 This figure, which comes from the IPCC and the CCSP and some other places have used it at various times, today is over here on your right, and this is 400 million years ago over here on your left. So this is deep time. This is real geology. And there's two things that's shown on this plot to start with. One, you'll see these blue things hanging down from the top at places like this and like this. This is glaciers. Back here, no glaciers. They're not here. And then what happened is at certain times the ice advanced towards the equator and then it melted away and then the ice advanced again. So you can think of this as back here where there's no glaciers. This is a time when it's hot, and this is a time when it's hot, and then in between this is a time when it's not hot. It's cold and down here it's not as hot. All right, now the other things shown on here are various ways to estimate how much CO2 was in the air. And what's down below is lots of CO2 and then there's actually a little dip right here that has become clear and here there's lots of CO2 and here it's low CO2 and here it's low CO2. And what you'll notice is that a hot goes with lots and not hot goes with not much and a hot goes with lots and not hot goes with not much. High CO2 gives you hot conditions. Low CO2 gives you cool conditions. And we understand that it is a CO2 causing the warmth, not the warmth causing the CO2. The mere existence of this correlation doesn't tell you that. That's described in the text and in a whole lot of scientific literature, but it takes more understanding to go from this correlation into causation. We will walk you through three times in the course, a really hot time when most of the things on the planet died, a hot belch that shifted a lot of living things around on the planet, and a cold time where Earth's orbits drove changes in the climate, but they were amplified by CO2.