https://skepticalscience.com/co2-higher-in-past.htm

"Another important factor is the sun. During the Ordovician, it would have been several percent dimmer according to established nuclear models of main sequence stars. Surprisingly, this raises the CO2 threshold for glaciation to a staggering 3000 ppmv or so. This also explains (along with the logarithmic forcing effect of CO2) why a runaway greenhouse didn't occur: with a dimmer sun, high CO2 is necessary to stop the Earth freezing over."

Civilization is dependent on the observed equalization of planetary energy input-output present for the past 10 000 years, along with stable CO2 levels. Both temperature and CO2 concentrations are breaking out of that envelope since 1930's, thus challenging poweful one-way threshold effects that will further release yet more greenhouse gases. Just this time without human intervention. It's kind of like lighting the fuse of a gigantic environmental bomb.