Strictly speaking that's not true. Listen man I don't really want to hold your hand and walk you through this.
Maybe you should actually read some of those studies?
[https://royalsocietypublishing.org/doi/10.1098/rsta.2013.009...]
Some of the recent ones (< 1 million years), co2 seems likely to be a feedback, not a forcing method - but there have been many times in the past where that is not a plausible explanation at all, with levels in the 2000 ppm range.
Maybe you should actually read some of those studies?
[https://royalsocietypublishing.org/doi/10.1098/rsta.2013.009...]
Some of the recent ones (< 1 million years), co2 seems likely to be a feedback, not a forcing method - but there have been many times in the past where that is not a plausible explanation at all, with levels in the 2000 ppm range.