Ice cores preserve annual layers of compressed snow containing trapped air bubbles (direct CO₂, CH₄), dust (wind patterns, volcanism), isotopes (δ¹⁸O, δD as temperature proxies), sea salt (ocean circulation), and sulfate (volcanic eruptions). The EPICA Dome C core reaches 800,000 years — eight glacial-interglacial cycles.
Three orbital cycles pace ice ages: eccentricity (95/125 kyr, ellipticity of orbit), obliquity (41 kyr, axial tilt 22.1°–24.5°), and precession (19/23 kyr, wobble of rotation axis). Before ~900 kyr BP, glaciations followed 41 kyr cycles. The "Mid-Pleistocene Transition" shifted to 100 kyr cycles — not fully explained, possibly linked to ice sheet physics.
CO₂ and temperature track tightly (r≈0.9) over the EPICA record. CO₂ varied from 180 ppm (glacial) to 280 ppm (interglacial) — until 1850. Current CO₂ (~420 ppm) exceeds any value in 800 kyr. Radiative forcing: ΔF = 5.35 ln(C/C₀) W/m². The CO₂ lag (~800 yr) during terminations means CO₂ is an amplifier, not the initial trigger.