Environmental Engineering Reference
In-Depth Information
Technical Note
.
: The Science of the
Greenhouse Effect
The energy radiated by an object is called
”
radiation. The name black-body was coined in the mid
“
black-body
s to describe objects that absorb all radiation that falls
on them and re
ect nothing. This can be confusing because
the same term has come to describe two things: the black
absorber, and the heat radiation from hot bodies like the
Sun, for instance, that are anything but black.
If an object remains at a uniform temperature, the total
amount of energy radiated away depends only on its size and
its temperature. The wavelength at the peak of the radiation
distribution also depends only on the temperature. Our Sun
has a surface temperature of about
F)
kelvin (over
and a diameter of about
times that of
the Earth), and radiates a huge amount of energy. The peak
in its radiation spectrum is at a wavelength of
miles (about
micron
(a micron is one-millionth of a meter), right in the middle of
the visible spectrum.
If we pretend that our Earth has no atmosphere and hence
no greenhouse effect, the same physics determines the
energy radiated as it does for the Sun. For the Sun we know
the temperature. For the Earth we calculate the temperature
required to radiate the right amount of energy. That is what
gives the temperature of
.
kelvin)
in the absence of the greenhouse effect. The peak wave-
length of the outgoing radiation is in the far infrared at
−
ºF (
−
ºC, or about
microns.
To calculate the actual effect, we have to know the
absorption of our
atmosphere
at
all
the
relevant
wavelengths.
Figure
shows the absorption versus wave-
length for water vapor, carbon dioxide, oxygen, and ozone,
and the sum of them all [
.
]. Also included is a rough
sketchofthedistributioninwavelengthoftheincoming
and outgoing radiation.
