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>>/2710/ > No sources cited > I'm right because I say so Just no. Look, the estimates of those ancient CO2 levels largely depend on carbon isotope ratios in plant fossils or ancient soils and their source. Did the incorporated carbon that was sequestered from the ancient atmosphere come from CH4 outgassing from marine methane hydrate or wet lands which has extremely negative δ13C of -70 to -50, from permafrost or fossil hydrocarbon sources like coal fires which is intermediate with -40 to -30 or abiotic sources like volcanoes which only have a δ13C of -10. It takes about 5-10x less biotic CH4 to explain a carbon isotope excursion than volcanic CO2. Means - a climate model fully blaming the δ13C excursion found in soils or fossils on volcanoes overestimates CO2 levels by factor 5 to 10. This contradicts the number stomata from fossilized leaves. The more CO2 in the air, the less stomata plants need but this also depends on water stress. And fossil stomata based CO2 estimates agree with a ar lower CO2 levels meaning larger fraction of non-volcanic CO2 in the air than previously estimated. Well it's not that easy but recent studies show that long term atmospheric CO2 levels of 4 - 6K ppm were last reached in the middle Cambrian about 500Mya ago. CO2 levels in the Jurassic most likely were around 1000 ppm except for the earliest stages (Toarcian) but there's a lot of uncertainty with paleosol data. The estimates depend on very complex and questionable models and often contradict each other. See the pink x marks in the first graph, that's data from paleosols. It's all over the place and maximum individual (!) values were found before the Triassic–Jurassic extinction event (215 Mya) with 3700 +/-1600 ppm CO2. But that was a huge catastrophe and there are also samples of the same age which point to far lower CO2 levels. The best estimate is around 2000 ppm for max Mesozoic CO2 levels. It’s all a clusterfuck. But it’s always been clear that such abnormally high CO2 levels definitely were not common during the Mesozoic and the outliers associated with negative carbon isotope excursions correlate with major extinctions, mainly the Permian-Triassic Mass Extinction Event (PTME, 252 Mya) where CO2 levels are estimated to have risen from near present-day level to around 2500 ppm within 75000 years. Maybe even 5000 ppm but that depends on how much CO2 was released from the Siberian Trap megavolcanoes and how much came from secondary sources like coal fires end methane hydrate. In any case CO2 levels rose enormously but it was the worst extinction event in the last 500Mya and totally fucked up the biosphere for tens of millions of years. CO2 levels fell rapidly within a few thousand years but remained high for the entire Mesozoic. The Carnian Pluvial Episode in the late Triassic (CPE, 233 Mya), the Triassic–Jurassic extinction event 201 (Mya) and the Toarcian Oceanic Anoxic Event (TOAE, 183 Mya) in the early Jurassic also were major catastrophes in earth's history associated with some of the strongest volcanic events ever. But carbon input from volcanoes also was previously overestimated and atmospheric CO2 levels are estimated to have stayed below 2000 ppm outside of major extinction pulses throughout the Mesozoic. Note that high CO2 levels are a huge problem for marine life because this shit doesn't stay in the air but dissolves in the oceans, acidifying them. No more calcified shells or corals which leads to marine mass extinctions. So nope, 4000 - 6000 ppm CO2 never were normal in the history of complex life and are incompatible with ecosystems as we know them. So there you have it, what else would I spend a Saturday evening with?