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Satellittmåling viser liten økt drivhuseffekt
- mai 15, 2021
- 18:54
- Geir Aaslid
- Kategorier: Atmosfærefysikk, CO2-utslipp
Klimanytt nr. 297. Redaktør: Ole Henrik Ellestad.
Forfattet av Ole Henrik Ellestad.
Satellittobservasjoner og beregninger av strålingseffekten av økte drivhusgasser viser liten økning, som utelukkende har positive virkninger. Da er det ikke klimakrise, men klimaforbedring.
I 200 år har laboratorieeksperimenter vist at gasser kan oppvarmes av stråling. I 160 år har vi visst at vanndamp er en meget sterk drivhusgass, CO2 bidrar litt, mens ozon, metan og lystgass i enda mindre grad bidrar til oppvarming (J. Tyndall 1859, KN 270). Teorien var velutviklet i midten av 1930-årene, de ledende meteorologer mente dette fra 1950-tallet (KN 194), og satellittmålinger og tilhørende beregninger viste dette fra 1970-årene (se nedenfor).
For hvert nytt molekyl som tilføres atmosfæren blir molekyleffekten mindre (logaritmisk utvikling). Mens de første 20 ppm (part per million) gir ca. 1.7 C vil 20 ppm på dagens nivå bare gi ca. 0.06 C. Så atmosfæren er nå tilnærmet mettet, som en våt svamp som kan ta opp bare litt mer damp. Studentene lærer det tidlig i kurset, alle innen feltet vet det, og det er omtalt langt inne i fagdelene av IPCC-rapportene. Men det står ikke i sammendragene og formidles ikke i medier og mange fagartikler. For de som gjør det betegnes fornektere.
Heller ikke Arrhenius tidligere beregninger eller datamaskinene med sine simuleringsmodeller fra 1970-80-årene endret den rådende vitenskapelige oppfatning. Ikke før IPCC ble etablert som en politisk, rådgivende organisasjon (1988). Da eskalerte klimaproblemene raskt til store, alarmerende høyder basert på store beregningsmodeller. Men drivhusgassenes strålingsbidrag kan beregnes meget godt for steder med kjente atmosfæriske forhold, minst like godt som med IPCC-modellene. Da er økt drivhuseffekt liten og fordelaktig.
Emisjon av jordstråling kan beregnes godt
Utstrålt energi fra jordoverflaten (langbølget infrarød stråling, ORL) til verdensrommet er siden 1883 beregnet med Stefan-Boltzmanns lov (KN 291) basert på den aktuelle overflatetemperatur (-90 til +60 C). Basert på kjennskap til atmosfæregassenes infrarøde spektrum kan absorpsjon og emisjon beregnes godt når strålingen passerer gjennom atmosfæren. Figuren under viser utstrålingen fra toppen av atmosfæren en skyfri dag over Guam (Stillehavet) i 1970 (326 ppm CO2). Energien er målt fra NIMBUS 4 satellitt (heltrukken, takkete linje, Michelson interferometer). Svakt prikket linje, som følger tett på den takkete, er beregnet emisjon med datidens database (HITRAN, US Air Force, i dag forbedret og supplert av databasen VAMDC). Kun H2O og CO2 er med i beregningene. Toppen betegnet O3 (ca. 1050 cm-1) skyldes ozon.
Så når man kjenner jordoverflatens temperatur og atmosfærens parametre kan man for klar himmel beregne atmosfæriske effekter fra absorpsjon og emisjon av infrarød stråling meget godt, i dag enda bedre enn over Guam.
Liten beregnet effekt fra økt CO2
Hva kan dette brukes til? Mengden drivhusgasser, atmosfæriske betingelser, utstrålingstemperatur fra bakken osv. kan endres og tilhørende effekter beregnes. Noen slike variasjoner for dagens situasjon er vist i neste figur som har stor likhet med den over. Den viser beregnet effekt av standard mengde drivhusgasser i atmosfæren (sort kurve med angivelse i hvilke frekvensområder gassene har effekt) opp til 86 km høyde (toppen av mesosfæren). Standard konsentrasjon i alle kurver ved havoverflaten er 7750 ppm H2O, 1.8 ppm CH4, 0.36 ppm N2O. Endringer i CO2-konsentrasjonen påvirker utstrålingen (Grønn kurve null CO2, sort kurve 400 ppm, og rød kurve 800 ppm). Blå kurve er beregnet utstråling fra jordoverflaten (Planck-stråling).
Energien som ut ifra figuren er tilført atmosfæren og kan lede til temperaturøkning, representeres ved arealet mellom utstrålingskurven fra bakken (Planck-kurven, blå) og utstrålingskurvene fra mesosfæren (takkede linjer sort, rød, grønn). Forskjellen skyldes atmosfærens avtagende temperatur med høyden (‘lapse rate’). Ved økt CO2-mengde emitteres ved høyere nivå som har lavere temperatur og derved mindre effektiv emisjon (Kirchhoffs lov 1859/Schwartzschilds ligning 1916). Effekten av dobling av CO2 er arealet mellom den røde og sorte kurve. Dersom det er temperaturinversjon i atmosfæren vil økt CO2-mengde lede til økt utstråling og avkjøling (Antarktis).
Dobling av nåværende mengder av CO2, N2O og CH4 øker samlet effekt bare med inntil et par prosent. Dagens konsentrasjoner av CF4/KFK og SF6 er meget små og gir ingen merkbar effekt. Selv med 100 ganger økning blir effekten mindre enn ved dobling av CO2-konsentrasjonen. Molekylenes spektra overlapper og målt totaleffekt er mindre enn summen av effekten av hvert av molekylene om de er alene. Spesielt gjelder dette for effekten av metan som dempes sterkt av vanndamp.
Fra områdene merket H2O ser man vanndampens store betydning og at økte vanndampmengder har liten effekt. Vanndampen foreligger i så store mengder at den med unntak av meget kalde områder er mettet.
Konklusjon
CO2 er en svak drivhusgass som over 200 ppm er på vei til å bli mettet med liten ytterligere effekt. Om CO2 var alene ville effekten av dobling være ca. 1 C for dagens situasjon. Det er ingen forsterkning, snarere er det påvist negative tilbakekoblinger som reduserer CO2s effekt. Ozon, metan, lystgass, og fluor- forbindelser har liten effekt (KN 270, Gavin Smith et al. 2019).
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