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Ice cores drilled from the Antarctic plateau contain air bubbles trapped as snow compacted into ice. Because each layer corresponds to a single year of snowfall, researchers can date the trapped air with remarkable precision and measure the concentration of carbon dioxide it contains. The longest cores extend back roughly 800,000 years. What makes these records so valuable is not simply their length but the fact that they combine a temperature proxy and a greenhouse-gas measurement in the same sample, allowing the relationship between the two to be examined directly rather than inferred from separate archives.

According to the passage, what is the main advantage of ice-core records?