Yes, that was the case about 500 million years ago. [0] The climate and planet was very different back then, and it took a long time for the CO2 to go down tha

Yes, that was the case about 500 million years ago. [0] The climate and planet was very different back then, and it took a long time for the CO2 to go down that much and for life to cause this/adapt to the changes. The life back then wouldn't be very happy if it was transported to today's planet either. In addition to the greenhouse effect, the location of the continents can also have a major influence on the climate due to the influence on ocean and air currents, the albedo, weathering speed of exposed rock in the tropics and different plant cover at different latitudes etc, thus it is hard draw good comparisons (only modelling is possible, and verification is hard). To be fair, I do not believe we are capable of life extinction on Earth. We are also not able to make the conditions on this planet worse than it is on any other planet we can reach so far. But we can cause a mass extinction easily and make life for ourselves much much harder. Our situation is so optimized for the current conditions that changes will cause great pain. (Just look at the locations of cities, or how agriculture is run.) If we developed with the climate of 500 million years ago we would be just fine with that. The problem is the fast and large change. [0] https://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth%27s_atmosphere#Past_concentratione https://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth%27s_at... PS: One more point: CO2 causes some cognitive issues in humans in concentrations above 1000 ppm, which often occurs in meeting rooms and means the windows need to be opened regularly. It's not bad, you just get a little tired and dumber... so you take a break and open the windows. Now imagine the CO2 concentration outside is 5000 ppm (currently at ~415 ppm). We did not evolve for this.