Greenhouse Gas Anthropogenic Greenhouse Gases The Role Of Water

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.

1 Anthropogenic greenhouse gases

The concentrations of several greenhouse gases have increased over time [2] due to human activities, such as:

According to the global warming hypothesis, greenhouse gases from industry and agriculture are partly or wholly to blame for recent global warmingGlobal warming is an increase over time of the average temperature of Earth's atmosphere and oceans. Global warming theories attempt to account for the documented rise in average global temperatures since the late 19th century and assess the extent to whi. Carbon dioxide is the subject of the proposed Kyoto ProtocolThe Kyoto Protocol is an amendment to the United Nations Framework Convention on Climate Change (UNFCCC), an international treaty on global warming. It also reaffirms sections of the UNFCCC. Countries which ratify this protocol commit to reduce their emis. Nitrous oxide and methane are also taken into account in the international agreements, but not ozone.

2 The role of water vapour

At least one IPCC TAR chapter lead author ( Michael Mann) considers mention of the effect of water vapor upon the Earth's greenhouse effect to be misleading as water vapor can not be controlled by humans [3]. Water vapor is a natural greenhouse gas and an increase in atmospheric temperature caused by anthropogenic gases will lead to greater evaporation of water from the tropical sea surface and an increase in the water vapor content of the troposphere. Whether the main impact is through heating due to trapping of IR radiation or through harder-to-determine effects through cloud changes, it is a definite part of the greenhouse gas equation even though not under direct human control.

3 Increase of greenhouse gases

Since the beginning of the Industrial Revolution, the concentrations of many of the greenhouse gases have increased.

As of 1994, Carbon dioxide is up 30%, from 278 ppvm to 358 ppvm; ten years later, the latest measurements from Mauna Loa show it to be at 379 ppvm [4]
Methane is up from 700 ppvb to 1721 ppvb [5]
Nitrous oxide 15%, from 275 to 311 ppvb
CFC-12 from 0 to 0.503 ppvb
HCFC-22 from 0 to 0.105 ppvb
Perfluoromethane from 0 to 0.070 ppvb
Sulphur hexafluoride from 0 to 0.032 ppvb

(Source: IPCC radiative forcing report -- 1994 -- ppvm : parts per million in volume)

4 Duration of stay and global warming potential

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 disasterous 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 vapour near the surface, which is 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 (approx 200-450 years) and its global warming potential (GWP) is defined as 1.
Methane duration stay is of 12.2 +/- 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
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 : IPCC, table 6.7

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