Yakir burned more than 100 Globe samples at 2,000 to 2,200°F in a super-oxygen-rich oven, letting the carbon in the paper combine with the oxygen to make carbon dioxide. He then measured the carbon from the exhaust and used a mass spectrometer to determine its isotopic composition, particularly a key marker of fossil-fuel burning, carbon-13, and compared with to carbon-12. In this experiment, he was piecing together the story of the air that the trees were "breathing in" by photosynthesis each year. When the ratio of carbon-13 to carbon-12 goes up or down in the atmosphere, so does the ratio in the trees. That's what was reflected in the newspaper snippets.