[MUSIC] The carbon cycle on Earth is amazing. You never would have believed it if you hadn't seen it for yourself. There's this uncanny stability. The climate of the Earth is very stable through geologic time, even though the Sun has been getting warmer through time. And there are other aspects, too, of the carbon cycle. Like the oxygen concentration of the atmosphere, which has this incredible stability. So we're gonna think of the carbon cycle as a series of these three reservoirs of carbon, each of which exchange CO2 with the atmosphere. So the atmosphere is kind of like the Grand Central Station of the carbon cycle. It's the smallest of these reservoirs, but it's the way that carbon goes from one reservoir to another. So in this lecture, we're going to focus on the solid Earth as it affects the atmosphere. Because this sort of sets the stage for why the three terrestrial planets, Earth, Venus, and Mars, are the way they are. [SOUND] So exchange of carbon between the solid earth and the atmosphere is governed by a chemical reaction called the Urey Reaction. So on this side we have a very simple chemical formula for an igneous rock. So this is a kind of rock that froze from lava or from magma, which is what they call lava if it's underground, a crystalline rock. Calcium silica O3, and here's carbon dioxide. And on the other side of the reaction we have calcium carbonate, which is limestone, and SiO2. These are the two main components of sedimentary rocks. So like all chemical reactions, there is an equilibrium, a state of lowest energy. And this particular reaction, because it has a gas, one of the chemicals is a gas, it has a very strong temperature dependence. So if you take a bunch of these elements and put them together at very high temperature, it favors this side of the chemical reaction. It wants the carbon to be free as a gas, if it's hot. The same way that there's more water vapor if it's warm. Because the free form wants to find itself in the higher temperatures. Whereas at colder temperatures, this side of the chemical reaction is favored. So this sets up a kind of a dynamic duality between the deep earth where it's very high temperature. Which basically wants all of the carbon to be in the atmosphere, like Venus. All of Venus's carbon is in the atmosphere. There's 70 atmospheres of CO2 on Venus, it's almost more like an ocean than an atmosphere, Venus. The low temperatures at the Earth's surface would really want carbon to be in the form of calcium carbonate. So the equilibrium according to the low temperature at the surface would only have about ten parts per million of CO2 in the atmosphere. Whereas the atmosphere in the natural world, before the fossil fuel era, was about 280 parts per million. And today it's just crossed the boundary of 400 parts per million because of human activity. So, the real CO2 in the atmosphere is kind of in between what the deep Earth wants and what the shallow Earth wants. So this is a diagram that shows the basic playing field for how the deep Earth and the surface Earth try to relate to each other in deciding what the CO2 concentration of the atmosphere should be. You have this sort of a cycle of carbon that goes out from the solid Earth and then back to the solid Earth. Coming out of the solid Earth, you have CO2 degassing from volcanoes and also the hot springs at the bottom of the ocean have a lot of dissolved carbon in them. And then, the CO2 reacts with igneous rocks. Here, I've written the Urey Reaction in a more simple way, I've gotten rid of the SiO2 on both sides, because that doesn't really, I mean it makes CaO look more like a real rock, but it doesn't really affect the carbon cycle other than that. So just stripping it down to its essentials, we have the igneous rock here, and the CO2 making the calcium carbonate. So those two reacting together to make carbonate, that's a process called weathering. And then the calcium carbonate is deposited on the bottom of the ocean and, eventually, as the ocean crust is subducted down into the Earth, the calcium carbonate gets carried down into the hot interior of the earth. Where the high temperature end says, I don't want that carbon to be as calcium carbonate anymore. I wanna cook that stuff off and remake some igneous rock and release the CO2 back to the atmosphere. So this sets up a whole cycle of carbon coming out of the earth and then going back into the earth. [SOUND] Now, the rate of weathering depends on rainfall, runoff. Because in order to dissolve a rock, you have to have water to dissolve it in. To a first approximation, every gallon of water that runs down off and into the ocean is carrying about as much dissolved rock as it can. And so, the way to make this chemical reaction happen more quickly is to have more gallons of water washing over the land. And, the rate of runoff from the land is a very strong function of the temperature. So, the biggest river in the world is the Amazon, it's in Brazil in the tropics. Much more, many more inches of runoff per year in the tropics than in the high latitudes. And the Earth's temperature ultimately depends on carbon dioxide. So the more carbon dioxide you have, the warmer it is, the faster you can do this weathering reaction. So this ends up being another beautiful negative feedback system, just like our kitchen sink. So we can recycle this analogy, just again and again for all kinds of different things. So in this particular application of the analogy, the water coming out of the faucet into the sink is the volcanic degassing rate. That's sort of what's driving the system. And then the rate of weathering depends on the water level in the sink. The rate of water going down the drain depends on the water level in the sink. The rate of weathering depends on the amount of CO2 in the atmosphere. So what happens then [SOUND] is that if you have some volcanic degassing rate. Some certain number of molecules of CO2 every year coming out of the Earth, and if the rate of weathering is higher than that, that means you're taking carbon dioxide out of the atmosphere faster than you're putting it in. Just like if it was going down the drain faster than coming out of the faucet. And so the CO2 would tend to drop until it approached and ultimately equaled, in an average way, the rate of degassing from volcanoes. Or if you started with no CO2 you'd have more coming out of the Earth than is going back into the Earth and so the CO2 would build up until it approached this equilibrium from the other side. So this sounds really cool. It sounds as though, this thermostat mechanism, this negative feedback, will clean up our global warming mess by absorbing our CO2. The trouble with that idea is this time scale, how long it takes to happen, is on the order of a million years. So it's not something that we can wait around for. [MUSIC]
