Greenhouse gas

Greenhouse gasesgaseous components ofatmosphere that contribute togreenhouse effect (see also global warming). The major natural greenhouse gaseswater vapor, which causes most (about 60%) ofgreenhouse effect on Earth, carbon dioxide (about 26%),ozone. The remaining fractioncaused by minor greenhouse gases which include methanenitrous oxide. Industrial greenhouse gases includeheavy halocarbons (chlorinated fluorocarbons), CFC, HCFC-22 molecules such as freonperfluoromethane,sulphur hexafluoride (SF₆).

Greenhouse gasestransparentcertain wavelengths ofsun's radiant energy, allowing thempenetrate deep intoatmosphere or allway toEarth's surface, where theyre-emitted as longer wavelength radiation (chiefly infrared radiation}. Greenhouse gasesclouds prevent somethis radiation from escaping, trappingheat nearEarth's surface wherewarmslower atmosphere. Alterationthis natural barrieratmospheric gases can raise or lowermean global temperature ofEarth.

The concentrationsseveral greenhouse gases have increased over time duehuman activities, such as:

• burningfossil fuelsdeforestation leadinghigher carbon dioxide concentrations,
• cattlepaddy rice farming, land usewetland changes,pipeline losses leadinghigher methane concentrations,
• useCFCsrefrigerationfire suppression systems.

According toglobal warming hypothesis, greenhouse gases from industryagriculturepartly or whollyblameglobal warming. Carbon dioxide issubject ofproposed Kyoto Protocol. Nitrous oxidemethanealso taken into account ininternational agreements, but not ozone.

At least one IPCC TAR chapter lead author considers mention ofeffectwater vapor uponEarth's greenhouse effectbe misleading as water vapor can not be controlled by humans.

Increasegreenhouse gases inatmosphere

Sincebeginning ofIndustrial Revolution,concentrationsmany ofgreenhouse gases have increased.

• Carbon dioxideup 30%, from 278,000 ppvb358,000 ppvb
• Methaneup from 700 ppvb1721 ppvb
• Nitrous oxide 15%, from 275311 ppvb
• CFC-12 from 00.503 ppvb
• HCFC-22 from 00.105 ppvb
• Perfluoromethane from 00.070 ppvb
• Sulphur hexafluoride from 00.032 ppvb

(Source: IPCC radiative forcing report -- 1994 -- ppvb : part per billionvolume)

Durationstayglobal warming potential

The greenhouse gases, once inatmosphere, do not remain there eternally. They can be withdrawn fromatmosphere:

• asconsequence ofphysical phenomenon (rain, condensation, remove water vapor fromatmosphere)
• asconsequence ofchemical phenomenon intervening withinatmosphere. This iscase methane, whichpartly eliminated by reactionradicals OH naturally present inatmosphere,give CO₂ (this effect due toproductionCO₂not included inmethane GWP)
• asconsequence ofchemical phenomenon intervening atborder betweenatmosphere andother compartments ofplanet. This iscaseCO₂, whichreduced by photosynthesisplants,whichalso dissolved inoceanend up giving bicarbonatecarbonate ions (CO₂chemically stable inatmosphere)
• asconsequence ofradiative phenomenon. For exampleelectromagnetic radiation emitted bysuncosmic rays break molecular bondsspecies inupper atmosphere. Some halocarbons disappearthis way (theygenerally too stabledisappear by chemical reaction inatmosphere).

The lifetimean individual moleculegas inatmospherefrequently much shorter thanlifetime ofconcentration anomalythat gas. Thus, becauselarge (balanced) natural fluxesand frombiosphereocean surface layer, an individual CO₂ molecule may last onlyfew years inair, on average; however,calculated lifetimean increaseatmospheric CO₂ levelhundredsyears.

Aside from water vapour nearsurface, whichhasresidence timefew days,greenhouse gases takeverytimeleaveatmosphere. Itnot easyknowprecision how longnecessary, becauseatmosphere isvery complex system. However, thereestimates ofdurationstay, i.e.time whichnecessary so thatgas disappears fromatmosphere, forprincipal ones.

Durationstaywarming capability ofdifferent greenhouse gases can be compared:

• CO₂ duration stayvariable (approx 200-450 years)its global warming potential (GWP)defined as 1.
• Methane duration stay12.2 +/- 3 years andGWP22 (meaning thathas 22 timeswarming abilitycarbon dioxide),
• Nitrous oxide hasduration stay120 years andGWP310
• CFC-12 hasduration stay102 years andGWP between 62007100
• HCFC-22 hasduration stay12.1 years andGWP between 13001400
• Perfluoromethane hasduration stay50,000 years andGWP6500
• Sulphur hexafluoride hasduration stay3 200 years andGWP23900.

Source : GIEC

See also:

• environmental agreements
• Global Climate Coalition
• global warming
• global warming controversy