geog 203- 2

Holecene

HOLOCENE EPOCH - TRANSCRIPT Slide 01: The Holocene is the current epoch. The Holocene started about 12,000 years ago. Slide 02: Don't forget, we are still in the Phanerozoic eon, we are still in the Cenozoic era, we are still in the Quaternary period. Look all the way up, that top light yellow rectangle near year 0: you'll see the Holocene epoch. Slide 03: Now, as we have been discussing, the Holocene is an interglacial within an overall Ice age called the Quaternary. The Holocene was preceded by a glacial that was called the Wisconsinian, and it will be followed by another glacial, some time in the future. But right now, the Holocene in itself is a period of relatively stable climate and mild conditions when compared to the overall cold Quaternary. Slide 04: The Holocene, is the very last yellow peak on the right hand side. Again, it started about 12,000 years ago and it is a warm time. The general amount of CO2 in the atmosphere during the Holocene, is about 270 or 280 parts per million of CO2. And if you compare that with the previous interglacial, for example, which is called the Sangamonian - look at the previous yellow peak, it is quite similar in terms of its size and magnitude. Slide 05: If you look at bit more closely, the Holcoene is in fact a little cooler than the previous interglacial, with CO2 concentrations a tad lower. Right? Remember that there is nothing unusual about our current interglacial in terms of its temperature. Naturally, it compares with other interglacials, and it's actually a bit cooler than the last interglacial. You know, the Holocene is a very typical interglacial. Slide 06: I'm going to show you a few maps to give you an idea of what happened since the last glacial maximum (21,000 years ago), all the way up until today. So 21,000 years ago, we've seen this before, the ice sheet at its largest. It covers Manhattan and parts of the northern states of the US, and almost all of Canada. Versus today, where the ice sheet is really retreated all the way to the top of the Arctic Archipelago of Canada, and also Greenland. Also, note the arrangement of the biomes, or the vegetation types, south of the ice margin, and how they've changed compared to today! Slide 07: 15,000 years ago, the ice sheet separated into two parts. There's the big dome on the East that we call the Laurentide dome. And there's the western dome that is much smaller. There's a corridor that formed between these two ice sheets. It is believed that people and animals migrated south from here; they used the land bridge that existed between Russia and Alaska, and then kept walking south. The biomes are shifting north as the ice sheet is retreating because the temperatures are warming. For example, the tundra is always very close to the ice margin, and as the ice sheet is retreating, the tundra follows. And then all of the other biomes follow as well, like the boreal and temperate forests, and then the grasslands, and so on. Slide 08: 12,000 years ago, the ice sheet is even smaller and it keeps retreating towards the north pole. Slide 09: 10,000 years ago, Canada still has quite a bit of ice, while all of the US is uncovered. The Great Lakes are not completely formed. They were probably a little more full back then because they were receiving so much meltwater from the ice sheet. The prairies that established back then are fairly close to their current location. Notice how the tundra keeps moving north though. Slide 10: At this point, you might wonder: When is the next glacial gonna start? We know that interglacials tend to last 10 to 15 thousand years each; they are pretty short compared to their glacial counterparts, which can last 80 to 90 thousand years. The sangamonian, again it's the second to last interglacial here at 105,000-125,000 ish, it lasted twenty thousand years, tops. So are we getting to the end of the Holocene anytime soon? Are we getting into an ice age? And that matters because, hey, I'm Canadian. I need to know if I need to tell my family to get down here! Haha! Ok so jokes aside, can we project when will the next glacial time begin? Slide 11: Well, we have many ways to predict when that will happen. First, we know that the pacing of glacial-interglacial cycles is closely tied to the Milankovitch cycles. We have all the mathematical parameters to calculate the Milankovitch orbital conditions and project those into the future. So all we have to do, is plot how future solar insolation will change and we can evaluate what will happen. Sounds easy... but it's not that straightforward. The figure that I plotted here, and for your information, I made it myself, not that I want to show off... this is nothing special! What I mean is that all this data is available online for free. You can go and check out the Milankovitch orbital parameters, you can download all that stuff and do any analysis you'd like. Okay so here you see solar insolation at 60 degrees north, which is a good latitude to study because it's where ice sheets would form. Sicne about 12,000 years ago, which is the beginning of the Holocene actually, solar radiation has been going down. This means that, following the peak in temperature during the early part of the Holocene, our climate should have been cooling, very slowly, going towards the next glacial. And in fact we have confirmation that the earth has been cooling. Slide 12: This figure here, for example, shows a temperature reconstruction for the past 2000 years in the Arctic. It's a compilation from dozens of sites from Alaska, Canada, Scandinavia, Northern Europe, and Russia. And what it clearly shows is a progressive cooling from calendar year 1 all the way until about 1850. This cooling is expected because of reduced solar insolation. But since 1850, there's an increase in temperature that has been recorded across the Arctic. See how the line reverts and start shooting up? This increase is going in the opposite direction of the solar radiation curve. I want to make this clear: solar radiation is not going up right now, it's still going down, like towards a glacial. But the temperatures have been going up. Something else must be causing this increase in temperature. We'll come back to this, but I'm sure you are already guessing: greenhouse gas is what's happening. Slide 13: The latest and best climate simulations that we have available are telling us that we might skip the next glacial stage altogether. There are 2 main reasons: one is natural - the Milankovitch cycles are not identical - you might have noticed from the figure with the yellow peaks and blue valleys that not every interglacial is created equal: some are longer, others are taller (or warmer), etc. The next glacial would naturally be small, and since glacial conditions don't only rely on Milankovitch - remember that the ice sheet feedbacks are also key for ice sheet growth - it's plausible that the cool conditions triggered by Milankovitch would have been insufficient for the ice sheet feedbacks to fully kick in. In other words, some models suggest that the next glacial might be a "failed attempt" into glacial conditions. Slide 14: The second reason is due to human activity - the extra CO2 in the atmosphere that comes from human activity is countering the natural cooling of Earth! So can all go to Canada and visit my family anytime! Slide 15: This is just a different plot showing the same CO2 information through the glacial-interglacial cycles. We said earlier that the Holocene had on average a CO2 concentration around 270-280ppm, and that was comparable, or even a bit lower, than the previous interglacial. But now look at the very end of the graph, on the right. Where are we right now? We're not at 270 or 280. We are over 400 ppm; all the way up there! Slide 16: To put this back into context, the last time Earth reached 400ppm was about 5 million years ago. Slide 17: Do we really matter this much? Is it possible that our CO2 emissions are large enough to impact the global carbon cycle? And the answer is YES. The answer is yes and I'm going to show you some key information that we have as scientists to say so. Yes, we are disrupting the global carbon cycle. Slide 18: We have been talking about how CO2 is important for the Earth multiple times already. CO2, and the carbon cycle in general, act as Earth's thermostat. Increasing CO2 concentrations very rapidly does have consequences for air temperature. And warmer conditions melt ice. The only two ice sheets we have now are melting. As a result of that, we have more meltwater, which contributes to sea level rise. But it's not only that: the oceans are becoming warmer too because they absorb some of the extra warmth. Water molecules expand in these warmer oceans; this process is called thermal expansion. So those are the two key reasons that lead to sea level rise: more water in the oceans from meltwater, and expanded water molecules because the water is warmer. Lots of biological processes are affected by CO2 concentration; we will cover those in another lecture. But for now, know that plant photosynthesis is sensitive to CO2 levels. Oceans are also sensitive to CO2 levels. Something else that happens as a consequence of very rapid changes in climate are changes in the atmospheric circulation. So now some places will be receiving more precipitation than before. So we see a lot of floods nowadays. And also some places will be receiving less precipitation than they used to. So really high aridity in other places like the American Southwest where we have been seeing these very dry arid conditions. And pretty dangerous droughts. Especially in places where we have a lot of agriculture. The website CO2now.org allows you to check out the historical change in CO2 concentration from the Mauna Loa observatory. Here's the curve from pre-industrial times, which starts around 270-280 ppm. And the ramping up all the way to about 408 ppm. Slide 19: So how do we increase CO2 emissions as humans? The two main sources are fossil fuel burning, so every time we take our car for example. And it's not only our cars. It's also a lot of industrial processes. The second main agent responsible for CO2 emissions is deforestation. When you have a big forest that takes up CO2 from the atmosphere for photosynthesis, the trees open their stomata, remove CO2 from the atmosphere, and they convert CO2 into biomass (leaves, bark, branches, roots, etc). When you deforest an area, these trees stop sucking up CO2 out of the atmosphere. But not only that!! If you cut them down and you burn them, all of the carbon that was stored into their biomass gets released back into the atmosphere! Slide 20: Here we go: the real CO2 evidence as to why human activity and specifically fossil fuel burning has been causing the increase in CO2 in the atmosphere. First of all, there's simply the timing. Around 1750, we discovered that we could take coal, burn it, and create a whole lot of energy from it. And this was an amazing time. And don't get me wrong, I think that fossil fuel, I mean my grandma is 87 years old and she's still super healthy you know, she would probably not be here if it weren't from fossil fuel and the energy it provides to develop the amazing technology and medicine around us. I say this, because I don't want anyone of you to feel bad or guilty about the Industrial Revolution or fossil fuel consumption. I have a car I put gas in it, and I use plastic. Okay? All of us do that. The thing is, now that we know that our actions are impacting CO2 concentration and that they have global effects on earth, it's time to find alternatives. Anyways, around 1750, we started burning coal for energy, and that led to the Industrial Revolution. Now look at CO2 concentration in the atmosphere, and here we have several sources of data including some ice cores from Antarctica, and also direct measurements from Hawaii in black here, towards the end. We see that CO2 has been pretty stable around 270-280. And then around 1750, it starts going up very rapidly. That's why we say that simply by looking at the timing of this increase in CO2, it coincides perfectly with the time when we started burning it. Slide 21: The second reason is more scientific. We all know now that we can use isotopes as tracers of environmental processes. And that is also true in the case of CO2 concentration. Coal comes from old plants. Slide 22: Millions of years ago, the world climate was warmer and land plants were larger. There were lots of big swamps like this one. When the trees die, they fall down into these wetlands and they start decomposing, but since these were wet environments, they didn't totally decompose. This dead plant material accumulated into thick layers over millions of years. And then these layers got buried down by younger sediments and soils and rocks. And because of compression and compaction, and they also got cooked a little bit under higher pressure and temperature conditions. Eventually, these layers vecame lignite and coal and anthracite deposits. Those are the layers that we mine today. We know that these old plants had been doing photosynthesis. Plants need to uptake CO2 from the atmosphere to live. But something special happens during photosynthesis: plants prefer to assimilate CO2 with light isotopes - CO2 molecules made out of carbon-12 rather than the ones made of carbon-13. Think about it: carbon-12 because is a lighter isotope. It's kind of easier for a plant to digest and metabolize. So, coal is very much enriched in carbon-12 and very much depleted in carbon-13. Every time we burn coal, we burn ancient plants. The CO2 emitted from coal is derived from the old biomass from these plants and it has preserved its ancient isotopic signature. The atmosphere has a lot of carbon-13. As we burn coal and other fossil fuels, we add a lot of carbon-12 to the atmosphere, and it's diluting the carbon-13. We can measure that quite well!! OK? We are changing the chemical signature of the CO2 that's in the air. It's like if I had a pot of blue paint, and this represents the atmosphere with carbon-13, and then I have a pot of red paint, and that represents coal with carbon-12. If I very slowly start putting some red paint into the blue paint, which is like burning the coal and putting the carbon into the atmosphere, well, slowly I'm going to start having kind of a purple sky. Slide 23: This is a very famous curve that has been published a while back already. Don't worry about the actual numbers; the concept is what matters. In 1800, the carbon isotope signature in the atmosphere is -6.5 per thousand, meaning the atmosphere is slightly enriched in carbon-12. But since coal is very enriched in carbon-12, as we burn coal over time, the isotopic signature of the atmosphere starts to drop. It's actually going down to about -8.3 parts per thousand or per mil because of that very process of transferring this old carbon that was contained into the atmosphere. There's no other process on earth that can explain this change in the isotope signature of the atmosphere. Slide 24: One last thing: you might have heard about the 2 degrees celsius tipping point that was suggested by the United Nations. I want to tell you just a little bit about this and where it's coming from. This figure shows temperature anomaly on the y axis. This means how abnormal our temperature is compared to pre-industrial times. The green line is showing you our baseline, which is pre-industrial. The blue line is showing you temperature change over the past 120 years or so. And we've almost reached one degree celsius increase in temperatures since pre-industrial time because of the increase in CO2 in the atmosphere. So we're almost at 1 degree C. Now why did they come up with this 2 degree C limit? The first climate models that have been developed were doing what we call a doubling of CO2. They were setting up experiments in which they would pretend thatCO2 was 2x 275ppm just to see what happens to Earth. This number is equal to 550ppm. That causes about 4 degrees celsius increase in temperature on average. And a lot of catastrophic and unpredictable things that happen, like ice sheets that are rapidly collapsing, really major changes in atmospheric circulation patters leading to big droughts and floods, and more. So, people said, well, we don't want to reach 550ppm or 4 degrees Celsius. It would be totally disastrous. The issue is that the Earth system is very complex and hard to predict; in other words, there's a lot of unpredictability and chaos, if you wish, in Earth's system. And we'll talk a lot more about these feedbacks and complexity in another lecture. So anyway, we know that 4 degrees is too much. So the scientists scaled back and said, ok well let's see what happens at 450ppm. This is about this 2 degrees Celsius increase. There are over 20 global climate models that are led by different research groups that are all independent. None of these models agree perfectly on what will happen at 450ppm or 2 degrees C increase in temperature. But what really matters, is that about 50 percent of the models say: yeah it's not too bad: nothing dramatic happens. You know, Earth is warming up, ice sheets are melting, sea levels are rising, but it's all going in a predictable matter. The other 50% of the models are saying the opposite: 450ppm leads to unpredictable stuff and catastrophic consequences. That's one of the reasons why people said, "Ok let's stay away from 2 degrees Celsius because we want to be a little conservative... We don't want to be living in a world where you don't know if a huge catastrophe will happen tomorrow!" This is pretty much how this 2 degrees Celsius limit came about. Some people are taking it back and suggesting that 1.5 C would be safer. Ok, that's all I have for today. I hope you enjoyed this

Earth Energy Budget

EARTH ENERGY BUDGET - TRANSCRIPT Slide 02 Let's get started with a quick reminder. Internal heat, as you recall, comes from convection and conduction of from the inner core. This inner energy makes its way to the asthenosphere and to the crust. Although a small portion of the inner heat reaches the crust, it really doesn't account for a whole lot of the energy that we get here on the surface. It's actually less than 1% of all the energy budget. More than 99% comes from the Sun, which is truly what's heating us up. Slide 03 Radiation is different from conduction and convection. Radiation is in the form of waves. And waves can travel through empty space. Which is why we can get heat from the sun even though we are not touching the Sun, or there's no actual convection happening. That is an important aspect of radiation. Slide 05 First of all, you probably know that the Sun emits ultraviolet, also known as UV. That's why we wear sunscreen. You can see them on the spectrum here, a little bit to the left of the visible light range. UV radiation falls in the spectrum of shortwave radiation. They are usually are the shortest waves that we receive here on Earth. They are the ones that the ozone layer is protecting us from, to some extent. The second type of EM radiation that the Sun is emitting is visible light radiation. The Sun is also emitting a fair amount of near Infrared (NIR) radiation, which would be located just to the right of the visible spectrum on that chart. Slide 06 Once solar radiation makes it here to the surface, three things happen to it: (1) it can be reflected back. Think about a white table cloth, and the sunlight would hit it and would reflect it back right away; (2) it can also be absorbed. Think about a black t- shirt you wish you didn't wear because it's 100 degrees outside and the Sun is hitting hard, and all of the heat is actually being absorbed onto your t shirt. Bad idea, and (3) the radiation can be refracted. Slide 07 Think about a light wave that crosses the air. That light wave hits a pond of water, or a glass of water for that matter. When the light wave hits that medium of different density, the heat wave will actually change its direction of propagation because it is entering the water, which has a different density when compared to the air. That's why a straw in a glass of water looks like it's broken. It's just an illusion because the light waves are changing direction because the air and the water have different densities. Do you know how much energy do we actually receive from the sun? I'm going to walk you through a few steps that explain how scientists can come up with this number. It's important to know these things because when we start talking about climate, past climates, modern climate, and future climates, we need to know what the natural state of our climate is and what the impact of the Sun is on it. Slide 08 To be able to calculate what we call the solar constant, we need two measurements. The first one is "L" in this equation, which is the luminosity of our sun. Now, in the equation we make the assumption that the Sun luminosity is held constant. In reality, and we'll see that later on, the Sun luminosity has changed throughout the Sun's history. But, you know if we're interested in a few thousand years, or even hundreds of thousands of years, the Sun luminosity is fairly constant. So it's fair to say that it's constant for now. The second measurement that you need is the distance between the Sun and the planet of interest, in our case, the planet of interest is Earth. But what I'm teaching you now, you can apply it to Jupiter, or Saturn, or Mercury if you wish. It's time to plug those 2 values into the equation. "4 pi d^2" is used here to represent the surface area of a sphere that would expand from the surface of the Sun to the surface of Earth. Let me explain. We are calculating the amount of radiation that comes from the Sun to the surface of our planet. To do that, think of a spherical cloud of radiation that starts where the sun is and that reaches our planet. Slide 09 The dashed line here represents the radius of that sphere of radiation. And the radius of that huge sphere is equal to the distance between the Sun and Earth. We have to do it this way, because the Sun radiates heat from its whole surface, and not just from one point. So, long story short, that's why we use the surface area of a sphere, 4 pi r^2, but we replace the radius r by the Sun-to-Earth distance d. Slide 10 The solution is 1366 W/m2. This number is called the solar constant. Slide 11 Just for fun... what's the solar constant on Pluto? And hey, I know that Pluto is not officially a planet anymore, but that cold rock still has a solar constant, right? So if I told you that Pluto's distance to the Sun is about 39 times greater than that of the Earth, and Earth is 150 million kilometers, I would multiply 150 million kilometers by 39 and I would, you know, calculate the radius part of that equation. What's your result? I get 0.89 W/m2. So, Pluto is really not getting a whole lot of solar radiation. Slide 12 Slide 13 What about mercury, which is the closest planet to the Sun? We should expect a greater solar constant for mercury than for Earth, and obviously than Pluto too. Mercury's distance to the sun is about 57 or 58 million kilometers. Slide 14 Its solar constant is 9167 W/m2. So it's really freaking hot on Mercury! Slide 15 Step #2 to determine how much energy we actually get on Earth's surface, is to determine the energy incidence. We know that Earth is almost a perfect sphere. And that it turns. Which means that we have nights and days. So only half of the planet is receiving solar radiation at any given time, which means that it's not true that the entire surface of the planet is getting radiation; only half of it. We need to take this into consideration when we're doing the math to calculate the incident solar energy. Slide 16 The way we do this is quite clever. Instead of trying to calculate half of the surface area of Earth, think of the shadow of Earth instead. Like on this figure here. See how exactly half of the sphere is projected onto that white wall? Now we simply need to calculate the surface area of a circle. Yay! 2D equations are much simpler than 3D ones. To estimate how many W/m2 of solar energy we actually get, we take the solar constant S, and we multiply it by "Pi r^2" which is the surface area of the circle, where "r squared" is Earth's radius, which I wrote down here for you: 6371km. Plug in the numbers, and you'll get 341 W/m2. That is another very important number. That is the true amount of solar radiation that we receive at the top of the atmosphere in the half of the planet with daylight. Slide 17 But that's not all! If earth was 100% white, like it were covered by a huge ice sheet, well 100% of this radiation would bounce back to space. So it would actually not be warm because all of it would be gone. Conversely, if earth was a perfectly black body, all of the solar radiation would get absorbed, and earth would be very warm. Slide 18 Earth is somewhere in between those 2 extremes, and the reflectance of earth is about 30% on average. We call this reflectance "albedo". Ice has a really high albedo, higher than about 90-95%. Asphalt, or a black t-shirt, have very low albedos. Very low reflectance. Slide 19 So, if we keep building up our equation to finally figure out how much energy we get on every square meter of our planet, we need to take into consideration the albedo. Obviously it changes from place to place. Antarctica has a very high albedo because it's white. The pacific ocean has a very low albedo because it is dark blue. A city has a low albedo because it's all black with a lot of concrete and asphalt. A crop of wheat has a pretty high albedo because it has a lighter color. On average, Earth's albedo is about 30% or .3. Let's plug this into our equation. Slide 20 We have the solar constant, and then we do the pi r squared thing because of daylight. Then, we multiply that by 1 minus the albedo, or 30%, and we get what we call the solar radiation reaching Earth's surface: 239 W/m2. Slide 21 You might've seen figures like this one before. It's a bunch of arrow showing you the amount of energy reaching earth, and then how it bounces back and forth all over the place. Well, the numbers will make sense to you now. And these arrows will hopefully make sense to you as well. In yellow here, you have incoming solar radiation from the sun. And what I've circled up here is our famous 341 W/m2. That's the amount of solar energy that reaches the top of the atmosphere. We know that because of the albedo, we lose 30% of it, which is 102 W/m2 that just goes straight back into space. Slide 22 Here. We don't ever really see it on earth. And so overall, the amount of energy we are concerned with is 239 W/m2. That's what's coming in. To calculate what's going out and get a true budget, we need to introduce one last concept. Because all bodies emit radiation: the sun emits radiation obviously, earth emits radiation, you emit radiation, your table emits radiation. Slide 23 Earth emits longwave infrared radiation. We need to take into consideration how much energy the earth is emitting and subtract it to make an energy budget. Slide 24 To calculate how much energy earth is emitting, we need to use what is called the black body radiation hypothesis. We assume that Earth is a perfectly black body, and that it emits the exact same amount of energy that it receives from the Sun. Okay, so this is the first line: energy emitted by earth equals energy absorbed from the sun. We know how much energy we get from the sun; we just calculated it 10 minutes ago. Below, I went through the math to get us down to 239 W/m2, which is what Earth should be emitting back. Makes sense. This is what the math predicts. This amount of radiation would correspond to a temperature for earth of 250 Kelvin. Maybe you remember from like 8th grade or something, but Earth's temperature is actually 288 Kelvin or 15 degrees C. And 255 Kelvin is -18 degrees Celsius. Obviously, Earth is not that cold!! So what is going on? Slide 25 Well, let's go back to this diagram here, I now circled on the right-hand side, which is the 239 of radiation that is emitted from earth in the beige arrow. But then look at how much IR (infrared) energy is actually being emitted by our surface and the atmosphere... Slide 26 it's actually really high. Way higher than 239. Like 356 + 169! We are emitting way above and beyond the energy that we're actually receiving. Slide 27 And the reason is the greenhouse gas effect. The planet emits radiation and that energy or heat gets kind of trapped in the atmosphere because of those greenhouse gases. That's why our actual measured temperature, is higher than what the mathematical model would predict. In the absence of our atmosphere, of and greenhouse gases, our temperature would be 255 K. But because we have greenhouse gases, our temperature is 288 K. That's great news. Slide 28 This is what it looks like mathematically. The temperature that is predicted by the black body hypothesis is 255 Kelvin. Slide 29 Earth's surface temperature as measured by thermometers etc. is 288 Kelvin. Slide 30 You can calculate the GHG increment, which is the difference between the two. Slide 31 288-255 = 33 Kelvin. We say that the greenhouse gas increment, or the amount of extra heat and temperature that is created, is 33 K. And that's a lot, and it's good for us. Otherwise we wouldn't be here. Slide 32 What's kind of cool is that we can go around and start checking out other planets and calculating their solar constant based on their distance from the Sun. We can also calculate their greenhouse gas increment. What's the greenhouse gas increment on Mercury? Find the temperature predicted (TP): 437 Kelvin. Remember that Mercury is very close to the Sun. So you know it's normal that it's a really warm place. Take the temperature observed (TOBS) and by observed, nobody has ever been to Mercury. It's way too hot but we do have probes and sensors that are giving us measurements of the temperature on different planets, and this is where these observed temperatures come from. The observed temperature on Mercury is 447. So what is the greenhouse gas increments on Mercury? I'll give you a few seconds to think about it. Slide 33 440-437 = 3 kelvin. You do the same exercise for Venus, The greenhouse gas increment on Venus is huge: 503 Kelvin. Slide 34 I want to finish the lecture by reminding you that the main GH gasses are: #1: water vapor! And the other 5 shown on the figure: CO2, CH4, CFCs, Ozone, and NOx. Water vapor is the most important greenhouse gas by far. CFCs are a human invention; there's been a ban on CFCs because it was destroying the ozone layer. Ozone here does not refer to the ozone layer; the ozone layer is great for us, but we have ozone that is in the lower atmosphere. It's created by natural processes and human activity. It's the ozone that creates smog in the city, including Houston or Los Angeles, this ozone is a greenhouse gas. Finally, nitrous oxide, or NOX, N-O-X, is another very important greenhouse gas that also comes from natural and human sources. Slide 35 Quick wrap up: we learned today that the main controls on a planet temperature are: the sun temperature, the distance from the Sun, the albedo, and the GH effect. And remember that we are making the assumption that sun temperature and luminosity remains constant over time.

MILANKOVITCH CYCLES

MILANKOVITCH CYCLES - TRANSCRIPT Slide 01 In a previous lecture, we learned about the amount of solar energy we are receiving on the surface of Earth. We learned the word solar constant, and we learned that this value is, well, constant, at 1366 W/m2, because it is based on 2 constant numbers: the distance between the Sun and Earth, and the Sun luminosity. We then learned that sunspots ever so slightly modify the so-called solar constant following 11-year cycles. We also learned that the Sun luminosity has actually not been constant over time, and that weathering, via its removal of CO2 from the atmosphere, has led to periods of icehouse and hothouse. We are concluding today by talking about Earth's orbit and its tilt. Earth's orbit changes over time, and affects the distance between the Sun and our planet... So the Sun-to-Earth distance is also not a real constant! It changes enough to induce changes in climate through time. Stay tuned for the details. Slide 02 Before we get started: here's a quick reminder. Earth is rotating on itself over a 24-hour cycle. That's why we have a succession of nights and days: the half of Earth looking away from the Sun is in nighttime, while the other half is facing the Sun during its daytime. In addition to that, our planet is rotating round the Sun. It takes about 365 days for Earth to compete an entire revolution along its orbit, around the Sun. Notice that when we are in summertime, which is on the left handside, in the northern hemisphere, Earth's north pole is tilted towards the sun. This indicates that it's summertime in the Northern hemisphere. In wintertime (on the right handside), Earth's north pole is tilted away from the Sun; we receive less solar radiation. That's why it's colder in winter than in summer. The tilt itself is the reason why there are seasons on Earth. If the Earth was straight up instead of tilted, there wouldn't be these differences in terms of solar radiation between seasons. Today, you will learn about three main cycles that have to do with Earth's orbit and tilt. Those are natural cycles that impact the amount of solar radiation that we receive on Earth. They are called the Milankovitch cycles. Milankovich is the last name of the Serbian person who came up with the astronomical equations about 100 years ago. Slide 03 The first one that's called eccentricity. It refers to the shape of Earth's orbit. It's actually not a perfect circle; it looks more like an ellipse. As it turns out, the shape of the orbit that Earth is following around the Sun changes slightly over time. It goes from being almost a circle, to being slightly oval, like in this photo here. This is hugely exaggerated, but hopefully you can appreciate the difference in the shape between these 2 orbits. It's easy to imagine that the amount of solar radiation that Earth receives when it follows the more circular orbit (on the left) is probably pretty regular throughout the year. But when Earth follows the more oval orbit (on the right), there seems to be a few months when Earth passes close to the Sun, and a few months when it's far away from the Sun. See that? This will affect the amount of solar radiation that we get on Earth through different seasons. For example, if it's summertime in the northern hemisphere when we are on the close pass with the Sun, our summers would be extra warm. Like we said, Earth's orbit is changing in shape over time. It's slowly morphing and moving from a circle to an oval, and then back into a circle. An entire cycle lasts 100,000 years. One way to think of it is that, when the cycle starts, the orbit is a circle. It takes 50,000 years to slowly morph into an oval. Then, it takes another 50,000 years to morph back into a circle. Every year, the orbit is ever so slightly different. Animation (just show the middle panel): http://iprc.soest.hawaii.edu/users/tobiasf/Outreach/ORBITAL_FORCING/Orbital_Forcing.html Slide 04 Let's look at the oval orbit (again, this is hugely exaggerated). It's asymmetric, meaning that at time 1, Earth is far away from the Sun (we call this aphelion) vs. at time 2, Earth makes a close pass near the Sun (we call this perihelion). Because solar luminosity is held fairly constant from year to year, it's the distance between Earth and the Sun that is changing the solar constant. We receive more solar radiation when the planet is closer to the Sun, and vice versa. Under these conditions, Earth spends some time in perihelion and in aphelion a little bit every year. Again, imagine that it's summertime in the northern hemisphere when Earth is in perihelion. The Earth would be tilted towards the sun, and it would be a really warm summer. However, if the rotation of Earth makes it such that our summertime in the northern hemisphere, so when we are tilted towards the sun, happens in aphelion, our summer would not be as warm because we'd be farther away from the Sun. So far, so good? These differences become very important over long periods of time. Slide 05 The second cycle is called the obliquity. Obliquity has to do with Earth's tilt. We all know that Earth is spinning on an axis. And that the axis is not straight up. The white line here shows the reference point, if Earth had 0 tilt. But the Earth is tilted. These days, we know that Earth is tilted at about 22.5 degrees. But the tilt of Earth also changes a little bit through time, from 22.1 degrees all the way to 24.5 degrees. This small back and forth follows a cycle of 41,000 years. One way to think of it is that, when the cycle starts, the tilt is 22.1. It takes about 20,000 years to slowly increase to 24.5. Then, it takes another ~ 20,000 years to go back down to 22.1. Every year, the tilt is ever so slightly different. Animation (just show the left panel): http://iprc.soest.hawaii.edu/users/tobiasf/Outreach/ORBITAL_FORCING/Orbital_Forcing.html Slide 06 So why does Earth's tilt matter? It's not changing the distance from the Sun (like eccentricity did), but rather it's changing the angle of Earth, and therefore the amount of solar radiation that different regions of Earth will receive. Let me explain. With a large tilt (left figure), when our planet is tilted towards the Sun during summertime in the northern hemisphere, the north pole and northern hemisphere as a whole is going to receive more solar radiation than under a lower tilt (like on the right figure). In the wintertime, when the north pole is facing away from the Sun, the northern hemisphere will receive even less solar radiation when the tilt is greater. In other words, high tilt increases the temperature difference between summer and winter; it increases seasonality. Slide 07 The third cycle is the precession of the equinox. This one is a bit weirder to visualize. It pretty much means that Earth spins around its own axis, it wobbles, like a top. Slide 08 This is not the daily spin that produces days and nights. It's a much slower kind of wobbly spin. Like a spinning top that starts slowing down, and it starts to wobble in a circular motion. It takes Earth 26,000 years to complete a full wobble. Animation (3:10 to 3:30 - no sound) https://www.youtube.com/watch?v=bR5df-BjQuU Voiceover Imagine that this top is Earth spinning, and that the bolt sticking out is representing Earth's axis. See how the direction in which the bolt is pointing changes slowly over time? It changes much more slowly than the pace at which the top spins. Animation (0:00 to 0:43 - with sound; don't show "the cause"; they are kind of wrong) https://www.youtube.com/watch?v=adzx547ptck Slide 09 Another example of this cycle here. Today, our north star is Polaris. Earth's axis points right to it. But 13,000 years ago, or should I say, half a wobble ago (don't forget that 1 cycle is 26,000 years), Earth's axis was pointing in a very different direction and our north star was Vega. One more thing to note: see how today, in this example, the northern hemisphere is pointing away from the Sun in perihelion (so it's winter) but that 13,000 years ago, the north pole was pointing towards the Sun in perihelion (so it's summer)?! This wobbling makes the seasons rotate through time! That's why this cycle is called the precession of the equinoxes. Animation (just show the right panel): http://iprc.soest.hawaii.edu/users/tobiasf/Outreach/ORBITAL_FORCING/Orbital_Forcing.html Slide 10 Altogether, the 3 Milankovitch cycles regulate the amount of solar radiation we receive on Earth, and they regulate our climate. Let's look at this first case study here. O the right handside: the northern hemisphere is tilted towards the Sun (so it's summertime). The tilt is small (so the northern hemisphere receives a smaller amount of radiation than if it was very tilted). And we are in aphelion, so farther away from the Sun. Those are the coldest summer conditions that we can get. On the figure, it says "ice growth" because this is what would happen: under the right circumstances, this orbital configuration can lead to the growth of ice in the northern hemisphere. Not so much because the summers are frigid, but rather because the summers are cool enough that the snow and ice that accumulated during the previous winter don't melt in the summer. So, the next winter, the ice accumulates even more, and the ice mass is growing. The next summer, well, not all of it melts either, and year after year after year, these ice masses build up and grow across the landscape. Because ice has a high albedo, and reflects solar radiation away, even more ice is forming under these cool conditions. Over tens of thousands of years of this, we end up in a glacial period, like the one that started 100,000 years ago and ended 21,000 years ago. We will get back to this glacial period in another lecture. What's important to remember here is that the Milankovitch cycles are responsible for making glaciations. They are the pacemaker, the regulator, the orchestrator of glaciations. Slide 11 Eventually, the northern hemisphere summertime happens during perihelion, which means it's warmer. When combined with a large tilt, the northern hemisphere gets the hottest possible summers. Meaning that now, the ice melts faster in summer than it can grow in winter. Over a few thousand years, much of the ice melts away and we eventually end up in an interglacial, like today. Slide 12 To conclude this lecture, a few maps showing you the extent of the last north American ice sheet and how different the climate was. The peak of the last glacial cycle, 21,000 years ago, is shown on the left. We call it the Last Glacial Maximum, or LGM. Almost all of Canada and the northern states (Washington, parts of Oregon, Montana, etc., all the way to New York City), were covered by miles of ice. Notice how the biomes were shifted southward. A biome is the main vegetation and forest type. Look, the tundra (in orange) went into Oregon, New York, and even into some pretty southern states, like the Carolinas. Parts of Florida were semi-deserts! So the world was a very, very different place. Mostly because of the Milankovitch cycles. And then today, on the map to the right, you can see that the ice sheet retreated. It's now confined to Greenland and parts of the Arctic Archipelago of Canada. The biomes are located in very different places, too! Slide 13 I'm going to conclude on this figure here. Every 100,000 years, give or take, there is an interglacial (in yellow) that is followed by a long glacial (in blue). The pacing, meaning the timing of the glacials versus interglacials, is explained by the Milankovitch cycles, mainly eccentricity and tilt. On the x-axis you have time, since 800,000 years ago, all the way until today, which is zero. And then on the y-axis you have atmospheric CO2 concentration recorded in ice cores, which is used to represent temperatures. More on this in another lecture. More CO2 in the air corresponds to warmer temperatures.

Quarternary Period

Slide 01 Here we are, in the Quaternary period. The Quaternary started 2.6 million years ago. We are still in the Quaternary today. And it is a time of very dramatic climatic change. First off, the Quaternary is considered an Ice Age. Not an extreme Ice Age - we are far from snowball Earth (we need for Earth to be fully covered by ice for that!) - and clearly today, there's a lot of ice-free land and continents and oceans. But, there are significantly large ice sheets on the planet right now. Also, the Quaternary is characterized by climate swings between "glacials" and "interglacials". Those are often referred to as "glacial cycles". During glacials, additional ice sheets form near the poles. During interglacials, they melt away. Thee cycles are related to the Milankovitch cycles. Right now, we are in an interglacial that is called the Holocene epoch. The Quaternary is considered one of Earth's Ice Ages because, all things considered, its climate is quite cold when compared to hothouse climate conditions that we have experienced throughout most of Earth's history. Throughout the Quaternary, there have been about 60 glacial cycles. At times, the polar ice sheets reached down to 40deg N à south of Manhattan NY! Substantial glaciers advanced and retreated over much of North America, Eurasia, parts of South America and Asia... and in Antarctica of course. These glaciations have profoundly modified the landscape over time because glaciers are very effective at carving out valleys, depressing the land, creating basins and lakes filled in by meltwater, moving sediment down slope, and also rearrange them in new landforms. We will cover more of this material during the geomorphology portion of the course. But for now, I imagine that you are familiar with the Great Lakes? Those 5 large lakes that border the US and Canada? They only started to exist about 20,000 years ago as a big glacier was retreating very slowly toward the north and the depressed land underneath the ice created these basins that were then filled with meltwater. Humans came about during the Quaternary. About 100,000 years ago, Homo sapiens had travelled from Africa northward and had reached Israel. The Quaternary is really the time for human development. One last thought before we move on: as you can imagine, the continental configuration has not changed a whole lot over the Quaternary - 2.6 million years is not enough time to move plate tectonics a whole lot! Slide 02: Ok here we go: the Quaternary in relation to the entire Earth history. Slide 03: As mentioned in the Intro, something that has been changing dramatically during the Quaternary is the climate. These glacial cycles are caused by the changing Milankovitch orbital parameters. We have these times with glaciers that are expanding in the northern hemisphere, and also in the southern hemisphere to some extent. And also in Asia a little bit, and then these glaciers are retreating every 100,000 years, which gives way to mild, interglacial conditions. This figure is showing you the pacing of these climatic cycles. This specific record goes back 800,000 years; it is based on an ice core from Antarctica. For older reconstructions, we need to rely on other techniques including the stomatal density of plants as discussed in a previous lecture or marine cores. On the y-axis, you have atmospheric CO2 concentration. On the x-axis, you have age or time. "0" is today. Follow the data with me, starting on the left: the red line goes up into a yellow peak, and then down into a blue valley, and back up again, and down, and up, and down, etc. This is the record of atmospheric CO2 concentration. When the red line is down, in those big blue valleys, those are the Glacial times. They are characterized by low CO2 in the atmosphere (~ 180ppm) and lots of ice on the continents. Remember this combination: low CO2 and high ice. Then, when the red line goes up into the yellow peaks, those are the shorter, milder, interglacials. And then the CO2 goes up to about as high as about 280 ppm, and the large ice sheets melt away. Obviously, the combination now is opposite: high CO2 and low ice. There's a pretty large difference between the maximum CO2 concentration during an interglacial and the minimum CO2 value of a glacial. Right? We go from about 180 ppm in the cold glacial all the way up to 280 ppm in the mild interglacial. In terms of temperatures in degrees Celsius between a glacier and an interglacial time, glacials are five to six degrees cooler than interglacials. Slide 04: This last yellow peak on the right, this is the current interglacial - this is now. It is called the Holocene. The Holocene started about 12,000 years ago - more on this later. For now, observe the red line, as it comes out of the last glacial cycle and goes up into our current interglacial. Just a little bit before it reaches today (year 0), this is the pre-Industrial period. The pre-industrial CO2 concentration in the atmosphere was about 270 ppm, similar to what we see during previous interglacials. So, this means that we are in a naturally warm time right now. We are not in a glacial cycle; we are actually in an interglacial cycle. So, it is normal that our climate is warm, compared to the past 100,000 years. Before we dig more into this, here are some additional thoughts to really understand how this works. There are 3 main factors that ultimately control glacial cycles. Slide 05: The first one are the Milankovitch cycles. They are the ones responsible for the pacing (or timing) of the glacial and interglacial cycles. Let's make sure that you can put all the pieces of this theory together. In the upper left diagram, the northern hemisphere is in aphelion, which means it's farthest away from the Sun during summertime. This means that, during summer in the northern hemisphere (see how the northern hemisphere is tilted towards the Sun = summer), we are as far away from the Sun as possible (that's called the aphelion), and it happens because Earth's orbit is not shaped like a perfect circle. It's an oval, and the Sun is not perfectly in the center of that oval. This means that on 1 side of that oval orbit, Earth passes closer to the Sun (perihelion), and on the other side, it passes farther away from the Sun (aphelion). OK. In addition, notice that the tilt is smaller. This means less seasonality, because less solar energy makes it towards the north pole, which means lower summer temperatures near the pole. Overall, the small tilt combined with being in aphelion during summer time means that this is the absolute coldest summers we can have in the northern hemisphere. Those are the perfect conditions for growing ice sheets. Why, you ask? Fair enough: ice doesn't form in summer!! BUT: ice melts in summer. So, think about the scenario where ice forms in the winter near the north pole, but then the summers are cool, so that the ice doesn't melt. Year after year after year, and century after century, a huge glacial ice sheet builds up. This big mass of ice builds up over tens of thousands of years, and leads to the full "glacial" conditions. The blue valleys on the previous figure. Eventually, as we keep orbiting around the Sun, the Milankovitch cycles slowly shift us into a new configuration. See the bottom right diagram. Here, the Northern Hemisphere summers are as warm as they can be: we have a large tilt towards the Sun, meaning that we have the greatest seasonality and warmest possible temperature. We are also in perihelion, so more solar radiation. During these times, all of the ice that has been building up in the northern hemisphere is melting very fast during the summer time. No matter how much ice you can build in winter, it's not enough. Think about it: it's much easier to melt ice than to build it. So, year after year, more and more is melting away in summertime, and eventually the ice sheets shrink as they retreat towards the pole, or they disappear altogether. For example, Greenland and Antarctica used to be much larger during the last glacial cycle, but they shrunk throughout our interglacial. Slide 06: The second very important factor in creating a glacial is the ice sheet itself. As an ice sheet builds up (because of Milankovitch), it has a cascading effect on its own environment; we call this a positive feedback. On the top part of the figure here, you see an undeformed land surface. As ice builds up on it, it does so laterally (it "advances" or "marches" towards the south), but it also builds up vertically as it gets thicker. Actually, these ice sheets are super thick. Like over 1 mile thick. As you can imagine, this is extremely heavy. As we know, the continental crust sits on top of the asthenosphere, and the asthenosphere is this kind of plasticky, viscous material that has the capacity of flowing. When we load up the crust with something very heavy like a huge ice sheet, it actually pushes the crust down, which deforms the land a little bit and depresses it like on the bottom figure, shown here. The asthenosphere is pushed away from under the crust as the crust depresses down. Now, this is very important! Check this out: in the example here, the ice sheet is 3.3 kilometers thick, but it's depressing the crust about 1 kilometer. So the actual height, or elevation of this glacier above the land, is just 2.3 kilometers. BUT: what's important to remember is that land depression doesn't happen right away; it is delayed by many thousands of years. OK. We all know that the top of a mountain is colder than its valley. That's why, even in the tropics, there are glaciers, but only at very high elevations! Back to our figure: as the ice sheet is building up, well, its top becomes colder and colder just by the fact that the ice sheet is growing in elevation. And because it's colder, it's able to create even more ice and it melts less in the summertime. Remember that the land doesn't depress right away, too, so it truly is very high for a while. Another positive feedback is that ice is white, which means it has a very high albedo, which means that it reflects sunlight. As ice sheets are building up, an increasing amount of sunlight bounces back into space, creating even colder conditions. So the albedo and also the elevation effect are two positive feedbacks that the ice sheet, as it's growing, is producing. Slide 07: this conceptual figure presents the elements we just discussed: the ice sheet growing, which leads to an eventual land depression (but that is delayed), and the ice sheet getting taller, creating higher and colder elevations, and they forgot the albedo in here, but that also contributes to an increase in ice mass (written here as a positive mass balance). Slide 08: The opposite happens when the ice sheet is starting to melt. It also has its own cascading effect and positive feedback that will make the ice sheet melt away very quickly. So here it is. At the top here, you have this big ice sheet and you have the land that is completely depressed underneath it. As the ice is melting, it is shrinking towards the left on the figure; a bit of meltwater is chilling at the front of the glacier because it's stuck in the depression left by the ice sheet when it depressed the land. The bedrock then starts rebounding. The asthenosphere is kind of picking up its space again underneath the crust, and pushing the crust back up. Don't forget that the reason why the ice sheet is melting is because the orbital conditions have changed due to Milankovitch cycles. What this means is that now we have higher summer insolation. As a result of that, the ice sheet is melting fast in the summer time. Because the ice is melting away, you now have this blue water. Dark blue water is absorbing a whole lot more solar radiation because of its low albedo. So the melting ice is sort of heating up its own environment and it's promoting its own ice melt. At the same time, the ice sheet is melting, and melting and melting some more, so the land rebounds up. BUT: that movement is delayed. This is critical: you remember how the ice sheets are kind of at lower elevation than they should be because the land is depressed? Well now, during the melting phase, this accelerates ice melt! Because the "mountain of ice" is not as tall as it should be, so it's not as cold up there. Thousands of years later, the bedrock will rebound. As the ice sheet is melting away, it's staying at this lower elevation. So it's warmer. So it' capable of meeting even faster. And so that creates a "more negative ice mass balance". And as a result of that, the ice is melting even faster. Slide 09: This is the conceptual figure showing the ice melt feedback loop. At the beginning of this presentation, I mentioned that there are 3 factors controlling ice sheet dynamics during glacial/interglacial cycles. We have now discussed Milankovitch cycles and ice sheet growth and retreat. The 3rd factor is atmospheric CO2 concentration. We will discuss this one in the next lecture. For now, let's talk a bit more about how different Earth is during a glacial vs an interglacial cycle. This is going to be fun! Slide 10: Let's look at the Last Glacial Maximum (or LGM). The LGM was just 21,000 years ago. It's the peak of the last deglaciation; after that, the ice started to melt away and eventually led to the current interglacial. Since 21,000 years is really not much time at all on the geological timescale, we have a lot of geological and biological evidence of what the world looked like when it was a at its coldest. This huge chunk of ice that's covering Canada and parts of the US is an ice sheet that is now completely gone. It's called the Laurentide ice sheet. The isolines drawn on it show you how many thousands of years ago that is disappeared. For example, the ice that was on top of Manhattan melted about 14,000 years ago, but the one in northern Canada only disappeared 6000 years ago. It takes a while to melt all of that ice! Slide 11: Just to situate you guys in time, I put a little red asterisk at the LGM. We are at the very bottom of the last blue valley. This last glacial is called the Wisconsinian. It's called the Wisconsinian because the ice sheet actually advanced as far south as Wisconsin; parts of Wisconsin were covered in ice. Actually, about 30% of the global land area was covered by ice. Slide 12: This image here is based on a geologic model, and shows you where we think the ice was located. So as you see, most of Canada, the northern states of the US, obviously Antarctica, Greenland, all of Scandinavia, and parts of Northern Europe as well as parts of Siberia, and western Russia were covered in ice. Also, there were large glaciers in Australia, New Zealand, and South America. Slide 13: So, how different was our world 21,000 years ago? First of all, there were huge ice sheets. These ice sheets are two to three kilometers thick. They cover really really, large areas. So those are not alpine mountain glaciers like the ones you might have seen if you've ever been in the Rocky Mountains or in Alaska even. If you've ever been to Glacier National Park or something like that, there's some pretty good and big glaciers over there. But those are not considered ice sheets because they are smaller than 50,000 square kilometers. Ice sheets are monstrous, gigantic sheets of ice literally, that are covering the landscape. They don't care if there's mountains or no mountains. They don't need anything, they just need space to grow and they need a source of water so that they can keep growing. That is the first big difference between the LGM and today, that, well, 30% of our land was covered by ice. Whereas, today we just have a small percent of our land that is covered by ice. Slide 14: A second major difference between 21,000 years ago and today is the sea level. Sea level was way lower at the LGM. This figure here is showing you a bunch of data from many places from around the world that hold natural records of past sea levels. And what you can see is that around 21,000 years ago during the LGM (on the bottom left of the figure), sea level was about 124 meters lower than it is today. Wait what? Why was the ocean so lower? Well, think about it. Where did all this sea water go? Oh yeah, it went into the ice. So all of this ice on the continents, it needs a source of water. Water was evaporating from the oceans and precipitating as snow onto the continents. And then forming ice over time. So that's why sea level was so much lower during the LGM. Eventually, sea level started going up and up and up as the glaciers were melting away, as the vast majority of this meltwater eventually makes its way back into the ocean. Notice that sea level has been fairly constant for the past 6000 years. Slide 15: A third very big difference in terms of continental configuration was that we had land bridges. This is my favorite slide. As a result of these lower sea levels, parts of the continental shelf, which today are underwater, were exposed to the air. On this map, look at the Bering Strait - today, there is a sea between Russia and Alaska. But back then, because the sea level was so much lower, you could have walked across the Bering strait, which is in dark color here on the map, and then make your way from Russia into Alaska. In fact, about 13 to 10 thousand years ago, this is exactly what happened. The Inuit people migrated this way. They were hunting and followed herds that crossed into North America. They have been living on the land for that long! They were actually hunting wooly mammoths and mastodons, and they made their way from Russia and Asia in general, and made their way into Alaska, and eventually down into the Americas. This is one of the great human migrations from Eurasia to the Americas. Thank you, Last glacial Maximum! There's a few other land bridges, though I'm not as familiar with them. On the map, Europe was attached to the UK. Looks like New Guinea as attached to Australia. Slide 16: Quite a few biotic changes also took place from the LGM to today. And by biotic, I am referring to plants and animals - things that live. I just mentioned to you woolly mammoths and mastodons. Those are extinct, of course. When we think about those, we often are left with the impression that they are very ancient animals that have gone extinct for such a long time. It was just literally you know 10,000 to 20,000 years ago that they went extinct. This is a photo from the movie Ice Age. I'm sure you recognize it. And no matter how silly this movie is, it's also very awesome because it helps us think about the glacial cycles. Well, I don't think the ice sheet margin looks just like this cliff. And there were definitely no trees growing right at the edge. But the fact that you have this dry, tundra environment and that it's very windy and harsh looking, those are fairly realistic representations of what we think the climate was like here in the northern US just south of the margin of the ice sheet back 21,000 years ago. And these animals were hanging out and grazing in the tundra. Think about this for a minute: there were no forests in Washington State back then. We were right there, at the southern edge of an ice sheet! The ecosystems you identify with the arctic were right here, in the lower 48! Slide 17: The fifth and last change that I wanted to discuss with you between the last glacial and today is atmospheric change in some greenhouse gasses. Here you have in blue and red CO2 concentrations. The red line is CO2 data from an Antarctic ice core. The line in blue is the temperature reconstruction based on the data. During the LGM, we had a much lower CO2 concentration in the atmosphere. As you guys know now, if we're in the blue valleys of the figure, we're always around 180-190 ppm, and then as the ice starts retreating away, the climate warms up, and we can record that through the CO2 concentration that really starts going up quite rapidly. It's not a perfect line between CO2 and temperature, but the general trend is the same: both go up during the deglaciation and reach maximum values during the interglacial. If you look on the right, we are around 8000 years ago, almost all the ice sheets are gone, we are in the Holocene (the current interglacial), and the CO2 is about 260-270ppm. The global temperature went up about 3.5 degrees C between the last glacial maximum and today. Slide 18: How do we know the size and volume of the ice that existed back then? We have 2 main indicators of how big the ice sheet was. First of all, we described earlier how the glacier will depress the crust down. We have multiple types of indicators to calculate the vertical crust motion based on physical models. Slide 19: This dynamic also leaves scars on the landscape, and we can use these marks to back-calculate the amount of ice weight that was needed to create those marks. Slide 20: Another type of geological data that allow us to delineate the limit of the ice sheet is called a moraine. We will talk about those in another lecture. All you need to know for now is that glaciers move around lots of rocks and sediments, and they dump a bunch of it at their edges. When the ice melts, those mountains of sediment stay in place, so we can map them today. Slide 21: In terms of ice sheet volume, we use oxygen isotopes H2O is what the water in the ocean is made out of. Oxygen has two main isotopes: oxygen 16 and oxygen 18. One of them is heavy, and the other, the lighter one, will get preferentially evaporated. So between oxygen 16 and oxygen 18, the oxygen 16 will get more easily evaporated away, so the H2O molecules made out of oxygen 16 preferentially leave the ocean and get into clouds; this water is enriched in oxygen 16 compared to the average. It precipitates on the continents as snow, which gets locked up into the ice sheets for thousands of years. The H2O molecules that are made out of oxygen 18 remain in the ocean water. This process happens over and over again for tens of thousands of years and there's huge amounts of water that is now transferred into the glaciers. As we know, the sea levels were 125 meters lower so this is a huge displacement of water. And all of this oxygen 18 is left behind into the ocean. If you look at a marine core of sediments, you actually can tell when the ratio between the oxygen-18 and oxygen-16 is changing. Slide 22: Here. See these shifts following the 100,000 -year cycle? It matches the glaciers growing and melting away. Just because of that signature of the evaporation of the water. Because the lighter isotope gets evaporated more easily. So that is a pretty neat way for us to be able to quantify how much water evaporated and created ice.

Past Climates

Slide 01 Here's a graph of carbon dioxide (CO2) concentration and how it's gone up and down over Earth's history. Note that the x-axis is not linear. As you can see, there were times with very high CO2 concentrations - up to 2000 parts per million (ppm) - which is much higher than today's CO2 concentration. Those time periods are called hot houses and, as their name suggests, they coincide with times during which Earth's temperatures were soaring. Conversely, cold periods are associated with low CO2 concentrations, such as during ice houses and glaciations. Also, note that during the past 800 thousand years, as seen as the blue line here, CO2 concentrations have been quite low when compared to the past 400 million years of Earth's history. Indeed, the Quaternary period is considered an Ice Age, and we are still part of it. The seesaw pattern that you can see within the Quaternary is caused by the swings between glacials and interglacials, with glacial times corresponding to valleys (i.e., low CO2 concentrations), and interglacial times corresponding to peaks (i.e., high CO2 concentrations). The past 11,700 years corresponds to the Holocene epoch, with a flat CO2 concentration around 270 ppm, until 1850 AD, where it starts rising with the start of the Industrial Revolution. We, in 2018, are at about 410 ppm, which is higher than anytime during the past 1,000,000 years. On the right end of the diagram are different CO2 scenarios (called RCPs, or Representative Concentration Pathways - we'll get back to those in another lecture) put together by the Intergovernmental Panel on Climate Change (IPCC) to project how much CO2 will be in the atmosphere by year 2500 based on different levels of human activity. I'm reviewing this info with you because what I want to do now is to dig a little bit further into the science of paleoclimatology. Clearly, understanding the past is very important to help us understand the present and prepare for the future. But how is it done? For CO2 reconstructions, you're going to learn about 3 types of measurements or methods today; note that there are more than 3. Slide 02 The first type of CO2 information comes from direct air measurements. Here's the Keeling curve; it is based on daily measurements of CO2 concentration in the air. The measurements started in 1958; the station is located in Hawai'i, on top of Mauna Loa (a volcano on the Big Island that is NOT erupting). You can go visit the observatory if you ever end up on the Big Island. The original instrument that was installed by Prof. Keeling himself is still there on display. Of course, there are many more stations around the globe that monitor CO2 concentration. But what makes Mauna Loa special, besides that it's the longest running record, is that it is located in the middle of the Pacific Ocean, far away from all the local vegetation signature. It's also very far away from pollution and other disturbances. There are also very strong winds up there, which means that we have a pretty clean "global" signature. Now let's look back at the Keeling curve. I see 2 patterns: (1) a general increase in atmospheric CO2 concentration over time, from about 315ppm to about 400pm, and also (2) oscillations at the annual scale - those are the small squiggles in the red line. Each mini peak is recorded during the month of May, and then it starts going down throughout the rest of the year, until it goes up again in winter time and peaks in May, etc. The reason why we have this annual cycle in CO2 concentration has to do with global biomass and the amount of CO2 that is used during photosynthesis: in May, which is spring time in the northern hemisphere, plants really start growing at the hemisphere scale, which means that collectively, they draw out a lot of CO2 from the atmosphere to do photosynthesis - we can see it as the red line goes down during spring, summer, and early fall every year = mini valleys on the graph. Then, during fall and winter, which corresponds to the mini peaks every year on the graph, CO2 concentration goes up again because photosynthesis dramatically slows down. You may wonder why the southern hemisphere isn't counterbalancing this trend; after all, trees in the southern hemisphere must also draw CO2 from the atmosphere to do photosynthesis during austral summer, which is during our winter, right? Well, the answer is yes, this process is happening. But there are so little trees in the southern hemisphere compared to the northern hemisphere - really because there is so much less land mass where trees can grow (the southern hemisphere has more ocean surface area than the northern hemisphere), that southern hemisphere photosynthesis does not affect the global atmospheric CO2 budget enough to be observed here. Slide 03 A quick close up to really appreciate the seasonal variations that we just described. Remember that the valleys correspond to the summer and early fall season (northern hemisphere) as a result of increased CO2 capture by plants for photosynthesis, while the peaks correspond to winter and early spring, i.e., when there is less photosynthetic activity. Slide 04 I also wanted to share with you a few other CO2 curves from around the world. First off, note that they are all very similar in terms of their trends. Now, see how the light blue curve - from Barrow, Alaska - has a much more pronounced annual cycle than the other ones... that's because the CO2 concentration over there is very much influenced by photosynthetic activity! In other words, trees growing during the summer time really have a strong impact on the data. You might not want to pick Barrow to represent the global CO2 concentration! The opposite is true from the Antarctic station - shown in black - where very little annual variations are observed. This is mostly because there is pretty much no photosynthesis happening there! Slide 05 I don't know about you, but I find these patterns fascinating. Let's get one step deeper into this subject here. We just said that there are annual cycles in atmospheric CO2 concentrations due to photosynthetic activity; this means that I would expect to see diurnal cycles (between night and day) too, since photosynthesis happens during daytime... And sure enough, this graph shows you just that! Look at these 5 days: the purple curve has clear valleys during the day time - when photosynthesis occurs - and peaks during night time. That said, the Mauna Loa site (in red) is not recording any diurnal cycle - that's because there is no local vegetation signature here. Which is good because we use Mauna Loa to represent the global state of CO2, and not what the vegetation on the Big Island! Slide 06 The Keeling curve and direct measurements are fabulous, but unfortunately, they only go back to 1958. So, if we want to go back in deeper time, we need other types of measurements. Let's talk about air bubbles from ice cores for a little while. Here in blue, you see the past CO2 concentrations are reconstructed using this method for the past 800,000 years. Since nobody was there to measure CO2 back then, we call those indirect measurements, or proxies. In the case of air bubbles, they are assumed to represent the chemical composition of the air at the time when the bubbles got trapped in the ice. We know that the deepest layers of ice were formed before the shallower layers (principle of superposition). Using very delicate instruments, scientists can extract mini samples of gas such as CO2 (but also CH4) from those tiny air bubbles that are trapped in the ice layers. They then measure the concentration of these gasses and reconstruct how they have changed over time. Slide 07 This is the CO2 record from the Dome C ice core, from Antarctica. Note the seesaw pattern, with the valleys corresponding to cold glacial times and the peaks corresponding to warm interglacial times. Slide 08 There are many of these records in Antarctica alone. I'm showing you this map because it's important to appreciate that there are many ice cores, and that they show very similar records of past CO2 concentrations. video showing Vostok research station and how ice cores are extracted Slide 09 The Vostok ice core record has been used to also reconstruct past temperature changes over time. So far, I've only talked about CO2 and CH4 concentrations. How do we get temperature changes out of an ice core? By measuring the stable isotopes of the ice layers! Snow that falls on the glacier and become snow have a specific isotopic composition. As you know, each molecule of snow, or H2O, is composed of 2 atoms of hydrogen and 1 atom of oxygen. Slide 10 Well, hydrogen atoms come in 2 main flavors: the ones with 1 electron and 1 proton, and the ones with 1 electron, 1 proton, and 1 neutron. The second type is heavier than the first because of that extra proton. We call it deuterium. Deuterium is a rare isotope. While most ice molecules contain 2 regular hydrogen atoms, once in while, an ice molecule will have one of its H atoms replaced by a deuterium atom. This will seem incredible at first, but we can trace this very small difference using mass spectrometry. Even more awesome is that the ratio of hydrogen vs. deuterium in the ice layers is a proxy for past temperature! Here's how this works: we know that to create glacial ice, we need snow to fall on top of the glacier. We also know that this snow comes from a source of water - in Antarctica, this is the ocean. When that water evaporated some time ago from the ocean to become snow, and then this snow fell on the glacier and eventually became ice, the temperature of the air played a huge role in determining the isotopic composition of the water molecules that evaporated. That's right: when the air is colder, the water molecules that contain deuterium tend to stay in the ocean and not get evaporated because they are heavier than the water molecules that only contain normal hydrogen. And since the air is cold, the process of evaporation is already hard enough! Whereas when the air temperature is warmer, evaporation happens faster, and the process is less "picky" in terms of its water, which means that even water molecules that have deuterium will evaporate. This process of fractionation between the 2 hydrogen isotopes follows a predictable and very well studied relationship with temperature, which means that if you know the isotope ratio of hydrogen and deuterium in a sample of Antarctic ice, you can back-calculate the air temperature that was needed to evaporate that specific water, which became the ice sample. Power to water isotopes!! Slide 11 Let's move on to the third type of CO2 measurement: the stomatal density of plants. This is also an indirect measurement of CO2, or proxy. This is a classic method that allows us to go well beyond any ice core. The study of ancient plant stomata allows scientists to reconstruct past CO2 levels from many millions, or even tens of millions of years ago. Slide 12 All you need is an ancient piece of plant. This one here is 160M years old! You can still see the stomata here: they are the small structures highlighted with black circles. Stomata are pores in plant tissue; they are similar to the pores of our skin, in a sense. They also act as the plants' mouth. Indeed, plants open their stomata to take up CO2 from the atmosphere (which is their food) and perform photosynthesis. But every time they open up, they also transpire water through those pores (like we transpire through the pores) of our skin. You might have never thought about this, but plants must make the difficult choice to open up their stomata to get food, knowing that they will also lose some water. Plants must walk the line between maximizing their CO2 intake AND minimizing their water loss. Slide 13 A figure showing you how this works. Slide 14 When there is lots of CO2 in the atmosphere, such as during a hot house, plants decrease the density of their stomata so as to minimize their water losses. That's right: every time they open their stomata, they are flooded by CO2 because there is so much of it in the air, and because they have less stomata, they also lose less water. Pretty smart!! Conversely, in an ice house, plants need to increase their number of stomata to get enough CO2 in and perform photosynthesis. They lose more water, alas, but that's the trade-off they go for. So now you see how scientists use stomatal density to estimate past CO2 concentration -- can look at ancient plant tissues and calculate stomatal density! Slide 15 Here are results from a study performed in the lab where a plant species was grown under different CO2 concentration treatments. See how the stomatal density decreases with increasing CO2!

Sun Luminosity

Slide 01 In a previous lecture, we learned about the amount of solar energy we are receiving on the surface of Earth. We learned the word solar constant, and we learned that this value is, well, constant, at 1366 W/m2, because it is based on 2 constant numbers: the distance between the Sun and Earth, and the Sun luminosity. We then learned that sunspots ever so slightly modify the so-called solar constant following 11-year cycles. Today, we will learn that the Sun luminosity has actually not been constant over time. If you recall, when Earth was formed about 4.5 billion years ago, the Sun was a very young star. We call it the faint young Sun because it was much more faint, or weak and pale if you wish, than it is today. Slide 02 This diagram is expressing this idea conceptually. This is not real data, but rather just a model describing how the Sun has become hotter and brighter throughout its history. At the bottom left, around 4.5 billion years ago, you see that the Sun luminosity was about 70% of what it is today. And then its luminosity and brightness and temperature have been increasing over time to reach today's values, and a luminosity of 100%. This doesn't mean that the Sun has stopped its growth; we know that it will continue to get brighter and its luminosity will keep increasing. We mark modern-day as 100% just as a reference, to compare how faint it was back in the beginning. Why has the Sun become warmer, hotter, and brighter over time? Slide 03 In short, because of nuclear fusion. The Sun (and all other stars) creates its own energy by fusion. Fusion happens is when two atoms that combine. And along with this combination there's a huge burst of energy that gets released. As the Sun is fusing its atoms of hydrogen together, this creates helium by the way, a huge amount of energy is produced, and so the Sun becomes hotter and hotter over time. Slide 04 OK, so it makes sense to say that the early Sun was fainter than today's Sun, thus the name "Faint Young Sun. But why do we call this phenomenon the Faint Young Sun Paradox? Well, the paradox is that, if we go into the data, we should see that Earth was frozen! It must have been sooo much colder than today... But we know that Earth was not a big block of ice, quite the opposite. Early Earth was in fact a hot place and rocks were melting and magma was erupting, etc. That's why we call it a paradox... the Sun was so much cooler, yet the Earth was not (in fact, it was probably even hotter than today!). Slide 05 Here's the explanation. On this second model here, the first blue line is a model that was run to show the Earth's temperature if there were no greenhouse gasses whatsoever in our atmosphere throughout Earth's history. At the beginning, temperatures at the surface of Earth would have been something like -40ºC. Here's our block of ice Earth. And then over time, if you follow that same line upwards, Earth's temperature went up slowly and would reach -18 degrees C today, which is Earth's temperature without the greenhouse gasses. Now if we use the amount of greenhouse gas effect that we have here on Earth today, we constrain the model such that today is in the top right in red on that second line, we put today's mean temperature at 15ºC. We keep that amount of GHG and back-calculate Earth's temperature back in time down to the beginning of Earth, we actually see that if we add greenhouse gasses, the Earth's temp would be -20ºC or something like that. Sorry guys, Earth would still have been a block of ice, even with GHG. So what happened? Why was Earth not frozen even though the Sun was so faint? Well, we have an interesting hypothesis as to what happened. The only way to maintain above-freezing temperatures while the Sun is faint is that there must have been a whole lot more GHG in the atmosphere. Geologists like to say that Earth's thermostat is first and foremost the carbon cycle. And specifically, carbon dioxide (CO2). I will explain in the next few slides. Slide 06 We all know what a thermostat is. You probably have one in your home. Let's say you set it at 75ºF. When the temperature outside of your house gets below 75ºF, your thermostat knows it and it kicks in to create more heat in your house to maintain that interior temperature at 75ºF. If the temperature goes above, the heat will switch off and the cooling will freshen the interior back to 75Fº. The life goal of a thermostat is to maintain, or regulate, a given temperature. What if I told you that CO2 in our atmosphere acts like a thermostat?! The concentration goes up when we need more heat, and it goes down if we need less heat. Nobody needs to set that thermostat; it's part of Earth's system. Slide 07 Here's a graphic representation of this concept. On the left, we have the early Earth. It's cold because of weak solar radiation (represented by the skinny squiggly red line) so we need lots of CO2 to maintain our preferred temperature (represented by the many red dots). Today, we have strong solar radiation (represented by the thick squiggly red line) so we need less CO2 (represented by fewer red dots in the atmosphere) to maintain our temperature. Where is that extra CO2 goes if we don't need it in the atmosphere? The answer is spelled out right there, on those schematics... The CO2 gets stored in the rocks. How is that happening? Through a natural process called chemical weathering. Slide 08 Let's start at the top here where it says "warmer climate". Warmer climates typically have lots of CO2 in the atmosphere. OK, a warmer climate leads to increases in precipitation (because with warmer temperatures we have more evaporation, and the warmer air can hold up more moisture, and then more precipitation can happen, so we have an overall intensified water cycle), and warmer temperatures combined with more precipitation lead to more vegetation growth. With all of these factors, we have what is called increased chemical weathering. Slide 09 Weathering is the breaking down and dissolution of rocks. Here for example, there's the continental crust. When it rains on those rocks, the rain water mixes up with the CO2 in the atmosphere, creating a weak acid called carbonic acid (H2CO3 on the figure), which is weak and not dangerous. Carbonic acid slowly attacks and degrades the rocks; the slowly dissolve away. Slide 10 Back here for a minute: lots of CO2 in the atmosphere leads to warmer temperatures, which leads to intensified hydrological cycle and more vegetation growth. This increases chemical weathering. But when the rain water combines with the atmospheric CO2, the CO2 gets captured from the atmosphere, it's transformed into carbonic acid, and it dissolves the rocks. That CO2 is now out of the atmosphere. Over time, this process cools the climate. Here's our thermostat! Slide 11 Back here again: H2O (rain) is combining with CO2 and creating H2CO3 (carbonic acid). This weak acid will then weather the rocks, or dissolve the rocks. And this is what we call chemical weathering. As we know, most of the continental crust on Earth is made of granite. And a lot of this granite is a silicate. You don't really need to know this, it's just if you're interested. Once the carbonic acid starts degrading those silicates into ions, these elements, including CO2, will be transported by the regional groundwater and rivers. Eventually these anions and cations will make their way into the ocean. What happens in the ocean is very cool. There are a lot of creatures in the ocean that take up these ions and use them to create their shells. Foraminifera and some plankton are good examples. Millions of organisms are grabbing these ions and using them to build up their shells. Over time, as these creatures die, their shells sink down towards the bottom of the ocean, where they accumulate on the ocean floor into layers of sediments. Over millions of years, these layers become sedimentary rocks. This is where the carbon gets stored during warm climates!! Isn't it incredible? These shells are formed with the carbon that is in the ocean water, but part of this carbon comes the land, which comes from the carbonic acid, which comes from the CO2 that was combined with rain water in the atmosphere. So now, the CO2 that was once in the atmosphere is trapped in sediments on the ocean floor. Over time, as this happens all over the planet on all the continents and oceans over thousands and thousands of years, Earth is actually able to bring down its temperature by drawing out CO2 from the atmosphere and storing it into sedimentary rocks. It's a lot more complicated than the thermostat in your house, but kind of the same idea, right? Now, you might ask, what happens when the climate gets cold enough? Let's say that the climate is at this equilibrium, like when your house has maintained 75ºF? Well, because there's less CO2 in the atmosphere, now the temperature is cooler, there is less precipitation, the vegetation is not as big, chemical weathering decreases, so we will remove less CO2 from the atmosphere, and so eventually, things kind of start balancing out and the weathering process slows down to maintain the right temperature. What happens next is one of my favorite geology topics; I think it's pretty cool. The issue is that Earth's thermostat did not work super well all of the time. Especially during the "Faint Young Sun" period of Earth's history. The thermostat actually broke a little bit. So sometimes, it was overshooting, like a pendulum that swings too far left and right. Slide 12 This issue led to 2 main kinds of climates on Earth: icehouse world and hothouse world. A couple times, Earth ended up being completely covered in ice! Those times are called icehouse worlds or snowball Earth. A few times, Earth ended up having no ice sheets at all! Those times are called hothouse worlds or greenhouse Earth. These huge swings in CO2 concentrations and temperature led to these dramatic changes in climate. Slide 13 Let's start with what I call a recipe for getting an icehouse world. First of all, icehouses only happened a very, very long time ago when the Sun was faint. It's highly unlikely to happen again. But back in the day of the faint young Sun, and remember, Earth had a lot of CO2 in its atmosphere to maintain adequate temperatures for life. About 650 million years ago, which was the last of the snowball earth events, all of the continents were located in the tropics. There were no continent near the poles, where Antarctica and northern Canada are today; just polar oceans. Most of the continental masses were close to the tropics. That is very important in this story! The equator is the warmest place on earth, right? It's where the solar radiation hits the most directly. Ok, there was a lot of CO2 in the atmosphere, it's hot, so the thermostat kind of wants to get rid of some CO2, and so the weathering cycle is very intense. Slide 14 There is a lot of CO2 being removed from the atmosphere through carbonic acid, and eventually a lot of CO2 is buried deep into the ocean. Eventually, we know that the cycle of weathering should slow down because CO2 is being taken out of the atmosphere. The issue here is that it did not slow down. It did not slow down because the tropics are always a warm place! So the weathering kept on going! So this whole idea that the weathering cycle will slow down does not work around the equator. Earth is in trouble! The thermostat is flawed!! Slide 15 The temperatures are starting to cool down but the weathering cycle is not slowing down. As a result of that, we start forming ice. The Earth is now cold enough that ice forms in the tropics. That's right. Ice in the tropics. Now, what's up about ice is that it has a really high albedo. A high reflectance. Slide 16 So now the weathering cycle keeps happening, we have these white patches of ice where the solar radiation comes in and then bounces back out to space, which cools our planet even more. As a result, there is a positive feedback loop where it's cold enough for ice to keep forming, which increases the albedo, which means that temperatures keep going down even more because the solar radiation is not being absorbed by land. Eventually, the Earth becomes covered in ice and that is what we call the snowball earth. How on Earth do we know all this? Slide 17 Uniformitarianism. The core concept and guiding principle of geology is to remember that "the present is the key to the past". We know a lot about glaciers, because we have a lot of glaciers that we can measure and observe. We know a lot about ice sheets because we've studied them in Greenland and Antarctica. We use this knowledge of modern-day Earth and apply it to the past. The present is the key to the past. To do this, we first need to assume that whatever is happening now, was happening in the past in just the same way. We assume that the same natural laws and processes that operate in the universe now have always operated in the universe in the past and apply everywhere in the universe. Slide 18 Here is an example of uniformitarianism. A few hundred or thousand years ago, when humans first saw the Grand Canyon, they didn't think, oh yes, a river has been carving down the rocks. They thought, oh my God what happened here?! This must have been a huge catastrophe! This is crazy! And for a while, geologists were thinking if the world's evolution as a very catastrophic type of process. Although it's true, catastrophic events that really change the landscape drastically do happen, lots of changes happen gradually, slowly. Even in the case of landscapes that look very dramatic, forces that play tend to be very slow and have been acting for millions of years. Like the Grand Canyon. Uniformitarianism is telling us that, hey, you know rivers, right? You've observed and measured them. You know how they operate today, how they carve their way down into the rocks. We can apply those principles to crazy-looking places like the Grand Canyon and understand what happened. Slide 19 Okay back to our snowball Earth story. How do we know that the Earth was covered in ice? You'll learn 3 different types of evidence that we have. (1) We have glacial deposits in tropical rocks. We can reconstruct how continents have moved and rotated through time. By dating rocks from 650 Ma and replacing them in their paleo-location, we can learn something from the geology from way back when. Ice in the tropics is rare. The rocks on this picture are from a real outcrop of those continental rocks that were located in the tropics, back during snowball Earth. These rocks contain dropstones, a proof of glacial activity. They are the rocks you see here that are embedded in these other rock layers. We know about dropstones because we've been observing them in modern day. Slide 20 This is a modern day dropstone. It's a rock that got detached from the land and that was taken away on an iceberg. A fairly common phenomenon. As you know, icebergs melt. Eventually, this iceberg is going to melt and this rock on top is going to drop at the bottom of the ocean. This is why it's called a dropstone. So where there is a lot of glacial activities, we find a lot of dropstones near the edges of the continents. Slide 21 All of these dropstones in this rock layer suggest that there must have been glaciers that were located in the tropics, because we found dropstones in sedimentary rocks from the area. Slide 22 The second evidence is the tropical distribution of the sites. There are so many sites and they are all over the tropics. So the fact that it was all over the place means that it was a global event. It wasn't just one glacier dropping lots of stones around an area. The locations were all over the tropics, suggesting that it was probably a really large event, probably covering a large part of Earth, if not all of it. Slide 23 And the third evidence is that these events are synchronous; all happening at the same time. We use radiometric dating to date the layers that contain the dropstones, and we know that they all belong to the same time period. Slide 24 After the ice house, or the snowball Earth, comes the hothouse or the greenhouse Earth. Here is the story, briefly. Imagine Earth as an iceball. What does it mean? There is no more weathering happening. Because all of the rocks are covered by glaciers, and ice sheets, and snow, and slush. There must be contact with the granite in order to make carbonic acid and dissolve the rocks. So, for millions of years, what it means is that CO2 is going to build up again in the atmosphere because volcanoes are still degassing (they are emitting CO2 and other gasses into the atmosphere). So slowly but surly, Earth is going to warm up again. Eventually, the greenhouse effect, as a result of this CO2 build up, starts to melt the ice. The thing is, when the ice starts melting, there are parts of the planet that are not white anymore. They would be like dark blue, showing the ocean. And so now the albedo goes the other way: low reflectance on those portions, which means that the albedo gets low, meaning there is more solar radiation that is absorbed by Earth's surface. More absorbed solar radiation means that the temperatures are heating up. Actually, temperature are rising so fast that the weathering cycle can't keep up! This leads to an increase of greenhouse gasses in the atmosphere and eventually all of the ice melts away. All of the ice. No more ice on the earth. The hot house is when there is absolutely no ice on earth. Slide 25 Like I just said, there is so much weathering going on during a hot house, but Earth cannot keep up with it. And just like we had proof of the ice house with the dropstones, we have proof that the snowball earth was followed by the greenhouse earth thanks to two different proofs. (1) cap carbonates. Geologists found layers of carbonates (made of calcium and carbon in areas right on the edge of past continents. The hypothesis is that there was so much CO2 that when it got in touch with rain, it created acid rain. As a result, there was very rapid and intense chemical weathering of the rocks, releasing large amounts of calcium (and other ions) towards the oceans, which was used by plankton but ALSO was deposited into distinctively textured layers of carbonate sedimentary rock. Those have been called cap carbonates. But it gets better: these cap carbonate sediments are found right on top of the sedimentary rocks that contained the dropstones! Makes sense!! That's also why we know that icehouse and hothouse followed each other pretty quickly. Slide 26 The last evidence is actually really cool. It has to do with iridium. Iridium is a cosmic particle that is not found naturally here on the surface of our planet. Well, we find iridium layers sandwiched between the dropstone deposits and the cap carbonate layers. The idea is that when we were in snowball earth, and these dropstones were being dropped into the ocean, etc., as we know, the Earth is covered in ice. But it was still being bombarded by meteorites from time to time, and getting iridium particles from the cosmos. This iridium must have been deposited onto the glaciers, and then nothing happened to it for millions of years. And eventually, when the ice melted quickly, all of this iridium is washed into the ocean and was deposited as a thin layer on top of the dropstone layers. And then cap carbonates started forming on top of the iridium layer as a result of the very rapid chemical weathering of the rocks. Slide 27 To recap: on the Y axis, there's atmospheric CO2 concentration is parts per million (ppm). So in 1 ppm, you have 1 molecule of CO2 with 1 million molecules of air. Right now, there's ~ 410 parts per million CO2 in the atmosphere. On the first panel to the left, we are back hundreds of million years ago. And you can see that the CO2 in the atmosphere is fluctuating like crazy. There were times with over 2000 ppm! That's about 5 times today's concentration. And those times were hot houses, when there was no ice on Earth and warm temperatures. If you look at this big peak around 200 million years ago, and you follow the line all the way until today, pretty much Earth's CO2 concentration has been pretty continuously going down and down and down. Maybe we have been needing less CO2 because the Sun has been getting stronger over time, that's part of the explanation. In the last panel to the right, we are in calendar years, from year 1000 to 2500. So it is actually showing you the predicted CO2 concentration for year 2500. At year 2000, we are at 400 ppm. The last time we were at 400 ppm was about... 5 million years ago at least. So it's been a very long time since Earth has had such a relatively high concentration of greenhouse gasses. And then, the future predictions that are made by scientists with different scenarios of how much more CO2 humans will put into the atmosphere are shown by these RCP scenarios. RCP 8.5 is the business-as-usual scenario, where humanity does not curb its emissions. By 2500 we would basically be back to 2000 ppm. And the last time we were there was 200 million years ago. Slide 28 I have no ask: do you think we in an ice house or a hot house? The answer is: an ice house. Not a snowball Earth, evidently. But we are still in an icehouse because we are living in a time when there are still ice sheets on the planet. A hot house is when there is no ice whatsoever. The time period we are in now is called the Quaternary. The Quaternary is characterized by periods of glacial and interglacial cycles; it is considered an Ice Age. Right now, we are in an interglacial (a period with less ice, but still with ice sheets: Greenland and Antarctica). But if you go back 21 thousand years ago, which sounds like a long time ago, but compared to Earth's history it's like, yesterday. Slide 29 Look at how big and extensive Earth's ice sheets were in the northern hemisphere only 21,000 years ago! They covered all of Canada, and even all the way down to New York City. Scandinavia was covered in ice.

Sunspots

Slide 01 In the previous lecture we learned about the amount of solar energy we are receiving on the surface of Earth. We learned the word solar constant, and we learned that this value is constant, at 1366 W/m2, because it is based on 2 constant numbers: the distance between the Sun and Earth, and the Sun luminosity. Today you will learn that actually, the solar constant varies a little bit over time and for different reasons. The first case study has to do with sunspots. Slide 02 This is a flare on the surface of the Sun; it's an eruption of plasma that happens in a sunspot. Here's what you need to know about sunspots: first, is they are regions of the sun with intensified magnetic activity. Second, they vary in size quite a bit. Smaller sunspots can be maybe fifteen kilometers in diameter, whereas the largest ones can be one-hundred and fifty thousand kilometers in diameter. Third, sunspots come and go. They can stick around for a few days, a few weeks, and then they disappear. But what is a sunspot? It's actually a "colder" region on the surface of the Sun. Now, I know they're called hot spots, but they are colder regions of the sun. The sun's temperature is about 5,700 Kelvin. And those sunspots are about 3,700 Kelvin. So, quite a bit cooler. But still ridiculously hot by Earth standards. Slide 03 Here are two more photos. Here's one that was captured using UV, and the other one is just showing you what sun spots look like. The sunspots are these dark areas on the sun. Slide 04 Something that will become very important in a few minutes is that around each sunspot, so the sunspot itself is the dark area, we have really bright areas that are called faculae. The faculae are extremely bright areas surrounding the sunspots. People have been fascinated by sunspots appearing and disappearing from the surface of the Sun for hundreds of years. Probably thousands of years to be honest. But really the first one who started making them more of an official case for science was Galileo, because he came up with the first telescope. He was able to actually look at the sun. Word of caution, do not take a telescope and stare at the sun, you're going to burn your eyes. There's a special filter that is required to look at the sun. The image I'm showing you here is a refracting telescope in Switzerland. This lab is the official sunspot counting lab. People started counting sunspots using this very telescope, on a daily basis in 1849. And every day, there's a person who counts the sunspots using the same telescope, even nowadays. You can trust me on this, there are way fancier telescopes, but they're still using that same old rickety telescope because they want to have as homogeneous data set of daily records of the sunspots numbers as possible. Slide 06 So how do we count sunspots? Well, there's two numbers that go into this equation. And this is really simple science. First of all, they count the number of sunspot groups, because they usually come in groups. And it's very difficult to pick apart the exact number of sunspots within each group. So what they do with that, as soon as they see a cluster, they assume there are ten sunspots in there. The equation goes like this: count the number of sunspot groups, multiply that number by ten. Then, count the number of individual, large sunspots. Add those 2 numbers and you get the total estimate of sunspots. Why does it matter? Well, I'm going to show you what the sunspot number has looked like since 1849. Slide 07 What do you notice? The graph starts in the upper left in year 1849. And if you continue the timeline, it goes all the way to 2015. Every dot you see is one daily measurement of the number of sunspots. Do you see the cyclicity in the data? How the number of sunspots goes up and down, over and over again? There is a predictable, measurable 11 year cycle going on. Slide 08 By the way, now we are in solar cycle 24. And see how our sunspot count is actually very small nowadays? How is that affecting solar radiation and our climate, if at all? Slide 09 So this figure here is going to help us answer this question. I'm going to explain it to you step by step. There's a bunch of different lines here. The first line to look at is the number of sunspots. It's the blue line. And here we have about 30 years worth of data, with roughly 3 sunspot cycles. Right? So the blue line goes up and down and up and down. When it's up we have more sunspots, and when it's down we have less. Second line that's important to look at is the red line. Because it's the average solar irradiance, so how much energy we're getting from the sun. It's the solar constant. We said that the solar constant was 1366 W/m2 in a previous lecture. But as you can see, in reality, it fluctuates around that number a bit. And see how it follows the sunspot observation quite well? The blue and red lines are pretty closely aligned. So, yeah, the sunspot number seems to be influencing the amount of irradiance we get here on earth. But look at the actual numbers: irradiance goes from 1365 to 1367, if that. It's not too wild, is it? We're not seeing really large variations in solar irradiance because of sunspots. It's very, very small changes. More sunspots, more solar irradiance. Many people have been wandering if there is a connection between the sunspot number and global warming. Could global warming be related to an increased number of sunspots?" Well...no. I just showed you a couple minutes ago, that we are in cycle 24 and that, if anything, we are experiencing lower sunspots than usual. But wait a minute: when we have more sunspots, we have more solar irradiance, but sunspots are dark cold spots on the Sun. So why will we get more irradiance when there are more dark cold spots?! It's because of the faculae, those really bright areas around the sunspots. They more than overcompensate for the dark areas, and that is the reason why we receive extra irradiance during those times. There was a time in our semi-recent history where people wondered if sunspots, if pushed to an extremely low or high number, might have caused past climate change. That's because we have a time period during the Little Ice Age, in the late 1650s, where there are pretty much no sunspots. That period is called the Maunder Minimum. Slide 10 I know, I told you that the Swiss only started counting the sunspots back in 1849. But the Chinese were counting sunspots way before that. And in fact, the record goes all the way back to 1600 and that is the record you're seeing now. See that, around 1635 and all the way to 1715, there's no sunspots, or a "sunspot minimum". We will talk about the Little Ice Age in another class, but just to start introducing you to these terms and these times in our recent history, the little ice age is a time when temperatures dropped over most of the world. In Western Europe, glaciers in the alps were growing. So many people said, "Hey, well maybe it's because there are less sunspots, and so we were receiving less irradiance." Maybe. But you must know that this hypothesis is controversial and that the sunspot minimum alone can't be used to explain the cold spells during the Little Ice Age. Some scientists a cascading effect...because when you receive less solar irradiance, you also receive less UV radiation from the sun. Less UV radiation leads to less ozone formation. And ozone is a greenhouse gas. The sunspots themselves influence the total irradiance that we get here on earth marginally, just a little bit, but the cascading effects of the change in irradiance that affects GHG concentration... this could have had a compounding effect on the climate. I don't want to ramble too much on this, but paleoclimate is my specialty so I want to tell you a few more things about the LIA. During the Little Ice Age there's also a general increase in volcanic activity on Earth. Most people tend to agree that this volcanism is what mostly caused the cool period. , When a large volcano erupts, it sends a lot of ash into the stratosphere. This dust and ash hang around and they prevent sunlight from penetrating into our atmosphere. Volcanoes have the effect of cooling our planet for one to three years depending on the intensity of the volcanic eruption. Slide 11 Slide 12 What you really need to remember is what sunspots are, the 11 year cycle, and that really, the sunspot number has a very small impact on solar irradiance. The potential impact of the sunspot number on our climate, if any, has more to do with potential cascading feedbacks and effects rather than the direct effect on the solar constant. And one more thing to conclude. If you want to go check out the northern lights, or auroras, in the Northern Hemisphere, go to like Alaska, or Scandinavia, or something like that, and make sure you go when we are at a sunspot maximum. Because that's when we get the best ones J