Hello, my name is David Schultz. Welcome to Our Earth, Its Climate, History, and Processes. In this lecture, I want to talk about the cryosphere. What is the cryosphere? The cryosphere is the ice that's on and in the Earth. Here are two maps, one looking at the Earth from the North Pole, one looking at it from the South Pole. You can see both have components of the cryosphere, but the distribution of that ice is quite different between the poles. In the North Pole, where there's relatively little land, we see ice sheets near the pole floating on the ocean, but in the areas surrounding the pole, we have this pink region of permafrost. In contrast, in the southern hemisphere, where we have a continent sitting at the South Pole, we have extensive ice sheets on land and sea ice extending off of that. Now, the current period, say, the last 2 million years where we've had glaciation on the planet, is not the only time when we've had ice sheets on the planet. Here's the most recent glaciation when you can see that although the southern hemisphere had a slightly enlarged region of ice. If you go back to 300 million years ago, to the Carboniferous-Permian period, you can see another era where extensive region of continent on the South Pole was associated with the anchor point for a large region of glaciation called the Karoo glaciation. And then if we go back to the period around 600 million years ago, just after the break up of Rodinia, we see again a, you know, large region of super continent with a lot of land mass in the southern hemisphere. And for about 150 million years, there were on and off periods of glaciation. Three different periods, the Sturtian, the Marinoan, and the Gaskiers glaciation, a much shorter period. You can see the evidence on for the glaciation of these three periods here plotted on a map of the existing continents. So you have to imagine that these places are now all spread apart on around the world. But nevertheless, you see the global scope of the glaciation on all the different continents during this time. Here's an artist image of what it may have looked like. You can see that the snowball Earth at this time was perhaps not exactly totally snow covered, that there were still open spaces where life could have flourished during this time. The kind of evidence that we have for this glaciation are what are called dropstones. This is one piece of evidence that we can use to infer the existence of glaciated periods. In this case, these are rocks that are carried by glaciers as they come off land and over the ocean. And then these icebergs will break off. And the melting debris from the iceberg will fall into weak sediments and deform them along the bottom just like this, and then later sediments will lie on top. And so, these dropstones are key indicators that this was a region that was once formally undergoing glaciation and iceberg carving and dropping dropstones into them. Going back even further to the, a previous snowball Earth in the Proterozoic right after the Great Oxygenation Event, the Huronian glaciation as well. So these periods of glaciation actually punctuate a much larger period on the planet that's relatively ice-free. And you can see from this graph here, looking back at the last 800 million years, the extent of ice sheets both in the northern hemisphere and in the southern hemisphere. Southern hemisphere's had a lot more ice, periods of glaciation in the southern hemisphere compared to the northern hemisphere. But largely, the planet has been ice-free through much of its time, although again, as I said, there are periods where ice extends off the poles. Now obviously, the global average temperature will control the existence and perhaps the extent of polar ice caps. So we want to ask the question, what controls the global average temperature? More specifically, how can we actually determine what the temperature was in the geologic past? If we just want to know about the last few 100 years, we can look at archives of meteorological data across the Earth's surface. Or we can look at tree rings. Dendrochronology is a great way to figure out the temperature in the past. If we want to know further back to tens, to hundreds of thousands of years, then we can look at stable isotopes. And you may remember a lecture by Ray Burgess early on about stable isotopes and their use. We can look at isotopes of oxygen in ocean sediments. We can look at isotopes of oxygen in ice cores or hydrogen isotopes in ice cores as well, both indicators of past global temperature. And so, if we can reconstruct the Phanerozoic temperature profile as measured by delta O18, in other words, stable isotopes of oxygen, then we can see the ups and downs of temperature. And when this temperature tends to be colder, these tend to be associated with periods of glaciation on Earth. Now, you may also recognize that the warm point in this graph, most recent warm point, is in the Late Mesozoic. And since that time, the Cenozoic has been characterized by declining temperatures. And as we've gone into the most recent 2 million year period, we've entered this period where we've had glacial and interglacial periods. But what has been responsible for these repeated glacial and interglacial periods? But before I answer that question, I want to look at the evidence for those cycles in global temperature. Here we've got two time series, one from an ice core and one from benthic foraminifera, fossils that are in sediments in the bottom of the ocean. And through the stable oxygen isotopes, we're able to assign temperatures associated with the relative uptake of stable oxygen into these systems. And what you see are cycles, sometimes around 100,000 years, sometimes around 40,000 years during this time. Turns out these cycles are quite similar to cycles in the orbital parameters of the Earth around the Sun. There are three different cycles. The eccentricity describes how elliptical or how circular the Earth's orbit is around the Sun. The obliquity describes the tilt of the Earth's axis. Currently, it's 23 and a half degrees, but it does vary over about a two degree range. The third one is precession and precession tells us the relative orientation of the North Pole relative to the fixed stars. And so as the Earth goes through its orbit precession is also occurring and may change the position of the Earth just slightly enough so that different amounts of energy from the Sun are hitting either the northern hemisphere or the southern hemisphere. Now, each of these three cycles has a characteristic time frame. The eccentricity is about 100,000 years, the obliquity is about 40,000 years and the precession is about 20,000 years. So, those are the three different factors that work their way into the Milankovitch cycles. Although it's a popular explanation for these Pleistocene glacial and interglacial cycles, unfortunately the Milankovitch cycles can't explain all the evidence. For instance, there's a switch between 100,000 year cycles and 41,000 year cycles about a million years ago. And it's not really clear what's going on to cause this change in the cycles. We also recognize that if you were to calculate the variations in solar radiation between the minimum and the maximum, then these differences are actually smaller than the response that we get on Earth. In other words, the amount of solar radiation just due to the Milankovitch cycles themselves cannot explain the differences between a glacial and an interglacial period. So it suggests that there, this might be some trigger, something to get the process going, but there are other feedbacks in the system that may kick in. And then the other problem is that we have glacial and interglacial periods that may start before the solar forcing actually kicks in. So, there's not necessarily a precise relationship between these cycles and what we observe on Earth. Of course, we also know that these Milankovitch cycles have not been confined to the last 2 million years. They've been occurring for quite a long period of time in geologic history. But if that's the case, then why is the Pleistocene so special? Is it possible that we're at a delicate climate balance where these changes in solar insolation are enough to force these feedbacks that I was just referring to to manifest themselves as glacial and interglacial periods? Now, I think that is kind of where we seem to be going, that the Milankovitch cycles may not explain everything. There's suggestive evidence, but there's certainly a lot of feedbacks built into the system that may lead to these glacial and interglacial periods. So, to summarize today's lecture, we've seen that glaciations have occurred throughout the geologic past, but they do seem to be unusual in Earth's history. Most of the time, Earth is without substantial amount of ice. We've also seen in this lecture that these glaciations may become so extensive that they may cover the Earth or nearly cover the Earth in ice sheets. Then finally, we saw that the Milankovitch cycles, the changes in the orbital parameters of the Earth, are an intriguing explanation for the Pleistocene glaciations in the last 2 million years, but they can't be the whole explanation. There must be some feedbacks within the system that amplify the small variations in solar radiation that we get on Earth to produce these glacial and interglacial periods.
