SELAMAT DATANG DI SITUS KAMI SEMOGA BERKESAN DAN BERMANFAAT.TERIMA KASIH

Jumat, 19 Juni 2009

Greenhouse gases in Earth's atmosphere

In order, Earth's most abundant greenhouse gases are:

When these gases are ranked by their contribution to the greenhouse effect, the most important are:[6]

  • water vapor, which contributes 36–72%
  • carbon dioxide, which contributes 9–26%
  • methane, which contributes 4–9%
  • ozone, which contributes 3–7%

The major non-gas contributor to the Earth's greenhouse effect, clouds, also absorb and emit infrared radiation and thus have an effect on radiative properties of the greenhouse gases.[7][8]

The contribution to the greenhouse effect by a gas is affected by both the characteristics of the gas and its abundance. For example, on a molecule-for-molecule basis methane is about eight times stronger greenhouse gas than carbon dioxide[9], but it is present in much smaller concentrations so that its total contribution is smaller.

It is not possible to state that a certain gas causes an exact percentage of the greenhouse effect, because the influences of the various gases are not additive. The higher ends of the ranges quoted are for the gas alone; the lower ends, for the gas counting overlaps.[8][7] Other greenhouse gases include sulfur hexafluoride, hydrofluorocarbons and perfluorocarbons. See IPCC list of greenhouse gases. 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.[10]

Although contributing to many other physical and chemical reactions, the major atmospheric constituents, nitrogen (N2), oxygen (O2), and argon (Ar), are not greenhouse gases. This is because homonuclear diatomic molecules such as N2 and O2 and monatomic molecules such as Ar have no net change in their dipole moment when they vibrate and hence are almost totally unaffected by infrared light. Although heteronuclear diatomics such as carbon monoxide (CO) or hydrogen chloride (HCl) absorb IR, these molecules are short-lived in the atmosphere owing to their reactivity and solubility. As a consequence they do not contribute significantly to the greenhouse effect and are not often included when discussing greenhouse gases.

Late 19th century scientists experimentally discovered that N2 and O2 did not absorb infrared radiation (called, at that time, "dark radiation") and that water as a vapour and in cloud form, CO2 and many other gases did absorb such radiation. It was recognized in the early 20th century that the greenhouse gases in the atmosphere caused the Earth's overall temperature to be higher than it would be without them.

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