Greenhouse effect

The "greenhouse effect" isprocess by whichatmosphere warmsplanet. Mars, Venusother planets havegeenhouse effect too, butsimplicityrestthis article will refer tocase ofearth.

When solar radiation reaches earth's atmosphere, somereflected backsomeabsorbed, but muchit passes throughreachessurface. There, most ofradiationabsorbed, which warmssurface. The surface radiates heat back at longer (infrared) wavelengths, andatmosphere absorbs somethis radiation. This warmsatmosphere,it eventually passes some ofenergy back tosurface [1].

The composition ofatmosphere means thatabsorbs more infrared radiation than visible sunlight. The atmosphere's effectsending energy radiated fromsurface back down outweighs its effectreducingamountsunlight which reachessurface. The resultthatsurface ofearthwarmer thanwould be inabsence ofatmosphere.

Itcommonplaceover-simplistic descriptions of"greenhouse" effectassert thatsame mechanism warms greenhouses (e.g. [1]), buteffectdifferent: see below. For this reasontermoften writtenquotes; such usage will be dropped from here on.

The term greenhouse effect may be usedrefertwo different thingscommon parlance:total greenhouse effect (see also climate change), or more looselyadditional (anthropogenic) greenhouse effect (see also anthropogenic global warming). The formeraccepted by all;latter ismatterdispute. This pageaboutformer.

Tablecontents

1 Controlling factors 2 Real greenhouses 3 Effectsvarious gases 3.1 Water vapour effects 4 Global warming 5 References

Controlling factors

Water vapor (H₂O) causes about 60% ofEarth's naturally-occurring greenhouse effect,others carbon dioxide (CO₂) (about 26%), methane (CH₄), nitrous oxide (N₂O)ozone (O₃) (about 8%), collectively known as greenhouse gases. This "greenhouse effect" occurs naturallyour atmosphereis responsible forearth's surface temperature which allows life on Earth.

Visible light fromSunpartially ablepass throughatmospherereachplanet's surface where muchitabsorbed, thereby warmingsurface [1]. The actual amounts absorbed at any placetime depend strongly onatmosphere (primarilyclouds),surface albedo (snow being reflective, oceans absorbing)latitude (higher latitudes havelonger atmospheric path lengththus more scatteringabsoption). Some ofheatradiated back at longer infrared wavelengths (the rest, assuming no long-term tmperature changes,moved withinatmosphere or oceans; there isnet fluxabsorbed energy fromequator topoles) andgreenhouse gases inatmosphere absorb somethis radiation, thereby warming upeventually passing some ofenergy back tosurface. The wavelengthslight thatgas absorbs isfunction ofquantum mechanically-determined energy levels thatcharacteristic ofdifferent gas molecules. It so happens that tri- (and more) atomic gases absorb stronglyinfra-red wavelengths, whichwhy H2OCO2greenhouses gases butmajor atmospheric constituents (N2O2)not.

The degree ofgreenhouse effectdependent primarily onconcentrationgreenhouse gases inplanetary atmosphere. For example, whileplanets Venus, Earth,Mars have similar amountsincident solar radiation,dense, carbon dioxide-rich atmosphereVenus causesrunaway greenhouse effectsurface temperatures hot enoughmelt lead,atmosphereEarth causesgreenhouse effecthabitable temperatures, andthin atmosphereMars causesminimal greenhouse effect.

The use ofterm runaway greenhouse effectdescribesituation obtaining on Venus, emphasisesinteraction ofgreenhouse effectother processesfeedback cycles. Venussufficiently strongly heated bySun that watervaporisedso carbon dioxidenot reabsorbed byplanetary crust. Asresult,greenhouse effect has been progressively intensified by positive feedback. OnEarth there issubstantial hydrospherebiosphere which respondhigher temperatures by recycling atmospheric carbon more quickly (in geologic terms;timescale forocean/biosphereremoveCO2 perturbationoforderseveral hundred years). The presenceliquid water thus limitsincrease ingreenhouse effect through negative feedback. This stateaffairsexpectedpersistat least hundredsmillionsyears, but, ultimately,warmingan aging Sun will overwhelm this regulatory effect.

Real greenhouses

The term 'greenhouse effect' originally came fromgreenhouses usedgardening, butismisnomer since greenhouses operate differently. A greenhousebuiltglass;heats up primarily becausesun warmsground inside it, which warmsair nearground,this airprevented from risingflowing away. This can be demonstrated by openingsmall window nearroof ofgreenhouse:temperature will drop considerably. It has also been demonstrated experimentally (Wood, 1909). Greenhouses thus work by preventing convection;greenhouse effect however reduces radiation loss, not convection.

Effectsvarious gases

Itharddisentanglepercentage contributions to"GH" effect from different gases, because thereoverlaps inIR spectrum ofvarious gases. However, one can calculatepercentagetrapped radiation remaining,discover:

Species                % trapped radiation
removed                radiation remaining

All                      0
H2O, CO2, O3            50
H2O                     64
Clouds                  86
CO2                     88
O3                      97
None                   100

(Source: RamanathanCoakley, Rev. GeophysSpace Phys., 16 465 (1978))

Water vapour effects

Water vapor ismajor contributor toEarth's greenhouse effect. Its effects vary duelocalized concentrations, mixtureother gases, frequencieslight, different behaviordifferent levels ofatmosphere,whether positive or negative feedback takes place. High humidity also affects cloud formation, which has major effects upon temperature butdistinct from water vapor gas.

The IPCC TAR (2001; section 2.5.3) reports that, despite non-uniform effectsdifficultiesassessingquality ofdata, water vapour has generally increased over20th Century.

Estimates ofpercentage ofEarth's greenhouse effect duewater vapor:

• 36% (table above)
• 60-70% Nova. Greenhouse - Green Planet [1]

Including clouds,table above would suggest 50%. Forcloudless case, IPCC 1990, p 47-48 estimate water vapour at 60-70% whereas Baliunas & Soon estimate 88% [1] considering only H2OCO2. Fortheoretical case if no other greenhouse gases were inatmosphere, Richard Lindzen estimated 98% (Global warming:originnature ofalleged scientific consensus. Regulation, Spring 1992 issue, 87-98 [1]).

Water vapour introposphere, unlikebetter-known GHG's such as CO2,essentially passivetermsclimate:residence timewater vapour inatmosphereshort (aboutweek) so perturbationswater vapour rapidly re-equilibriate. In contrast,lifetimesCO2, methane, etc,long (100'syears)hence perturbations remain. Thus,response totemperature perturbation caused by enhanced CO2, water vapour would increase, resulting in(limited) positive feedbackhigher temperatures. In response toperturbation from enhanced water vapour,atmosphere would re-equilibriate dueclouds causing reflective coolingwater-removing rain. The contrailshigh-flying aircraft sometimes form high clouds which seemslightly alterlocal weather.

Global warming

In recent years some researchers seegreenhouse effect assignificant contributing factor tocurrent global warming, due toincreased concentrationsome greenhouse gases inatmosphere asresulthuman activity. Such climatologistsconcerned that increased levelsgreenhouse gases inatmosphere might cause more heatbe trapped. The hypothesis thatman-made increasegreenhouse gas concentration would lead tohigher global mean temperature was first postulated inlate 19th century by Swedish chemist1903 Nobel Laureate Svante Arrhenius (see global warming), although, his peers largely rejected that theory. The theory that human greenhouse gas emissionsconnected withobserved heating ofEarth's atmosphere in20th century has steadily gained adherents inpopular community since1980s, toextent that many bodies aroundworld have strongly endorsed it. Automobile exhausts, coal-burning power plants, factory smokestacks,other waste vents ofindustrial age now pump six billion tonscarbon dioxideother greenhouse gases intoearth's atmosphere each year. Concentrationshuman-influenced greenhouse gases inatmospherecurrently at approximately 25% above pre-industrial values. Thisconsiderably higher than at any time duringlast 420,000 years,periodwhich reliable data (from ice cores) exists. From less direct geological evidence, itbelieved that values this high were last attained 40 million years ago. Sincelast Ice Age,Earth has hadlower temperature than usual, so discussion about recent warming sinceLittle Ice Age continues. See also:

• emissions trading
• Kyoto Protocol

References

• Kiehl, J.T.,Trenberth, K. (1997). Earth's annual mean global energy budget, Bulletin ofAmerican Meteorological Society 78 (2), 197–208.
• Wood, R.W. (1909). Note onTheory ofGreenhouse, Philosophical Magazine 17, p319–320. Fortextthis online, see http://www.wmc.care4free.net/sci/wood_rw.1909.html