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...