This figure was prepared by Robert A. Rohde from publicly available data.
This graph shows the natural cycle of CO2 over the last 400,000 years. Your "normal" level can be set, depending on which part of the cycle you find yourself. The cycle is due at least in part to the 100,000 year wobble in the Earth's orbit (Malinkovitch cycles).
Something is taking us to the highest CO2 in more than 400,000 years.
Co2 is a indisputably a greenhouse gas.
A runaway greenhouse effect is very likely unless the global community takes drastic action to decarbonise, stop forest burning, and sequester atmospheric CO2.
Our descendants would not want to live in a +6*C world.