Greenhouse gas
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Greenhouse gases (GHG) are gaseous components of the atmosphere that contribute to the greenhouse effect. 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 between 9-26%; and ozone, which causes between 3-7% (note that it is not really possible to assert that such-and-such a gas causes a certain percentage of the GHE, because the influences of the various gases are not additive. The higher ends of the ranges quoted are for the gas alone; the lower end, for the gas counting overlaps). [1] [2].
Minor greenhouse gases include, but are not limited to: methane, nitrous oxide, sulfur hexafluoride, and chlorofluorocarbons - see complete IPCC List of Greenhouse Gases.
The major atmospheric constituents (N2 and O2) are not greenhouse gases, because homonuclear diatomic molecules (eg N2, O2, H2 ...) do not absorb in the infrared as there is no net change in the dipole moment of these molecules.
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Anthropogenic greenhouse gases
Human activity contributes to the greenhouse effect primarily by releasing carbon dioxide, but other gases, e.g. methane, are not negligible [3].
The concentrations of several greenhouse gases have increased over time [4] due to human activities, such as:
- burning of fossil fuels and deforestation leading to higher carbon dioxide concentrations,
- cattle and paddy rice farming, land use and wetland changes, pipeline losses, and landfill emissions leading to higher methane concentrations,
- the use of CFCs in refrigeration systems. The use of CFCs and other halons in fire suppression systems and various manufacturing processes.
According to the global warming hypothesis, greenhouse gases from industry and agriculture are partly or wholly to blame for recent 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.
The role of water vapor
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Water vapor is a natural greenhouse gas which, of all greenhouse gases, accounts for the largest percentage of the greenhouse effect. Water vapor levels fluctuate regionally, but in general humans do not produce a direct forcing of water vapor levels. In climate models an increase in atmospheric temperature caused by the greenhouse effect due to anthropogenic gases will in turn lead to an increase in the water vapor content of the troposphere, with approximately constant relative humidity. This in turn leads to an increase in the greenhouse effect and thus a further increase in temperature, and thus an increase in water vapor, until equilibrium is reached. Thus water vapor acts as a positive feedback (but not a runaway feedback) to the forcing provided by human-released greenhouse gases such as CO2 ([5], see B7). Water vapor is a definite part of the greenhouse gas equation even though not under direct human control: IPCC TAR chapter lead author (Michael Mann) considers citing "the role of water vapor as a greenhouse gas" to be "extremely misleading" as water vapor can not be controlled by humans [6]; see also [7].
The IPCC discuss the water vapor feedback [8].
Note that is it not really possible to assert that such-and-such a gas causes a certain percentage of the GHE, because the influences of the various gases are not additive. The 1990 IPCC report says "If H2O were the only GHG present, then the GHE of a clear-sky midlatitude atmosphere... would be about 60-70% of the value with all gases included; by contrast, if CO2 alone was present, the corresponding value would be about 25%".
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 280µL/L. From the same ice cores it appears that CO2 concentrations have stayed between 260 and 280µL/L during the entire preceding 10,000 years. Some studiesIndustrial Revolution, the concentrations of many of the greenhouse gases have increased. Most carbon dioxide was released after 1945. Those with the largest radiative forcing are:
| Gas | Current (1998) Amount by volume | Increase over pre-industrial (1750) | Percentage increase | Radiative forcing (W/m2) |
|---|---|---|---|---|
| Carbon dioxide | 365 ppm | 87 ppm | 31% | 1.46
|
| Methane | 1,745 ppb | 1,045 ppb | 150% | 0.48
|
| Nitrous oxide | 314 ppb | 44 ppb | 16% | 0.15
|
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| Gas | Current (1998) Amount by volume | Radiative forcing (W/m2) |
|---|---|---|
| CFC-11 | 268 ppt | 0.07
|
| CFC-12 | 533 ppt | 0.17
|
| CFC-113 | 84 ppt | 0.03
|
| Tetrachloromethane | 102 ppt | 0.01
|
| HCFC-22 | 69 ppt | 0.03
|
(Source: IPCC radiative forcing report 1994 updated (to 1998) by IPCC TAR table 6.1 [12][13]).
Duration of stay and global warming potential
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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 (condensation and precipitation 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 partly eliminated by reaction with the hydroxyl radical, OH·, which is naturally present in the atmosphere, to produce CO2 and water vapor (this effect due to the production of CO2 is not included in the methane GWP).
- 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 form 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 cosmic rays break molecular bonds of species in the upper atmosphere. Some halocarbons are dissociated in this way which releases Cl· and F· as free radicals with disastrous effects on ozone (halocarbons are generally too stable to disappear by chemical reaction in the atmosphere).
The lifetime of an individual molecule of gas in the atmosphere is frequently much shorter than the lifetime of a concentration anomaly of that gas. Thus, because of large (balanced) natural fluxes to and from the biosphere and ocean surface layer, an individual CO2 molecule may last only a few years in the air, on average; however, the calculated lifetime of an increase in atmospheric CO2 level is hundreds of years.
Aside from water vapor near the surface, which has a residence time of 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.
Duration of stay and warming capability of the different greenhouse gases can be compared:
- CO2 duration stay is variable (approximately 200-450 years) and its global warming potential (GWP) is defined as 1.
- Methane duration stay is 12 +/- 3 years and a GWP 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
- Tetrafluoromethane has a duration stay of 50,000 years and a GWP of 6500
- Sulfur hexafluoride has a duration stay of 3,200 years and a GWP of 23900.
Related effects
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Carbon monoxide has an indirect radiative forcing effect by elevating concentrations of methane and tropospheric ozone through chemical reactions with other atmospheric constituents (e.g., the hydroxyl radical, OH) that would otherwise destroy them. Carbon monoxide is created when carbon-containing fuels are burned incompletely. Through natural processes in the atmosphere, it is eventually oxidized to carbon dioxide. Carbon monoxide concentrations are both short-lived in the atmosphere and spatially variable.
See also
- environmental agreements
- Global Climate Coalition
- global warming
- global warming controversy
- greenhouse effect
- carbon sink
- Kyoto Protocol
References
- ^ Friederike Wagner, Bent Aaby and Henk Visscher (2002). "Rapid atmospheric CO2 changes associated with the 8,200-years-B.P. cooling event". PNAS 99 (19): 12011-12014. DOI:10.1073/pnas.182420699
- ^ Andreas Indermühle, Bernhard Stauffer, Thomas F. Stocker (1999). "Early Holocene Atmospheric CO2 Concentrations". Science 286 (5446): 1815. DOI:10.1126/science.286.5446.1815a "Early Holocene Atmospheric CO2 Concentrations." Science. Accessed on May 26, 2005.
- ^ H.J. Smith, M Wahlen and D. Mastroianni (1997). "The CO2 concentration of air trapped in GISP2 ice from the Last Glacial Maximum-Holocene transition". Geophysical Research Letters 24 (1): 1-4.