Causes of Climate Change - Climate Resilient Agriculture for Ensuring Food Security - page 19

Table 2.1 Atmospheric concentration, lifetime, and global warming potential (GWP) of major greenhouse gases
(IPCC 2007b )

Lifetime GWP
(years) Greenhouse gas Atmospheric concentration (100 years)

Carbon dioxide 387 ppm Variable 1

Methane 1,780 ppb 12 25

Nitrous oxide 319 ppb 114 298

CFC 11 250 ppt 45 4,600

CFC 12 533 ppt 100 10,600

HCFC 22 132 ppt 11.9 1,700

HFC 23 12 ppt 260 12,000

carbon dioxide and enhanced by emissions of
other greenhouse gases.
The sunlight enters a greenhouse through the
transparent glass or plastic and heats the plants,
but the heat emitted by the plants in the form of
infrared radiations cannot pass through the glass
or plastic roof and walls of the greenhouse. As a
result, temperature inside the greenhouse rises.
The phenomenon is known as “greenhouse
effect.” In a similar manner, the earth's atmo-
sphere, which acts like the glass or plastic roof
and walls of a greenhouse, allows most of incom-
ing sunlight to pass through and heat the surface.
But the heat radiated by the heated surface cannot
pass freely into the space because of the presence
of a number of gases such as carbon dioxide,
methane, ozone, nitrous oxide, and water vapor
in the atmosphere. Consequently, the average
temperature of the earth's atmosphere is increas-
ing - a phenomenon which is commonly known
as global warming. It has been found that carbon
dioxide contributes 60 %, methane 15 %, and
nitrous oxide 5 % to the global warming (IPCC
2007b ). Also, as a heat-trapping gas, methane is
25 times and nitrous oxide is 298 times more
effective than carbon dioxide (Table 2.1 ).
The buildup of greenhouse gases in the atmo-
sphere has led to an enhancement of the natural
“greenhouse effect” (Fig. 2.1 ). It is this human-
induced enhancement of the greenhouse effect
that is of concern because ongoing emissions of
greenhouse gases have the potential to warm the
planet to levels that have never been experienced
in the history of human civilization. Such climate
change could have far-reaching and/or unpredict-

able environmental, social, and economic
consequences.
The earth would be much colder if not for the
“greenhouse” gases that provide a blanket that
warms the atmosphere. Some of the gases in the
atmosphere transmit the short-wave radiation
from the sun to the earth, warming its surface.
Some of this warmth is emitted in the form of
long-wave (infrared) radiation from the earth to
the atmosphere, and some of the gases in the
atmosphere absorb and reemit radiation of this
wavelength, effectively enhancing the warming
of the lower atmosphere. These gases are called
greenhouse gases because their effect is similar
to the function of a glass greenhouse that heats up
as infrared radiation is trapped by the glass. The
main greenhouse gases are water vapor, carbon
dioxide, methane, and nitrous oxide, all of which
occur naturally in the atmosphere. Most of these
human contributions to climate change have
occurred over the last 200-300 years, following
the agricultural and industrial revolutions.

2.3 Greenhouse Gases

The three main causes of the increase in green-
house gases observed over the past 250 years have
been fossil fuels, land use, and agriculture
(Fig. 2.2 ) (IPCC 2007a ). Six main greenhouse
gases are carbon dioxide (CO 2 ), methane (CH 4 ),
nitrous oxide (N 2 O), hydrofl uorocarbons (HFCs),
perfl uorocarbons (PFCs), and sulfur hexafl uoride
(SF 6 ). Water vapor is also considered a green-
house gas.