Every year, snow falls on the Antarctic ice sheet and, in the coldest, driest interior regions, never fully melts. Layer accumulates on layer, each one slowly compacting under the weight of the snow above it, trapping tiny bubbles of the atmosphere present at the moment it was sealed off from the world above. Drill down far enough, and those layers become a continuous, dated archive of the atmosphere stretching back hundreds of thousands of years. The deepest ice cores extracted to date, from sites including Dome C in East Antarctica, reach depths of nearly three kilometres and preserve an atmospheric record extending back roughly 800,000 years — through eight complete glacial cycles, long before anything resembling modern humans existed.
Reading a Bubble of Ancient Air
When researchers extract an ice core, they cut it into sections and analyse the gas trapped in air bubbles directly, measuring the concentration of carbon dioxide, methane, and other atmospheric gases exactly as they existed when that layer of snow sealed shut. This is different from, and generally considered more reliable than, most other paleoclimate proxies, because it involves measuring actual ancient air rather than inferring atmospheric composition indirectly from tree rings, coral growth bands, or ocean sediment chemistry. The ice itself also carries information: the ratio of different oxygen isotopes in the water molecules that make up the ice varies in a well-understood way depending on the temperature at the time the snow fell, allowing researchers to reconstruct a temperature record alongside the atmospheric gas record, layer by layer, year by year in the most recent, best-resolved sections.
What 800,000 Years Shows
The resulting record shows Earth's climate cycling repeatedly between ice ages and warmer interglacial periods, driven primarily by slow, predictable variations in Earth's orbit and axial tilt — the same Milankovitch cycles responsible for periods like the Green Sahara. Throughout this entire 800,000-year record, atmospheric carbon dioxide concentrations remained within a relatively narrow band, never naturally exceeding roughly 300 parts per million even during the warmest interglacial peaks. Current atmospheric carbon dioxide levels, driven by human emissions, now sit above 420 parts per million — a concentration this ice core record indicates has not existed at any point across nearly a million years of Earth's climate history, reached over the course of roughly two centuries rather than the many thousands of years natural cycles typically require for comparable shifts.
Confirming the Pace of Change
What the ice core record adds that shorter instrumental records cannot is a sense of pace. Natural transitions between glacial and interglacial states, though sometimes surprisingly abrupt in geological terms, still generally unfolded over centuries to millennia. The current rate of atmospheric carbon dioxide increase, clearly visible in the most recent, high-resolution sections of ice cores compared against the preceding 800,000 years, is unprecedented in the entire record — not merely the concentration reached, but the speed at which it has been reached. This is one of the primary reasons ice core data, gathered by international collaborations including the British Antarctic Survey, carries such weight in climate science: it is not a model or a projection, but a direct physical record of what the atmosphere has actually done, repeatedly, for most of the last million years.
A Library Written in Ice
Older, deeper ice — beyond the current 800,000-year record — is now being actively pursued by international drilling projects targeting sites believed to preserve ice up to 1.5 million years old, which would extend the direct atmospheric record across an important transition in Earth's glacial cycle timing that remains poorly understood. Every core extracted is, in a real sense, irreplaceable: a physical sample of a specific year's atmosphere that exists nowhere else on Earth, and that grows more scientifically valuable, not less, the longer the current period of rapid atmospheric change continues.