Humans are not the primary source in the tricky sense that someone looking to deny the evidence couldn't deny.

But they are the primary source of the net emissions or, in other words, the increase of CO2 levels.

It's a bit like if your neighbor produces just enough grain for a year's worth of food for his family of 10 and you steal just enough for yourself. Technically, you aren't the primary source of the decrease in food, but you are the primary (and only) source of the fact that your neighbor's food supply are lower than needed.

https://www.youtube.com/watch?v=OWXoRSIxyIU

http://whatsyourimpact.org/greenhouse-gases/carbon-dioxide-e...

https://www.skepticalscience.com/human-co2-smaller-than-natu...

https://www.netl.doe.gov/research/coal/carbon-storage/carbon...

You can prove it to yourself by asking what happened to all the carbon in coal and oil extracted since the 18th century. It tracks the rise in atmospheric CO2, as you would expect, since we burned it.

I've been pointing this out to obstinate nerds for years. This really isn't complicated: human-generated CO2 does not magically disappear, and we know exactly how it interacts with infrared radiation. Just put these two simple facts together.