Based on the assumption we'll hit double the CO2 levels compared to preindustrial times (280 ppm) by 2100. We're currently at 425 ppm, rising around 2.5 ppm per year now. Throw in all the extra methane, and we might be flirting with 475 ppm equivalent.
Three points:
1: Methane levels 21,000 yr ago were not nearly as high as they are today, and I don't know if the study took this into consideration.
2: 4C of warming is catastrophic no way you slice it. All coral reefs will go extinct, the Arctic will permanently lose it's ice sheets, sea levels will eventually rise 50-75' as vast swaths of Greenland and Antarctica melt away, the permafrost will thaw, all coastal cities will flood, vast areas of farmland will be inundated by the ocean, the Amazon will die and burn, most boreal forests will die and burn, and half the planet will become uninhabitable. There's a good chance global trade comes to a screeching halt and most advanced civilization collapses.
2: Due to positive feedback loops like the burning of the Amazon and the boreal forests and the thawing of the permafrost, there's very little guarantee that CO2 levels will stop rising by 2100, even if humans transition to 100% renewables. If they keep rising for centuries to come, and CO2 levels eventually level out at 650-700 ppm, 5-6C of warming is back in play.
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