[cogitator]

Well, 19th century average atmospheric CO2 was 335ppm.

19th century average atmospheric CO2 never got higher than 300 ppm.

Current value is 384.

Recent Greenhouse Gas Concentrations

[(384-280)/280] x 100 = 37.1%. Sorry for rounding up.

[aruanan]

19th century average atmospheric CO2 never got higher than 300 ppm.

Many of the 19th century readings were higher than 400 ppm. A few were as low as a little over 250 ppm. The average for the 19th century readings was 335 ppm (Slocum, G., Has the amount of carbon dioxide in the atmosphere changed significantly since the beginning of the twentieth century? Month. Weather Rev., 1955(October): p. 225-231) These were actual measurements of CO2 in the atmosphere at the time, not from ice samples.

What your graph doesn't show is that the values were "corrected" because their actual CO2 content was found to be too high for what was expected for that time. For instance, the original Siple graph showed CO2 values of almost 330 ppm in 1891. Later the shallow ice values were moved arbitrarily about 8 decades to the right to make them match up with the values from Mauna Loa.