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Sometimes air gets trapped in unusual places. When these types of things are found, that air can be tested with modern equipment.

"Ancient air pockets changing the history of Earth’s oxygen"

https://www.sciencedaily.com/releases/2016/08/160822174234.h...

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Thanks, though that study makes no mention of CO2, and concerns instead oxygen from 500m, to 1b years ago, which seems a little far off to be applicable to the order of the around one hundred and fifty years ago being asked about. Have I missed something?

My comment only explains how it's done. I'm sure you can understand that the article I mentioned was just an example. If you're looking for a specific CO2 measurement from air captured exactly one hundred and fifty years ago, I'll leave you to Google that yourself.

Well, here's the only solid thing with referenced figures that came up. Chart on page one is quite the ride, as I'd have expected for early science:

https://21sci-tech.com/Subscriptions/Spring%202008%20ONLINE/...

Everything else I found was unfortunately hand-wavy "scientific consensus says" and the ubiquitous "280ppm" coupled with "pre-industrial" repeated again without reference, nor apparent empirical basis.

Not convincing, which for "the science is settled" I'd expect a bit of a higher standard.

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