I'm trying to figure out how much a given oil project (4 billion barrels over its lifetime) will increase atmospheric $CO_2$ concentration, and the numbers I get don't seem to make much sense. I calculated a barrel of oil to contain about $5.97*10^23$ molecules of carbon. And given an atmosphere containing $1.75*10^44 molecules$, the 4 billion barrels gives a negligible increase of 0.122 ppmv of $CO_2$ if its all burnt. Hopefully someone here can help explain where I went wrong. Even using larger numbers like the amount of petroleum produced since the start of the industrial revolution seems to translate to a far too small $CO_2$ ppmv increase.