How do they know that, exactly?
I'm actually very curious to know.
what, measure co2 levels? they take ice cores, the air bubbles in them have been trapped for super long periods of time and they do measurements on them
that's for stuff from a long time ago, for recent years they just have it on record
edit: oh and they can use tree rings too, not very familiar with that method though, i could try to explain but it wouldn't make sense. if you want ill look it up. there's probably several more methods heh