What Is Global Warming?
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Temperature of the Earth
The temperature near the surface of the earth is determined by the balance between solar
radiation energy and heat reflected from earth to outer space. Sunlight warms the surface
of the earth, which then cools down when the heat (infrared rays) is released from it. If
heat exchange were this simple, then the surface of the earth would cool down rapidly as
soon as solar radiation stops, with only heat reflection continuing. In reality, however,
heat-absorbing atmospheric gases absorb a certain percentage of heat (infrared rays)
reflected from the surface of the earth. The atmosphere, thus warmed, radiates infrared
rays out toward space and back toward the surface of the earth, warming the latter. In this
manner, the earth surface temperature is kept at around 15 (global average), realizing
an environment suited for the existence of humans, animals, plants and other life forms.
The natural process of heating the surface of the earth by sunlight is called the
"greenhouse effect," and the infrared-absorbing gases in the atmosphere are called
"greenhouse gases." If not for greenhouse gases, the temperature of the earth would drop
by over 30 to about -18 .
Fig. 1 Mechanism of Greenhouse Effect
Source:What Will Happen to the Earth? What Do We Do for the 21st
Century? (Environment Agency)
Human Activities and Global Warming
Since the Industrial Revolution at the end of the 18th century, there has been a marked
increase in the emission of greenhouse gases (mainly carbon dioxide), in proportion to
industrial expansion.
As well, artificial greenhouse gases that did not exist before the Industrial Revolution,
such as CFC (chlorofluorocarbon) and sulfur hexafluoride, have been produced, and are
being released into the atmosphere. Increased greenhouse gases destroy the heat
exchange balance, keeping greater amounts of heat in the atmosphere and on the earth
(i.e. intensifying the greenhouse effect) and raising the temperature near the surface of
the earth to undesirable levels. This phenomenon is called global warming.
Greenhouse Gases

Carbon dioxide
Carbon dioxide is a greenhouse gas that has been increasing remarkably, mostly as a
result of the combustion of fossil fuels such as coal, petroleum and natural gas. The
atmospheric concentration of carbon dioxide, which was about 280 ppmv (ppmv is a unit
for ratio of volume in parts per million) before the Industrial Revolution, has reached 358
ppmv (about 1.3 times) in 1994. If no other measures than those taken at present are
introduced, the figure is expected to reach 500 ppmv in 2050 and 700 ppmv in 2100, and
to continue to increase for many centuries to come.
The atmospheric concentration of carbon dioxide can be stabilized by controlling, on a
global scale, artificial carbon dioxide emissions to a level sufficiently below the present
level.
Fig. 2 Atmospheric Concentration of Carbon Dioxide and
Contribution of Greenhouse Gases to Global Warming
Atmospheric carbon dioxide
concentration
Contribution to global warming by
greenhouse gases from human activities
(1992)
Source:What Will Happen to the Earth? What Do We Do for the 21st Century?
(Environment Agency)

Methane
Methane forms naturally in swamps, lakes and marches, and is also generated as a result
of human activities such as livestock farming, paddy field cultivation and underground
disposal of waste materials. The atmospheric concentration of methane increased from
about 700 ppbv (ppbv is a unit for ratio of volume in parts per billion) before the
Industrial Revolution to some 1,720 ppbv (about 2.5 times) in 1994, as human activities
have expanded. Although low in concentration, methane accounts for about 20% of
contribution to global warming by greenhouse gases, in consideration of the 20-year total
sum of its intensity of greenhouse effect (global warming index), based on the fact that its
atmospheric residence time is 56 times longer than that of carbon dioxide.

CFC (Chlorofluorocarbon)
CFCs are artificial gases that did not exist in nature before the Industrial Revolution.
They have been in wide use as coolant gases in air conditioners and refrigerators, and as
industrial cleansing agents. In recent years, CFCs have come to be known as an ozone
layer destroyer, but they are also greenhouse gases.
CFCs known as "alternative CFCs," which deplete the ozone layer less, or not at all, are,
however, greenhouse gases. For example, the greenhouse effect intensity of an alternative
CFC called HFC 134a is about 3,400 times that of carbon dioxide in terms of 20-year
total effect, based on its atmospheric residence time. For this reason, despite their small
volume, such gases account for about 10% of contribution to global warming by all
greenhouse gases.

Nitrogen suboxide
Nitrogen suboxide is a byproduct of the combustion of organic substances and nitrogen
fertilizers. The atmospheric concentration of nitrogen suboxide, about 275 ppbv before
the Industrial Revolution, had increased to 312 ppbv by 1994 (about 1.1 times). The
intensity of its greenhouse effect (global warming index), in terms of the 100-year total
effect based on its atmospheric residence time, is 310 times that of carbon dioxide, but its
contribution to global warming is only abut 6% because of its low concentration.