An introduction to climate change - Climate Change: Biological and Human Aspects - page 3

Perhaps the best way to illustrate the greenhouse effect is to consider what it
would be like if the Earth had no atmosphere. This is not as difficult as it might
first seem. We only have to travel 384 400 km (238 856 miles) to the Moon and see
the conditions there. On that airless world (its atmosphere is barely above vacuum
at one trillionth [10 − 12 ] of the Earth's) the daytime temperature is 390 K (117 ◦ C),
while at night it drops to 100 K (

28 ◦ C).
During the lunar day, sunlight either is reflected off the Moon's rocky surfaces or
is absorbed, warming the rocks that then re-radiate the energy. The total amount of
incoming radiation equals that outgoing. However, at the Earth's surface the average
global temperature is higher, about 288 K (15 ◦ C). The Earth's atmosphere keeps the
planet warmer than it would otherwise be by some 43 K (43 ◦ C). This 43 K warming
is due to the Earth's atmospheric greenhouse. It is perfectly natural. This warming
effect has (albeit to a varying extent) always existed. It occurs because not all the
thermal radiation from the Sun falling on our planet's surface gets reflected back out
into space. The atmosphere traps some of it just as on the Moon it is trapped in the
rocks that are warmed. However, more is trapped on Earth because the atmosphere is
transparent to some frequencies (the higher frequencies) of thermal radiation, while
opaque to some other, lower, frequencies. Conversely, rock on the Moon is not at all
transparent so only the surface of the rock warms and not the strata deep beneath.
The reason that some of the light reflected from the Earth's surface, or radiated
as infrared radiation from the lower atmosphere, becomes trapped is because it has
changed from being of the sort to which the atmosphere as a whole is transparent to
that to which the atmosphere is opaque. There are different types of light because
photons of light can be of different energy. This energy ( E ) of electromagnetic
radiation (light, thermal radiation and other rays) is proportional to its frequency
(

173 ◦ C), giving a median of some 245 K (

− −

) or colour, with the constant of proportionality being Planck's constant ( h , which
is estimated to be 6.626

ν

10 − 34 J/s). Therefore, the atmosphere is transparent to
some frequencies of light but not others. This transparency mix allows some higher-
energy light into the blanket of atmosphere surrounding our planet, but hinders other
wavelengths, especially lower-energy infrared (heat-level), from getting out. The
exact mathematical relationship between the energy of a photon of light (or any other
electromagnetic radiation) was elucidated, long after Fourier, in 1902 by the German
physicist Max Planck. It can be expressed in the following simple equation.

×

E = h v .

E ( energy ) is measured in joules and v ( frequency ) in hertz .

When sunlight or solar radiation is either reflected off dust particles and water droplets
in the atmosphere or, alternatively, off the ground, it loses energy. As a result of the
above relationship between energy and frequency, this reflected light is now at a
lower energy, hence lower frequency. As stated, the atmosphere, although transpar-
ent to many higher frequencies, is opaque to many of the lower thermal frequen-
cies. The atmosphere traps these and so warms up. Consequently, the atmosphere
acts like a blanket trapping lower-frequency radiation (see Figure 1.1). It functions
just as the glass of a greenhouse does by allowing in higher-frequency light, but
trapping some of the lower-frequency heat; hence the term greenhouse effect. This