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So how do we have readings going so far back, like even in the late 1800s? Is this just an assumed average for back then?
Here is the Mauna Loa data line from 1958, matching the top figure.
https://upload.wikimedia.org/wikipedia/commons/c/c5/Mauna_Loa_CO2_monthly_mean_concentration.svg
The rest has been matched and synchronized to it using other sources. I'm not a climate scientist, but I would guess the best sources are ice samples from polar regions where it accumulates from top and melts from bottom. CO2 dissolves in water and when snow falls and turns into permanent ice in such places, it captures a snapshot of that period's atmospheric gas content, among that the CO2 level.
Tree rings are another used source for historical estimates
Usually measurements in ice core drillings.
Oh yea..didnt think of that. But i have heard of that method for lots of things!
old trapped air
