Antarctic ice holds an 800,000-year, unbroken record of Earth's air

Each year's snowfall traps a tiny bubble of that year's atmosphere. Drilling the Antarctic ice sheet and counting down through the layers gives a continuous CO2 and temperature record stretching back 800,000 years — and CO2 has never been close to today's level in all that time.

— Dieter Luthi et al., High-resolution carbon dioxide concentration record 650,000-800,000 years before present — Nature, 2008