system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know that the atmosphere bubbles trapped in glaciers accurately represent past climates?

Do we have anything to compare these levels against, or can we be sure that we can account for any distortions?
gpt
Yes - we have several complementery proxies for paleo-atmospheric conditions; among the main ones and in no particular order:

Sulphur isotopes rations in evaporites (Temperature);

Stable isotope ratios in fossil material (Temperature);

Paleo depth variation data from brachiopod assemblages; re: Boucot-assemblages (Temperature/sea-level changes);

Poleward or equatorialward Migration of foraminifer assemblages (temperature);

Dendrochronology (Tree-rings records) for Temperature and rainfall patterns;

Stomatal count records (atmospheric CO2 concentration);

Sulphur isotopes in Sedimentary sulphides (Global temperature, oceanic conditions);

Stable isotope and micro gaseous inclusion records in speleothems such as stalagtites (Temperature, athmospheric composition);

Palynological records (Temperature, climatic and rainfall patterns through detailed floral characterisation of fossil pollen assemblages).

The awesome thing is that these various disparate methods give mutually reinforcing and correlateable data. Each has it's strengths and weaknesses: if you are interested in year to year variation in the recent past, you'd best look at dendrochronology, speleothems and ice-core data; if you want big-picture trends over several tens of millions of years you should focus on stable isotopes in evaporites, sulphides and carbonates.