Energy and Earth - Fundamentals of the Physical Environment - page 32

which are its outputs. From our privileged vantage point
we could measure, with suitable equipment, the inputs to
and outputs from Earth. We could therefore draw up a
simple model of the globe as an energy system ( Figure
2.10 ) showing the inputs and outputs, but that would give
us no idea of what happens inside. It would be a picture
of the globe as a black box system. It would be the simplest
view of the system we could obtain, but it would tell us
nothing about the internal components or subsystems, or
about the relationships between them. We would see only
what enters and leaves the globe.
Let us start by looking at those energy flows we can
examine. Without doubt, the main input of energy to the
global system comes from the sun. Compared with the
solar contribution, all other inputs are negligible. Small
amounts are received through reflection from the moon,
and extraterrestrial material passes into the atmosphere
and even down to the surface as meteorites, but we can
ignore these contributions. Just as the input of energy is
dominated by radiant energy from the sun, so the output
of energy from Earth is almost entirely radiant energy,
although this time with somewhat different properties.
Much of the energy has been radiated or emitted by Earth
and its atmosphere, but some is solar radiation reflected
from clouds or from Earth's surface without any major
modification. As the overall energy level of Earth is not
changing, we can assume that there must be a balance
between the energy input to and energy output from the
globe as a whole.

Exogenetic energy
We can look at the details of solar radiation input to Earth
in a more meaningful manner. This input is termed
exogenetic because it originates outside the Earth system.
Because we know the mean distance of Earth from the
sun we can work out, from law 5 above, how much
radiation Earth should receive. This amount is the solar
constant and has a value of about 1,370 W m -2 at the top
of the atmosphere. Satellite observations have shown that
the solar constant increased about 0·1 per cent between
1996 and 2001, with a decrease from 2002 to 2006 as part
of the solar cycle evident in sunspots. A 1 per cent increase
would be adequate to cause an increase of 0·5 C in global

temperature. By measuring how much radiation reaches
the top of the atmosphere, and knowing the size of the
sun, as well as Earth's mean distance, the emission
temperature of the sun can be determined from law 3. For
the photosphere, or visible light surface of the sun, this
value works out to about 6,000K. This figure enables us
to determine at what wavelength most radiation will be
emitted from the sun from law 4, that is:

m = 2,898/6,000 = 0·48 μm

From Figure 2.2 we can see that this value is in the middle
of the visible part of the spectrum. Note that it is the
wavelength of blue light.
From the radiation laws it has been possible to
determine how much radiation Earth ought to receive, as
well as the amount and properties of solar radiation.
Similar calculations can be made for Earth when we are
considering outputs.
The input of energy to Earth at its mean distance from
the sun is only an average value, for changes are taking
place all the time. For example, Earth is rotating on its axis
once in twenty-four hours, it is orbiting the sun about
once in 365 days and, as its axis of rotation is at an angle
of about 23·5

Reflection and
emission from Earth

Solar input

to the vertical, the distribution of radiation
at the top of the atmosphere is constantly changing. Over
even longer periods of time the nature of Earth's orbit and
its angle of tilt also change, thus affecting the amount and
distribution of radiation over Earth. These, however, are
important only on a time scale of thousands of years and
will be discussed more fully in Chapter 9.
The sun also emits energy in what is called corpuscular
radiation (sometimes referred to as the solar wind ), which
is composed primarily of ionized particles and magnetic
fields. There is a connection between variations in the
strength of the solar wind and activity on the surface
of the sun. This activity is most clearly seen in the form

Earth

Figure 2.10 A black box model of Earth's energy system.