Carbon is measured in atmosphere in PPM, Parts per million, raising it a few doesn't matter much. IE if you had a pool and dropped a few thousand grains of sand into it, there wouldn't be much affect. Now imagine instead of a pool, it's a river that has been naturally flowing and changing over the course of the last few thousand years.
The fear is from feedbacks in that carbon creating more water vapor (Strongest and largest portion of our greenhouse gasses) which simply hasn't occurred as they predicted.

Moreso, carbon and heat has been far more variable than we previously thought. If you produce more carbon, vegetation is able to grow larger due to the higher amounts of food available to it. IE there are ferns in the cretaceous period (iirc) that went extinct because there wasn't enough carbon to support them.

Carbon cannot be good?

Deforestation isn't good. Dumping toxic waste in the ocean isn't good. Trying to ban cheap, affordable energy which has allowed our species to achieve unprecedented prosperity isn't good.

And yet nuclear is ignored as a green energy solution (despite 20,000 consecutive years of operation with only 3 incidents, even the worst of which didn't create a lifeless irradiated wasteland as some feared). Deforestation and toxic dumping is hardly ever discussed next to levying carbon taxes and giving more subsidies to green energy.

The jury is out about carbon. But many are proclaiming it to be settled and ignoring every other tangible concern.