This graphic shows the CO2 concentrations over the past 1 million years or so as derived from an ice core from Antarctica. There are bubbles trapped in the ice that capture the atmosphere at the time those bubbles were enclosed by the ice which allows us to find out with good accuracy what the CO2 concentrations were. These results have been reproduced via other proxies but the ice core recorders are the best.
In any event, you can see that CO2 concentrations have oscillated between 290 and 190 PPM fairly cyclically due to the glacial - interglacial cycles which are driven by Milkanovitch orbital cycles.
Today's concentration is 395 PPM which is far above the peaks in the previous cycles. Over a 100 ppm difference that was never experienced in the past 1 million years even though ocean temps have been warmer during that time. Direct observations show that the ocean is still gaining CO2 as well.
Also, the X axis of that graph is time but it is time on a very large interval. In other words, the CO2 in the atmosphere changed very slowly in the past when it rose. It fell relatively quickly when compared to the rise but those drop are still far slower than the rise we have seen in the past 100+ years (especially the past half century).
If someone wants to come up with a theory that gain we have seen in atmospheric CO2 concentrations is not due to human emissions they must explain several things in relation to the record:
1. How an ocean that is still taking up CO2 today would not be taking it up if humans were not emitting CO2
2. The timescale difference - Why is this increase so fast?
3. Why did CO2 concentrations not rise in the past 1 million years to levels we have seen today?
Someone who could come up with a theory that did answered those questions without violating any other chemical or physical property of the natural world would likely be in a position to shake up the earth sciences quite a bit.