Ice Cores: 800,000 Years of Climate in Trapped Air

Snow that never left

In the interior of Antarctica and Greenland, summer melt is rare enough that each year's snowfall survives to be buried by the next. Over centuries the weight of accumulating snow compresses the layers below into firn and then solid ice, trapping tiny bubbles of the atmosphere that was present when each layer closed off from the air above. Drill a vertical core through that ice and you are pulling up a continuous, dated archive of the atmosphere — the deepest cores from East Antarctica (EPICA Dome C, and the more recent Beyond EPICA project) now reach back roughly 800,000 and, in newer cores, well over a million years.

Two independent thermometers in the same core

Ice cores carry two separate temperature proxies. The first is the ice itself: water molecules made with the heavier oxygen isotope ¹⁸O evaporate less readily and rain/snow out preferentially over lighter ¹⁶O as an air mass cools and travels poleward, so the ratio δ¹⁸O (or the analogous deuterium ratio, δD) left in polar snow is a direct function of the temperature at which it condensed — colder means more depleted in the heavy isotope. The second is the trapped air itself: bubbles pinched off from the firn preserve actual samples of ancient atmosphere, from which CO₂, methane and nitrous oxide concentrations can be measured directly, not inferred.

δ18O(‰) = ( (18O/16O)_sample / (18O/16O)_VSMOW - 1 ) × 1000
  more negative δ18O  ->  colder condensation temperature
  bubble CO2 (ppm)    ->  direct atmospheric sample, no proxy needed

Because the ice and the air in its bubbles are not sealed at exactly the same depth — air keeps diffusing through the porous firn for centuries after the ice above it has closed — cores carry a well-characterised gas age – ice age offset (denoted Δage) that scientists must correct for before lining up the temperature and CO₂ records precisely.

Counting years: layers, and where layers run out

Near the surface, annual layers are visible as alternating summer/winter bands in dust content, visible stratigraphy, or seasonal isotope cycles, and can be counted like tree rings — this gives near-exact, calendar-year dating for the last few thousand years. Deeper down, layers compress to millimetres and become impossible to count individually, so chronologies switch to ice-flow modelling (physically simulating how the ice thins as it flows and is buried) cross-checked against known reference horizons — volcanic ash layers from dated eruptions, and matched patterns of atmospheric methane variability that appear near-simultaneously in both polar records.

The headline result: CO₂ and temperature move together

Plotted over 800,000 years, Antarctic temperature and atmospheric CO₂ track each other astonishingly closely through eight glacial-interglacial cycles, with CO₂ oscillating between about 180 ppm in ice ages and 280 ppm in interglacials — never approaching the 420+ ppm measured directly at Mauna Loa today. Careful dating of the terminations shows Antarctic warming beginning roughly a few centuries before the CO₂ rise, which is consistent with the accepted mechanism: orbital changes (Milankovitch cycles) trigger initial warming, which releases CO₂ from the ocean and amplifies the warming through the greenhouse effect — a feedback loop, not CO₂ acting as the sole first cause of each cycle's onset.

What the simulation shows

This simulation stacks core layers from the surface downward, letting you scrub through depth and watch a synthetic δ¹⁸O and CO₂ trace evolve together, with glacial and interglacial bands shaded to show how tightly the two proxies co-vary once you get past the noisy top few metres of firn compaction.

Frequently asked questions

Try it live

Everything above runs in your browser — open Ice Core Record and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.