How was it possible that CO2 levels were higher nearly 650,000 years ago? Recently I've been working on a debate for climate change and I'm working on a rebuttal statement but then I came across many different sites that claimed that there were higher CO2 levels before there was even industrialization or a substantial amount of CO2 being made from humans. I just need an explanation on how this happened.
The explanation is that human activities are not the only processes that affect atmospheric composition.  

Last time CO2 levels were higher than today was 10-15 million years, during the Miocene epoch. During the Miocene, sea level estimated to be 100 feet higher and the global average surface temperature estimated to be 10°F warmer than today.





References:

Miocene Sea level graph.

Temperature over time graph.

www.climatecentral.org

Coupling of CO2 and Ice Sheet Stability Over Major Climate Transitions of the Last 20 Million Years