Solar radiation interacts with the surface of the earth in
several ways. Some portion of this energy is reflected back into space by
the earth's atmosphere, another portion is dispersed and scattered by the
molecules in the atmosphere and a large portion penetrates through the
earth's atmosphere to reach the surface of the earth. The radiation
reaching the earth's surface is largely absorbed resulting in surface
warming (Figure 1).
Figure 1. The greenhouse effect.
Much of this absorbed energy is eventually re-radiated in longer
infrared wavelengths. As it leaves the earth, it once again interacts with
the atmosphere. Some of this re-radiated energy escapes to space, but much
of this re-radiated energy is reflected back to the earth's surface by
molecules in the earth's atmosphere. This phenomenon is similar to the
warming that occurs in an automobile parked outside on a sunny day
(Figure 2).
Figure 2. A parked car in the sun creates a localized greenhouse
effect.
The molecules responsible for this phenomenon are called greenhouse
gases, i.e. water (H2O), nitrous oxide (N2O),
methane (CH4), and carbon dioxide (CO2) because they
act like the glass in a greenhouse, trapping re-radiated energy. Without
these gases most life on earth would not be possible, as the surface
temperature of the earth would likely be about 60°F colder.
In essence, greenhouse gases act like an insulator or blanket above the
earth, keeping the heat in. Increasing the concentration of these gases in
the atmosphere increases the atmosphere's ability to block the escape of
infrared radiation. In other words, the earth's insulator gets thicker.
Therefore too great a concentration of greenhouse gases can have dramatic
effects on climate and significant repercussions upon the world around us.
Climates suitable for human existence do not exist simply above some
minimum threshold level of greenhouse gas concentration, rather they exist
within a finite window - a limited range of greenhouse gas concentrations
that makes life as we know it possible.
