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Greenhouse Gases
(GHGs)
Greenhouse gases (GHGs) are gaseous components of the atmosphere
that contribute to the "greenhouse effect". Although uncertainty
exists about exactly how earth's climate responds to these gases,
global temperatures are rising. Some greenhouse gases occur
naturally in the atmosphere, while others result from human
activities. Naturally occuring greenhouse gases include water
vapour, carbon dioxide, methane, nitrous oxide, and ozone. Certain
human activities, however, add to the levels of most of these
naturally occurring gases.
The "Greenhouse
Effect"
When sunlight reaches the Earth's surface, some is absorbed and
warms the earth. Because the earth is much cooler than the sun, it
radiates energy at much longer wavelengths than the sun. Some of
these longer wavelengths are absorbed by greenhouse gases in the
atmosphere before they are lost to space. The absorption of this
longwave radiant energy warms the atmosphere (the atmosphere also is
warmed by transfer of sensible and latent heat from the surface).
Greenhouse gases also emit longwave radiation both upward to space
and downward to the surface. The downward part of this longwave
radiation emitted by the atmosphere is the "greenhouse effect." The
term is in fact a misnomer, as this process is not the primary
mechanism that warms greenhouses.
The major natural greenhouse gases are water vapor, which causes
about 36-70% of the greenhouse effect on Earth (not including
clouds); carbon dioxide, which causes 9-26%; methane, which causes
4-9%, and ozone, which causes 3-7%. Note that it is not really
possible to assert that a certain gas causes a certain percentage of
the greenhouse effect, because the influences of the various gases
are not additive.
Other greenhouse gases include, but are not limited to, nitrous
oxide, sulfur hexafluoride, hydrofluorocarbons, perfluorocarbons and
chlorofluorocarbons.
The concentrations of several greenhouse gases have increased
over time. Human activity raises levels of greenhouse gases
primarily by releasing carbon dioxide, but human influences on other
gases, e.g. methane, are not negligible. Some of the main sources of
greenhouse gases due to human activity include:
* burning of fossil fuels and
deforestation leading to higher carbon dioxide
concentrations;
* livestock and paddy rice farming, land use and wetland
changes, pipeline losses, and covered vented landfill emissions
leading to higher methane atmospheric concentrations. Many of the
newer style fully vented septic systems that enhance and target the
fermentation process also are major sources of atmospheric
methane;
* use of CFCs in refrigeration systems, and use of CFCs and
halons in fire suppression systems and manufacturing processes.
According to the global warming trend, greenhouse gases from
industry and agriculture have played a major role in the recently
observed global warming. Carbon dioxide, methane, nitrous oxide and
three groups of fluorinated gases are the subject of the Kyoto
Protocol, which entered into force in 2005. Methane, nitrous oxide
and ozone-depleting gases are also taken into account in the
international agreements, but not ozone. Note that ozone depletion
has only a minor role in greenhouse warming, though the two
processes often are confused in the popular media.
Increase of greenhouse
gases
Based on measurements from Antarctic ice cores, it is widely
accepted that just before industrial emissions began, atmospheric
CO2 levels were about 280 parts per million by volume. From the same
ice cores it appears that CO2 concentrations have stayed between 260
and 280 parts per million during the preceding 10,000 years.
Since the beginning of the Industrial Revolution, the
concentrations of many of the greenhouse gases have increased. Most
of the increase in carbon dioxide occurred after 1945. Those with
the largest radiative forcing are:
Gas |
Current (1998) Amount by
volume
|
Increase over pre-industrial
(1750)
|
Percentage
increase
|
CarbonDioxide |
365 ppm |
87 ppm |
31% |
Methane |
1,745 ppb |
1045 ppb |
150% |
Nitrous Oxide |
314 ppb |
44 ppb |
16% |
|