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.
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.