Values are globally-averaged monthly means determined from marine surface sites. Units are mole fractions expressed as parts per million (ppm) for carbon dioxide and parts per billion (ppb) for methane and nitrous oxide. Note that x-axes differ in scale for each graph. Data plotted are available from the U.S. National Oceanic and Atmospheric Administration (<a rel="nofollow" class="external free" href="https://gml.noaa.gov/ccgg/">https://gml.noaa.gov/ccgg/</a>).
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