That's a very misleading sentence.
There is something called the carbon cycle, much like the hydrologic cycle, by which carbon goes in and out of the atmosphere, with pit-stops in plants, animals, etc. The total amount of carbon in the system changes little if any over time.
Carbon added by burning fossil fuels, OTOH, increases the total carbon in the system.
It's a little like saying you're only adding 1% to the water behind a dam. This may be true, but if it causes the dam to collapse, that's a pretty important 1%.
Humans are at present the cause of pretty much all of the increase of carbon in the ecosphere.
I'm agnostic on what the effects will be, but that we are causing the increase is just a fact.
The amount that human activity increases the carbon in the whole eco system doesn’t amount to a fart in a hurricane. Volcanic eruptions, otoh, really DO affect it; but God CREATED it as a self-correcting SYSTEM.
Don’t be agnostic. There is no Greenhouse effect occurring whatsoever.