outputs of which about 36 per cent are
in the short wavelengths derived from reflected insolation, and about 64 per cent in the
long wavelengths, largely from emission by the atmosphere. Taken together, the
difference between the incoming radiation and the outgoing radiation is Earth's
net
radiation
.
