It's very hard to look back that far and compare to the measurements today because our rate of rise now is easily measured across years, whereas the data for millions of years ago is via the geological and fossil records, which records time on a longer scale. I recall that when they've modeled possible events which would sharply raise the C02 levels, it takes a very significant like the end of an ice age or huge amounts of volcanic activity.

From the fossil record we know that we last had something similar to the current levels about 15-20 million years ago, when our ancestors were just great apes. It was indeed a few degrees warmer and the sea level was higher. https://www.sciencedaily.com/releases/2009/10/091008152242.h...

We do have very good levels from the more recent history from ice cores, which shows that the current rate of change is unequalled: https://www.climate.gov/news-features/understanding-climate/...