Greenhouse gases are
gases in an atmosphere that absorb and emit radiation
within the thermal infrared range. This process is the
fundamental cause of the greenhouse effect. The primary
greenhouse gases in the Earth's atmosphere are water
vapor, carbon dioxide, methane, nitrous oxide, and
ozone. In our solar system, the atmospheres of Venus,
Mars and Titan also contain gases that cause greenhouse
effects. Greenhouse gases greatly affect the temperature
of the Earth; without them, Earth's surface would be on
average about 33 °C (59 °F) colder than at present.
Since the beginning of the Industrial revolution,
the burning of fossil fuels has substantially increased
the levels of carbon dioxide in the atmosphere.
In order, Earth's most abundant greenhouse gases are:
water vapor carbon dioxide atmospheric
methane nitrous oxide ozone
chlorofluorocarbons
The contribution to the
greenhouse effect by a gas is affected by both the
characteristics of the gas and it's abundance. For
example, on a molecule-for-molecule basis methane is
about eighty times stronger greenhouse gas than carbon
dioxide, but it is present in much smaller
concentrations so that its total contribution is
smaller. When these gases are ranked by their
contribution to the greenhouse effect, the most
important are:
In addition to the main
greenhouse gases listed above, other greenhouse gases
include sulfur hexafluoride, hydrofluorocarbons and
perfluorocarbons. Some greenhouse gases are not often
listed. For example, nitrogen trifluoride has a high
global warming potential (GWP) but is only present in
very small quantities.
Ice cores provide evidence
for variation in greenhouse gas concentrations over the
past 800,000 years. Both CO2 and CH4 vary between
glacial and interglacial phases, and concentrations of
these gases correlate strongly with temperature. Direct
data does not exist for periods earlier than those
represented in the ice core record, a record which
indicates CO2 levels staying within a range of between
180ppm and 280ppm throughout the last 800,000 years,
until the increase of the last 250 years.
Since
about 1750 human activity has increased the
concentration of carbon dioxide and other greenhouse
gases. Measured atmospheric concentrations of carbon
dioxide are currently 100 ppmv higher than
pre-industrial levels. Natural sources of carbon dioxide
are more than 20 times greater than sources due to human
activity, but over periods longer than a few years
natural sources are closely balanced by natural sinks,
mainly photosynthesis of carbon compounds by plants and
marine plankton. As a result of this balance, the
atmospheric concentration of carbon dioxide remained
between 260 and 280 parts per million for the 10,000
years between the end of the last glacial maximum and
the start of the industrial era.
It is likely
that anthropogenic warming, such as that due to elevated
greenhouse gas levels, has had a discernible influence
on many physical and biological systems. Warming is
projected to affect various issues such as freshwater
resources, industry, food and health.
The main
sources of greenhouse gases due to human activity are:
1. Burning of fossil fuels and deforestation leading
to higher carbon dioxide concentrations. Land use change
(mainly deforestation in the tropics) account for up to
one third of total anthropogenic CO2 emissions. 2.
Livestock enteric fermentation and manure management,
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
sources of atmospheric methane. 3. Use of
chlorofluorocarbons (CFCs) in refrigeration systems, and
use of CFCs and halons in fire suppression systems and
manufacturing processes. agricultural activities,
including the use of fertilizers, that lead to higher
nitrous oxide (N2O) concentrations.
Water vapor
accounts for the largest percentage of the greenhouse
effect, between 36% and 66% for clear sky conditions and
between 66% and 85% when including clouds. Water vapor
concentrations fluctuate regionally, but human activity
does not significantly affect water vapor concentrations
except at local scales, such as near irrigated fields.
Information
obtained from the Wikipedia web site. Wikipedia the free
encyclopedia.
