 We want to know the long history of the climate, what happened thousands, even millions of years ago, but our thermometer records are very short and we can get a global estimate for only 150 years. How do we find out what happened before that? We have to turn to natural archives, what we call proxies, like these tree rings and corals and ice cores, and they can tell us about how the climate changed in the past. The layers of sediment in a lake basin, as long as they're relatively undisturbed, it's like reading a book and with the oldest part of the story at the bottom, and we can read the story of what happened from looking at the various kinds of organic molecules and the sediments themselves, and so then we can reconstruct what that climate history was like in this spot. We can compare our record for the last 3.6 million years with cores taken from Antarctica and from other parts of the world.