The amount of carbon dioxide in the atmosphere is measured at Mauna Loa, Hawaii, by the National Oceanic and Atmospheric Administration. Could the rising carbon dioxide be caused by the volcano?

Long-term records from Alaska, Hawaii, Samoa, and the South Pole all capture the rising trend of CO2. The yearly “sawtooth” pattern is caused by plant growth. During the growing season, plants use CO2 for photosynthesis. Over the winter, plants decompose and return some CO2 to the atmosphere. This seasonal cycle is largest in the Northern Hemisphere, because it has more land area than the Southern Hemisphere. But regardless of the seasonal swings, the long-term increase of CO2 is similar around the globe.
NASA
The amount of carbon dioxide (CO2) in the atmosphere is measured by many different methods, all around the world. By using more than one approach, scientists can be sure they’re measuring a global trend, rather than a local variation.
Full article in Ask NASA Climate.
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