by Hentet: Fra tv2,dk
Danske forskere med opsigtsvækkende klimastudie – Danmark kan blive helt forandret om få år
Flere forskere, som TV 2 har talt med, er skeptiske over for beregningerne. En kalder studiets dystre forudsigelse ”meget provokerende”.
I tusindvis af år har varme havstrømme holdt det danske klima relativt lunt. Men de nærmer sig næsten med sikkerhed et totalt kollaps.
Det kan give Danmark et klima som i Canada, Alaska eller Novosibirsk, og det vil højst sandsynligt ske om kun 34 år.
Sådan lyder konklusionerne i nye beregninger fra Københavns Universitet, som er fagfællebedømt og tirsdag blev udgivet i det anerkendte videnskabelige tidsskrift Nature Communications.
Det er en meget provokerende konklusion
Steffen Malskær Olsen, klimaforsker ved DMI
Bag arbejdet står søskendeparret Peter Ditlevsen, klimaprofessor ved Niels Bohr-instituttet, og Susanne Ditlevsen, som er professor i statistik. Og med deres arbejde modsiger de både den seneste forudsigelse fra FN’s klimapanel og flere af deres forskerkolleger.
– I første omgang tænkte jeg, at det her kan ikke være rigtigt. Og selvom jeg nu sætter min røv i klaskehøjde, håber jeg stadig, vi tager fejl, siger Peter Ditlevsen til TV 2.
Havstrøm holder Danmark lunt
Har de to forskere ret, risikerer Danmark ifølge Peter Ditlevsen et mærkbart koldere klima.
Vil man forstå, hvordan det hænger sammen, må man først vide lidt om et globalt system af havstrømme med det mundrette navn ’den termohaline cirkulation’.
I Atlanterhavet fører havstrømmene kort fortalt tropernes varme havvand mod europæiske kyster, og sender Nordatlantens kolde vand tilbage den anden vej.
Sat lidt på spidsen virker havstrømmene derfor både som en gigantisk radiator, der gør vejret varmere i Nordeuropa, mens klimaet i troperne afkøles.
En af motorerne er forskelle i temperatur og saltindhold mellem det varme tropevand og det iskolde hav syd for Grønland. Men den motor er et skrøbeligt maskineri.
– Den er begyndt at svinge mere og mere i styrke, og det er udtryk for, at den bliver mere og mere ustabil, siger Peter Ditlevsen.
– Det er næsten sikkert, at det vil ske i dette århundrede
Forskere har i årevis frygtet, at klimaforandringerne kan stikke en kæp i hjulet på havstrømmene – blandt andet fordi den smeltende indlandsis sender enorme mængder koldt ferskvand ud i havet.
Den gængse opfattelse hos blandt andre FN’s klimapanel (IPCC) har længe været, at havstrømmene vil blive svækket over en lang periode. Men et egentligt kollaps er meget usandsynligt i den nærmeste fremtid, lyder det.
Den forudsigelse er ifølge Peter Ditlevsen behæftet med fejl.
– Vi siger, at det næsten er sikkert, at det vil ske i dette århundrede. Forskellen er, at IPCC bygger sine forudsigelser på klimamodeller, og det er der store usikkerheder ved, siger han.
Derfor har de to forskere i stedet set på data for overfladetemperaturer i det nordvestlige atlanterhav fra 1870 til 2020, som ifølge Peter Ditlevsen er en slags “fingeraftryk” for de dybe havstrømme.
Ved hjælp af en række nye matematiske værktøjer har de fremskrevet den udvikling for at komme med et ”mere robust estimat” for, hvornår den termohaline cirkulation vil kollapse i Atlanterhavet.
Danmark kan få klima som Canada
Holder Peter Ditlevsen og Susanne Ditlevsens forudsigelse stik, vil havstrømmen i Atlanterhavet med 95 procent sikkerhed begynde at kollapse mellem 2025 og 2095.
Mest sandsynligt starter det i 2057, altså om 34 år. Men ifølge Peter Ditlevsen er det svært at forudsige, hvor hurtigt det vil gå.
Det er meget friskt at komme med et årstal
Steffen Malskær Olsen, klimaforsker ved DMI
– Modeller fortæller os, at det er et spørgsmål om få årtier, så det er også mit bedste bud, siger han.
På samme måde er det ifølge Peter Ditlevsen svært at spå om, præcis hvor koldt det kan blive herhjemme, hvis beregningen holder stik.
– Vi vil højst sandsynligt opleve koldere vintre og nok også en del koldere, end hvad den globale opvarmning har gjort det varmere, siger Peter Ditlevsen.
Tror du, at vi kan få samme klima som i for eksempel Canada, der ligger lige så nordligt som Danmark?
– Ja, det tror jeg på. Det er det mest sammenlignelige.
En beregning med store begrænsninger
Alligevel ligger Danmark relativt heldigt placeret, hvis den termohaline cirkulation forsvinder.
Her vil den globale opvarmning og et kollaps af havstrømmene nemlig trække temperaturerne i hver sin retning. Men i Atlanterhavets tropiske egne er det en helt anden historie.
– Den varme, vi ikke får herop, bliver jo i troperne. Så får vi endnu mere opvarmning i troperne, er der milliarder af mennesker, der lever i et klima, som kan blive rigtig, rigtig svært, siger han.
Spørger man klimaforsker med speciale i havstrømme ved Nationalt Center for Klimaforskning under DMI, Steffen Malskær Olsen, ville et pludseligt kollaps af den termohaline cirkulation rigtigt nok give et ”ekstremt” klima.
Men efter at have læst et preprint – altså en tidlig udgave – af forskningsartiklen tvivler han ligesom flere andre forskere, TV 2 har talt med, på Peter Ditlevsen og Susanne Ditlevsens forudsigelser.
– Det er en meget provokerende konklusion baseret på et datagrundlag, som har store begrænsninger. Det er min umiddelbare vurdering, siger han til TV 2.
– Jeg må jo give han ret i, at der er en usikkerhed
I løbet af de seneste år er forskere ifølge Steffen Malskær Olsen blevet mere og mere i tvivl om, hvorvidt overfladetemperaturerne i det nordvestlige Atlanterhav overhovedet er forbundet til den termohaline cirkulation.
Men det er netop den sammenhæng, hele studiets forudsigelser er baseret på.
– Det lægger en enorm usikkerhed ind over hele studiet, som ellers er meget fint statistisk. Og med hele den usikkerhed, så er det meget friskt at komme med et årstal, siger Steffen
Derfor mener forskeren stadig, at IPCC’s forudsigelse om, at et kollaps stadig er meget usandsynligt, er det bedste bud.
Alligevel kalder han studiet for vigtigt. For det viser, at vi med den rigtige data kan bruge avancerede matematiske modeller til at forudsige for eksempel et kollaps af havstrømme.
– Det er en meget vigtig diskussion, for pludselige ændringer vil få meget store konsekvenser. Så vi skal stadigvæk arbejde med at afdække den risiko, siger Steffen Malskær Olsen.
Spørger man Peter Ditlevsen, fremhæver han også sammenhængen mellem overfladevandet og de dybe havstrømme som den største usikkerhed ved beregningerne.
Men ifølge ham har vi ikke bedre tal til rådighed:
– Jeg må jo give ham ret i, at der er en usikkerhed. Jeg havde jo været lykkelig, hvis vi havde direkte målinger af havstrømmene, men det her er så at sige det bedste, vi har, lyder det fra forskeren bag studiet.
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