That is the key unproven assumption. No one knows how much is too much and when the effects of human activity may cause real global consequences. All we have are a bunch of computer models made by bombastic scientists, claiming that the end is near.
The other poster was noting that human addition of CO2 to the atmosphere is trivial in magnitude compared to natural sources. That is a true statement.
I agree we don’t fully understand the process. There are no doubt both positive and negative feedback effects and we don’t known which are which.
However, if I have a “churning” of 1T tons of carbon in and out of the atmosphere every year, to pick an arbitrary number, it makes no difference in the concentration. We change the location but not the amount.
Adding a net 1B tons (arbitrary figure) by burning fossil fuels, while an insignificant amount compared to that being churned, is nevertheless significant because it increases the total. Whether that is significant for temperature over the long run remains to be determined, as will whether the effects will on net be positive or negative.
But that the concentration is increasing cannot be effectively disputed, not that the primary cause of the increase is the burning of fossil fuels.
There is a point beyond which CO2 density does not contribute further to the “greenhouse effect”,
that point has long passed, and global temperatures are going down.