That guy's argument is nobody knows the level of CO2, or what was the level in the past. He clearly knows his audience. They don't want to be informed. It's much easier for his reasoning that his audience won't fack check him.
That assertion about past levels of co2 is completely wrong, but it's no surprise that hill billy idiots deny the facts.
The fossil record of minute sea creatures called forams shows the changes exactly over millions of years and the changing levels correlate perfectly with known climate type, also available from the fossil record. It's literally carved in stone - or written at least.
Seawater acidity levels vary exactly with atmospheric co2 levels. More co2, more acidity and if you drill out fossil foram cores from the rock, you can measure the isotropic concentration of boron in the samples.
As a calibrating check, you can also drill out ice cores from ancient deposits such as those on Greenland, and or Antarctica and the bubbles in the ice contain the actual ancient atmosphere whose components can be measured.
These atmospheric content studies can easily be compared with the other fossil records to see what kinds of plants were growing, and what animals there were, so we know what the temperature levels were. If you find crocodile remains in Greenland and the Arctic Ocean floor, which you do, you know it was warmer in the past.
Trump's army of ignorant bigots, will, like him, say anything that comes into their heads (not a lot) to deny FACTS. They don't believe in anything that doesn't fit with their view of the world, and will cherry pick, outright lie and do anything to assert that their flat earther views are the correct ones.
Past high co2 levels (far above the present) were caused by huge upheavals of volcanic activity which affected large areas of the planet. The fossil record of these outpourings from under the earth are also there in the geology with massive, ancient lave flows like the Deccan Traps there under our feet.
Some researchers think that intense volcanism could have contributed to the dinosaurs' decline well before a comet or asteroid impact sealed their fate.
www.amnh.org
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