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25
begin shortly before the end of this century. At the same time, the
effects of elevated CO2 levels on atmospheric temperatures will have
increased to new high levels. Figure 1 shows the combined effects of
these warming trends.
Therefore, temperatures could reach their highest level in several hundred
years shortly after the year 2000, and they will reach their highest level in the
last 125,000 years by mid-century [1:7-11].
Figure 1. Combined Effect of the 180-Year Cycle and Increased CO2
Concentrations. Source: Harold W. Bernard. The Greenhouse Effect
(Cambridge: Ballinger, 1980), 10.
III. MECHANISMS OF THE GREENHOUSE EFFECT
For the mean global temperature to stay constant, the earth-atmosphere system
must be in radiative equilibrium with the sun. In other words, the incoming
solar radiation must match the outgoing thermal radiation from the earth. Of
the incoming solar radiation, 35% is reflected back into space. The reflectivity
of the earth is its albedo. The albedo is taken into consideration when the total
energy flux of the earth-atmosphere system is calculated. Of the remaining
65% of solar radiation that is not reflected back, 47% is absorbed by the
surface and 18% is absorbed by the atmosphere. For the temperature of our
system to remain constant, this energy that is absorbed by the atmosphere
must be radiated back out. This radiation primarily takes place in the 5-micron
to 30-micron range of wave lengths, which is in the infrared portion of the
electromagnetic spectrum. A micron is one millionth of a meter [2:755].
Natural Greenhouse Effect
The effective radiating temperature is the temperature the earth should have
for the amount of solar radiation it absorbs. Calculation of the effective
radiating temperature gives a value of -200° C. However, the observed mean
global temperature is 140° C. The difference of 340° C is caused by a natural
greenhouse effect that takes place in the atmosphere [11] . As the earth tries to
lose heat into space, the atmosphere absorbs infrared radiation emitted by the
surface. Specifically, the atmosphere allows 50% of the incoming solar
radiation to reach the surface but only 10% of the longwave radiation from the
surface to escape. This causes the temperature of the earth-atmosphere system
to increase. The magnitude of the greenhouse effect is defined as the
difference between the upward infrared radiation from the surface and the
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