Thegreenhouseeffect-amolecularanalysis
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HelmutUllmanninitialsituation
Withitsatmosphere,planetearthpreventsitfromgettingtoohotduringthedayandtoocold atnight.Theradiatedsolarenergyisfilteredwhenitreachesthesurfaceoftheearth andisemittedwithadelay.Thiscreatesagreenhouse-likeclimateinthelowerlayersof theatmosphere,wherelifecoulddevelop.Disturbancesinthefunctioningofthe greenhousecouldseverelyaffectagloballyanddigitallynetworkedpopulationofbillions.
Inthelast150years,globaltemperatureshaveincreasedbyabout1degree.Duringthe sameperiod,theconcentrationofcarbondioxideintheairincreasedfrom280to400ppmby volume.ThisisblamedontheIndustrialRevolutionwiththeuseofcoal,oilandgas.Whatis notsaid:TheLittleIceAgeendedinthemiddleofthe19thcentury.Thiscoolingphase aftertheMiddleAgeswascharacterizedby1to2degreeslowertemperaturescompared tothepresent,especiallyinthenorthernhemisphere.
ThemeasuredCO2-temperature-timerelationshipleadstothesuspicionthatman-made CO2emissionsarethecauseofthetemperaturerise.Thegreenhouse(TH)effectbecame thesubjectofuncertainclimateextrapolationsuntiltheendofthiscenturywithpredictions ofaclimatecatastrophe.Asaresult,zeroemissionstargetswillbeenforcedwith politicalpower,whichwillleadtodeindustrializationandimpoverishmentofsociety.
Emotionalcounter-reactionsleadopponentsofclimateprotectionpolicytodenialofthe THeffectandthegreenhouseeffectofCO2.Ascientificexplanationwouldberequired. However,relevantresearchinstitutionsliveonstatefunding,whichinfluences theresearchgoal.Hereanattemptismadetoobtainarealpicturefromtheanalysis ofknownphysico-chemicaldataandtheirrelationtotheCO2molecule.
Theradiationbalance
Attheouteratmosphereoftheearth,ataheightofapprox.20km,thesolarradiation hasapowerdensityof1.35kW/m2(=solarconstant).Onitswaythroughtheatmosphere, solarradiationisweakenedbyreflectionandabsorption,dependingon
pathlengthandangleofincidence.Atthesurfaceoftheearth,theradiationoutputisamaximum of1kW/m2,dependingongeographicallatitude,altitude,weather,dayandseason.At higherlatitudes,thepeakvalueislimitedtomiddayandthemonthsofJuneandJuly.Atthe 60thdegreeoflatitudethisisupto900W/m2insummerandupto200W/m2inwinter.
Thebalancebetweensolarandterrestrialradiationisassumedtobebalanced.
Withitsatmosphere,theoceancurrentsandthewatercycle,theearthformsakindof thermostat.Thespectrumofthesun'selectromagneticradiationincludeswavelengths fromIRthroughvisibletoUVwithincreasingenergy.Onaverage,about58%oftheenergy remainsattheearth'ssurfacefromtheentirespectrum,wherebythespectraldistribution hasshiftedinfavorofthelong-wavecomponents(1).
Thehighest-energyUVradiationisweakenedthemost.Short-waveUVradiationis absorbedinchemicalreactionswithtracesofoxygenduringtheformationanddecomposition ofozoneinthestratosphere(2).OnlyaboutatenthofUVradiationreachestheearth'ssurface.
Visiblelightisreflectedfromcloudsandparticulatematterintheatmosphereandfrombright areasoftheearth'ssurface,onlyhalfofwhichreachestheearth'ssurface.
Thermalradiation(IR)isabsorbedbywatervapour,aerosolsandtracegases(CO2,CH4 andothers)intheatmosphere.OnlyabouthalfoftheIRradiationreachestheground.The absorbedradiationisabsorbedbysolidsinlatticevibrationsandthusconvertedintoheat. Thisenergyistransferredasheat(kineticenergy)toairmoleculesonroof surfaces,concretemotorways,housewalls,farmlandandheatedwater,andispartially reflectedbackasIRradiation.
Heatedairislessdenseandrises.Heatspreadsthroughconvection,aircurrents,highandlow pressureareasintheatmosphere.Inordertorestoretheenergybalancebetweenincoming andoutgoingradiation,theheatwouldhavetoescapeintospacewiththegas molecules,buttheseareheldtoearthbygravity.Theradiationbalancecanonlybe compensatedforbyconvertingitintoIRradiation.
Thebalancebetweenincomingandoutgoingradiationisbalancedaslongasthereareno changesinthesolarorplanetaryparameters,whichhashappenedseveraltimesinthehistory ofourplanet:solaractivity,earth'sorbitorearth'saxishavechanged,airand
seacurrentsaswell.Therewereiceagesandwarmperiods,grasslandsbecamedeserts.But evenwithconstantcosmicparameters,theradiationbalanceisbalancedneitherdailynorannually, sometimesonlyaftermillionsofyears.
Greenhousegasesandgreenhouseeffect
Whilethetemperatureatthetopoftheatmosphere,accordingtosatellitemeasurements,is-18
C,anaverageof+15Cismeasuredontheearth'ssurface.Thedifferenceof33degreesis calledthegreenhouse(TH)effect.Thewatercyclewithlatentheat(inmillielectronvolts,meV)is likelytomakethelargestcontributiontotheTHeffect:
heatoffusion
6.0kJ/mol=62meV/molecule
Heatofvaporization40.7kJ/mol=422meV/molecule
Theremainingthermalenergyisdistributedamongallgasesintheair.
Heatismovementofmolecules.Moleculesaresetinmotionbycollisionwithothermolecules, solidsontheearth'ssurface,bodiesofwaterorgasmolecules.ForthediatomicgasesN2andO2with exclusivelystatisticallydistributedkineticenergy,themaximumabsorbableenergy(molarheatcapacity Cp)(3)isaboutthesame.
Incontrasttothemaincomponentsofair,thediatomicgasesN2andO2,3-andpolyatomicgas moleculessuchasH2O(upto3%intheatmosphere)andtracegasessuchasCO2,CH4and others(totallessthan0.1%byvolume)intheatmosphere)duetotheirmolecularstructure,rotation, oscillationanddeformationvibrationsintheirspectrum,whichareexcitedbyIR radiationofcertainfrequenciesandcanalsoemitIRphotonsagain(4).Inadditiontothekinetic energy,theycanabsorbinternalenergyaccordingtothenumberandsizeoftheresonance frequencies(5).Asaresult,themolarheatsincreasefasterwithtemperaturethanforthe2atomgasesN2andO2.ThemolarheatcapacityCpisthemeasureofthemaximumthermalenergy thatagascanabsorb.
ThedistributionoftheTHeffectof33degreessolelyonwatervaporandthetracegases CO2,ozone,nitrogenoxidesandmethaneleavesthelargeproportionoftheheat containedinN2andO2unconsidered.Theheatretentioninthegreenhouseearth iseffectedbyallgascomponentsaccordingtotheirmolarheatcapacityand concentrationintheatmosphere.
TheenergystoredintheatmosphereincreasesduetoIRabsorptioninthepolyatomic gases,whichiswhytheyarereferredtoasgreenhouse(TH)gases.
WatervaporandtheTHgasesdonotdistinguishbetweenIRingressandemanation.They alsoabsorbIRradiation.Asaresult,thesolidsurfaceoftheearthisheatedlessand consequentlylessheatistransferredtotheair!
Amongthegasmoleculeswithstatisticallydistributedenergy,therewillbeenoughN2 andO2moleculeswhosekineticenergyisclosetothevibrationalenergyoftheTH molecules,whichcanthereforestimulateanoscillationintherangeoftheIRquantaby collidingwiththem.TheTHgasesabsorbheatradiatedfromthesunandradiatedfromthe earth'ssurfaceandemititinalldirectionsasIRphotons.TheTHgasesplayanimportant roleinradiativebalance,actingastransmittingantennaethatradiateheatintospace.
Theroleofcarbondioxide
Thewatercyclefunctionslargelyindependentlyofhumaninfluences,whichiswhycarbon dioxidehasbeendeclaredthemainculpritforglobalwarming.TheCO2contentinthe atmosphereiscurrently0.04%,theanthropogeniccontributionisestimatedat1/3ofthat. TheCO2moleculehasabsorptionbandsintheIRspectrumat2.7,4.3and15µm,with thelatterbandhavingagreaterwidth.Duetotheirlowerconcentration,othertracegases havehardlyanyinfluenceontheheatcapacityoftheair.
WhatinfluencedoesthelowCO2concentrationhaveonincreasingthemolarheat capacityofair?Let'sassumetheCpofairat25Ctobe29.1J/molK(thisappliestoN2,themain componentofair,buttheincreaseisimportantinthefollowing).
Thetemperaturedependenceoftheheatcapacityiscalculatedaccordingto(3).
CpCO2increasesmorewithTthanCpN2.CpforthemixturewithXvppmCOisthen:
ItfollowsfromTab.AandB:TheincreaseintheCO2contentoftheairfrom0vppmto1000 vppmatconstanttemperature(Tab.B)hasaboutthesameeffectforCpairasaTincrease byapprox.2to3K(Tab.a).Thisissignificantlylessthanwouldsufficetojustifythemeasured increaseinToverthepast150years.
AcontributionoftheCO2totheincreaseoftheheatcapacityoftheairispresent,butsmall.
CO2playsanimportantroleinIRradiationintospace.Let'snotforgetthatincreasingCO2levels alsodecreasetheIRenergyhittingthesolidsurfaceoftheearth,reducingheattransfertotheair molecules.
ThequestionofcauseandeffectremainsunexplainedintherelationshipbetweenCO2 concentrationandtemperature:DoestheincreaseinCO2concentrationcauseaTincrease, asassumedbythepoliticalsideasthebasisforaction?OrdoesaT-riseinthesurface temperatureoftheoceansnotonlyevaporatemorewater,butalsoreleaseCO2fromthe seawater?TheamountofCO2intheseaisassumedtobemanytimesgreaterthantheamount intheatmosphere.
Dayandnight,vegetationanddecompositionphasesinnature,aswellasproduction inhumansocietycanbringregionalandlocaldiscontinuitiesintotheclimate.
Energyistakenfromtheinfluxfromthesunandstoredinnaturalorindustrialproducts.The energystoredintheproductonlyreturnstotheenvironmentwhenitisbrokendownbybacteriaor whenitisincinerated.Soitisunderstandablethatalong-termdefinedconnectionbetweenCO2 concentrationandglobaltemperaturecouldnotbefound.Inaddition,themeasurementofan averageglobaltemperatureissubjecttolargeuncertainties(6).
TheCO2-temperature-timerelationshipmeasuredoverthepast150yearsledtothesuspicion thatman-madeCO2emissionswerethecauseofthetemperatureriseofaround1degree. ThefactthattheLittleIceAge,whichischaracterizedby1to2degreeslowertemperaturesin thenorthernhemisphere,alsoendedinthemiddleofthe19thcentury,isnotmentioned
was.
Theradiativebalanceoftheearthissubjecttofluctuationsduetonumeroussolar,planetary aswellasglobal,ecologicalandanthropogenicinfluences,whichrepeatedlycauseclimate change.
Theearth'satmosphereabsorbsalargepartofthesolarradiationaswellas theearth'sradiationintheIRrange,whichcreatesagreenhouseeffect.Theheatretentioninthe greenhouseearthiscausedbyallgascomponentsoftheatmosphere.Thelargestamount ofenergyiscontainedintheatmosphericpartofthewatercycle,followedbythemaincomponents oftheatmosphere,nitrogenandoxygen.Therestoftheenergyiscontainedintracegases.
Theso-calledTHgasescontainsimilaramountsofkineticenergypermoleculeasallothergas moleculesintheatmosphere.Inadditiontotheothertypesofmolecules,theystoreIRquanta inmolecularvibrations.Theyaretheonlycomponentsofairthatcanradiateheatintospace aselectromagneticradiation.AgrowingCO2contentincreasestheheatcapacityoftheair slightly,butatthesametimereducestheIRradiationontheearth'ssurface.
ThequestionofcauseandeffectintherelationshipbetweenCO2andtemperatureremains unanswered:istheatmospherewarmingupasaresultofrisingCO2concentrations,oris risingtemperaturereleasingadditionalCO2fromthesea?
Theprocessesintheheatbalanceoftheatmosphereareextraordinarilycomplex,making theirquantitativeassessmentdifficult.Thelevelofknowledgedoesnotprovideabasisfor thecurrentlypracticedclimateprotectionpolicywithitsseriouseffectsonthe economyandlife.
*)Prof.Dr.sc.nat.HelmutUllmann,Diplom-Ing.forradiochemistry,earlierworkatthe CentralInstituteforNuclearResearchandlaterattheTUDresdenwithenergyprocessesand thermodynamicsofmaterials.(HisaddressisavailabletoAGEU.)
(2)U.Feister,Earth'sozonesunglasses,Teubner,Leipzig,1990,p.51
(3)G.Kortüm,Introductiontochemicalthermodynamics,VerlagChemie,Basel1981
(4)C.Czeslik,H.Seemann,R.Winter,BasicKnowledgeofPhysicalChemistry,Vieweg andTeubner,GWVFachverlage,Wiesbaden2010
(5)earthobservatory.nasa.gov/features/EnergyBalance/page7.php
(6)K.Tägder,CriticalAssessmentofGlobalMeanEarthTemperature,ageu-die realisten.com/archives/3452,April14,2019