Atmospheric Science, the weather, and global warming

[jdramj]

Originally Posted by Frogberto

If you begin with the year 1750, generally accepted as the beginning of the Industrial Revolution, when the standard measure for CO2 levels was 278ppmv, add the known level of human-generated CO2 for that year.....

Ok not to be one of those "naysayers", but I'll be one. Exactly how does one measure the amount of CO2 in 1750?