Here https://royalsociety.org/news-resources/projects/climate-cha...
It's been below 300 ppm for at least the last 800,000 years, had now increased to over 400 ppm in a pattern that directly maps to the industrial revolution, radiocarbon dating traces the carbon in the atmosphere to "old" carbon like that from fossil fuels, and the known sources of human emissions adequately explain the increase.