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As the most abundant greenhouse gas in our atmosphere, CO2 has become a direct proxy for measuring climate change. Its levels have varied widely over the course of the Earth’s 4.54 billion year history, partly driving swings in our planet’s average temperature. 
Find out more about atmospheric CO2’s history here.
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While we have some understanding of CO2 levels throughout our planet’s 4.5 billion years of existence, the most reliable data covers the last 800,000 years. Changes in past atmospheric carbon dioxide concentrations can be determined by measuring the composition of air bubbles  trapped in ice from Antarctica. Drilling and extracting ice cores up to 3 kilometers (over two miles) long  has provided us with detailed information about the “recent” composition of our atmosphere.
The data from 20,000 years ago to 2000 years ago was reconstructed using marine sediments, one of the major natural carbon sinks.
Most of the past 2000 years’ records were found using lake sediments and tree rings, while the last 35 years of CO2 fluctuations have been precisely measured thanks to scientists at Mauna Loa observatory.

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We use this record as a baseline to compare current events to, and the post-industrial upward trend in CO2 concentrations is evident. Unfortunately, the trend is recent enough that the results have yet to fully kick in. The time lag between CO2 emission and their pollution and warming effect is around 50 years, and whatever changes we observe now are only the tip of the iceberg. 

References
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