Botkin Environmental Science Earth as Living Planet 8th txtbk

20.7 The Major Greenhouse Gases 443
atmosphere, resulting in high internal fluxes. For example,
in terms of the figure, the amount of IR absorbed at Earth’s
surface from the greenhouse effect is approximately 88
units, which is about twice the amount of shortwave solar
radiation (45 units) absorbed by Earth’s surface. Despite
the large internal fluxes, the overall energy balance remains
the same. At the top of the atmosphere, the net downward
solar radiation (70 units, 45 units 25 units) balances the
outgoing IR from the top of the atmosphere (70 units).
The important point here is recognizing the strength
of the greenhouse effect. For example, notice in the figure
that of the 104 units of IR emitted by the surface of Earth,
only 4 go directly to the upper atmosphere and are emitted.
The rest is reabsorbed and reemitted by greenhouse
gases. Of these, 88 units are directed downward to Earth
and 66 units upward to the upper atmosphere.
All this may sound somewhat complicated, but if
you read and study the points mentioned in Figures
20.15 to 20.17 carefully and work through the balances
of the various parts of the energy fluxes, you will gain
a deeper understanding of why the greenhouse effect is
so important. The greenhouse effect keeps Earth’s lower
atmosphere approximately 33°C warmer than it would
otherwise be and performs other important service functions
as well. For example, without the strong downward
emission of IR from the greenhouse effect, the land surface
would cool much faster at night and warm much
more quickly during the day. In sum, the greenhouse
effect helps to limit temperature swings from day to
night and maintain relatively comfortable surface temperatures.
It is, then, not the greenhouse effect itself but
the changes in greenhouse gases that are a concern.
20.7 The Major Greenhouse
Gases
The major anthropogenic greenhouse gases are listed in
Table 20.1. The table also lists the recent rate of increase
for each gas and its relative contribution to the anthropogenic
greenhouse effect.
Carbon Dioxide
Current estimates suggest that approximately 200 billion
metric tons of carbon in the form of carbon dioxide (CO 2 )
enter and leave Earth’s atmosphere each year as a result of
a number of biological and physical processes: 50 to 60%
of the anthropogenic greenhouse effect is attributed to this
gas. Measurements of carbon dioxide trapped in air bubbles
in the Antarctic ice sheet suggest that 160,000 years before
the Industrial Revolution the atmospheric concentration
of carbon dioxide varied from approximately 200 to 300
ppm. 12 The highest level or concentration of carbon dioxide
in the atmosphere, other than today’s, occurred during
the major interglacial period about 125,000 years ago.
About 140 years ago, just before the major use of fossil
fuels began as part of the Industrial Revolution, the atmospheric
concentration of carbon dioxide was approximately
280 ppm. 13 Since then, and especially in the past few
decades, the concentration of CO 2 in the atmosphere has
grown rapidly. Today, the CO 2 concentration is about 392
ppm, and at its current rate of increase of about 0.5% per
year, the level may rise to approximately 450 ppm by the year
2050—more than 1.5 times the preindustrial level. 13
Table 20.1
MAJOR GREENHOUSE GASES
TRACE GASES RELATIVE CONTRIBUTION (%) GROWTH RATE (%/YR)
CFC 15 a -25 b 5
CH 4 12 a -20 b 0.4 c
O 3 (troposphere) 8 d 0.5
N 2 O 5 d 0.2
Total 40–50
Contribution of CO 2 50–60 0.3 e –0.5 d,f
a W. A. Nierenberg, “Atmospheric CO 2 : Causes, Effects, and Options,” Chemical Engineering Progress 85, no.8 (August 1989): 27
b
J. Hansen, A. Lacis, and M. Prather, “Greenhouse Effect of Chlorofluorocarbons and Other Trace Gases,” Journal of Geophysical
Research 94 (November 20, 1989): 16, 417.
c Over the past 200 yrs.
d H. Rodha, “A Comparison of the Contribution of Various Gases to the Greenhouse Effect,” Science 248 (1990): 1218, Table 2.
e
W. W. Kellogg, “Economic and Political Implications of Climate Change,” paper presented at Conference on Technology-based
Confidence Building: Energy and Environment, University of California, Los Alamos National Laboratory, July 9–14, 1989.
f H. Abelson, “Uncertainties about Global Warming,” Science 247 (March 30,1990): 1529.