Climate is always changing. It's never really in a steady state. Climate has changed many times in Earth history, and of course is changing rapidly now. Now we have to remember that climate change always has a reason, it doesn't just happen all by itself. There has to be some underlying cause of climate change. We of course know why climate is changing now, say over the past 100 years, that's clearly us. Our activities loading the atmosphere with greenhouse gases. But climate changes in the past, I know it's the same thing, they all had a cause, it may have been a different cause. Well, clearly it was a different cause and us humans weren't around. But it's very valuable to understand some of these past climate changes, to see how things have changed, and try to get a grasp on why things have changed. So let's spend a little time now talking about climates of the past, the study of paleoclimatology. Now, why is paleoclimatology important? I've already hinted at it. First, it tells us what the climate system is capable of doing, even without human activities, what is it capable of doing? Also, very important, the causes of climate change. I just emphasized again, all climate change has a reason, it doesn't just happen all by itself. It can also inform us of what the future may hold. If we understand past climate changes and some of the consequences and impacts of that, we might get a better handle on what we can expect in the future. It gets back to the issue that one of the keys to understanding the future is understanding the past. Now, to get a sense of the antiquity of the Earth and some of the climate changes that have occurred, it's usually just take a look at the geologic timescale. Right now Earth is of course, very ancient, and that was at 4.25, 4.28 billion years, I think that's the best date out there, something like that about how old the Earth is. But just look at Earth history, there was the Precambrian, long, long, long ago, the Paleozoic, the Mesozoic, that was the age of the dinosaurs, the Triassic, Jurassic, and Cretaceous. Then the dinosaurs die out and we move into the Cenozoic, a more modern era, and of course, there's many ages within that. Right now we're in what's called the Holocene, which is the most modern area. The modern era is called the Holocene. There are some who say we've moved out of the Holocene into something new and that's called the Anthropocene, the age of humanity. But the point is if you look at this geologic timescale on the big divisions between them, certainly the big ones, they're very much tied into climate change because that's how we can differentiate between these times. These markers often are really stating climate changes in the past, some really big ones. Now, what might have caused some of these really long climate changes in the past over like tens to hundreds of millions of years? Well, continental drift certainly is one of them, because with continental drift we're changing the very configuration of land and oceans and how much ocean we have, how much land we have, this sort of thing that can very much influence our global climate. Because when we talk about climate change, the real thing to always remember is we're looking for something that alters the energy budget of our planet. If we have more longwave radiation going out to space than shortwave coming in, we cool. If it goes the other way, we warm up. So what can cause those sorts of things? In the distant past, things like continental drift, sure, that could be one of these things that can drive it. Now, here's the question, is continental drift still occurring? The answer is very, very much yes. Anything you look at from the San Andreas Fault and the earthquakes in California to the North Atlantic Ridge and the spreading of the continents, in other words, North America and Europe spreading further apart as time goes, these are all evidence that our planet is very geologically active and continental drift is still occurring, absolutely. Now, looking at some of these past changes, pretty remarkable, some of the things we have evidence for. Snowball Earth, 700 million years ago or thereabout, it seems that the Earth experienced episodes of global cooling called Snowball Earth, where the entire planet was Arctic. There is evidence for this. Now, what could have caused Snowball Earth? Let's ask another question. Well, we're not quite sure. Yeah, extended vulcanism might have been part of it that we have a whole series of volcanic eruptions going on, putting junk up in the upper atmosphere and reflecting solar radiation away. Continental drift? Yeah, that might be it. The sun got dimmer. That we got less energy from the sun. Really the answer is here is, all of these are contenders. We don't know what really caused Snowball Earth because we had nobody back there looking at it, to tell us what was going on. But again, it clearly had a cause. The fact that we don't exactly understand the cause doesn't say that there was no cause, but it's remarkable that the earth could have done something like that, going into something like a Snowball Earth situation. Now of course there's catastrophes, the end of the dinosaurs, 65 million years ago. A big meteorite hit the Earth somewhere down at the Yucatan Peninsula, is very good evidence for that and well, there is something that'll cause climate change in a hurry and yeah, catastrophes. We see these at the geologic record and they can cause tremendous climate changes. If we move into the little more modern era, we have strong evidence over the past several million years, ice ages and interglacials between them were paced by what we call Milankovitch cycles, variations in earth orbital geometry. We find, for example, that the Earth's orbit about the sun is not circular, it's ecliptic. That means at one time in the year we're further from the sun then another time of the year we're closer to the sun. Of course, that's going to influence how much solar radiation you get. Also Earth's tilt varies. Now that Earth's tilt with respect to the ecliptic is 23.5 degrees right now, but it's changed. It's gotten smaller, it's gotten bigger. Now if it gets bigger, that means seasons are more extreme. In the Northern Hemisphere, a bigger tilt says, well, colder winters, but much warmer summers. Of course, everything is reversed in the Southern Hemisphere. But also the Earth basically precesses, it wobbles on its axis. This tells us what's important here is it affects the time of year at which summer occurs. In other words, do we have the summer solstice, one where, tilted more strongly towards the sun in the Northern Hemisphere, does that occur at the same time when we are closest to the sun or furthest from the sun? These effects influence how much solar radiation you get at the top of the atmosphere at different latitudes and at different times of year. We find that the ice ages of the past, over the past several million years seemed to have been paced by these, triggered by these. But what happens also, of course, is that if we have a climate forcing of any kind, there are feedbacks that kick in, albedo feedback. You warm things up, snow and ice melts, exposes the darker underlying surfaces, and you absorb more solar radiation or we warm it up, and that means the atmosphere can hold more water vapor, but water vapor is a greenhouse gases. There's all feedback, some of them acting fairly quickly like a water vapor feedback, others lasting much longer or taking much longer to kick in like carbon cycle feedbacks. But yeah, Milankovitch cycles are definitely something that we can associate with climate changes. Now, of the last ice age, we had peaked somewhere around 25,000 years and we've had a series of ice ages and interglacials. At 25,000 years ago, a lot of North America was covered by a big ice sheet, the Laurentide Ice Sheet. Part of Northern Europe was also covered with ice, sea level was 120 meters lower than today. Pretty remarkable. But these ice ages and interglacials, including the most recent interglacial period that we had peaking 25,000 years ago, seems to have been paced by these Milankovitch forcings. Going even a little more of going definitely more recently, the Little Ice Age of the 17th and 18th century. Well, this evidence that this could be caused by a number of things like volcanic eruptions where the sun got dimmer. Now, art is very interesting in that it provides us some paleoclimate information. What I'm showing here is a picture or a piece of art by Avercamp called "The Pleasures of Winter", which is a scene from England. Well, right now, we don't see people ice-skating in England outside in the winter. But back then apparently, it was cold enough that happened. So it's interesting. A source of paleoclimate information is art, interestingly enough. I mentioned the causes of the Little Ice Age. Well, we think that solar variability played a part in that. There was a period called the Maunder Minimum back then when the Little Ice Age was occurring of a minimum number of sunspots. We know that when you have fewer sunspot, we get a little less solar energy from the sun, so that will be a cooling agent. Now, also, during these solar cycles or when we have a solar minimum, it's also changing the quality of the solar radiation that comes in, notably the ultraviolet radiation. That starts to implicate the stratosphere because of course, what happens in the stratosphere is the stratosphere absorbs ozone, is going to affect stratospheric temperatures, and that could in turn, in fact, influence the climate down at lower levels in the atmosphere. We know we have an 11-year sunspot cycle that's part of just the overall solar cycle. We can actually see that affecting climate records if you look closely. But there was this period back then where there were fewer sunspots, the Maunder Minimum, and it is very good evidence that this is part of the Little Ice Age. Now, I mentioned also extended volcanic activity could have played a part in this. If we have a single volcanic eruption go off like Pinatubo, I think that's the one I'm showing here on the right, then well, that can last a few years. The effects on the climate can last a few years. But a whole series of them going off, yeah, there's evidence that that was a cause too, so the Little Ice Age seems to be a combination of solar variability, less sunlight getting in, and extended volcanic activity at the same time. Now, let's take a look at Arctic climate over the past 60 million years. There's only so much we can say about it, but we can say a few things. This is a bit of a complicated figure showing here, but it's showing on the x-axis, time, on the left, 60 million years ago, to the right, all the way over, basically to the present. That blue line at the bottom low with all the squiggles, that's an estimate of temperatures over the Arctic. What we can see is if we were around 50 million years ago, 60 million years ago, back during the Paleocene, it was quite warm apparently on the Arctic. But then we can see that temperature started to slowly drop during the Eocene and then what we call the Oligocene. It was around 40 million years ago that there's the first evidence of sea ice. Things continued actually to cool off through the Miocene and onward, and that's when we started to see the first evidence of perennial sea ice, that is sea ice that last through the melt season, multi-year ice. So an overall cooling period. But of course, right now, things are very different. Things are warming up quickly. But remember what the axis here said. This is over millions and millions of years, so what's happening over the past one million years isn't even showing up here, because what's happening now, of course, is a very different thing. This is looking at Arctic climate over the past 2,000 years from various reconstructions. On the top is the carbon dioxide record and it's showing that we are going along with the past 2,000 years, carbon dioxide levels stay fairly steady and wham. They're well over 400 parts per million right now. Of course, that's a big driver of climate because carbon dioxide is a greenhouse gas. The one in the middle on the red, that's showing an estimate of temperature, so we're going along over the past 2,000 years, and wham. The one on the bottom is showing sea ice extent as well as we can reconstruct. We're going along, things aren't changing much. Then wham, we fall off the edge and we're losing it quickly. What we're seeing is overall, there was this cooling, overall cooling. We had a series of ice ages that we'll talk about later. Some interglacials between them, but still, there was an overall cooling across the Arctic until recently where things really turned around, and that's us. Here's the carbon dioxide level measured at the Mauna Loa Observatory. This one here is going through January 1st of 2020. We're up to something around 410 parts per million. The baseline, we were probably a couple of thousand years ago, around 280, something like that. But it's really gone off the charts and that's awesome because the thing is, climate never changes all by itself. Whatever the climate changes were in the past, they had to have a cause. We don't necessarily always understand the cause because we had no one back then measuring things, but we certainly know what has been going on on our planet over the past 100 years. Thank you.
