CO2 vs smog are 2 different things. my guess is that in the 1700's there were more trees to take in more CO2. with progress in any civilations, trees will be cut and so there will be less CO2 being used by trees, thus the increase.
the only way that we would be in trouble is for every tree on the planet, or a high percentage, to be cut down. trees are CO2 filters.
im more concerned with the toxic chemicals that some of these landfills just let spill out on the ground and pollute the drinking water than i am with CO2 emissions.