We know what global temperatures are like now, from direct measurement around the globe. And we know quite a lot about what temperatures were like over the past few hundred years thanks to written records. But what about further back than that? How do we know what temperatures were like a thousand years ago, or even hundreds of thousands of years ago? There is, of course, no written record that far back in history – but there is a chemical record, hidden in the ice of Antartica and Greenland.