It isn’t too much of a stretch to imagine given the amount of CO2 we put into the atmosphere through the burning of fossil fuels (almost 30 billion tonnes), that this may have a causal link to the amount of CO2 that remains in the atmosphere; so let’s explore the empirical evidence that demonstrates that the rising CO2 levels is from Fossil Fuels.
Firstly, it is important to note that the carbon atom has several different isotopes (eg – different number of neutrons). For instance, Carbon 12 has six neutrons, whilst carbon 13 has seven.
As plants have a lower C13 to C12 ratio than the atmosphere, if rising atmospheric CO2 were from fossil fuels, the ratio of C13 to C12 in the atmosphere should fall as the fossil fuels are burnt.
And indeed, this isotope ratio falling trend is exactly what is observed: http://www.bgc.mpg.de/service/iso_gas_lab/publications/PG_WB_IJMS.pdf
This is directly measured evidence that demonstrates that fossil fuels are the source of the rising CO2 in the atmosphere.