Atmospheric carbon dioxide measurements at Mauna Loa, Hawaii. These measurements show the increase in carbon dioxide since 1960. The sawtooth appearance of the curve is the result of annual cycles resulting from seasonal variation in CO2 fixation by the Calvin cycle in terrestrial plants (see insert). Much of this fixation takes place in rain forests, which account for approximately 50% of terrestrial fixation.
[(Left) stillfx/age fotostock. (Right) Data from http://www.esrl.noaa.gov/gmd/ccgg/trends]