Significant moves in either direction could have a huge impact on human life. Like ok you may have had events in the past at very high levels of co2 that resulted in an increase of life on the planet, but going back to those levels may not be good for humanity specifically. Increasing global temperatures 5C+ or whatever the number is would be extremely risky for us. To me it seems desirable to limit the amount to what it has been when humanity has been healthiest.