BioPHYSICAL chemistry

of year, but we will assume that this energy is constant over time. In the
absence of any atmosphere, the heat from the solar energy required to
set the Earth’s temperature at a specific temperature is based on how
much solar radiation is required to reach the Earth and what fraction is
absorbed. The presence of the atmosphere alters how much solar radi-
ation heats the Earth (Figure 2.9). Upon reaching the atmosphere, part
of the incident solar energy is reflected whereas the remainder penetrates
the atmosphere and reaches the Earth’s surface. The fraction of sunlight
that reaches the surface heats the surface while another part is reflected
back to space. While returning to space, a fraction will escape while part
is reflected back. The trapping of a certain fraction of the solar radiation
by the atmosphere, termed the greenhouse effect, results in an increase
in the average temperature.
The efficiency of energy trapping by the atmosphere depends upon the
composition of gases. The atmosphere is composed of primarily nitrogen
(78%) and oxygen (21%), with the remaining portion contributed by argon
(0.9%), carbon dioxide (0.03%), and several trace gases. The atmosphere
contains other naturally occurring components including water vapor,
which contributes about 60% to the natural greenhouse effect. There are

CHAPTER 2 FIRST LAW OF THERMODYNAMICS 41

The natural level of greenhouse gases

leads to a temperature of 15C

The addition of greenhouse gases

results in trapping of the reflected solar

radiation and consequently an increase

in temperature

Figure 2.9Global warming arises from the trapping of solar radiation by gases in the atmosphere.
After the radiation from the sun enters the atmosphere, some is reflected back into space.
Greenhouse gases limit how much of the reflected light leaves the earth.