 I'm Bill Ruderman. I'm a emeritus professor at the University of Virginia and a long-time employee previously of Columbia University's Lomontdority Geological Observatory. I work on past climates, on timescales ranging from tens of millions of years to a few thousands of years. Well, basically there are two factors, two and a half factors in the orbital cycles. One is very simple. It's the tilt of the orbit, whether the Earth and particularly the high northern latitudes or ice sheets grow, whether it leans well towards the sun, which keeps ice sheets from growing, or whether the tilt is smaller and not so much towards the sun which reduces the sunlight into high latitudes and allows ice sheets to grow. So it's the amount of tilt. The other factor is that the Earth, when it orbits around the sun, it wobbles, it precesses. So the orbit is not perfectly circular, it's eccentric to a small extent, and so you can have a summer occur with the Earth in the northern hemisphere tilting towards the sun, but you're far away from the sun, or you can have one at a time when you're in a part of the orbit where you're tilting towards the sun and getting a lot of sunlight. So that also affects climate. So together those make cycles of 100,000, 41,000, 23,000 years. By the timing of the CO2 changes, which you can see from measurements of air bubbles in ice cores, basically CO2 is high when climate is warm during the times when ice sheets are small, we're in interglacials, and CO2 is lower when we're in times of glacial climate when the ice sheets are on North America and Scandinavia. So ice sheets and CO2 go together and they're coupled in some way. There's a bit of a chicken and egg argument that the CO2 changes lead and push the ice sheets to the ice sheets go first and bring the CO2 along with them. My opinion is that the ice sheets go first and that the CO2 is an amplifier. There is a myth that global temperature leads CO2, but in recent work shows that's not so. They seem to be very closely coupled. Temperature is most easily measured in the Antarctic region, and the Antarctic region is an early responder that changes in the orbit. The real question is whether CO2 leads, well, you can ask whether CO2 leads global temperature, and the ice sheets are playing a role in global temperature, very strong role, and so if the ice sheets determine temperature for a large part of the northern hemisphere, and that probably lags CO2 very slightly. So you have two things going on, insulations making the ice sheets grow and shrink. The ice sheets are affecting CO2, but CO2 can react faster than the push it gets from the ice sheets. I was part of the Climap project that mapped global temperature at the time slice of 21,000 years ago, and Climap got a good estimate of perhaps 4 to 5 degrees centigrade cooler than today. Then later came arguments how much of that was because the ice sheets were there and they were chilling the northern hemisphere, and how much of that was because the ice sheets had caused CO2 and other greenhouse gases to drop, and that was cooling temperature. As I recall, it ends up about 50-50. So CO2 is an amplifying feedback, and it's a big one. It does not lag global temperature. I think, no, pretty effectively that when CO2 values in the atmosphere are low, there's carbon that's not in the atmosphere. It's not on the land because the ice sheets have gotten rid of forests and it's dried out the climate, and it's not in the surface ocean because the surface ocean is coupled to the atmosphere in such a way that if the atmosphere has lower CO2, then the surface ocean has lower CO2. So it has to be in the deep ocean, and that's known firmly. But why and how it gets there is still a question. There's two, three, four competing hypotheses, and it may be one of those messy situations where several things together make that happen. But we know that it happened, but we don't know how. I'd say it's in flux. If you go back to the 1900s, right up to the turn of the century, everyone believed that climate during the Holocene was warm because of natural factors. The orbital forcing had pushed us out of the last glaciation into this interglaciation, and the orbital forcing hadn't yet got to the point of pushing us back into the first step of a glaciation. So in 2003, I suggested that humans had played a role in that and had kept the climate warmer, starting around 7,000 years ago and building, building through the millennia, even before the Industrial Revolution. So that's ten years ago that I made that suggestion, and there's been a real vigorous debate on that. I'd say there's at least 50 papers published, and the literature is scattered all over the place in different journals, and so there's very few people that are up to date on the current state of the argument. I'm up to date, and in my opinion, we're just about to go from a majority paradigm that this interglaciation was natural until the Industrial Revolution to the view that humans started taking control increasingly and that we wouldn't be in a full interglaciation if it hadn't been for humans, early farmers. It depends on latitude. If you're in the tropics, it would be a little bit cooler, but if you're up in the Northern tier, let's say Siberia, Alaska, Brooks Range, Northern Rockies, Canadian Archipelago, Norwegian Alps, you'd be under permanent snow cover, which would have turned into the beginnings of ice sheets, small ice sheets. So up there, it would be radically different, and one effect that would have projected farther south is that it would be cooler in the summer at Northern mid-latitudes, cool enough that you couldn't grow crops as far north as you do now. I decided I wasn't going to be a good engineer, and I wasn't going to be a good architect in college, and I thought, well, what else do I like? I like geology, so I took a geology, leaned towards a geology major. I was taking a paleontology course, and on the last day of the class, the professor brought in a vial of planktonic form and every little sand size, guys about like my size of my little finger, and a paper that showed that there had been cycles in climate going back several hundred thousand years, and that these little guys in the vial were the way that you could tell that, and I just had one of those, I want to do that, and that was it. I kept making decisions that headed me in that direction. There was a real a-ha moment. I think the scientists who monitor what the skeptics or deniers are saying and shoot down their arguments are performing a useful service for those that haven't quite decided and are watching the arguments. I think most scientists, a huge majority of climate scientists, have more or less the attitude. It's not even worth trying to convince them. It's the public that you've got your eye on. It's such a small number of people and they're so entrenched. There's a few that have respectable reputations, but most don't. I guess I would say there are people who work like myself back on longer timescales and we know what the planet is capable of in terms of its temperature response, and we know to a first approximation the reasons why those changes have happened. There's not a lot of mystery left looking back at climate. As you come into the last 10,000 years, you see climates being very stable, maybe cooling a little bit because of these orbital factors. Then you see this unprecedented takeoff, too fast and increasingly too large to be explained by anything we know from the natural climate system. I guess I would say nature can't be doing it. I've spent my life, good part of my life, helping define how climate has changed in the past and myself and lots of other people have shown why. They're using half of the fact that we have defined these past climate changes, which is true, and they're natural. They're leaving out the most important part of the argument, which is that we basically understand those changes. That leads to the argument I just made. None of those causes in the past can explain a fast and soon to be huge warming.