An ice core is a record of the atmosphere because snow that never melts traps air as it compacts, and the deepest Antarctic cores reach back some eight hundred thousand years. What they establish is not merely that carbon dioxide and temperature moved together, which was known, but the range within which both stayed across eight glacial cycles — a range that current concentrations have left entirely. The correlation itself is often misused in both directions. Through the glacial cycles carbon dioxide mostly followed temperature, driven by orbital changes and ocean chemistry, which sceptics cite; it also amplified the warming it followed, which is why the cycles were as large as they were.