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Naturlig nedkjøling fikk isbreene til å vokse
Det naturlige klimasystemet har mekanismer som kan utløse brå klimaendringer. Ny forskning antyder at spontan nedkjøling kan ha fått arktiske isbreer til å vokse lenge før den lille istiden, uten påvirkning utenfra.
For noen hundre år siden rykket breene frem i hele Europa, mens klimaet ble kaldere. Dette er den såkalte lille istid, og skylden legges vanligvis på vulkanutbrudd og et minimum i stråling fra solen i det 13. århundre.
Denne teorien er imidlertid vanskelig å få til å stemme med nyere data som viser at arktiske breer begynte å vokse mye tidligere. En ny studie fra Bjerknessenteret for klimaforskning antyder at denne overraskende regionale effekten kan ha blitt utløst av spontane klimaendringer.
Arktiske breer begynte å vokse flere hundre år før andre breer i Europa
– Jeg har studert de prehistoriske endringene i breene i Arktis i noen år nå og lurt på hvorfor den lille istiden der har ligget flere hundreår foran endringene ellers i Europa, sier artikkelens hovedforfatter, geologen Willem van der Bilt.
Vulkanutbrudd og endringer i solinnstråling er viktige triggere i naturlig klimaendring. I denne perioden var det ingen store variasjoner, og dermed er det ikke innlysende hvorfor det skulle inntreffe store endringer i klimaet. På grunn av det antatt stabile klimaet har tidsintervallet mellom år 700 og år 1000 blit kalt for middelalderens stille periode.
– Å sammenstille mer og mer data som viste den samme trenden brakte meg ikke nærmere noen forklaring, sier van der Bilt.
– Heldigvis tvang noen av kollegene mine meg ut av komfortsonen ved å snakke med modellører om å forklare mønstrene i disse dataene.
Resultatet er en ny studie, som nylig er publisert i tidsskriftet Quaternary Science Reviews, der forfatterne utfordrer ideen om at klimavariasjoner alltid krever påvirkning utenfra.
Kombinerte modeller og empiriske tilnærminger
– Klimamodeller har simulert brå endringer som utvikler seg spontant, sier en av forskerne bak artikkelen, fysikeren Andreas Born.
– Men ofte har disse eksperimentene blitt sett på som klimamodelleringens stygge andunger.
Det er ikke tilfellet denne gangen. Ved å sammenligne geologiske bevis med modellberegninger, ble Born og van der Bilt overbevist om at den tidlige fremveksten av de arktiske breene under den lille istiden kunne forklares med spontan avkjøling.
– Det vi mener, er at disse endringene ikke krevde eksterne mekanismer, men oppsto tilfeldig, forklarer van der Bilt.
– I dette tilfellet var det endringer i atmosfæren øst for Grønland som satte i gang rask vekst av sjøis, legger Born til.
Sjøisen spiller en viktig rolle i å kjøle ned det arktiske klimaet ved å reflektere solstråling og hindre havet i å tape varme til atmosfæren. Slik kan vekst av sjøis ha ledet til kaldere forhold, som igjen kan ha intensivert veksten av is. Denne kjeden av selvforsterkende hendelser, kjent som en tilbakekoblingsløkke, medførte en endring fra unormale værforhold i et lite område til en fase med vekst av breer og sjøis i og rundt hele Nord-Atlanteren.
– Ting eskalerte brått. Innen noen tiår hadde avkjølingen bredt seg hele veien fra Grønland til Svalbard, sier van der Bilt.
Dette karakteristiske endringsmønsteret fanges opp av geologiske data så vel som modellresultater, demonstrert gjennom en statistisk analyse utført av en av de andre artikkelforfatterne, ph.d-kandidaten Kristian Haaga.
Van der Bilt og Born argumenterer for at arbeidet deres understreker hvorfor klimaforskere bør kombinere modellering og empiriske tilnærminger.
– Modellberegninger representerer mulige scenarioer for klimaendring. Å sammenligne disse med geologiske data kan fortelle oss i hvor stor grad de representerer virkeligheten, sier Born.
– Og motsatt, ved å definere nøkkelkomponenter i klimasystemet hjelper modellene oss med å forstå dataene våre, tilføyer van der Bilt.
– Og det er nettopp det som skjedde med denne artikkelen. Vi demonstrerer at simuleringer av spontane endringer ikke er så hypotetiske som tidligere antatt, mens vi tilfører en forklaring på en gåtefull fase med betydelige klimaendringer.
Referanse
van der Bilt, Willem GM, Born, Andreas & Haaga, Kristian A (2019) Was Common Era glacier expansion in the Arctic Atlantic region triggered by unforced atmospheric cooling? Quaternary Science Reviews, 222, p. 105860.
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