S ECTION 2
Atmospheric Composition
The atmosphere is a gaseous mixture of several
constituents, some of which are relatively constant, while
others are quite variable. The composition of the atmosphere
is shown in Table 2.1. By volume, the air in the atmosphere is
composed almost entirely of nitrogen and oxygen (together
equaling 99%). These two constituents and many others
(argon, neon, helium, krypton, hydrogen, and nitrous oxide)
represent the vast majority of the atmosphere and are
relatively constant in their concentration. Several components
vary as a result of natural or anthropogenic processes. The
most variable constituent is water vapor, which varies
naturally as a result of the hydrologic cycle. Carbon dioxide
varies both naturally (due to photosynthesis and respiration)
and anthropogenically (due to the burning of fossil fuels). Of
particular importance to climate and the hydrologic cycle are
those gases that are “radiatively active” (i.e. water vapor,
ozone, methane, carbon dioxide, and to a lesser extent
oxygen).
A radiatively active gas is one whose molecular structure
is such that it absorbs (and emits) radiative energy (this will
be discussed in much more detail in the next chapter).
Radiatively active gases are often referred to as “greenhouse
gases”. The primary hypothesis for anthropogenic climate
change has to do with the combined facts that 1) humans
T ABLE 2.1. P RIMARY C ONSTITUENTS OF THE
S TANDARD A TMOSPHERE IN THE T ROPOSPHERE
GAS
% BY VOLUME
Nitrogen (N2)
78.084
Oxygen (O2)*
20.946
Argon (A)
0.934
Carbon Dioxide (CO2)*
0.040
Neon (Ne)
0.00182
Helium (He)
0.000524
Methane (CH4)*
0.00016
Krypton (Kr)
0.0014
Hydrogen (H2)
0.00005
Nitrous Oxide (N2O)
0.000035
Ozone (O3)*
Water (H2O)*
0 - 0.000007
0-4
*radiatively active gases
have added a significant amount of carbon dioxide to the
atmosphere and 2) it is a greenhouse gas. In fact, water vapor
and methane are more radiatively active, but carbon dioxide
gets the most attention because it is the one that is increasing
rapidly due to anthropogenic sources. The vertical profiles of
some of these constituents will be discussed below. The clear
signature of both natural and anthropogenic effects on carbon
dioxide can be seen in measurements. An observatory on
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in Figure 2.1 which is attributable directly to the
anthropogenic source from the burning of fossil fuel. As shown
in Figure 2.1, this source has led to a dramatic increase in
CO2 concentrations of approximately 25% in the last 50 years.
Moreover the trend is not linearly increasing, but accelerating
under present emission conditions. A major topic of ongoing
research is what impact does this significant increase in
carbon dioxide in the atmosphere have on the global climate,
weather patterns, and extremes.
F IGURE 2.1 Time series of measured atmospheric CO2 at
Mauna Loa Observatory in Hawaii shown in red along with a
moving average of the long-term trend shown in black (from
esrl.noaa.gov/gmd/webdata/ccgg/trends/co2_data_mlo.png).
Mauna Loa, Hawaii has provided relatively long-term records
of the carbon dioxide concentration (Figure 2.1). Two key
modes of variability are seen in the data. The first is the
natural seasonal variability, where CO2 decreases due to
increased uptake by deciduous plants in the Spring/Summer
season and increases due to respiration as a result of fallen
leaf (and other biomass) decay during the Fall/Winter. This
natural pattern should have some interannual variability, but
is expected to be relatively steady in terms of the long-term
average. The second mode of variability is the clear trend seen
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