We've already talked about the importance of paleoclimatology as a science. How looking at the past can tell us what the climate system is capable of. Looking at the past also helps us understand where we are today and where we might go in the future. Now, we know something about Arctic climate going back say 60 million years, but a lot of those records are sparse, there is lot of uncertainty in them. As we get more towards the present, we know more. The records are more clean, there's more of them. Now, let's focus today on what happened over the past million years. We know quite a bit about what happened over the past million years. Now, we do know that there were a series of ice ages and interglacials period with a lot of land ice when sea level was a lot lower to end today, and interglacials like the one we're living in right now. Now we know that these ice ages and interglacials were paste or linked to these Milankovitch forcing exist periodic variations in earth orbital geometry that affected how much solar radiation you get at different latitudes and at different times of years. It's also clear that feedbacks amplified the changes on the short term, the water vapor feedback, this albedo feedback. On longer timescales, we start to speak of carbon cycle feedbacks that affect how much carbon dioxide is in the atmosphere. But also Milankovitch forcing doesn't explain everything, there's always uncertainties. Now, Here is a record of climate from the EPICA ice core, and this was going back basically almost 800,000 years ago. On the bottom, what I'm showing is deuterium isotope temperature proxy. This is what it is, it's an estimate of temperature changes based on deuterium isotopes, and that deuterium is an isotope of hydrogen. You see how it's going up and down. Now, so on the high-end that's where it was warm, those are the interglacials. On the bottom-end of it where it's low, those were the glacial periods and this probably roughly 100,000 years cycle here. Now, we are of course in a present day interglacial. Now, on the top is showing we constructed carbon dioxide level. Now, we can actually get carbon dioxide levels in the atmosphere from trapped bubbles in the ice cores. It's an amazing thing that we can do that but we can. One of the things we see is that the records of temperature and the records of carbon dioxide are pretty much sync to each other. When there's less carbon dioxide in the atmosphere, it's colder. When there's more carbon dioxide in the atmosphere, it is warmer. Now, over this ice core record, we're seeing that the carbon dioxide levels range from about a 180 parts per million during these glacials and up to maybe 280 parts per million during the interglacials. Now, this record doesn't show the modern period what's been happening over the past century or so. What we know, is carbon dioxide levels compared to what we see here are off the chart, they're like at 410 parts per million as I make this video basically off the chart. Here's a great example of how looking at paleoclimate records really puts the modern day in perspective that we are way above any levels of carbon dioxide that we had in the atmosphere over at least a 100,000 years or likely longer. Now, what's going on here? We know that today carbon dioxide is acting as a forcing. We put more in, the climate warms. But what we see in the paleoclimate record is that, it also acts as a feedback. If we have something that makes it a little cooler, eventually a feedback kicks in which takes carbon dioxide out of the atmosphere and largely puts it in the ocean, less carbon dioxide in the atmosphere, it cools. Then something happened, a climate forcing, that gives us a bit of warming. Some of that carbon comes out of the ocean, goes back into the atmosphere and helps to further warm things up. Here's an example of carbon dioxide can act as both a climate forcing as we've been seeing today and a feedback as we've seen in the past. Now, I mentioned that these ice ages and interglacials had been paced by these Milankovitch cycles. These are these periodical variations in Earth's orbital geometry which relate to the eccentricity of our orbit. That is, how far out it is from our circle. If it's a highly eccentric orbit at one time of the year, we're considerably closer to the sun than the other time of the year. There's also obliquity of the ecliptic. Basically, this is saying Earth's tilt. Earth's tilt right now is at 23.5 degrees. If it's bigger, which it has been in the past, seasons are stronger. If there's more tilt, Northern Hemisphere summers are warmer than they are today, but also that means that winters are cooler. There's also this precession effect, the Earth wobbles like a torque, and the importance here is that this tells us things like, well, was the Northern Hemisphere solstice when we're tilted most strongly towards the sun? Did that occur when we were closest to the sun or furthest from the sun? All of these factors influence how much solar radiation you get at the top of the atmosphere at different latitudes and at different times of the year, very much like climate forcing. Now, let's think of today. As of the year '20, when is the Earth actually closest to the sun on the year 2020? The answer is January 4th. That may surprise some people. We're actually closest to the sun in the middle of winter. That hasn't always been the case though, but it is right now. Now, let's look at the Eemian. The Eemian was the most recent interglacial before ours. It peaked about 125,000, 128,000 years ago. These are reconstructions of Arctic temperature on the left and the extent of the Greenland ice sheet on the right. The little circles and squares are showing the sites where a lot of the paleoclimate information was obtained from, ocean cores, lake cores, that thing. This is basically temperatures compared to today, it's the idea, and you see that on the left, it's basically all in the red, warmer than today. It was a very, very warm period back then, very little ice. Now, we look on the right and that's the extent of the Greenland ice sheet and its elevation back then, about 125,000 years ago. We find that the Greenland ice sheet was considerably smaller than today. It was still there, but it was considerably smaller than today. Makes sense because sea level was actually was contributing to fairly high sea levels back in those days. But this is the information that we can get during the Eemian. Now, the Milankovitch forcing associated with the Eemian, what was going on if you can take a look at this figure a little bit more off line, is when we got into the Eemian, what was happening is the Northern Hemisphere, summer solstice basically, when the Northern Hemisphere, and we're tilted most directly toward the sun, was at a time pretty much when we were closest to the sun at the same time, and also when Earth's tilt was fairly large, so unusually warm Northern Hemisphere summers occurring at a time when we're also closest to the sun. Perfect way to get rid of ice sheets and go into an interglacial because it was a previous ice sheet, then we move into the Eemian that passed into interglacial. Then the Milankovitch forcings changed and we started to slide into the next ice age. The next ice age peaked something around 25,000 years ago. Back then, there was a huge ice sheet that covered a lot of North America, covered pretty much all of the Arctic, except for example, you see a good part of Alaska was not covered. There were much more extensive glaciers and an ice cap on the Brooks Range, but a lot of that was ice-free. But you had a big ice sheet over a lot of the continent. We see the Greenland ice sheet was bigger. There was an ice sheet or a bigger, I guess you'd call it an ice sheet or an ice cap covering basically all of Iceland. What about Europe? Yeah, something called a Fennoscandian Ice Sheet. Same idea. A lot of ice there. Now, interestingly, there's evidence of an ice sheet covering what's now parts of the Bering Sea. How could that happen? Because it's ocean now. Well, the sea level was a whole lot lower and ice in fact, could develop over what was then land. How much lower was sea level during the last glacial maximum compared to today? I think I gave it away earlier, but we'll see if you've been listening. The answer is, as much as 120 meters lower. Wow, 363, but fairly close to 400 feet lower. That's because all of this ice was locked up in these immense ice sheets over North America, and over Europe, sea level was much lower back then. Now the period back then, if we got towards that most recent glacial period, Pleistocene Megafauna, mastodons, and things like this, different kinds of rhinoceros, some really very impressive big animals back then, woolly mammoths and the such, we find evidence of these animals sometimes frozen in peatlands and things like that, basically complete bodies of these things, but pretty interesting time to live if you could. Of course we did have humans around back then who hunted these sorts of things. Now one of the fascinating things we find in the Greenland ice core records are what we call Daansgaard-Oeschger Cycles and Heinrich Event. This figure is just illustrating some of these features. Daansgaard-Oeschger event, there's evidence for about 20 of these things going back to 80,000 years. Now what was going on here apparently, well not apparently, the records are quite clear. There were these periods where we're going long, and then suddenly it gets very warm over a period of only 10 years or so, over about a decade, suddenly temperatures go way up, and then temperatures fall over a couple of thousand years, only to see another huge spike in temperature, and then temperatures fall again, and then another one, and another one. The coldest periods of these, there are also these things called Heinrich events. Some of these Daansgaard -Oeschger events at the coldest parts of them are associated with these things called Heinrich events. These Heinrich events, well, what it represents is vast Armadas of icebergs traveling down the Atlantic. The evidence for that is things like dropstones and things like that in ocean cores. How did these stuff get there? The only it could get there is if there were icebergs there. If you think of it, we're having these very strongly warm period, we have cooling after that, and then during these very cold periods of some of these, we have these vast Armadas of icebergs coming down the Atlantic. You wouldn't want to be having some shipping bag, then of course we weren't doing that, but pretty impressive these big and rapid changes in climate that we have seen in this ice core record. It's another indication of what the climate system is capable of doing. Why did we have these? We're still debating that. It looks like it ended up dealing with episodes of freshening of the North Atlantic, where we have episodes of freshening, where the ice sheets started to melt. That basically puts a very stable lid of freshwater on top of the ocean and the northern North Atlantic. That can keep warm waters from the South from moving up, so temperatures then decline, that shuts off, and suddenly temperature's warm again because that heat flow from the ocean increases suddenly. There's still some ideas about that, there's still some uncertainties about that, but we're looking into it as much as we can. Here's just a timeline of selected Daansgaard-Oeschger events. So They try to put them all together to see the history of these things. What you can see on the left side of it. This is looking at the time relative to the start of the event, 200 years before to 1,000 years later, and Del O 18, that's just an analysis of oxygen isotope ratios. But basically the more less negative that Del O 18, the warmer it is. What you see here from these different events, if you put one on top of the other, that's very rapid warming, followed by this cooling thereafter. Fascinating what was happening back then. I hope I've given you a little bit of an idea of some of the things that had been going on over the past million years, and how the Arctic is a rich source of paleoclimate information, and how a lot of these events very intimately involve the Arctic. We'll learn more about that in the next video.
