The Earth's Climate
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Greenhouse gases
The
most abundant natural greenhouse gases are water vapor, which accounts
for half of the natural greenhouse effect, and carbon dioxide, which
is responsible for another quarter even though it represents only 0.03%
of the molecules that make up the Earth's atmosphere. Other gases, including
methane, ozone and nitrogen protoxide, also contribute to the natural
greenhouse effect, but to a lesser extent.
Human activity results in the release of large quantities of greenhouse
gases into the atmosphere. These gases of anthropogenic origin are in all
likelihood responsible for the climatic trends observed since 1975. They
include:
- Carbon dioxide (CO2), which mostly comes from the combustion of fossil
fuels (petroleum, coal, natural gas) used for transportation and heating,
and secondarily from deforestation;
- Methane (CH4), which is abundant in wet zones, either natural or man-made
(like ricefields), and is also produced by the digestion of ruminants,
landfills, and losses during the extraction, transport and use of natural
gas;
- Nitrogen protoxide (N2O), which is used in fertilizers;
- Halocarbons, including the now-infamous CFCs, which in most cases do
not exist in nature. These gases were used for many years in the industrial
production of substances like cooling and propellant gases.
The amount of time a gas spends in the atmosphere is called
its "retention time." Water vapor only stays for a few days, but most of the other greenhouse gases remain for much
longer periods, ranging from a decade in the case of methane to thousands of years
for certain halocarbons.
