You can't have "average CO2" either.

"AIRS data have shown that carbon dioxide is not well mixed in the atmosphere and that its transport around the globe is greatly influenced by the mid-latitude jet streams. The AIRS data have also discovered a belt of mid-tropospheric air containing enhanced concentrations of carbon dioxide, appearing most strongly between 30 and 40 degrees south latitude during the southern hemisphere winter season. This belt had not previously been seen in any chemistry transport model and has only recently been confirmed via in situ aircraft measurements. The process by which this belt arises is not yet understood."

https://web.archive.org/web/20130329200800/http://airs.jpl.nasa.gov/composition/composition_files/shapeimage_3.png

CO2 distribution on earth, NASA image. What ppm or % you measure depends where you are. We don't actually average tem, we assume the whole world is the same as the collection location. Snicker.

https://web.archive.org/web/20130329200800/http://airs.jpl.nasa.gov/composition/