Discussion
Assertion (A): Ice cores provide direct measurements of past greenhouse gas concentrations rather than proxies for them.
Reason (R): The trapped bubbles contain the actual ancient air.
Answer: D
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason is exactly why the distinction holds. As snow compacts into ice the pore spaces close off and trap samples of the atmosphere of the time, so measuring carbon dioxide or methane in those bubbles measures the ancient air itself rather than inferring it from something correlated with it. The one caveat is that the air is younger than the ice enclosing it, since the pores stay open for some centuries near the surface.
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