About 50 Ma ago [CO2]_atmos was about 1000 ppm. The climate was radically different than it is today. This article in the Atlantic summarises the results of a study on a terrestrial proxy of temperature, which suggest that land surface temperature was greater than previously believed. This is interesting from a climate change perspective because business as usual (SSP2/SSP3) fossil fuel consumption means that we may reach this by 2100 CE. We will likely not experience Eocene-like conditions (described in the article) by 2101, because the ice sheets will not have yet melted, but it may be bad news bears for the planet as we know (and love) it.
Image credit: https://www.deviantart.com/mickeyrayrex (Excellent stuff!)