Anonymous
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Just to add to what's already been said- about 0.038%, give or take. See also reference 1, a graphical depiction of measurements taken at Mauna Loa. The regular variation is due to seasons, i.e.: reduced CO2 from spring/summer growth of plants, and an increase in the winter.
Daniel O
0,035 % of the volume of the atmosphere is CO2. And it increases every year with 4% due to burning of fossil fuels.
Edit: The information was 10 years old so today it is 0,035 * 1,04^10 % (I don't have a calculator right now so you have to work it out by yourself)
Anonymous
The average concentration as measured at Mauna Loa in 2000 was 369 parts per million by volume, or 560 parts per million by weight.
Pinolera
1%
iandanielx
It is exactly 0.04%...I just measured it.
It actually varies depending on where you are, since we add to it based on population and industry.