Perhaps not that high. Regardless, entering that period was marked by a mass extinction.* This source [0] states it well: "The end-Permian mass extinction, th

Perhaps not that high. Regardless, entering that period was marked by a mass extinction.* This source [0] states it well: "The end-Permian mass extinction, the largest biological crisis in Earth history, is currently understood in the context of Siberian Traps volcanism introducing large quantities of greenhouse gases to the atmosphere, culminating in the Early Triassic hothouse. In our study, the late Permian and Early Triassic atmospheric CO2 history was reconstructed by applying the paleosol pCO2 barometer. Atmospheric pCO2 shows an approximate 4× increase from mean concentrations of 412–919 ppmv in the late Permian (Changhsingian) to maximum levels between 2181 and 2610 ppmv in the Early Triassic (late Griesbachian)." [0] https://pubs.geoscienceworld.org/gsa/geology/article/50/6/650/612995/Five-million-years-of-high-atmospheric-CO2-in-the https://pubs.geoscienceworld.org/gsa/geology/article/50/6/65... * looks like we can quibble on 2k ppm or 5k ppm, the salient part is the global mass extinction part. Whether that was a correlation or caused by CO2 is not defined, at any rate it is not really speaking to a very habitable earth for humans.