Researchers can track fluctuations in atmospheric CO2 by looking at ice cores drilled in Antarctica, Greenland, and some tall mountains in lower latitudes. The cores contain trapped air bubbles as fluffy snow was compressed into ice by the weight of newer, fresher overlying layers.
Some of these climate chronicles go back hundreds of thousands of years. But high-resolution data, tracking centennial-scale or shorter variations, have been lacking, in part because of contamination from techniques used to extract CO2 from the ice.
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