Environmental Engineering Reference
In-Depth Information
energy radiated would be blocked and only the surface
temperature would determine the energy out
ow needed
to balance the energy from the Sun. If the system became
out of balance, the temperature would change to bring it
back. Too much heat leaving would cool the surface; too
little would allow it to heat.
Assuming the entire surface of the Earth is the same,
ignoring the difference between the day and night sides,
ignoring the cold poles compared with the rest and
assuming that nothing blocks the outgoing heat, the aver-
age temperature required to radiate enough to balance the
incoming solar energy is
C). A fancier calcu-
lation taking into account the things ignored in this
simple calculation, but continuing with the assumption
that nothing in the atmosphere blocks any of the radiated
heat, gives a number only a few degrees higher.
The average temperature of
F(
the Earth is actually
Fis
entirely caused by the greenhouse effect, which traps
part of the energy that would be radiated from the
surface in its absence. The surface temperature has to
increase so that the part of the radiated energy that can
get through will carry enough energy to keep the system
in balance. Without the greenhouse effect the Earth
would be a frozen ball of slush. With it we have, on
average, a comfortable world, capable of supporting
diverse life forms.
Over the history of the Earth, the average temperature
has varied considerably as the amount of greenhouse gases
in the atmosphere has changed and as the output of the
Sun has changed. Today, the concern about global
warming focuses on human activity that causes an increase
F(
C). The difference of about
þ
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