Greenhouse gases are
gaseous components of the
atmosphere that contribute to the
greenhouse effect (see also
global warming).
The principal greenhouse gases are water vapor,
carbon dioxide,
methane,
nitrous oxide and
ozone.
Industrial greenhouse gases include the heavy halocarbons (
chlorinated fluorocarbons), CFC, HCFC-22 molecules such as
freon and
perfluoromethane[?], and
sulphur hexafluoride[?] (SF
6).
Greenhouse gases are transparent to certain wavelengths of the sun's radiant energy[?], allowing them to penetrate deep into the atmosphere or all the way into the Earth's surface. Greenhouse gases and clouds prevent some of infrared radiation from escaping, trapping the heat near the Earth's surface where it warms the lower atmosphere. Alteration of this natural barrier of atmospheric gases can raise or lower the mean global temperature of the Earth.
The concentrations of several greenhouse gases have increased over time, mostly due to human activities, such as:
- burning of fossil fuels leading to higher carbon dioxide concentrations,
- cattle farming leading to higher methane concentrations,
- the use of CFCs in refrigeration systems.
According to the global warming hypothesis, greenhouse gases from industry and agriculture are partly or wholly to blame for global warming. Carbon dioxide is the subject of the proposed Kyoto Protocol. Nitrous oxide and methane are also taken into account in the international agreements, but not ozone.
Since the beginning of the Industrial Revolution, the concentrations of many of the greenhouse gases have increased.
(Source:
IPCC radiative forcing report --
1994 -- ppvb : part per billion in volume)
The greenhouse gases, once in the atmosphere, do not remain there eternally. They can be withdrawn from the atmosphere:
- as a consequence of a physical phenomenon (rain, condensation, remove water vapor from the atmosphere)
- as a consequence of a chemical phenomenon intervening within the atmosphere. This is the case for methane, which is eliminated by reaction with radicals OH naturally present in the atmosphere, to give CO2
- as a consequence of a chemical phenomenon intervening at the border between the atmosphere and the other compartments of the planet. This is the case for CO2, which is reduced by photosynthesis of plants, and which is also dissolved in the ocean to end up giving bicarbonate and carbonate ions (CO2 is chemically stable in the atmosphere)
- as a consequence of a radiative phenomenon. For example the electromagnetic radiation emitted by the sun and the cosmic rays "break" molecules in the upper atmosphere. Part of halocarbons disappear in this way (they are generally too stable to disappear by chemical reaction in the atmosphere).
Aside from water vapour, which is evacuated in a few days, the greenhouse gases take a very long time to leave the atmosphere. It is not easy to know with precision how long is necessary, because the atmosphere is a very complex system. However, there are estimates of the duration of stay, i.e. the time which is necessary so that the gas disappears from the atmosphere, for the principal ones of them.
Duration of stay and warming capability of the different greenhouse gases can be compared:
- CO2 duration stay is variable (some say 200 years) and its global warming potential (GWP) is 1.
- Methane duration stay is of 12.2 +/- 3 years and a GWP of of 22 (meaning that it has 22 times the warming ability of carbon dioxide),
- Nitrous oxide has a duration stay of 120 years and a GWP of 310
- CFC-12 has a duration stay of 102 years and a GWP between 6200 and 7100
- HCFC-22 has a duration stay of 12.1 years and a GWP between 1300 and 1400
- Perfluoromethane has a duration stay of 50,000 years and a GWP of 6500
- Sulphur hexafluoride has a duration stay of 3 200 years and a GWP of 23900.
Source : GIEC
See also: