Environmental Engineering Reference
In-Depth Information
means that the earth should largely be a frozen ball of ice, not the green-
blue planet we see in satellite images from outer space.
The factor that we have neglected to consider in our calculation is the
earth's atmosphere. Or more directly, we have neglected to consider the ef-
fect of certain gases—water vapor (H
2
0), ozone (O
3
), methane (CH
4
), and
carbon dioxide (CO
2
)—in the earth's atmosphere. These gases together
make the atmosphere opaque.This opacity does two things. First, it prevents
a good amount of (but not all) the solar radiation from striking the planet
by reflecting that radiation back to outer space, which in turn reduces the
amount of potentially harmful radiation like ultraviolet rays from striking
the earth.This is why we worry about holes in the ozone layer of the at-
mosphere.
Nevertheless, some radiation has wavelengths small enough to pass be-
tween the gas molecules and reach the earth's surface.As the earth's surface
heats up, it becomes much warmer relative to the surrounding air and so
reradiates heat energy back toward outer space.The energy that is emitted
is now in the form of a longer-wave radiation called infrared radiation that
has difficulty penetrating through the layer created by the atmospheric gases.
This energy would be completely lost were it not for the absorbing capac-
ity of the gases.The energy absorbed by the gases is then reradiated back
to the earth resulting in a moderation of the earth's climate.We have all
experienced evidence of this moderating effect during the course of our
daily lives. For example, summer nights with clear skies often require one
to don a sweater because the temperatures are much colder than summer
nights with opaque (cloudy) skies, even though daytime temperatures could
have been identical.
We apply this basic principle of physics whenever we warn society not
to leave their pets in cars with closed windows on hot summer days or when
we construct greenhouses for growing plants. Consequently, the energy
absorbing and moderating effect of
these gases on the planet is meta-
phorically called the
greenhouse effect
.
The gases that contribute to this ef-
fect are accordingly called
greenhouse
gases
. The greenhouse effect caused
by the presence of these gases is what
leads to an estimated steady state
temperature of 15° C for the planet.
Consequently, the energy absorb-
ing and moderating effect of these
gases on the planet is metaphori-
cally called the
greenhouse effect
.
The gases that contribute to this ef-
fect are accordingly called
green-
house gases
.
