Vostock Ice Cores
Ice cores are unique since they trap air, enabling direct records of past changes in atmospheric trace-gas composition. This enables us to measure the effect humans may have had on the environment by looking at climatic conditions and cycles in the past, and comparing them to today.
Drilling at Vostock station, East Antarctica, has allowed the extension of ice records of previous atmospheric composition and climate, up to the past four glacial-interglacial cycles. This stretches back approximately 160,000 years and highlights changes between glacial (colder) and interglacial (warmer) periods. Besides this 100,000 year cycle (between glacial periods), the CO2 record seems to exhibit a cyclical change with a period of some 21,000 years.
For more information on the extraction see: https://cdiac.ess-dive.lbl.gov/trends/co2/vostok.html
CO2 records have reached an all-time high https://climate.nasa.gov/climate_resources/24/graphic-the-relentless-rise-of-carbon-dioxide/
Wikipedia: Vostock Ice Cores (see the Ice Core Dating section)