In "An inconvenient truth", Al Gore states that the level of atmospheric carbon dioxide goes up and down every year. When it's summer in the northern hemisphere, where there is the majority of the vegetation " the leaves come out and they breathe in the carbon dioxide and the amount in the atmosphere goes down". On the winter the vegetation exhales carbon dioxide and so the levels increase. This is what a college professor of him found out. He started making these measurements in 1958. By the middle sixties, when the teacher shown Al Gore's class, the graphic showed clearly that the levels were going up.
