Welcome to the first lecture of our Earth's future. Today we're gonna be talking about the basics of climate, how climate works. We are also gonna look through a specific example, which is ice cores. But before we do any of that I want to make sure we're all on the same page regarding science. How do you know when you're doing science as opposed to something else? There are a couple of different ways to look at it. Most of you probably learned the scientific method in school. That was the idea that you observe something and you generate a hypothesis and you figure out how you're going to test it. You do the test, you look at the results and you come to a conclusion. Science pretty much never works that way. Science is really messy. Science involves going out and gathering data. And you're usually working with limited information. So how are we gonna define science? Well, the way I like to do it is to say science is that which is falsifiable. A statement is scientific if it can be proven to be false. Note that I didn't say proven true. You can't really prove things to be true. It's sort of like in a court of law. People are either judged guilty or not guilty. No one's ever judged innocent. So how do we know if a particular hypothesis is scientific or not? I'm gonna work through some examples that might seem kind of silly, but they're a good way to start thinking about this. So let's take the hypothesis, I am taller than my sister. There are two questions we wanna ask about it. One, is it science? Second, is it true? Well, is it science? I think so. You can measure people's height. In this case we have a couple of methods we could use. You could have us stand back to back, or someone could measure both of us and then we would know who was taller. You could also check our driver's licenses. But if you did that, you would find out that I was taller. The other methods would show that she is taller. That's because I lied on my driver's license. So just because information's out there doesn't mean it's accurate. Now let's look at a related example. I'm stronger than my sister. It sounds like the same kind of statement, but it really isn't. And that's because it's a poorly posed question. What does stronger mean? Maybe I can do more pushups, but she can run further. I'm fairly sure that she could take me in a fight. So we have the problem of the poorly posed question. Often in science, the hard part is getting to a really good question. Once your question is well defined, finding the answer can be a lot easier. Now let's think about a related question. I am taller than my brother. It sounds like the first example, doesn't it? But it isn't, because I don't have a brother. So that's one of those statements that sounds good, but when you think about it, it doesn't actually make any sense. Another example would be the statement, the number 5 is happy. It has all the components of a reasonable sentence, but it doesn't mean anything. So you have to look at more than the format of the question or the statement. You have to really think about the results. To go back to the first example, I'm taller than my sister, you might know that it's a scientific statement but have a lot of trouble determining it's truth value. I haven't told you my sister's name or where she lives. So it may be that you don't have enough information to answer the question. That's okay. That makes it hard to determine the truth value, whether it's true or false. But it doesn't impact whether or not it's science. I wanna be clear that a lot of really important information and wonderful questions aren't science. Something can be true or false without being science. For example, my niece Sophie is the cutest two-year-old in the world. I know the truth value of that statement. That's definitely a true statement. But it's not science because there is no objective scale of cuteness, and even if there were, I haven't actually checked. So we'll know we're doing science when we can prove a statement to be false. With that in mind, I want to introduce you to the science behind climate. So what is climate anyway? It's easy to confuse climate and weather. They both deal with things like temperature and rainfall, wind. But the big difference is time scale. Weather is what happens every day, whether it's sunny out or rainy. Climate is the long-term average of weather. One way to think about it is that you dress for the weather, but you build your home for the climate. We're going to consider climate on multiple spatial scales and also multiple scales within time. But why do we have climate in the first place? Why isn't it just the same temperature everywhere? We're all the same distance from the Sun. So the first thing to look at when we're trying to understand how the climate system works is the Sun. We're far enough away from the Sun that it's rays of lights basically come in parallel to each other. If you're near the equator, the light hits you head on. The light is making a 90-degree angle with the surface of the Earth. That means you're getting a lot of sunlight in a fairly small area. But if you look further north or further south, you'll see that the Sun is making a different angle with the surface of the Earth. That means that same amount of sunlight is being spread over a larger area. When you go all the way up to the poles, that sunlight is being spread over a really large area. That's the fundamental reason we have climate to begin with. There's an unfair distribution of sunlight in the world. The tropics have extra and the poles don't have enough. It is the goal of climate to recirculate that. It's the goal of climate to even out this uneven heating. So the uneven heating of the Earth by the Sun is the first thing that we have to consider. The Earth is gonna work to correct that imbalance. Both the atmosphere and the ocean move heat from the equator to both poles. It's not quite accurate to talk about moving heat. Heat is really a property of something, but it's enough to give you an idea of what's going on. Warm water moves north and south from the equator. Warm air moves north and south from the equator. So you would think we could just have a simple circulation cell. Warm air rises at the equator, moves north and south, sinks and cool air returns. But there's a problem. The Earth is spinning. So if you imagine trying to roll a ball back and forth on a carousel, you'll realize that trying to send something in a straight line on a spinning Earth is never going to work. Because of the rotation of the Earth, everything gets deflected. It moves to the right in the northern hemisphere, and to the left in the southern hemisphere. This is called the Coriolis effect, and it makes this business of redistributing heat much more difficult. The Sun is a 150 million kilometers away from us, but we pretty much understand how radiation works. As you get further and further from the source of radiation, the amount of energy you receive drops off. Based on that, we can calculate an average temperature for the Earth. When I teach climate to undergraduates I make them do this. It's always really frustrating because when they do it they come up with an answer that seems way too cold. Based on the distance between the Sun and the Earth, it should be an average of -18 degrees Celsius here on Earth. That's -1 Fahrenheit. You know that can't be right. If that were true, the water on Earth would be frozen, and if there were life it would look a whole lot different than it does now. It's in fact much warmer than that. Why? Why do we get this extra boost of heat so that our average temperatures is a balmy 59 degrees Fahrenheit where you can have liquid water? Well, that's the greenhouse effect. The name greenhouse effect can be a little bit misleading because it's not exactly how greenhouses work. To begin with, you've gotta understand that everything radiates. Me, you, the chair I'm sitting on, everything. The Sun radiates at a variety of frequencies. We can think about solar energy, heat and light, as just being this range of frequencies. Solar energy peaks at the range that we call the visible spectrum. That works out really well. That's why we can see the Sun. As the Earth becomes warmed up, it radiates too. The Earth radiates its peak frequency in infrared, what we call heat. So you have solar radiation coming into the Earth and warming it up. And then you would think that the radiation from the Earth would just escape back into space. But that's not what happens. Our planet is covered in an atmosphere. That atmosphere is mostly made out of nitrogen, which is really isn't our concern in this class. But the atmosphere also contains water. Water, you might be surprised to learn, is the most important greenhouse gas. We're going to be talking a lot about a different greenhouse gas, carbon dioxide. Carbon dioxide isn't important because there is so much of it. It's important because it's such a strong greenhouse gas. So what is a greenhouse gas? Well, when the Earth's radiation reaches a certain level in the atmosphere, it makes the molecules of those gases vibrate. When that happens the energy is absorbed by those molecules and then re-radiated in all directions. So some of the radiation escapes to space, but some of it goes right back to the Earth. And then the process repeats itself. The Earth gets warmer, radiates out, the greenhouse traps some of that heat, and send some of it back to Earth. That's why the Earth is warmer than you would think just looking at our distance from the Sun. With just that basic information about the climate system, understanding the difference in heating between the tropics and the poles, understanding the rotation of the Earth, and understanding the greenhouse effect, you can explain a lot of the variability we see in climate. But that's for today. In order to really understand climate, we're going to have to consider how it changes in time. It's actually easier than you might think, to go back and see what the climate was like in the past. One of the ways that we do that is through ice cores. An ice core is exactly what it sounds like. You take a hollow drill bit, send it down into the ice, and pull up a narrow core of ice. [NOISE] The ice has layers like tree rings, and using isotopic dating we can figure out how old they are. But that's not the really interesting part. The great part is that air bubbles get trapped inside the ice, so we can actually look at prehistoric air. But there's something even stranger that goes on within an ice core. The ice itself varies with time. This is one those observations that requires some pretty complex technology. If you think about all the water in the world, you can kind of think of every water molecule as being identical. But it turns out that there are some water molecules that are a little heavier then others. The reason isn't so important here. But there are oxygen atoms that have a little bit of extra weight to them. This is a very small effect. But with good enough equipment, you can tell. Is your water the usual type, with just a little bit of the heavier version? Or do you have unusually heavy water, or unusually light? You might wonder, well, why do we care how heavy the water is? When water molecules evaporate from the ocean, the lighter isotopes, the lighter molecules, evaporate first. They weigh less, they're easy for the atmosphere to pick up and they're easier for the atmosphere to transport. When it comes time for that moisture to come out of the atmosphere as snow, it's easier for the heavy stuff to fall first. Over time, what's going to happen is that the ocean, the liquid water, is going to get heavier and heavier, because more of the heavy water is left behind whereas more of the lightweight water is transported to glaciers. So when we see isotopically light ice, which just means it has a slightly higher percentage of the lightweight kind of water, we know that this was a time when there was a lot of ice on land. When we see heavier ice, ice that only has a little bit of that lighter weight stuff compared to what it usually has, we know that this was a time when there was less ice on Earth. So even though we're only looking at an ice core in a particular location, it can tell us something about the conditions in the whole world. This basic concept has been used to figure out past climate. One of my favorite ice cores is called GISP2. There's a really cool model of it here at the Museum of Natural History, and you can see the video that goes along with that exhibition. This was 3,000 meters of ice core that let us go back 100,000 years in time. And the results were not what anyone expected. We thought that all the changes in climate would be slow and gradual, because a lot of us imagine that climate might move in that way. But it turns out that's not the case at all. Climate can change abruptly. Climate can change on the order of tens or hundreds of years. Those results challenged a lot of the existing science about climate. In science, though, that's a good thing. When you come up with results that are really surprising, that means you're doing something worth doing. That was one of our first windows into the details of past climate. We had a big picture before, but we didn't have this year-by-year variability that we now have. What we can see from the ice cores is how sensitive the Earth's climate really is. Now you might think, this is just one ice core, it's in Greenland. I don't live in Greenland. But it turns out that if you compare the results from the Greenland ice core to results from ice cores taken around the world, they all match. Not perfectly, but really, really close. Still, why should you care about climate in the past? Well, if you imagine climate on some other planet, climate that never changed, you would say, well, that's a really robust system. That's a system that isn't going to change no matter what I do. But climate on Earth isn't like that. Climate on Earth can have a really big response to just a really small push or forcing. That's a topic that we'll get into more in a later lecture.
