 I have to apologize for this one. We've done something to you that may be a little bit confusing. Today is over here on your left and this is 400,000 years ago over here on your right. So old is over here on the right and young is over here on the left and time goes this way. What's shown here first of all is the temperature in Antarctica. That's this blue curve down below here and what you'll notice is it sort of goes down and up and down and up. This is not temperature in the globe, this is temperature in a place in Antarctica which is called Vostok, but if you blur your eyes it is sort of temperature on the globe. And what you'll notice is that this really really really does not look like a random curve. It's sort of warm, cold, warm, cold, warm, cold, warm, cold, warm, cold. And you can see this just going tick, tick, tick, tick, tick. And if you look carefully you'll see some other faster sort of tick, tick, tick, tick, tick running down below here. There are techniques that people have worked out for you analysis to tell you what are the wiggles that went into making this curve. If you was on there this is what you get. There's a big tick, tick, tick, tick, tick at about 100,000 years spacing. There's one at 41,000 years and there's a couple at 23 and 19,000 years. And this is a remarkable thing because these are features of Earth's orbit and they were predicted decades before they were discovered. Molankovic and other workers before him said we know that sunshine on the planet is being varied by these things. And when you climate scientists finally get a good enough record you will find a peak under each of these arrows. And this one actually is an interaction of these two so they really predicted that one too. And so when it happened people actually found that. There's really no question that we could need to worry about that the ice ages are driven by features of Earth's orbit. They are not driven by CO2 or the brightness of the sun or continental drift. They're driven by wiggles in Earth's orbit. But these Earth's orbit wiggles have very little effect on the total sun that reaches the planet. All they do primarily is move the sun around. So some places we'll be getting more sun, other places we'll be getting less sun. And what's really strange I show you here is sort of mid-summer sun at the South Pole and when you see here when it was very cold mid-summer sun was actually high at the South Pole. It turns out that temperatures at the South really do depend on sunshine at the South. But they also follow sunshine in the far North. In fact the whole world follows sunshine in the far North. When ice was growing in Canada the whole world got colder in concluding places that were getting more sun. When ice was melting in Canada the whole world warmed up including places that got less sun. Now that's weird. Some places listened to their sun, some places ignored their sun. How did that happen? Well you'll notice the second curve up here. This is CO2 in the atmosphere. When the ice grew in Canada a huge number of things changed on the planet. Dust to the ocean, ocean circulation, winds, sea level, a bunch of things. And it shifted some CO2 from the air into the ocean. When the ice melted on Canada these things changed back and it shifted CO2 out of the ocean and into the air. If you try to explain why the temperature in Antarctica really wasn't following the sun in Antarctica the temperature at the equator wasn't following the sun at the equator. If you ignore the CO2 no one has ever explained it successfully. If you include the effects of the CO2 it all makes sense. And so the ice ages are caused by features of Earth's orbit but they're globalized by CO2. And that helps us to understand that CO2 really does have an effect. Now, how's we then look at the future? This is the same plot as you saw before except I've squeezed it down to show you the level that we will go to. People taking this course are likely to see us go off of this page if we don't change our behavior. Some of you are likely to live that long. And so it is, you know, this was important to this but we may be going here. Now it is indeed true that as the CO2 gets higher it takes more to make a big difference but we are making a very big change to the atmosphere in something that we have very, very high confidence will affect the climate.