 This is a wonderful, but a little bit disturbing story of some really smart people living in southwestern Colorado, ancestral Puebloans, at a place called Mesa Verde. They lived there for hundreds of years and left shortly before Columbus got to the New World. A lot of their farming was done on top of a mesa, and they lived in caves down the side of the mesa, and these wonderful structures made mostly out of stone. Here is a modern ladder with a person for scale, and what I want you to notice here, in addition to the great new buildings, is up here on top. This fairly clearly was a granary. This is where they kept their food late in their existence there. It's probable, not certain, but it's probable, that this is a little bit like a modern person putting the cookies way up on the upper shelf so they won't eat them before they're ready to, except this wasn't the cookies. This was the food, and that might be something you'd do if food was really getting scarce. Now that structure we just looked at, if you go behind it, you find it's actually built in a cave, and this is sandstone, and there's little bits of shale, and water sort of percolates down through here, and then it hits the shale, and it seeps out, so it's a little bit damp back there, and a generally dry climate. And if you look around back here on this rock, do you see things like this? Little holes that they dug in the rock, so that when water came seeping out, it would fall into these holes, drip in, and you could take a cup and get a little bit of water. And that's what you do if you're really thirsty, because that doesn't look like the best water for us, but if that's all the water you have, that's what you do. Now they built these wonderful structures, and some of them had windows between, say, your house and mine, or your room and mine, and late in the occupation it appears that they walled the windows off. And usually when you're walling the window off, you're trying to keep something out. And it is possible that they were not getting along with their neighbors quite as well as they might once have if they're walling off the windows within the structure. Now if you go up on top where they grew the corn, this actually was built by the ancestral Puebloans hundreds of years ago, and it stops erosion, so there's a little tiny stream comes through here, but this little dam of rocks catches a little bit of soil, where you could grow a little bit of corn, and it traps a little bit of water and a little bit of soil. And so you're looking at people that really were shepherding what they had conserving, and apparently they needed this. Now what you see then is people who were living on the edge, hard to get enough food, hard to get enough water, life was getting hard. Now if you look around the structure it's mostly made of stone, because there weren't many trees, but there's a little bit of wood in the houses there at Mesa Faraday. You know anything about wood, you know that it has layers, and the layers when the tree is happy it grows a thick ring, and when the tree is unhappy it grows a thinner ring, and you can see different thicknesses of rings in this, and you can take cores, and you can count the layers, and count how thick the layers are, and in a place like Mesa Faraday where it's very dry, a happy tree is one that has rain, and an unhappy tree doesn't. It's also possible if you have a living tree you can take a core from it, and you can see the pattern of thick and thin rings in that core. There might be dead trees nearby, and you can see the pattern of thick and thin rings in that, and you can match them up between trees, and you can go back to the wood in the archaeological sites, and you can find the same pattern, and so you can make a tree ring record which is longer than the life of any given tree with the thickness of the ring telling you how much it rained. So now what can you do? Archaeologists and tree ring people got together, and they made the plot that you're going to see here. The year 800 is on your left, the year 1200-1300 there, which is sort of the crash, and so first of all you have in green here a history of how many people could live in the area, and this is actually done just down the road from Mesa Verde at a place called Longhouse Valley that had less trade, so it was sort of easier to work on, and what they did is they looked at the trees and they said how thick a layer is is how much it rained. Rain also tells you how much corn could be grown, and corn is how many people could live there. So the green curve is from trees and from knowledge of the people, how many people could live in this place. Independently, archaeologists went in and they looked at the classical things, how many people were living there, you know, how many burials and how many houses and that sort of thing, and so they looked at how many people were living there and that's this curve. And what you'll notice is sort of within the scientific uncertainties of all this, these are the same record. How many people lived there and how many people could live there are essentially the same. When it got wet, population went up. When it got dry, we don't know whether they died or whether they left or whether they quit having babies, but the population went down. And the rains came back and so did the people and then the rains left and at some point the people said we are out of here, we're going to become environmental refugees and we're going to go someplace that rains more and they left. And so these are very smart people doing amazing things but they were controlled by the climate ultimately and it got them in the end. Now this is more recent. This is Oklahoma from the year 1900 to the year 2000 and the red curve here now is a history of drought and down is really dry and this thing right down here is the dust ball and that made great literature but it made lousy living and you've probably seen the pictures of the the okies headed for California to get away from the dust. But look at the population history of Oklahoma and you'll see that while the dust bowl kicked them and then World War II, some probably going off to war, it's not nearly the disaster it was for the the ancestral Puebloans. Now these are also smart people but these are people that when the drought hit some of them got in gasoline-powered cars and they drove away but some of them had food delivered in gasoline-powered cars or in diesel-powered trains. They had diesel-powered well drilling that they could drill a well and they could run motors or run windmills to pump water out and so they had more tools and they had more stuff and those were fossil fueled. And more recently when other droughts have occurred it really hasn't affected the population much at all. Little tiny changes but really not too much. And so what you can see is our having fossil fuels and having machines and having trade has greatly insulated us from what otherwise would be disasters for even very smart people.