fossil fuels are natural, the burning of them in the quantities we do is not.

I quoted an article already but here's another:

we humans are a significant contributor to global warming. But, in reality, the CO2 we’re breathing out is part of a natural cycle, by which our bodies convert carbohydrates from CO2-absorbing plants into energy, plus water and CO2. As such, we’re not adding any extra CO2.

which fully supports your statement