memes

When ice forms, it traps small airpockets in it, so we take samples of ice from the polar circles kilometres deep, date it, crush it to release the air pockets and measure the air contents. Then we can see how different the CO2 amounts were to now.

Source: there's a fuck ton of different articles that talk about this, and this is the first one I found of Google, search yourself for more.

Its ice core data, this nature article describes it if you can access it somehow. "Air bubbles in ancient ice cores" is what NASA says

Maybe spend a few minutes googling this stuff because it's been answered many times over and it just looks like you're deliberately trying to poke holes in what is ultimately very well established scientific fact.

To give you a TLDR though it's ice cores. You look at the air trapped in ice cores, the deeper you drill the further back in time you go because the ice and therefore the air trapped inside is older.