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https://www.klimarealistene.com/2022/06/04/megatorke-med-sosial-kollaps-etter-siste-istid/
Megatørke med sosial kollaps etter siste istid
- juni 4, 2022
- 19:57
- Geir Aaslid
- Kategorier: Aftenposten, Ekstremvær
Klimanytt nr. 321. Redaktør: Ole Henrik Ellestad.
Forfattet av professor Einar Sletten, medlem av KVR.
Periodar med megatørke har ført til klimakriser og sosial kollaps etter siste istid utan variasjonar i atmosfærens CO2-innhald. Det syner at naturlege variasjonar gir ekstremeffektar som vi må handsame slik Bjørn Lomborg hevdar. CO2-tiltak hjelper ikkje.
Innleiing
Klimadebatten er prega av påstandar om ekstremeffektar og vert søkt knytt til auka drivhusgassar. Men i hovedrapporten frå 2013/14 konkluderer IPCC at det er ingen auka tendensar til ekstremeffektar. Dei få tilfelle i dei få åra som er gått, endrar ikkje den statistikken. Tørke finn stad også i kaldare periodar.
Men i år er det tørke fleire stader og da dukkar det stadig opp artiklar om emnet som t.d. den i Aftenposten 14.mai 2022: «Dette har ligget inne i klimaberegningene for fremtiden i lang tid. Det er noe vi venter vil opptre oftere, sier Helge Drange ved Bjerknessenteret for klimaforskning. Tørken vil også bli verre».
Men historien viser noko anna. Tørke inntrer også i kaldere perioder, og i fleire tusen år under den varmeste perioden etter siste istid (Holocen) var Sahara steppelandskap med rikt dyreliv. Naturlege variasjoner forårsakt av ulike fenomen dominerte som vist i utklippet fra 1977. Det var året den naturlege kalde perioden endte, og folk var redde for ei ny istid. Men så kom ei ny 20-års oppvarming (som i årene 1920-40).
Megatørke indikatorar og årsaker
Megatørke er periodar med langvarig ekstrem tørke. Holocen er tidsrommet etter den siste istida, ca 12 000 år. Avansert paleoklimatologi kan identifisere klimaendringar i denne perioden ved å analysere data frå sediment, pollen, iskjerner, treringar, stalangmittar i dryppsteinhuler, uran/thorium isotop- analyser etc. (Termometeret blei først brukt av Galileo Galilei i 1592). Analysene viser at periodar med megatørke har oppstått brått, styrt av naturlege verhendingar som t.d. El Niño Southern Oscillation (ENSO), Pasific Decadal Oscillation og monsunar som gir stor regional nedbør andre steder. Megatørke er historisk vel dokumentert for alle kontinent.CO2 -nivået under Holocen var relativt konstant (ca. 260 ppm iflg. IPCC). Ein kan slå fast at det er ingen samanheng mellom langvarige tørke og endringar i CO2–nivået. Weiss presenterer i boka si 9 døme på megatørke over klimatiske periodar. Dei førte til sosial kollaps og regional fråflytting.
Eksemplar på megatørke
Khmer Emperiet (Kambodsja) med hovedstaden Ankor med omlag 175000 innbyggarar hadde eit blomstrande kulturliv dokumentert med funn av fantastiske tempelbygg. To alvorlege tørkeperiodar, 1345-65 og 1401-25, med etterfølgande storflaum førte til fullstendig kollaps og fråflytting.
Maya kulturens sosiale kollaps (800-1100 f.Kr) er blitt flittig omtalt gjennom tidene. Noen få forskarar på 1980-talet foreslo at megatørke kunne vere årsaka. Men det er først etter at sedimentkjerner frå Lake Chichancanaba og prøver frå stalagmittar i Yok Balum hula blei analysert (1995-2012), at hypotesen blei endeleg verifisert.
Thetihuacan var den største byen i Mexico på 8-hundretalet f.Kr. Den dekka 21 km2 og hadde eit folketal på 125000. Byen var senter for økonomi, politikk og kultur. Storheitstida varte i sju århundre før byen på mystisk vis blei avfolka i løpet av 600-800 f.Kr. Mange teoriar har blitt foreslått, men først i 2012 blei det publisert paleoklimatiske data som viste at langvarig megatørke hadde funne stad og ført til kollaps.
Hyppig megatørke og flaum har herja i alle kontinent, til dømes i åra 1875-78 då India, Kina, delar av Afrika og Sør-Amerika hadde ekstrem tørke som førte til at 50 millionar døydde av hungersnaud; “the worst humanitarian disasters in human history” (Deepti Singh, Washington State University). Katastrofen vert forklart med ein heilt uvanleg intens ENSO som spreidde varmt vatn over Stillehavet og varma opp lufta. Den var kraftigare enn dei sterke El Niños i 1997-98 og 2015-16. Mike Davis, University of California, nemner i boka, Late Victorian Holocausts, at slike ekstreme verhende er sjeldne, men kan komme att!
Grønland fikk etter kvart to større samfunn (Vesterbygd og Austerbygd) med, på det meste, fleire tusen innbyggarar. Kvifor vart busetjinga borte på 1400-talet? Mange meiner at kulde var ein avgjerande faktor, men nye bevis tyder på at noko heilt anna også kan ha bidratt før den kaldaste delen av Lille istid (1645-1720). Innsjøen Lake 578 ligg sør på Grønland. R. Bradley, Universitetet i Massachusetts, samla paleoklimatiske sedimentdata frå sjøen. Konklusjon: langvarig tørke, ikkje kulde var sannsynleg årsak til fråflytting allereie på 1400-talet.
Midt-Vesten. I nyare tid (1930 åra) var det høge temperaturar og ekstrem tørke på prærien (The Dust Bowl). Dette førte til enorme skader i delstatane Colorado, Nebraska, Kansas, New Mexico, Texas og Oklahoma. Store mengder jord frå 400 000 km2 blåste bort. Det blei stor folkevandring av bønder vestover mot California.
Konklusjon
Dei kraftigaste tørker har funne stad utan medverknad av auka CO2. Det er difor viktig som Bjørn Lomborg påpeikar, med ein beredskap for å handsame effektene av naturlege klimavariasjonar.
Referanse: Megadrought, Collaps and Causality. Oxford University Press (2017). Editor: Harvey Weiss.
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