Analysis of the air enclosed in polar ice (∼0.1 ml per g of ice1) provides one of the most promising ways of discovering atmospheric composition over the past 100,000 yr. Until now, because of an apparent but not well understood enrichment of CO2 in the trapped air, all attempts to reconstitute the ancient atmospheric CO2 content from polar ice have failed2–6. We have obtained CO2 contents that can reasonably be considered representative of present or past atmospheric contents using a new method of air extraction. The results reported here, based on the CO2 analysis of two deep Antarctic cores including the last climatic interchange, strongly suggest that during the coldest part of the last Ice Age (20,000–15,000 yr ago) the atmospheric CO2 content was half (0.016%) that of today's level (0.033%).
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Delmas, R., Ascencio, JM. & Legrand, M. Polar ice evidence that atmospheric CO2 20,000 yr BP was 50% of present. Nature 284, 155–157 (1980). https://doi.org/10.1038/284155a0
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