Thus, net CO2 from natural causes must average very close to 0 on long time scales.
PS: If you look into it natural carbon sequestration increases slightly as atmospheric CO2 increase which is why things end up in balance. Natural carbon sequestration is also why there are huge sources of coal and oil to begin with.
Which is to say, the natural net contribution to CO2 in the atmosphere is negative. If we somehow put a stop to natural processes involving CO2 and just carried on with human activity, the rate of CO2 accumulation would go way up.
This is the relevant bit for how much emitted CO2 has accumulated in the atmosphere:
"From 1870 to 2014, cumulative carbon emissions totaled about 545 GtC. Emissions were partitioned among the atmosphere (approx. 230 GtC or 42%), ocean (approx. 155 GtC or 28%) and the land (approx. 160 GtC or 29%)."
If you want to double-check against increasing CO2 concentration, the mass of the entire atmosphere is about 5.15e18 kilograms, so one part per million is about 5e12kg or 5 gigatonnes.
The preindustrial CO2 concentration was roughly 280ppm. We're now at about 400ppm, so that's 120ppm or about 640 gigatonnes more CO2 in the atmosphere today. Things are a bit confusing here because for some reason emissions are measured in gigatonnes of carbon alone, not CO2, so you need to multiply emissions by 3.67 (the mass ratio of CO2 to just C) to get CO2. Taking the cited 230 GtC added to the atmosphere and multiplying by 3.67 gets us 873.46 gigatonnes, which is roughly in the same ballpark, considering this is an off the cuff internet comment using random googled sources.