Either that, or there already exist measurements of the relative level of CO2 up into the atmosphere (and CFCs, a much larger molecule which presumably would see more of an effect). (realclimate.org)
The researchers were permitted by the National Park Service to collect one new rock sample to analyze its magnetic properties, and added this to existing measurements made over 30 years ago from a few other sections of the tower. (scientificamerican.com)
Either that, or there already exist measurements of the relative level of CO2 up into the atmosphere (and CFCs, a much larger molecule which presumably would see more of an effect). (realclimate.org)