Paleoclimate reconstruction from ice cores — 800,000 years of climate history
Ice cores from Antarctica (EPICA Dome C: 800,000 years) and Greenland contain annual layers — visible as light/dark bands of summer/winter ice — plus trapped air bubbles preserving ancient atmosphere. Isotopic analysis of ¹⁸O/¹⁶O ratios (δ¹⁸O) records past temperatures: colder periods preferentially evaporate lighter ¹⁶O, depleting ocean water and leaving warmer periods enriched in ¹⁸O. The Vostok and EPICA cores revealed eight glacial cycles driven by Milankovitch orbital forcing (100 ka, 41 ka, 23 ka cycles) with tight coupling between CO₂ (ranging 180–280 ppm) and temperature — a 100 ppm change corresponding to ~10°C global temperature variation. Modern atmospheric CO₂ at 422 ppm lies far outside this 800,000-year natural range.