Climate Science 101: Fundamentals of Climate Science

1273.1.Climate Science 101 - Fundamentals - Justin Bandoro

[00:00:00:18] Thanks again everyone for coming out here. So I'll be focusing on the climate science part of these lectures. I'll be doing a talk today and another talk tomorrow. The one tomorrow will build on what we're going to learn today. And introduce myself, I'm Justin Bandoro. I'm a PhD student in the Program in Atmospheres, Oceans, and Climate. So that's course 12 here at MIT. And my area of focus is on atmospheric science and has to do with the layer of the atmosphere called the stratosphere, which we'll learn more about today.

[00:00:35:29] So the purpose of these lectures is to develop a broad understanding of the Earth climate system. So referring to what Christoph is showing with his whole model, we're going to be learning about the science for the Earth system part of that-- so the Earth system, so focusing on that. And so today, we're going to focus on the first bullet point, which is to develop a broad understanding of it. And then tomorrow, we'll look at how the climate system can respond to different natural and human caused changes.

[00:01:06:16] And like you said, I'm not focusing on the policy, economics, or governance. These lectures are on the science. And today's topics-- we're going to touch on the history of climate science, and then go into the structure and composition of Earth's atmosphere, and then look at Earth's energy budget and how greenhouse gases can affect Earth's energy budget and warm the surface, and then look at different lengths of variability in the climate system that range from anywhere from a year to hundreds of thousands of years. And on the last topic-- will be on the emissions of greenhouse gases and their long-lived persistence in the atmosphere.

[00:01:50:09] So this is a cool animation developed by scientists at NASA. So this is going back to 1850 and looking at the temperature change since 1850. And as it goes around in the circle, it will repeat. It's showing the months around the circle, and then showing the temperature change since 1850. So you can see, it's going out. And the color code is-- purple is relatively colder, and yellow is warmer. So you can see, as it progresses.

[00:02:21:25] There's two important things to note. The first is that some years you can see it contract inwards and comes back outwards, which shows the variability in the system. So it's not saying that every year is getting warmer and warmer-- other years where it spreads out and goes inwards. But you can see the overall trend where the Earth has warmed globally. So, yes, this is a global average. It is globally around 0.8 degrees Celsius. So today, we're going to try to understand what could be causing this.

[00:02:53:11] So to start off with the definitions of climate-- the popular definitions could be-- it's the average of weather. The other one is it's what you expect, and weather is what you get. But here, the climate is the statistics of weather. So over time, you accumulate more and more information-- you can get the mean of the weather, but also the variability. Yes, so it's aggregations over time scales in more than one year and so that the seasonal cycle is not considered.

[00:03:28:07] And examples of climate variability are of what most of you have probably heard of is El Nino, La Nina. So I'll talk more about this later, and that just has to do with the warming or cooling over the Eastern Pacific Equatorial Ocean. And that affects weather all across the globe. And then, another length scale is the Little Ice Age. So that was around in the 1600s, where the Dutch were skating up canals to work.

[00:04:01:10] And this is just an example of where they could be periods that are colder or warmer than others. And then, along a longer time scale, you can have these glacial cycles, which are anywhere from 20 to 100,000 years. And for an example of this, this is where a glacier is covered, a large part of the Northern-- or the North Hemisphere-- or sorry, North America and reach all the way down here to Boston.

[00:04:30:21] To start off with, we'll just dive into the history of climate science and the Greenhouse Effect. We'll get into the science behind it, so it's just that some gases in the atmosphere absorb infrared radiation, and they re-emit it back down to the surface, which causes a warming effect. But this was actually first known in the late 1700s by John Fourier.

[00:04:59:05] And he first understood was going on, that these gases would absorb it and re-emit it. So it would get warmer. But it wasn't first quantified until John Tyndall in the mid 1800s, where he built an apparatus that could actually measure, quantitatively measure, how much absorption there is from certain greenhouse gases.

[00:05:27:27] And then, continuing on-- this was Svante Arrhenius. And in the late 1800s and early 1900s, he actually-- without any of the global climate models we have these days-- was able to figure out that, or estimate, any doubling of the percentage of carbon dioxide in the air would raise the temperature of the Earth by four degrees. So he knew this, or he was able to estimate this in the late 1800s. And surprisingly, this number here, which we'll learn about in tomorrow's lecture of climate sensitivity, is pretty in the middle of what estimates we have today.

[00:06:14:05] And lastly, this man-- Milutin Milankovitch-- in the early 1900s-- he solved the mystery of the Ice Ages. So ice ages occur because in the Northern Hemisphere, it receives more or less solar insulation during the summer season. And the reasons for this have to do with Earth's eccentricities. So if you think about how circular Earth's orbit is-- because it's not a perfect circle. But how circular it is, or the eccentricity, changes with a period of 100,000 years. And there is also the obliquity.

[00:06:57:14] So because we have seasons, Earth's axis isn't exactly perpendicular to its orbit. So it's tilted, and this tilt varies around 2 and 1/2 degrees. It's around 23 degrees, but it can vary up to 2 degrees. And that has a period of around 41,000 years.

[00:07:18:04] And lastly, there's also precession. So that's how much it wobbles around its axis. So you can think of it as spinning a top. And when the top is about to die, you notice that it starts wobbling around. That's what's called precession. So all of these together-- the combination of all these 41,000, the 20,000, the 100,000 year cycles together-- that can explain the Ice Ages, because it has differences in how much solar insulation the Earth is receiving in the summer season in the Northern Hemisphere.

[00:07:54:29] What was obliquity again?

[00:07:56:19] Oh sorry-- so obliquity-- so Earth's axis isn't perpendicular, so it's actually tilted. So when it's going around the sun, it's actually tilted.

[00:08:06:13] Well, it'll stay that constant tilt.

[00:08:07:20] Yeah, so it stays in that tilt, but then, the amount of that tilt varies.

[00:08:12:04] Oh, it varies. So that's the obliquity factor.

[00:08:14:14] Yeah, so it varies by around two and a half degrees. And that will-- if you think about it-- depending on which season you're in, you'll receive more sunlight than depending on if it was tilted in the other direction. Great.

[00:08:33:23] So we'll start with the structure of the atmosphere. So the atmosphere is divided, commonly, into four different layers. And so the troposphere, which is also called the weather layer-- that goes from the Earth's surface to about 10 kilometers. What happens? The cloud tops-- they only get up to 10 kilometers here. And right under here is usually the level that planes fly at.

[00:09:09:06] So this is showing temperature with respect to height. Kilometer is on the left, mile is on the right-hand side. And you can see temperature decreases with height in the troposphere. And then, the second layer, which contains both the stratosphere and the mesosphere, is the middle atmosphere. And the stratosphere-- what's interesting to note is that the temperature turns around and starts increasing back with height.

[00:09:36:05] And this has to do with the ozone layer, that you all have probably heard of, which peaks around 30 kilometers in the stratosphere and ozone absorbs solar UV radiation. And that's what causes this layer to warm up.

[00:09:54:03] And then, you notice these things called the tropopause, the stratapause and mesopause. These are just points where the temperature turns around, so it turns from cooling with height to-- or warming with height. And then, in the mesosphere, it decreases back again. Then finally, way up over 90 kilometers, we have a thermosphere, where temperature turns back around and increases with height.

[00:10:20:06] And this has to do with the interaction with the charged particles from the sun that heat up this area. But for these lectures, you don't really have to consider the mesosphere and the thermosphere. And we'll only be focusing on the troposphere and the stratosphere, because that's what's important for climate.

[00:10:43:05] And another thing to note is the atmosphere is very thin compared to the radius of the Earth. So if you look up here, going from 0 to 130 kilometers, the radius of the Earth is around 6,000 kilometers. So it's very small compared to the radius of Earth.

[00:11:00:14] Sorry, which region are the satellites?

[00:11:04:13] So the satellites-- depending on which orbit they're in, they can-- some of them are right at the low end of the-- above the mesopause. And some of them can be way up higher.

[00:11:21:00] So this is showing the temperature climatology of the Earth. So it's showing the average temperature for a given month. So this is going through from January to December. So this is averaged over 40 years. And the color bar-- this is well below freezing. And then, hot and red is maybe around 20 to 30 degrees Celsius. So it's showing how much-- just the average temperatures over the globe.

[00:11:48:23] And what you notice is that this belt of warm temperatures around the tropics-- it shifts northward or southward depending on the season. And then, in certain areas-- like in Siberia-- you can see changes of around 50 degrees Celsius from summer to winter. While in the tropics, some of these regions only have a change of around 3 degrees Celsius. So you get more extreme variations in high latitudes in the Northern Hemisphere.

[00:12:21:21] And the other thing you might notice is that in the Southern Hemisphere, the changes aren't as drastic. As you can see over here, they're very drastic. It's not as drastic over the Southern Hemisphere, and that's because a large portion is ocean. And in the Northern Hemisphere, is where you have more of the land. So the ocean damps out the seasonal variability in the Southern Hemisphere.

[00:12:46:23] And this is showing the same thing but for sea surface temperatures. And you'll notice again, you have the warm belt over the tropics, but it's not zonally symmetric. So what that means is there's regions where the ocean is much warmer than others. And other features to note-- you can see the Gulf Stream coming up here for North America, that brings warmer water up our East Coast, and over to Europe-- and the Kuroshio Current in Japan. So this is just to show broad view of the average, what we think contains average temperatures on land and also in the ocean.

[00:13:31:01] So next part we're going to look at is atmospheric composition. So this is a pie chart, and blue is nitrogen, N2. And red is oxygen, O2. And the first thing that stands out to you is that most of the atmosphere is nitrogen and oxygen. So 78% nitrogen, 20% oxygen. And you only have this tiny sliver here. 1% is argon, and you have this tiny sliver is here with water vapor, which is 0.4%, and these trace gases, which include carbon dioxide, methane, and other inert gases-- and they also have ozone and nitrous oxide.

[00:14:11:19] Sorry, this is everything below stratosphere?

[00:14:14:29] Yes, there's not much above the stratosphere. 80% of the masses in the troposphere-- so there's not much above there.

[00:14:24:15] But is the composition the same going through the--

[00:14:28:00] I'm going to touch on that next. You're right. So concentrations of nitrogen and oxygen are relatively constant with height, but you can get very drastic variations. So the top one here shows water vapor concentrations, which are also called mixing ratios. And it's a log scale down here. And it has temperature, which is the solid black curve. And then water vapor is the dotted curve. And you can see, water vapor-- because this is a log scale-- the concentration of water vapor drops exponentially with height. And this has to do with temperatures cooling.

[00:15:09:20] So the amount of water vapor you can hold in the air depends on the temperature. So as it gets colder, water vapor precipitates out of the atmosphere. And so that's why these concentrations drop.

[00:15:25:09] And another one that changes with height is ozone. And so like I was referring to before, we have the low ozone layer here, and it peaks in the stratosphere. And this is what's called good ozone, because it protects us. You also shouldn't confuse it with the ozone from pollution in the troposphere, which is bad for health effects. So that's just another example that-- nitrogen and oxygen are relatively constant with height concentrations. Some of these gases vary in the vertical.

[00:16:07:28] And it also varies temporally as well-- so not just in height but over time. So this is showing from CO2 from the Mauna Loa Observatory in Hawaii-- concentrations of carbon dioxide. That's also measured in parts per million or mixing ratios. That's how much you get given volume. And you can see, this is going back from late 1950s, when it started, up to present day. And you can see the increase in CO2 over time.

[00:16:38:06] But what you also notice-- the black curve is the average. So it's like a low-pass filter or just averaging over years and moving through it. But we can also see, there's these variations that are occurring every year. And this has to do with-- in the Northern Hemisphere, as I was pointing out before, there was more land in the Northern Hemisphere. So in wintertime there, CO2 levels peaked, because you have less photosynthesis. So that's why you can see that it's also changing seasonally, as well as overall increasing trend. And it's getting up over 400 parts per million.

[00:17:20:29] So the next thing we're going to talk about is Earth's energy balance. So we have the sun, and it emits a luminosity. The power or energy that it emits is 3.9 times 10 to the power of 26 watts. To put that into perspective, you're household light bulb puts out around 100 watts. So just showing the order of magnitude of what's coming out of the sun. You can also think of this in terms of radians, which is the flux of energy for area.

[00:17:55:27] So if you just looked at a certain area, right on the outside of the sun, which is the photosphere, you'll measure 6.4 times 10 to the seven watts per meter squared. So that's how much energy is passing through the surface in surface area.

[00:18:12:04] That's based on the surface of the sun--

[00:18:14:04] Surface of the sun. So this is the radiance-- just the outside edge of the sun, which is called the photosphere. And then, because the inverse square law, which says that intensity drops as a factor of 1 over the radius squared-- so as you get further away, the intensity or the radiance diminishes, as you get farther away. So the separation between the Earth and the sun is around 1.5 times ten to the power of 11 meters.

[00:18:44:08] And you can see, once it gets to the top of the atmosphere, the radiance or intensity reaching the Earth is 1370 watts per meter squared. And this value is referred to as the solar constant. And this, as we'll see later-- the output from the sun also has an 11-year cycle with it. Yes?

[00:19:09:22] Is that what actually reaches the surface of the Earth?

[00:19:12:11] No, sorry, this is what's at the top of the atmosphere. I'm going to touch on that next. So the total absorb radiation-- so if you take that solar constant and-- if you think about it, the surface that the sun sees is just a circle, at any given time. It just sees a circle. And the area of a circle is pi r-squared. And you can use the radius of the Earth, and you can multiply the radius of the Earth by the solar constant. And that will give you the total absorb solar radiation. But you also have to take into account the albedo.

[00:19:51:24] So the albedo is a fraction of incident or incoming solar radiation that is reflected back to space. And this can be from clouds, can be from ice. So the average value is around 0.3, so 0.3 of the incoming solar radiation is reflected. So it never touches the Earth's surface. So you have to take this into account. So you do 1 minus ap, so that's going to give you 0.7. So this gives you the total absorb solar radiation.

[00:20:23:01] And then, you have to think about that total absorb solar radiation as distributed all over the Earth. So if you think about Earth model it as a sphere, which is pretty good, the area of a sphere is 4 pi r-squared. So if you take this value and divide it by four pi r-squared, this gives you the absorption per unit area-- so the energy per unit area that's averaged over the whole planet.

[00:21:02:24] So we can think about what temperature the Earth would be if we didn't have an atmosphere. So if we didn't have an atmosphere, we can estimate Earth's surface temperature by using Stefan Boltzmann's law, which has to do with the black bodies. So black body is a theoretical body that it's a perfect emitter and a perfect absorber. So all incident radiation upon it, it absorbs it, and then, it re-emits it. So that's why it's called a black body, because it has no color.

[00:21:38:17] And so if you think about it-- what Stefan Botlzmann's law tells you for a black body is that f is the radiation that's emitting is proportional to this constant times the temperature of the body to the power of a four. So this is a total absorption per unit area. And so Earth has to be in balance, so that what's absorbed is emitted. If you take [INAUDIBLE] and you plug in sigma t to the power of four, and you solve for the surface temperature, which is the effective temperature of the Earth, this value comes out to around 255 Kelvin or minus 18 degrees Celsius.

[00:22:29:15] So if we didn't have an atmosphere, this would be the temperature. And as we all know, that's way too cold. And the actual observed surface temperature is 15 degrees Celsius. So this tells you the atmosphere has to be important, because else it would be much colder than it is now. So what can be contributing to this?

[00:22:54:29] So coming back to John Tyndall, he first measured the infrared absorption of atmosphere gases. So I keep talking about infrared, or IR, so why do you keep focusing on this spectrum of radiation? And this has to do with-- again, coming back to the black bodies. Depending on the temperature of the black body-- so [INAUDIBLE] showing the sun at around 6,000 Kelvin.

[00:23:31:14] So Earth, which is around 303 Kelvin. So the peak in the wavelength of emission on the black body is inversely proportional to its temperature. So the sun, which is very hot, emits at a very small peak wavelength. So this is showing you the visible spectrum of light. And its peak is right in the middle, right at the edge of the spectrum of visible light.

[00:24:02:23] While Earth-- since we just saw-- if you take the Earth as 303 Kelvin, around 15 degrees Celsius, its peak is around in this region here, which is the infrared-- so just under 10 microns, it peaks at. So that's why we're interested in infrared absorption or absorption of atmospheric gases.

[00:24:31:08] That peak at 303 degrees Kelvin-- is that [INAUDIBLE]?

[00:24:35:03] Sorry, so this is showing the black body spectrums for two different objects-- one that's at 6,000 Kelvin, which is-- it's hard to tell here that's the red one. And then, this black one or blue one--

[00:24:48:08] Oh, it's two different curves.

[00:24:49:00] It's two different curves, yes. So this one, which is blacker blue is at 303. And this one, red one, is at 6,000 Kelvin.

[00:25:03:07] So his main conclusions were that nitrogen and oxygen are transparent in both infrared and solar radiation. So these spectrums we see here-- both nitrogen and oxygen-- they don't absorb from any of the solar radiation, and they don't absorb any of the outgoing infrared radiation from the planet. However, there are certain molecules, like the trace gases I was telling you before, that only make up a fraction or a small fraction of Earth's composition that are incredibly important, because they absorb in infrared.

[00:25:44:13] So this is water vapor, carbon dioxide, ozone, and some other gases. And he speculated how fluctuations in water vapor and CO2 could affect Earth's climate. So this is a complicated figure, but I'll walk you through it. So at the top again, this is similar to what I just showed. The solid red line is showing the black body spectrum of the sun that you would see at the top of the atmosphere.

[00:26:19:27] So this is if you're right at the edge of the atmosphere, and you're looking at the sun, what spectrum you would see. So don't focus on this part yet. The red curve is-- you'd see at the top of the atmosphere from the sun. And again, you can see it peaks in the visible. And there's a little [INAUDIBLE]. And it goes from the UV. It goes down.

[00:26:38:12] And then, these three curves here are the black body spectrum that you'd expect if you were standing at Earth's surface and looking down. So if you're just looking down at Earth's surface, what would you see coming up? And these are the blue curve, which is around 303 Kelvin. And then, the other one is showing 210 versus the black one, which is at 310 Kelvin.

[00:27:03:19] And then, we come back to these solid filled parts. So for the one on the left-hand side, this is showing what you would see at the surface of the Earth if you're looking up. So this is telling you how much of the black body spectrum is absorbed by atmospheres and the gas as the solar radiation comes down to the surface. And you can see here, this is just showing the total percent that's absorbed and scattered. And then, it shows that for each gas or scattering process.

[00:27:40:13] So a large portion, as you can see, is absorbed by oxygen and ozone. And so this is in the UV, so the very small wavelengths. So that's what's absorbed. So it doesn't get down to the surface.

[00:27:53:13] And then, there's also what's called Rayleigh scattering. So this is scattering, because these wavelengths are very similar to the size of molecules in the air-- so nitrogen, oxygen. So they scatter the incoming radiation, so it doesn't even get to the surface and scatters it back outwards. And so you can see, if you look at the sun, you can think of it as these gases taking chunks out of the black bodies. So they are taking these chunks out of it. So what we see at the surface ends up being this.

[00:28:31:16] That Rayleigh scattering?

[00:28:33:19] Yes, it scatters at the very small wavelengths.

[00:28:40:17] Now you have those charts listing oxygen and ozone and methane.

[00:28:45:19] Yes, first I'm just trying to point out what absorbs the solar radiation. So you can see, some of these molecules-- water vapor absorbs some of the incoming solar radiation, as well as-- carbon dioxide has a little band here. But a large portion is absorbed from oxygen and ozone and scattered back out from the atmosphere.

[00:29:12:11] I'm trying to understand the difference between Rayleigh scattering, because you mentioned that's from--

[00:29:15:12]

[00:29:15:21] Sorry, yeah, so Rayleigh scattering--

[00:29:16:24] --the size of the molecules. How is that different from--

[00:29:17:25] It's not absorption.

[00:29:19:09] Oh.

[00:29:19:21] So here, yeah, I'm showing total, so it's taking the total absorption and scattering. So absorption would be oxygen and ozone absorbing the incoming solar radiation. But scattering is just when-- you can think of it as bouncing off and just reflecting.

[00:29:37:04] So it's sent away from the Earth.

[00:29:37:15] Yes, so it's sent away, so it never reaches the surface. And this is because it's preferential, it's smaller wavelengths. And coming back, on the right-hand side, you can see-- so this blue solid curve is what's seen at the top-- so say if you're at the top of the atmosphere, and you're looking downwards, what spectrum would you see? And can see, it's very different from what you'd see at the surface. And this has to do with water vapor.

[00:30:12:14] So you can see, water vapor absorbs very strongly. It's actually the strongest greenhouse gas in the infrared. So it absorbs a lot. But you also have carbon dioxide, and then other constituents like methane and nitrous oxide, which absorb, and IR.

[00:30:37:00] So this part here is what's called the atmospheric window, so some of the infrared radiation is allowed to escape. But that's only 15% to 30% of it, while almost all of it is being absorbed by the atmosphere. And like I said, water vapor is the strongest absorber. And this is the same that John Tyndall concluded. And another important feature is that the carbon dioxide and the water vapor bands don't completely overlap. So any increase in here, in carbon dioxide, will absorb much more of this outgoing infrared radiation.

[00:31:21:19] So now, with this information on greenhouse gases, we can go back to that model, or that simple model we had with no atmosphere, and then add in an atmosphere and see what happens to the Earth's surface temperature. So we have to make some assumptions to do this. The first one is that the atmosphere is completely transparent to solar radiation. So all incoming solar radiation gets down to the surface. And then, the atmosphere is also a opaque to infrared radiation, so that all outgoing infrared radiation is absorbed by the atmosphere.

[00:32:01:12] So the infrared emission is from the surface and from the atmospheric layer. And here in this model, it's a simple model, we're only considering one layer, so a single slab of the atmosphere. So you can think about it like this. So coming back, this is the same value we had as the radiation that gets down to the surface of the Earth.

[00:32:22:24] And then Earth absorbs that, and then, it radiates at some temperature outwards. Then, you have the atmosphere here, which absorbs all the outgoing infrared radiation and radiates it back, both downward and upward. And then, you have to take that into account when we're going to calculate the surface temperature of Earth.

[00:32:44:07] But just to figure out what temperature the atmosphere should be at-- so this value, as we found out before, was 255 Kelvin. And so in balance, the radiation entering must equal to the outgoing radiation. So this means, just by that, the temperature of the atmosphere is now the effective or emission temperature of the planet.

[00:33:12:24] So we know the temperature of the atmosphere. And then at the surface, we have this balance, so that the radiation the surface is emitting is equal to both that of the downward from the atmosphere and also the incoming solar radiation. And if we rearrange this equation, we can solve for the surface temperature, and we get a value of around 30 degrees Celsius. And this is better than before, but it's a bit too hot. And this is because of the assumptions we made.

[00:33:50:21] So assumptions again-- like I said, there's a window where some of the IR radiation is allowed to escape. And not all the solar radiation that comes down to the surface is absorbed by the atmosphere. And you also have other processes that can transport heat from the surface that I'll touch on later, that are convection and conduction.

[00:34:18:03] You mentioned there the temperature of the atmosphere, and then, you talked about the temperature of the Earth. But which is controlling that? Is it the temperature of the Earth or the atmosphere, or is it a combination of both?

[00:34:27:08] So you can think of it as-- these all have to be in equilibrium. So the incoming radiation has to equal to the outgoing radiation. And through this simple assumption, this means the temperature of the atmosphere now has to be equal to the same value we had before, in the case with no atmosphere. And then, why-- because the atmosphere radiates both upward and downward and longwave or infrared radiation.

[00:34:59:13] The surface now has-- in addition to the solar radiation coming in-- the atmosphere radiating infrared radiation back downwards as well. So you have to take this into account. And that's what causes the temperature of the surface to go up to what we had before.

[00:35:23:03] So we can think about this in terms of energy budget of the atmosphere. So this is more complicated than that simple one we had before, but I'll walk you through it. So this value at the top is the incoming solar radiation. So this at 370 watts per meter squared, divided by four. 340 watts per meter squared is coming in. 79 of the watts per meter squared is absorbed by the atmosphere-- the oxygen, ozone, and some other gases.

[00:36:00:19] And then, you know, I was talking about the albedo before, so this is what's represented here. So around 100 watts total is reflected back out to space. So this has to do from both clouds. So there is a contribution coming from both clouds, then, there's a contribution from the surface, from ice, snow, and other areas that reflect the incoming radiation back to space. So out of the 340 total, 100 is lost or reflected back out to space, which means that only 161 is absorbed by the surface.

[00:36:44:23] And then, you can see-- so it's coming down. And then, you can also see that at the surface, it's radiating 398 watts per meter squared, back up into the atmosphere. And then, what's happening is that it's absorbed by the atmosphere. It radiates it back downwards, and then balance. These are the values of both of them. And then, you have this portion here, which is the atmospheric window, where some of the IR radiation is allowed to escape back up. And then, the total outgoing thermal radiation is around 240. And if you add the 240 up with the 100, you get back up, so that they're roughly equal. So the incoming is equal to the outgoing.

[00:37:32:00] And then, you also have these processes that-- radiation isn't the only way you can have energy coming up from the surface. You can also have evaporation and, also, sensible heat, which is a conduction from just having these layers in close contact with each other. Energy is transferred. So this is just the overview of the Earth's energy budget. And these are all measurements that were made, so either from satellites or ground-based instruments that can measure the fluxes of total radiation.

[00:38:09:09] And so we can see here that if we add greenhouse gases to the atmosphere, what happens is that it's going to be radiating more energy back down to the surface. And then, in turn, the surface has to adjust by warming up to [INAUDIBLE] background.

[00:38:29:18] All right, we'll move on to the next section, which has to do with climate variability. So there are two types of natural climate variability. The first one is external forcing of the climate system. So this has to do with changes in the orbit of the Earth, which affects the amount of solar radiation impinging on the hemispheres during the summer. And like I said before, the Milankovitch cycles can range from 10,000 to 100,000 years.

[00:39:09:16] And what's also considered external, even though it's actually in the climate system-- it's inert but it's also these large volcanic eruptions. So eruptions from volcanoes will send sulfur dioxide up into the atmosphere that will condense onto particles. And you can think of these as little mirrors in the atmosphere that will reflect radiation back. And I'll touch on these two in a little bit.

[00:39:40:04] Another one is the solar variability. So the output from the sun-- that 1370 value that I was talking about before isn't constant. It has approximately an 11-year cycle. And then, the second source of climate variability is internal. So all of these-- you can think of these as external. So if these didn't happen, the climate system-- it wouldn't change.

[00:40:10:21] But because of internal climate variability, even without these, you get these non-linear interactions in a complex system. And I'll talk more about The ENSO, or El Nino Southern Oscillation. Sorry, no, this is El Nino. Then, there's also another mode called the NAO, which is the North Atlantic Oscillation. So these are just intrinsic, internal variability in the climate system.

[00:40:42:12] So, yes, as I was talking about before-- how volcanic eruptions can affect the climate-- this is a photo taken from Mount Pinatubo in 1991. You can just see all of the emissions. And what happens is that when you have these large volcanic events, it inputs sulfur dioxide up, up right at the top of the troposphere and into the lower stratosphere. And then, these condense onto particles and form sulfuric acid. And the, these sulphuric acid particles are like-- like I said before-- tiny mirrors. And then, they reflect the incoming solar radiation back out to space.

[00:41:28:02] And this is showing an example of how far you can see into the atmosphere. So it's called optical depth from a satellite, before and after the eruption of Pinatubo. So you can see, just a month after, you have the sulfuric acid particles all over focused on the tropics. But then, as you get later and further out, it covers the entire globe. And this causes cooling of Earth's surface.

[00:42:07:27] So what this is showing is-- the solid black line is observed Earth's temperatures. And then also with it is this red line, which is a model, and then lots of simulations from this model. I'll be talking more about global climate models in tomorrow's lecture.

[00:42:28:06] But what you can see here is these lines are volcanic eruptions. So Mount Agung from Indonesia in the 1960s. You have El Chichon. And then you also have Pinatubo. If you look here, you can see a drop in global surface temperatures following these eruptions, because these effect the Earth's radiation budget, where less radiation is coming down to the surface because of these eruptions.

[00:42:59:22] And then, the other climate force or mode of external variability is from the sun. So it was discovered that you have these sunspots that you can see on the sun-- and that the number of sunspots you see are related to-- so this is showing the sunspot number from 1600 to present day. And you can see the sunspots-- there's some sort of cycle on it-- these ups and downs going up. And these are 11-year cycles.

[00:43:42:19] So this is showing the sunspot numbers along with the incident solar radiation. You can see it varies, because it's going up and down. And this is from the 1970s to present day. So you can see the incoming solar radiation is related to the sunspot number. So this is also another mode of external variability, where either that or changing the energy budget by how much radiation is coming into the planet.

[00:44:16:14] And although it's hard to see-- so this is again showing what I had before, but then, this is the global surface temperature. And if you take an 11-year moving average, you can see that you have these ups and downs in Earth's temperature that are related to incoming solar radiation.

[00:44:39:18] And then lastly, on intrinsic climate variability or internal climate variability, you have what's called the El Nino or La Nina oscillations. So this is showing the average sea surface temperature anomalies for December 1982 to February 1983. And you have a large warming of the Equatorial Eastern Pacific. And then, you compare that to this down here, where you have large cooling of Equatorial Pacific.

[00:45:16:26] And these warming and cooling phases can affect the weather in North America but, also, all around the world. And because you have easterly-- from the East-- winds blowing from East to West-- and then, during the El Nino phase, you weaken these winds. And this is related with enhanced precipitation across the Equatorial Pacific. And in the US, because that's where we are, you have more rain in the South and cooler winter temperatures in the Southeast.

[00:45:58:18] And then, in the La Nina phase, which is the cool phase, you have stronger winds along the equator. And that's related to a reduced precipitation across this region. But in the US, you have less rain in the South, and winter temperatures are warmer in the Southeast. And this is because the atmosphere-- you can think of it like if you hit a bell-- so if you ring a bell in one part of the atmosphere very strongly, you're going to hear it elsewhere in the atmosphere.

[00:46:30:15] So that's basically what this is. Because these are such large changes in ocean temperatures that it effects the atmosphere and circulation in North America and, also, all the way as far as Europe.

[00:46:52:22] Wait, so are those the same thing as the high end of the cycle? Is one of those the low end of the cycles?

[00:46:58:19] Yes, so this is the El Nino phase, where you have warm temperatures over the sea surface. And this is La Nina phase, where you have cooler temperatures over the surface. And here is showing the related, all over the world, changes. So with the El Nino phase, which we just had in 2016, you have much warmer-- it shows you where it's warmer in the world.

[00:47:26:18] And then, La Nina phase-- it shows you where it's cool and, also, precipitation-- where it's more wet and more dry. And this is just to show you that you can have these fluctuations in temperatures, precipitation, and other conditions that are intrinsic to Earth and not related to the external changes and external forces. So these will happen without any changes in external radiation coming into Earth.

[00:48:00:00] And what's the primary drive? What causes those--

[00:48:03:09] That's a very good question, and it has to do with just these modes. So you have these changes and upwelling and downwelling in the ocean. And so if you get enough stress on the ocean surface over time, it's going to accumulate and cause changes in circulation, which will cause both of these events. yes, up there?

[00:48:32:19] Yeah, also, it looks like when the middle part is cooling or heating, the rest of the ocean is still opposite.

[00:48:39:10] Yes, that's true.

[00:48:40:21] So doesn't that balance it out somehow?

[00:48:43:16] Well, you have to think in terms of what's causing-- they are called teleconnections, or teleconnection patterns, and circulation. So you have to think about how it changes the mean state of the climate. And in here, the mean state climate are easterly winds. And so if you change the winds, that will change precipitation. And so these guys are also changing with it, but it's predominantly what's going on in the tropics that's affecting it, because even though this looks kind of large, it's right in here where you get the most warming and cooling of the sea surface temperatures.

[00:49:31:00] How long does it [INAUDIBLE]

[00:49:35:18] Unlike the sun that I was showing you before, where it's relatively 11 years, this changes from two to four years. So you can have a year or two where it's in the El Nino phase. Then, you can have another year or two where it's in the neutral, so it's normal. But then, you can have the year, where it's La Nina, and then, the next year can be El Nino.

[00:49:55:29] So it's not predictable like the sun, where you have the 11-year cycle. It's still has two to four year cycle in it. So that's why in 2016, where we had the record-breaking temperatures all over the world, that was the El Nino phase and where it's predominantly associated with warmer temperatures. So that was helping push it way up above the records.

[00:50:24:22] And so the last part I'll quickly go through is changes in greenhouse gas concentrations over time. So what this is showing-- the green is carbon dioxide. The orange is methane. And then, the red is showing nitrous oxide. And it's showing changes in these three gases' concentrations. And you can see some lines. These lines are when we had direct measurements, so we can directly measure these species. But these circles are what we have to rely on beforehand from ice core.

[00:51:02:17] So these gases were present in the 1850s. And then, you have these bubbles and ice core data. And if you drill further down into the ice, you can go further back in history, and you can get observations from back here, that are estimated from the ice core data. So you can see, will all three of these greenhouse gases, they've been increasing with time-- with the CO2 from fossil fuels, methane. You also get it from combustion but, also, from agriculture-- you can get methane. And then, N2Os, also from the agriculture, [INAUDIBLE] fertilizer where it's increased as well.

[00:51:56:18] So this is just showing that all three of these guys have increased with time. And if we look at the focus on CO2-- so this is showing gigatons of CO2 per years. So the emissions of CO2, using the same thing that I had, over the same time period I had before-- and you can see, it's showing the changes due to forestry. So that's deforestation. If you burn trees and cut down trees, you'll increase the amount of CO2 in the atmosphere.

[00:52:30:25] But since the 1900s, the large fraction of this is from fossil fuel combustion. Cement use, which also releases CO2 and glaring or fracking from natural gas. So you can see, this is showing the global anthropogenic CO2 emissions. And compared to 1750, and just showing you the cumulative CO2 emissions. So if you integrate these values over time up to 1970 and to 2011, you can see how much of it is from fossil fuel emissions and from land use changes.

[00:53:16:22] And one question you might ask is how do we know that the increase in CO2 in the atmosphere is related to fossil fuel emissions. So you can use this method of isotopes. So if you look at the atom of carbon, there's different isotopes of it, where you have different neutrons. So you can have different neutrons and in the atom. And so you have C12, which has 12 neutrons-- C13, which has 13 neutrons-- And C14, which has 14 neutrons. And carbon is present in all living things. And life has a preference for lighter C12 carbon. And this is because when plants breathe CO2 and photosynthesis, they prefer C12. So plants absorb C12.

[00:54:11:13] And when they die, they sediment. And then, eventually, and same with all other living organisms, this is what eventually goes to the fossil fuels that we're pumping out of the ground or these past living things. If you look at the C13 to C12 ratio, what we would expect is that it should be decreasing as we're pumping more C12 into the atmosphere if we're burning fossil fuels.

[00:54:51:25] If you look at observations-- so this again showing global emissions of carbon. And then, right here, it's flipped, so that going up is decreasing. So if you look at the C13 to C12 ratio, it's decreasing with time. So it shows that we're changing the ratio in the atmosphere. And this is from the emissions of fossil fuels, or the combustion of fossil fuels.

[00:55:23:14] So this is, again, showing the carbon cycle and how much perturbation or changes in the global carbon cycle are caused by human activities. And you can see, we're putting out 34.1 petagrams, which is one times 10 to the power of 12 kilograms of carbon into the atmosphere. But the atmosphere has only taken 16.4 of this number. And this is because the ocean takes up the carbon dioxide from forming bicarbonate.

[00:55:58:28] And the ocean is becoming more acidic by taking up the CO2. But there is a level, of how much you can take up. But it's continuing to take it up. And you can also see that the land is taking-- the plants are respiring some of the extra CO2 we're putting up into the atmosphere. Around 16.4 of it is left in the atmosphere.

[00:56:26:04] And this is showing that figure but over time. So if you look at focus on the top first, this is what I had before with gigatons of carbon or CO2 going up the that we're emitting. And if you look at how much of it is going where-- so the dark blue is ocean, so it's showing how much the oceans are taking up. And then, the light blue is the atmosphere, so you can look at the atmosphere burden of CO2. And then, also, the land sink-- so much CO2 the land is taking up. And you're going to see, over time, a larger portion of it is going into the atmosphere. And the ocean is taking up some of it as well.

[00:57:03:27] And the importance of this-- so this is showing for CO2-- if we keep ramping up CO2-- so from 1800s to-- so this done with model simulations, where CO2 is increased up to-- so right now, we're just at this 400 parts per million level. And then, if you keep increasing it to 550, 650, 750, 850-- and then, once you reach this peak, you cut off all CO2 emissions. So you are not emitting anymore into the atmosphere. What happens? How long does it take for the CO2 to go back down?

[00:57:46:14] Even here, if you cut it off at this level, it decreases. But then, it still stays high, and much higher than it was in the pre-industrial time. And you can see that the more CO2 we keep putting into the atmosphere, the longer it's going to take to get rid of all of this.

[00:58:05:03] And this is because there are different processes that can take out the carbon from the atmosphere. Like I was saying, you can have the photosynthesis. And this is a short time scale, from one to 100 years, but the oceans can take it up. But the sedimentation of the carbon that first created fossil fuels takes thousands and thousands of years. So this is calcium carbonate sedimentation and, also, silicate weathering. But this is just to show that all the CO2 we're putting into the atmosphere is going to take a very long time before it can even get back down to this level we had before.

[00:58:47:13] Could you go to the previous slide?

[00:58:49:25] Yeah.

[00:58:50:11] Why is there a big variability between the three [INAUDIBLE] over time?

[00:58:55:10] So what I didn't mention in this is that the ocean part in here is-- we measured the land in the atmosphere. And the ocean is inferred from the rest of it, from the variability. But the variability has to do with-- like I was talking about before-- this has to do with the land use changes, where you have these large deforestation events, coupled with the atmosphere, where you can have-- like I was saying-- the El Nino and other variations that affect plant growth. But, yes, that's what the interannual variation is due to.

[00:59:36:22] And, yes, so this is just to show that it will take a very long time to remove all this carbon dioxide, which is the important greenhouse gas from the atmosphere. And every gigaton more carbon we're putting up, it's going to take a long time to remove it.

[00:59:58:25] So in summary, these trace gases of water vapor, carbon dioxide, methane, and nitrous oxide-- even though they are a trace and make up a very small portion of the atmosphere, they're opaque to outgoing to infrared radiation, and they're responsible for the greenhouse effect. And then, because of the Greenhouse Effect, the surface must warm to be in balance.

[01:00:22:22] So if we put up more of these concentrations are changing-- are increasing, decreasing-- the surface of the Earth has to respond by cooling or warming-- and that the variability of the climate system can span anywhere from one to 100,000 years and increase in CO2 since the pre-industrial levels-- is from fossil fuel emissions and the removal of CO2 from the atmosphere is a very slow process.

[01:00:53:02] And that's it for today. And I'm leaving up here some resources and good books, if you want to look into this more, because I don't have time to touch on everything in detail today. But here are a few books you can look at. And you can also check out the Global Change website and look at our educational resources we have online if you want to learn more. And tomorrow, I'm going to be talking about how both human and natural caused forcings and how the climate system responds to it and how we can identify the temperature changes that we're seeing are related to human activity.

[01:01:34:05] And I'll take any questions for a short amount of time, because I went over, but I'll take a few questions. Getting back to that El Nino and its reverse, so what is the fundamental cause of that?

[01:01:47:03] So that has to do with changes in ocean circulation, where you have-- because it was first measured-- because you had one station here and another station here. And they measured changes in temperature. And there is a connection between both of them. And that has to do with the upwelling and downwelling changes in the ocean circulation.

[01:02:16:03] And this is because the changes in those circulations are from accumulated stresses of wind. I was talking about the easterly winds-- if you keep stressing the ocean over time, it'll all accumulate until it'll just flip, and it reaches these different equilibria-- or these were stable. But to get from one stable equilibria to another, it's like a sudden shift. So you just get a shift into these perturbations.

[01:02:43:12]

[01:02:44:14] If you look at some of the resources I put up, it'll give a more in depth view of [INAUDIBLE] climate variability in here as well. Or see me after, and I can point you towards some links on it. It's not an easy process. You can do the whole course on just trying to understand that modeling. Yes.

[01:03:10:06] You remember that you can trace the increase in the CO2 in the atmosphere and anthropogenic sources, because the C13-C12 ratios going down. What is it going down in? Just in the atmosphere itself-- like probes in the atmosphere?

[01:03:26:15] Right, you can take a sample of air. So this is only since the 1980s, when you had technology to look at the isotope ratios. So if you take a sample of air, and you can measure how many have C12, how many are C13, how many are C14, you can see how this ratio of C13 to C12 is changing over time. And because, like I said, life has a preference for lighter carbon because of photosynthesis, and the planets preferentially absorb C12-- and that's what all the carbon and the fossil fuels is.

[01:04:12:20] So for burning fossil fuels, we're emitting more C12 into the atmosphere. And so if you keep increasing the amount of fossil fuel combustion, we're going to get a higher ratio of C13 to C12.

[01:04:27:25] What you're not saying is fossil fuels have lower C13 ratio, is that right? You don't actually say what the ratio is for fossil fuels.

[01:04:42:16] So, yes-- if you look at the C13 to C12 ratio, it has higher C12. So this ratio, if you're measuring C13 to C12 in the atmosphere, it should be falling as we're increasing fossil fuel combustion, because we're putting in more C12 into the atmosphere, because that's more in fossil fuels-- the C12-- because like I said, the fossil fuels got there from an existing life beforehand.

[01:05:21:19] And you say there is a preference for C12. I mean, is that like mostly C12 or is it like 2% more?

[01:05:26:27] I'm not an expert in plant biology or this area, but there are people who are experts in this area. And photosynthesis, preferentially, takes up CO2 that is lighter-- in the lighter isotope of CO2.

[01:05:47:18] But just in a gross way, is this talking about a 50% increase in that, or is it just a few percent?

[01:05:55:17] I do not know this. I'm not going to say that, but I can look it up. If you want to talk to me later, you can look it up.

[01:06:07:24] Does the Earth magnetic force have anything--

[01:06:10:28] Oh, so changes in the magnetic field? So that slide I had way before of the atmosphere-- so I didn't point it out, but there's something called the magneto sphere, which is above the thermosphere. So it's the magnetosphere, and that's where the magnetic-- like, where the flips,

[01:06:43:18] magnetic dipoles, and magneticism is. Yes, so that's taking a lot of these ions that are coming from the sun. And I'm not sure how much changes in the magnetic dipole of the Earth affects the climate. I'm not sure how much it, but I don't think it's negligible compared to what we're talking about here within the troposphere and stratosphere.

[01:07:21:10] Yes, come tomorrow, and we'll talk about greenhouse gas, or how we can-- forcing mechanisms and feedbacks in the climate system.

[01:07:33:25] [APPLAUSE]