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Lesson
3 : Climate Change
Sources
of
Global Climate Changes
Climatic changes defined as significant changes in the mean
values of atmospheric variables or invariability occur much less
often. Climatic changes can be either natural (as it occurred in
the Ice Age) or human-made (e.g. from burning fuels which
produce CO2 (Obasi, 1989).
People now have the capability to influence climate
locally, regionally and globally. Man-made effects on urban
climate and on local air pollution are well known but those
local effects usually have relatively little impact on food
production. Regional effects, such as transboundary acid rain,
may have serious impacts on forests and other vegetation. By far
the most important potential effects on food production caused
by mankind are the global climate changes that may result from
increasing CO2 and other greenhouse gasses.
Greenhouse Gases
Trace gases such as water vapor, carbon dioxide, methane,
chloroflourocarbon’s, tropospheric ozone, nitrous oxide create a
greenhouse effect by trapping heat near the earth’s surface. The
concentration of many of these gases are increasing in the
atmosphere (Figures 1-2). Because of these increases, the gases
are expected to trap more energy on the earth’s surface and in
the lower atmosphere, in turn causing increases in temperature
or global warming (Silver and DeFries, 1990; Rosenberg, 1989).
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1
Light Energy Absorbed
and Converted to Heat
Energy
Heat Trapped
Light
Energy
Carbon Dioxide and
Other Greenhouse Gases
Absorption by Carbon
Dioxide and Other
Greenhouse Gases
2
Incoming Light
Energy
Atmospheric
Warming
Outgoing
Injured Energy
EARTH
FIGURES 1-2 The greenhouse effect. A car or greenhouse under the sun gets
hot because light energy entering through the glass is absorbed and
converted to heat, infrared radiation which cannot escape through the
glass. The trapped, heat
makes the temperature rise. Similarly, the
earth’s atmosphere is heated by light coming through, and outgoing
infrared radiation is absorbed by carbon dioxide and certain other gases
causing warming. The greater is the concentration of carbon dioxide or
other greenhouse gases, the greater is the warming or greenhouse effect.
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Atmospheric concentration of CO2 alone has increased by
about 25 percent since coal, oil, and gas became the primary
sources of energy to fuel the industrial revolution (Figure 3).
Carbon dioxide concentration is currently increasing by about
0.4 percent each year (Kemp, 1990; Silver and DeFries, 1990;
Rosenberg, 1989; Hansen, 1989). CO2 is also released as forests
are cleared and organic matter is burned or allowed to decay.
CO2 Concentration (ppm)
355
350
345
340
335
330
325
320
315
310
58
60
62
64
66
70
72
74
76
78
80
82
84
86
88
90
YEAR
Figure 3: Concentration of atmospheric carbon dioxide in parts per
million of dry air (ppm) versus time for the years 1958 to 1959 at
Mauna Loa Observatory, Hawaii. The dots indicate monthly average
concentration. (Reprinted, by permission, from C.D. Keeling et al.
1989. “A Three Dimensional Model of Atmospheric CO 2 Transport Based on
Observed Winds: Observational Data and Preliminary Analysis,” Appendix
A, in Aspects of Climate Variability in the Pacific and the Western
Americas, Geophysical Monograph, vol. 55, Nov. Copyright  1989 by the
American Geophysical Union.)
Methane (CH4) also known as natural gas is produced through
bacterial activity in bogs and rice paddies and in the digestive
tracts of ruminative animals and insects such as termites. Most
atmospheric methane comes from biological sources. It is present
today at roughly 1.7 parts per million and is increasing at a
rate of about 1.1 percent each year (Silver and DeFries, 1990;
Blake, 1989).
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Chloroflourocarbons (CFCs) are a group of synthetic
compounds used in refrigeration, insulation, foams and other
industrial purposes. Apart from their role as greenhouse gases,
CFCs which rise to the upper atmosphere, or stratosphere,
release free chlorine which catalyzes the breakdown of ozone,
the protective layer that shields the earth from ultraviolet
radiation. The two most common CFCs or CFC 12 or Freon 12 which
per molecule has 20,000 times the capacity of CO2 to trap heat
and CFC 11 or Freon which has 17,500 times the capacity. Both of
these compounds are long-lived and they increase in the
atmosphere at a rate of about 5 percent per year (Silver and
DeFries, 1990; Blake, 1989).
Nitrous oxide (N20) is produced naturally, through microbial
action in the soil and in response to the spread of agriculture,
burning of timber, decay of crop residue, and combustion of
fossil fuels. Agricultural use of mineral fertilizers containing
nitrogen presumably accelerates its the rate of release.
Atmospheric concentration of N20 increases by about 0.25% per
year (Silver and DeFries, 1990; Blake, 1989).
Tropospheric ozone (03) is produced through the reactions
involving hydrocarbons and nitrogen oxides, all released through
the combustion of fossil fuels used for motor vehicles in and
industry (Fig. 4). Concentration of 03 appears to be increasing
at many location nowadays (Silver and DeFries, 1990).
Table 9 summarizes the main characteristics of greenhouse
gases (GHG). Also shown are the projected concentration of the
various GHG in 2030, its potential influence on changes in
climate and surface temperature (Bach, 1989).
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PRIMARY POLLUTANTS
Particulates
Refuse Incinerators
Industrial Stacks
Secondary Pollutants
Ozone, PANs
Hydrocarbons Carbon Monoxide
Acids Sulphuric Nitric
Nitrogen Oxide
Sulphur Dioxide
Motor Vehicles
Coal – burning
Power Plants
Figure 4: (a) The prime sources of the major air pollutants are depicted in this
illustration. (b) Vehicles are the prime source of hydrocarbons and nitrogen
oxides because in each cylinder of a gasoline engine a fuel-air mixture is
(1) taken in, (2) compressed, (3) ignited, and (4) exhausted about 25 times
each second during normal operation. The high-pressure combustion causes the
production of nitrogen oxides, moreover, combustion is often incomplete
causing emissions of hydrocarbons and carbon monoxide.
Acid Rain
Many other gases from time to time become constituents of
the atmosphere. These include sulfur dioxide, oxides of
nitrogen, hydrogen sulfide and carbon monoxide, and a variety of
more exotic hydrocarbons, which even in small quantities can be
harmful to the environment. Increasing industrial activity and
the continued reliance on fossil fuels as energy sources, has
cause a gradual, but steady, growth in proportion in sulfur and
nitrogen oxides in the atmosphere over the past 2 to 3 decades.
These gases, in combination with atmospheric water of water
vapor, are the main ingredients of acid rain (Fig. 5),
considered by many to be the major environmental problem facing
the Northern Hemisphere at the present time (Kemp, 1990;
Cicerone, 1989).