In the last video, we talked about climate changes that had occurred over the past million years and that emphasize what we had, was this series of ice ages and interglacials warm periods between the ice ages. These were pretty clearly paced by Milankovitch effects, it's variations that earth orbital geometry. We got them to our most recent interglacial the one before today, about 125,000 years ago called the Eemian when the Milankovitch forcing was favorable for getting rid of any ice, that might have been there in the Northern Hemisphere. We're at a situation where, Northern hemisphere, we were closest to the sun during the Northern hemisphere solstice. Also, Earth's tilt was fairly big and so that means that Northern hemisphere summers were rather warm, then that started to change and we slid into another ice age, and that ice age peaked about 25,000 years ago. Then basically a similar thing happened, the Milankovitch forcing change in such a way to favor getting rid of those big ice sheets and get us into the interglacial that we now live in and what we're calling the Holocene. Now, let's think first about the decay of that Laurentide ice sheet. These are various reconstructions of how big that Laurentide ice sheet was, as we move out of the peak of the ice age into the present. From left to right, it's showing what was going on between about 12,000 and 11,000 years ago, 9,000-8,700 years ago, and onwards to the more recent period to getting closer to today, and we can see how it progressively decayed. There was still a fair bit of ice over the Northern part of North America, including a lot of the Arctic, even say, 11,000 years ago. But then it continued to decline, there were these big huge lakes that developed in front of the decaying ice sheet. A big one called Lake Agassiz, which we'll talk about in a minute. You can see now for a while, say 9,000 years ago, it looks like the Greenland ice sheet was still connected to ice over the Canadian Arctic Archipelago, around Ellesmere Island. Then that separated out and 7,000 years ago, there was still some remnant ice over parts of the Canadian Arctic Archipelago and the Greenland ice sheet. Then we got towards the situation more moderate, that we see today where we have really just the Greenland ice sheet left, but then we have ice caps over some of the Arctic Islands, notably Baffin Island or Ellesmere Island. Sea level, of course, rose pretty quickly as we got rid of that Laurentide ice sheet and so here's a reconstruction of post-glacial sea level rise starting about 24,000 years ago. That's a little bit after the maximum size of the Laurentide ice sheet, the peak of the last ice age. On the y-axis is the change in sea level in meters, and you can see, we were going along with a maximum ice and sea level was 120, maybe even 130 meters lower than today. As the ice sheet melts, all that water gets dumped back into the ocean and sea level rises to where we are today. Now you can see looking 1,000 years ago, pretty much it's been flat since about 6,000 years ago. What this doesn't show, of course, is the very modern period in this analysis where we know that sea level is rising again, because of what's happening to Greenland, Arctic glaciers and ice caps, glaciers and ice caps all over the world. The Antarctic ice sheet, thermal expansion of the ocean, sea level is certainly on the rise again, and that's us. It was an interesting time that we're going along and coming out of the last ice age, the bird are starting to chirps, spring is coming around, we're getting out of the ice age, it is going to be nice again. Then something like 12,000 years ago or so, there was this event called the Younger Dryas cold interval. We're warming up, things are looking good, we suddenly slam back into near Ice Age conditions for a couple of thousand years. So, boy, what a rude awakening, right coming out of the ice age, things are looking good and then along comes the Younger Dryas, this cold interval. It's named after this Dryas flower, which I show on the right here. On the left I'm showing records from various different sources, the GISP2 ice sheet, dome, sea ice sheet. You can see these things and basically always we're going along and we get this very cold period and then we come out of it. What was going on back then? Well, the best ideas of this is that it was something that involve freshening in the northern North Atlantic. I talked about these Dansgaard Oscar event, where it looked like there were episodes of strong freshening in the northern North Atlantic. That basically put a freshwater lid on top and it basically prevented what we call deep water formation, which then prevented the warm waters from the South from moving up into the North. So things got cold because we reduce greatly that ocean heat transport somehow. That was the idea of getting into these cold periods during these advanced Dansgaard Oscar cycles, and then suddenly we came out of them very quickly over a period of 10 years. It looks like something similar was going on with this Younger Dryas. Some people call the Younger Dryas as the latch of the Dansgaard-Oeschger cycles. Others, I guess, don't. But there have been various ideas put forth of how this could happen. I noted that there were these big glacier, these big lakes in front of the receding ice sheets, proglacial lakes, we call them. One idea was this Lake Agassiz, one of these big ones, suddenly catastrophically drain through the St. Lawrence Seaway which became open at that time, and it drained all this water through the seaway up into the North Atlantic, and that put that freshwater lid on it, and that's what gave you, you've got into this very cold period and then we got out of it slowly. The problem we had is that the geomorphic evidence for it isn't all that strong. There's different ideas out there that there was a very catastrophic drainage of Lake Agassiz but it went through the Mackenzie River, the Mackenzie River drainage into the Arctic Ocean and then found its way into the North Atlantic, and that is what caused that cooling associated with the Younger Dryas. There's still another idea out there of an asteroid impact. There's evidence that an asteroid hit Northwest Greenland at about the right time to account for the Younger Dryas. Big asteroid impact, throwing a bunch of stuff into the stratosphere and things like that, could that have been it? There's these different ideas out there about what caused the Younger Dryas, we still don't know. See, that tells you that there's a lot we do know and we can see in the paleoclimate records, there was certainly this bizarre event called the Younger Dryas, but what exactly caused it? It's still not entirely clear. We come out of the Ice Age, we go into the nasty Younger Dryas, we eventually come out of that, and we really fully start to come out of the last Ice Age. We got to something called the Holocene Thermal Maximum. That was basically the maximum warmth of the Holocene. Now, if you look at Greenland temperatures based on oxygen isotope analysis, it looks like this figure here. If you go back 8,000 years ago, the ice sheets that were remaining were certainly declining very quickly by that time, the remnants of them, and then we got into this warm period, the Holocene Thermal Maximum, and then things started to cool down again to what we called the neoglaciation. The question I would have is, was that Holocene Thermal Maximum and the subsequent cooling link to the Milankovitch cycles? The answer is absolutely yes from what we can see. When we get to that Holocene Thermal Maximum, the Milankovitch. Cycles were set up in a nice way to give us really warm northern hemisphere summers. That the winters would have been cold because we had a lot of tilt, that doesn't matter so much, it seems like it's the summer warmth that is really the key here. Summer warmth, getting rid of the remainder of the ice sheets and getting to that Holocene Thermal Maximum. Now, people have done an analysis of this Holocene Thermal Maximum for the Arctic North America. It's quite interesting. Figures on the left are showing the onset and the termination of the Holocene Thermal Maximum for different regions of the North American Arctic. Now, what happened here is the Holocene Thermal Maximum, came on fairly quickly, fairly soon over places like Alaska, but it came on much, much more recently, much later, that's shown in the yellows, over Eastern North America. At the same time, the Holocene Thermal Maximum ended fairly early over like the area around Alaska, but ended fairly recently over the North Eastern part of North America. In other words, the onset and the terminus was not the same everywhere. Now, the argument for this, at least in part, is proximity to the decaying ice sheets. The decaying ice sheets, the remnants of it, hung on out in that Eastern North America region, so it stayed cold there until fairly recently, and then that thing melted away and so we got our Holocene Thermal Maximum later in those areas where the remnants had been remaining. It wasn't the same everywhere, it depends on where you were, where that Holocene Maximum was. But then, we started to go into this neoglaciation period, a cooler period. The argument is someday, we slide into a new ice age, according to the Milankovitch forces but we're not there yet, of course, and the question is, are we in control now? Now, here's an analysis of Arctic climate change over the past 2,000 years. I think I showed this slide in an earlier video but it's nice to repeat it. On the top is showing the carbon dioxide record from about 2,000 years ago onward. We're going along, there's not much going along, we get to the modern period like 100 years ago, it goes up. That's a classic hockey stick kind of diagram where, for most of the period, at that straight part, that's the stick part, and then we go up, that's the blade, the classic hockey stick diagram. The middle one is showing Arctic temperatures, same thing, going along, going along them, wham. Then Arctic sea ice, varying, varying, varying, then suddenly down. Within this record, you can pick out some warm and cool periods over the past 2,000 years or so. One of those being the Little Ice Age where it once cooler than today. I've shown this slide figure as well before. It's an image or it's a photograph of a piece of art that was written during the Little Ice Age that lasted 16th-19th century, 17th-18th, depends on the source and depends on where you were. But people ice-skating right in England, which you don't see anymore. People were drawing what they saw back then. There was definitely this Little Ice Age, there's no doubt about that, the records are fairly clean. I mentioned during some previous videos, the little ice caps that I had studied as a young grad student back in the early 1980s. That's when those ice caps probably formed during that cool period of the Little Ice Age. Why did we have a little Ice Age? Probably linked in part to extended volcanic activity as I'm showing here, I'm just showing you a couple of big volcanic eruptions. The one on the right, I believe, is Pinatubo which certainly did affect climate in the early 1990s for a couple of years. But here, we're talking about an extended period of volcanic activity. There's good evidence for that. But also a solar variability. Some people say, "You climate people, you don't talk about solar variability at all." No, we talk about it all the time. There was a period that was called the Maunder Minimum roughly associated with the timing of the Little Ice Age when there were a few sunspots. We know that there's a link between sunspot numbers and how much energy comes out of the sun, and also the quality of the radiation that comes from the sun how much of it is in the ultraviolet, for example. All of these things can influence our climate. The Little Ice Age seems to have been some combination of these sorts of things, solar variability and probably extended volcanic activity. Here's a question. I kind of already gave it away, but we'll see again if you were listening. Are the effects of solar variability evident in the modern instrumental record of global temperature? Say the records we have over the past 120 years or so? The answer is yes. Yeah, you can. I mean, the variations are fairly small, smaller than we had back associated with the Little Ice Age for sure. But yeah, if you analyze global temperature records carefully, you can see the effects of solar variability. It's there. It's right there in the data. Now, I'm ending here with a couple of figures here from the modern period. Now here's our global temperature record over the instrumental record starting at about 1880. This one's going through 2019. I could add 2020 here soon. As you can see, we're going up and down, up and down, a lot of variability from year to year but overall up. There's natural variability in climate, the effects of Mount Pinatubo are in there, the effects of solar variability are in there, these effects of El Niño cycles that are in there. But overall, we are going up, and that is us. Are we now really in control of the future of the earth? That's the real question. But here's this Arctic amplification. Remember, the globe is warming up, but where are the biggest temperature changes in the Arctic? Remember, this was something that was predicted back in the late 19th century that we would see this. Sure enough, there it is. Lot of it has to do with the reduction in sea ice cover, effects of cloud cover, effects of changes in atmospheric circulation, and how much heat is being brought into the Arctic. But there's your Arctic amplification, very clear as a bell. This is a record that looking at temperature changes from 1961 through 2018. The question is have we entered a new geologic age or era? Have we entered what some people are calling the Anthropocene? The age of humanity where I suppose if there are aliens that came down to earth millions of years from now and started digging down, they'd see things like piles of tires or something like that. They would see the impact of humanity. Are we in control of the future of our planet at this point? That's really the big question out there. Thank you.
