Figure 15.18: The Earth’s heat budget, showing the amount of energy coming into and going
out of the Earth system and the importance of the greenhouse effect. The numbers are the
amount of energy that is found in one square meter of that location. ( 11 )
of the atmosphere is a pleasant 15oC (59oF). Without insulation, daytime temperatures
wouldbeveryhighandnighttimetemperatureswouldbeextremelylow. Thisisthesituation
onalloftheplanetsandmoonsthathavenoatmosphere. IftheEarthdidnothaveinsulation,
temperatures would likely be too cold and too variable for complex life forms.
TherearemanyimportantgreenhousegasesintheatmosphereincludingCO 2 ,H 2 O,methane,
O 3 , nitrous oxides (NO and NO 2 ), and chlorofluorocarbons (CFCs). All of these gases are
a normal part of the atmosphere except CFCs, which are human-made. However, human
activity has significantly raised the levels of many of these gases; for example, methane lev-
els are about 2 1/2 times higher as a result of human activity. Table15.2shows how each
greenhouse gas naturally enters the atmosphere.
Different greenhouse gases have different abilities to trap heat. For example, one methane
molecule can trap 23 times as much heat as one CO 2 molecule. One CFC-12 molecule (a
type of CFC) can trap 10,600 times as much heat as one CO 2. Still, CO 2 is a very important
greenhouse gas because it is much more abundant in the atmosphere than the others.
Table 15.2:Greenhouse Gas Where It Comes From
Carbon dioxide Respiration, volcanic eruptions, decomposi-
tion of plant material; burning of fossil fuels