The Air Trapped in Antarctic Ice

Look closely at a slice of deep Antarctic ice and you'll see it studded with tiny bubbles, like champagne caught mid-fizz. Each one holds a mouthful of ancient sky.

The mechanism is pleasingly simple. Snow falls, doesn't melt, and gets buried by the next season's snow, and the next, for millennia. Under the accumulating weight, the buried layers compress into ice—but before they seal shut completely, they trap pockets of the air that was drifting over Antarctica at the moment of burial. That air stays put. A bubble two kilometres down has been sitting there, unopened, since long before anyone invented the wheel.

This turns an ice sheet into a filing cabinet of atmospheres. Drill a core, saw it into sections, crush each section in a vacuum chamber, and you release air that no one has breathed since the Bronze Age. Feed it into a mass spectrometer and you get direct measurements of the carbon dioxide and methane that were actually present—not a model, not an estimate, but the genuine article, decanted from a bubble.

The results reach astonishingly far back. Cores from Vostok and Dome C in East Antarctica have unrolled a continuous record stretching past 800,000 years, capturing eight full ice-age cycles. Australia's Million Year Ice Core project, drilling at a spot called Little Dome C, is chasing air older than a million years—old enough to catch a puzzling shift in the planet's climate rhythm that no one has fully explained.

Reading these bubbles requires a certain care. The deeper layers are thin, squashed by pressure into annual bands only a millimetre or two thick, so a metre of very old ice can hold centuries. And gas moves slightly through firn—the granular, not-yet-sealed snow near the surface—before the bubble closes, so the air is always a bit younger than the ice surrounding it. Glaciologists spend a great deal of effort accounting for this gap.

The upshot is a graph that has become one of the most quoted in science: carbon dioxide bobbing between roughly 180 and 280 parts per million across those glacial cycles, tied tightly to temperature, and never once breaching 300 in nearly a million years—until the last two centuries, when it shot past 420.

There is something quietly astonishing about it. A bubble the size of a pinhead, sealed while mammoths still walked, kept its little parcel of prehistoric weather intact through eight ice ages, all so that someone with a saw and a vacuum pump could finally let it out.