On a quick search:
https://earth.org/data_visualization/a-brief-history-of-co2/
shows 2K ppm about 400M years ago.
One of the things we know for certain is that it was very cold at the end of the Pleistocene, and then experienced a rapid warm-up during the first half of the Holocene. During the late Pleistocene, sea levels were around 120 meters lower than they are now, and ice sheets about 2 miles thick covered much of present day Canada and the northern US. All of that ice melted during the 1st half of the Holocene, giving us present day sea levels which have remained relatively unchanged in the intervening 9000 years (give or take a little).
All of the light blue areas on this map are sea depths between 0 and 300 feet. Take a good look. All of that area was dry land 20 thousand years ago, and there had to be massive heating to melt the 10K foot thick ice sheets that existed in most of the northern latitudes at that time. It all melted without us. Note that present day UK was in fact part of the European landmass at that time, and there were no fjords in Scandinavia, and no Baltic sea. Papua/New Guinea and Australia were part of the same landmass, and there was no Indonesian archipelago, nor Malaysian peninsula. Tampa and Miami were probably a 100 or more miles from their respective present-day coasts. But long before that, we know that part of Florida was under water, and had been for a very long time. A special kind of rock, called oolyte, undergirds Miami. Oolyte only forms under water over a very long time period.
If you work out the math on the ice melt as a result of all that warming, you come to an average sea level rise of about 1 meter every 75 years, which is leaps and bounds faster than what the worst prognosticators are saying today. If all the world continental glaciers melted tomorrow, it still wouldn't even raise the seas by as much as they rose during the first half of the Holocene, between about 18K and 9K years ago. And if the rest of the glaciers melt in the next 1000 years, it won't be the first time, and certainly won't be the last.
Ecclesiastes 1:9
What has been will be again,
what has been done will be done again;
there is nothing new under the sun.
No, not one thing.