Air From Another Age

Snow that falls on central Antarctica may remain instead of melting. Later snow buries it, pressure turns it into ice, and closing pores trap samples of the surrounding air. A bubble is not labeled with a year, but it preserves gases from an atmosphere far older than any instrument record.

Drilling brings the layers up in order, with younger ice above older ice. Earlier Antarctic cores produced a continuous climate record reaching about eight hundred thousand years. A newer core from Little Dome C reaches at least 1.2 million years, although researchers must still analyze and date its deepest sections carefully. “Oldest” therefore describes both an achievement and an active measurement problem.

Two kinds of evidence come from a core. Air bubbles let researchers measure gases such as carbon dioxide and methane. The ice itself holds indirect temperature clues. Ratios of heavier and lighter forms of oxygen and hydrogen vary with the conditions in which snow formed.

The gas record and the temperature clues do not share exactly the same age at a given depth: air is sealed only after snow has already begun becoming ice. Scientists model that age difference and compare several dating clues. An ice core is a powerful archive not because every layer speaks plainly, but because many physical records can be tested in one ordered column.