Mauna Loa is not the only source of historic CO2 levels, though it has the oldest data. You can see all the other ones that NOAA keeps track of through their da

Mauna Loa is not the only source of historic CO2 levels, though it has the oldest data. You can see all the other ones that NOAA keeps track of through their data interface [1]. Most of the data sources report very similar numbers, especially in the trendlines. [1] https://gml.noaa.gov/dv/data/index.php?category=Greenhouse%2BGases&parameter_name=Carbon%2BDioxide https://gml.noaa.gov/dv/data/index.php?category=Greenhouse%2...