Quote:
Originally Posted by jem
Among other methods - for ice cores it’s the isotopic composition of gasses trapped in tiny bubbles frozen in the ice. Another method is tree rings - although they only really work back a few thousand years, and ocean sediment cores.
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Also layers of rock sedimentation and radiometric dating. The rock literally is laid as eras pass, and geologists can tell what the atmosphere was like to form it. (We knew something happened at the time the dinosaurs went extinct via this method, long before we found the crater).