Dotted vertical lines coinciding with January 1 each year are included to highlight the seasonality of trends in concentrations of each gas. Scatter points are globally-averaged monthly means determined for 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. 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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