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 its rays of light basically come in parallel to each other. If you're near the equator, the light hits you head on. It's 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 going to 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 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 negative 18 degrees Celsius here on Earth. That's negative 1 Fahrenheit. You know that can't be right. If that were true, the water on Earth would be frozen and if there were a 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 temperature 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 got to 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. That's not what happens. Our planet is covered in an atmosphere. That atmosphere is mostly made out of nitrogen, which 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's 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 sends 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.